CREEP DEFORMATION OF AMORPHOUS POLYMER …repository.um.edu.my/1126/1/Chua Ngeah Boon_MSC_Dissertation 200… · creep deformation of amorphous polymer (polyurethanes) chua ngeah

CREEP DEFORMATION OF AMORPHOUS

POLYMER (POLYURETHANES)

CHUA NGEAH BOON

DISSERTATION SUBMITTED IN FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE

OF MASTER OF SCIENCE

FACULTY OF SCIENCE UNIVERSITY OF MALAYA

KUALA LUMPUR

APRIL 2008

ABSTRACT Polyurethanes (PU) based on two different molecular weight, poly(tetramethylene

oxide) polyol, PTMO-1000 and PTMO-2000 and three different diisoscyanates, TDI

(toluene diisocyanate), IPDI (isophorone diisocyanate), HDI (hexamethylene

diisocyanate) with chain extender BDO (butanediol) have been synthesised by a

two-step polymerization. The effect of soft segment chain length and the effect of

hard segment structures on creep behaviour of four different type polyurethanes;

PU1-TDI, PU2-TDI, PU1-IPDI and PU1-HDI have been investigated. The PUs

obtained were characterized by Dynamic Mechanical Analysis (DMA), Differential

Scanning Calorimetry (DSC), X-Ray Diffraction (XRD) and Scanning Electron

Microscopy (SEM). The creep studies mainly involve variable load constant-

temperature creep measurement. Outstanding creep resistance was observed at

the long-term creep test at room temperature for PU1-TDI, PU2-TDI and PU1-HDI.

No failure occurred for all these three PUs until the end of the creep test. The

mechanical studied reveals that PU1-TDI has the highest tensile strength whereas

PU1-HDI showed the highest modulus properties. The increase of molecular

weight of polyol causes an observed decrease in tensile strength and elastic

modulus and leads to increase in strain at maximum. From the DMA and DSC

results, PU1-HDI show the lowest Tg value indicated that the degree of microphase

separation of PU1-HDI is the highest which will lead to the better mechanical

properties. PU1-HDI exhibits multiphase morphologies whereas the morphologies

of the PU1-TDI and PU2-TDI are relatively homogenous. The XRD results indicate

that crystallinity decreased from aliphatic to aromatic based diisocyanates of PUs.

I

ABSTRAK Polyurethanes (PU) disintesis dengan menggunakan kaedah pempolimeran dua

langkah berasaskan berat molekul poly(tetramethylene oxide) poliol yang berbeza,

PTMO-1000 dan PTMO-2000 dan tiga jenis diisoscyanates yang berbeza, TDI

(toulene diisocyanate), IPDI (isophorone diisocyanate), HDI (hexamethylene

diisocyanate) dengan rantai pemanjangan, BDO (butanediol). Kesan daripada

panjang rantaian segmen lembut dan struktur segmen keras terhadap sifat

rayapan empat jenis PU dikaji; PU1-TDI, PU2-TDI, PU1-IPDI dan PU1-HDI.

Sampel PU yang diperolehi dicirikan dengan menggunakan Analisis Mekanikal

Dinamik (DMA), Kalorimetri Pengimbasan Pembezaan (DSC), Penyerakan Sinar-X

(XRD) dan Mikroskopi Elektron Pengimbas (SEM). Kajian rayapan terutamanya

melibatkan pengukuran rayapan beban berubah dengan suhu malar. Kerintangan

rayapan yang baik dapat diperhatikan melalui kajian rayapan jangka masa panjang

pada suhu bilik bagi sampel PU1-TDI, PU2-TDI dan PU1-HDI. Tiada kegagalan

diperhatikan bagi ketiga-tiga sampel PU sehingga tamat kajian rayapan. Kajian

mekanikal menunjukkan bahawa sampel PU1-TDI mempunyai kekuatan tegangan

tertinggi manakala sampel PU1-HDI menunjukkan sifat modulus tertinggi.

Penambahan berat molekul poliol menyebabkan penurunan kekuatan tegangan

dan modulus kenyal dan seterusnya menyebabkan peningkatan maksimum pada

terikan. Daripada keputusan-keputusan DMA dan DSC, sampel PU1-HDI

menunjukkan nilai suhu kekaca yang terendah menandakan bahawa darjah

pemisahan fasa mikro bagi sampel PU1-HDI adalah tertinggi dan memberikan sifat

mekanikal yang lebih baik. Sampel PU1-HDI mempamerkan morfologi pelbagai

II

fasa yang mana morfologi bagi sampel PU1-TDI dan PU2-TDI adalah

homogenous. Keputusan XRD menunjukkan sifat kehabluran PU berasaskan

diisocyanate alifatik berkurang kepada aromatik.

III

ACKNOWDLEGEMENTS I would like to express sincere gratitude to my supervisor, Dr. Zurina Osman for her

incessant motivation, support and valuable guidance.

Special thanks are given to my ex-supervisor, Dr. Abu Bakar Ahmad for his

valuable suggestion, ideas, great patience and encouragement to make this work

started to roll.

I would also like to thank Miss Thamil Selvi Velayutham for her valuable

suggestions and helps. Gratitude is also extended to the lab assistance, Mr. Salleh

Zakaria for his helps and advices.

The generous financial support, which was received from Sponsored Research

Unit, Institute of Research Management and Consultancy, University of Malaya,

had made this study possible.

Last but not least, I would like to thank my parents, siblings, Kwong Chen and

friends, who gave moral supports and cares. It was so wonderful to have those

people in my life in accomplishing my master course.

IV

CONTENTS

Page

ABSTRACT............................................................................................................................I

ABSTRAK............................................................................................................................ II

ACKNOWDLEGEMENTS ...............................................................................................IV

CONTENTS ......................................................................................................................... V

List of Figures ................................................................................................................... VII

List of Tables ........................................................................................................................ X

List of Abbreviations ..........................................................................................................XI

CHAPTER 1: INTRODUCTION ....................................................................................... 1

1.1 Overview....................................................................................................................... 1

1.2 Scope and Objective of the Study................................................................................. 2

1.3 Dissertation Layout....................................................................................................... 3

CHAPTER 2: LITERATURE REVIEW ........................................................................... 5

2.1 Description of Polyurethane ......................................................................................... 5

2.1.1 History and Development ...................................................................................... 5

2.1.2 Properties and Applications ................................................................................... 6

2.2 Mechanical Properties of Polymeric Materials........................................................... 10

2.2.1 Stress-Strain Behaviour ....................................................................................... 11

2.2.2 Creep Properties................................................................................................... 17

2.2.3 Interpretation and Application of Creep Data...................................................... 24

2.3 Polymer Characterization ........................................................................................... 29

2.3.1 Dynamic Mechanical Analysis (DMA) ............................................................... 29

2.3.2 Differential Scanning Calorimetry (DSC) ........................................................... 32

2.3.3 X-Ray Diffraction (XRD).................................................................................... 35

2.3.4 Scanning Electron Microscopy (SEM) ................................................................ 40

CHAPTER 3: EXPERIMENTAL METHODS ............................................................... 43

3.1 Materials Description.................................................................................................. 43

3.2 Sample Preparation..................................................................................................... 43

V

3.3 Conventional Static Creep Tests................................................................................. 47

3.3.1 Testing Apparatus ................................................................................................ 47

3.3.2 Specimen Preparation .......................................................................................... 48

3.3.3 Static Creep Test Procedure................................................................................. 50

3.4 Tensile Tests ............................................................................................................... 51

3.4.1 Testing Apparatus and Test Procedure ................................................................ 51


3.5 Dynamic Mechanical Analysis (DMA) ...................................................................... 54

3.5.1 Testing Apparatus ................................................................................................ 54


3.6 Other Characterization Methods............................................................................. 56

3.6.1 Differential Scanning Calorimetry (DSC) ........................................................... 56

3.6.2 X-ray Diffraction (XRD) ..................................................................................... 58

3.6.3 Scanning Electron Microscopy (SEM) ................................................................ 60

CHAPTER 4: RESULTS AND DISCUSSIONS.............................................................. 62

4.1 Static Creep Test Results ............................................................................................ 62

4.2 Tensile Test Results.................................................................................................... 74

4.3 Dynamic Mechanical Analysis (DMA) Test Results ................................................. 80

4.4 DSC Results................................................................................................................ 88

4.5 XRD Results ............................................................................................................... 93

4.6 SEM Results ............................................................................................................... 96

CHAPTER 5: CONCLUSION AND RECOMMENDATIONS................................... 100

REFERENCES ................................................................................................................. 104

VI

List of Figures Figure Page

...............9 Figure 2.1: Applications of polyurethane with different form density and stiffness.

.......................................................................11 Figure 2.2: A typical Stress-Strain Diagram.

Figure 2.3: Deformation in polymer material. (a) Stretching of polymer molecules. (b)

Straightening out of coiled molecular chain. (c) Intermolecular slippage...........................14 Figure 2.4: Classification of polymeric materials on the basic stress-strain diagram. (a)

Hard and brittle (b) Weak and brittle (c) Hard and strong (d) Hard and tough (e) Soft and

weak (f) Soft and tough......................................................................................................16 ........................................................18 Figure 2.5: Tensile creep curve with various stresses

...........................................19 Figure 2.6: Characteristic Creep Curve with linear time scale.

..............................................23 Figure 2.7: Creep of a viscoelastic under a constant stress.

.....................27 Figure 2.9: Creep modulus versus time which derived from isometric curve.

.....................................28 Figure 2.10: Construction of isochronous curve from creep curve.

................................................................................................30 Figure 2.11: DMA structure.

.......................................31 Figure 2.12: An ideal DMA scan showing the types of transition.

.....................................................33 Figure 2.13: A schematic diagram of a DSC apparatus.

Figure 2.14: A schematic DSC curve demonstrating the appearance of several common

features seen for polymers. ................................................................................................35 ...........................................38 Figure 2.15: Geometry of the diffraction of X-rays in a crystal.

.....................................................41 Figure 2.16: A schematic diagram of a SEM apparatus.

........42 Figure 2.17: Secondary signals that are generated during electron beam irradiation.

.............................................................................47 Figure 3.1 Creep testing system set up.

...................................................................49 Figure 3.3 Dimension of ASTM D638 Type IV.

..51 Figure 3.4 Diagram showing gripping surface and effective gauge length for creep test.

..............................................................53 Figure 3.5 Instron 4443 Tensile Testing Machine.

............................................................................55 Figure 3.6: Perkin Elmer Diamond DMA.

..........................................56 Figure 3.7 Dynamic mechanical analysis specimen geometry.

...................................................................................57 Figure 3.8: Mettler Toledo DSC822e

...58 Figure 3.9: The DSC temperature program used for all samples during the experiment.

...................................61 Figure 3.11: Leica Stereoscan 420 Scanning Electron Microscopy.

VII

Figure 4.1: Creep strain of PU1-TDI as a function of (a) linear and (b) logarithmic time at

various stresses. .................................................................................................................66 Figure 4.2: Creep strain of PU2-TDI as a function of (a) linear and (b) logarithmic time at

various stresses. .................................................................................................................67 Figure 4.3: Creep strain of PU1-IPDI as a function of (a) linear and (b) logarithmic time at

various stresses. .................................................................................................................68 Figure 4.4: Creep strain of PU1-HDI as a function of (a) linear and (b) logarithmic time at

various stresses. .................................................................................................................69 ..............70 Figure 4.5: Isochronous curve of PU1-TDI obtained from the data of Figure 4.1.

..................................................71 Figure 4.6: Isometric curve of PU1-TDI at strain = 545%.

.....................................................72 Figure 4.7: Effect of stress on creep modulus PU1-TDI.

...............................................72 Figure 4.8: Effect of stress on creep modulus for PU2-TDI.

...................................................73 Figure 4.9: Effect of stress on creep modulus PU1-IPDI.

..................................................73 Figure 4.10: Effect of stress on creep modulus PU1-HDI.

Figure 4.11: Stress-strain curve of two different molecular weight of soft segments in

polyurethanes. ....................................................................................................................78 Figure 4.12: Stress-strain curve of three different diisocyanates structure in polyurethanes.

............................................................................................................................................79 Figure 4.13: Loss factor (tan delta) versus temperature of two various molecular weights of

soft segment in polyurethanes. ...........................................................................................84 Figure 4.14: Loss factor (tan delta) versus temperature of three different diisocyanates

structure in polyurethanes...................................................................................................85 ..............................................86 Figure 4.15: Storage modulus as a function of temperature.

..............................................87 Figure 4.16: Dependence of Loss modulus on temperature.

.........................................................................90 Figure 4.17: DSC thermogram of PU1-TDI

.........................................................................90 Figure 4.18: DSC thermogram of PU2-TDI

........................................................................91 Figure 4.19: DSC thermogram of PU1-IPDI

.........................................................................91 Figure 4.20: DSC thermogram of PU1-HDI

..............................................92 Figure 4.21: DSC thermograms of 4 various polyurethanes.

...............................................................................94 Figure 4.22: XRD pattern for PU1-TDI.

...............................................................................94 Figure 4.23: XRD pattern for PU2-TDI.

..............................................................................95 Figure 4.24: XRD pattern for PU1-IPDI.

...............................................................................95 Figure 4.25: XRD pattern for PU1-HDI.

....................................................97 Figure 4.26: SEM micrograph of PU1-TDI (Mag.=500x).

VIII

....................................................98 Figure 4.27: SEM micrograph of PU2-TDI (Mag.=500x).

...................................................98 Figure 4.28: SEM micrograph of PU1-IPDI (Mag.=500x).

...................................................99 Figure 4.29: SEM micrograph of PU1-HDI (Mag.=500x).

IX

List of Tables Table Page Table 2.1: Characteristic Features of Stress-Strain Curves as It Relates to Polymer

Properties [15].....................................................................................................................15 ...........................................................45 Table 3.1: The materials used in synthesizing PUs.

.......................................................46 Table 3.2: Compositions of the prepared PU samples.

.......................................................46 Table 3.2: Compositions of the prepared Pu samples.

.............................60 Table 3.3: The specifications of XRD Diffractometer -Siemens D5000.

Table 4.1 Tensile properties of two different molecular weight of soft segments in

polyurethanes. ....................................................................................................................77 ..77 Table 4.2 Tensile properties of three different diisocyanates structure in polyurethanes.

.........................................................................................................81 Table 4.3: DMA Data.

......................................89 Table 4.4: DSC and DMA Thermal Transitions of polyurethanes.

....................................................96 Table 4.5: Scherrer length calculated from XRD results.

X

List of Abbreviations

PU Polyurethane

PTMO Poly(tetramethylene oxide)

TDI 2,4-Tolylene Diisocyanate

IPDI Isophorone Diisocyanate

HDI Hexamethylene Diisocyanate

BDO 1,4- Butanediol

DMA Dynamic Mechanical Analysis

DSC Differential Scanning Calorimetry

XRD X-Ray Diffraction

SEM Scanning Electron Microscopy

T Glass Transition Temperature g

Tangent Delta tan (δ)

Loss modulus E”

Storage modulus E’

XI

Chapter 1: Introduction

Creep Deformation of Amorphous Polymer (Polyurethanes) Page 1

CHAPTER 1: INTRODUCTION

1.1 Overview

Polymers, example polyethylene, polypropylene and polyurethanes are being

utilized in an increasing number of long term, load bearing applications, used under

severe high-temperature, and high-pressure conditions.

The mechanical properties of polymers are time-dependent, in order to intelligently

design a product for a specific applications, it is necessary to know how these

materials will respond to stress and deformation well below the nominal yield stress

or strain of the material which may be applied to the product for periods of years or

decades.

Properties that are particularly important for extended service are creep resistance

and stress relaxation. When an instantaneous load is applied to a plastic part, the

resulting deformation can be roughly predicted by the tensile modulus of the

plastic. Deformation then continues at a slower rate indefinitely until the part

ruptures. Upon removal of the load, some portion of the part's original dimensions

will be recovered. Some portion of the deformation will remain permanent. This

time, temperature and load dependent deformation is called creep and it is a result

of the viscoelasctic nature of plastic materials. In the other words, creep occurs



when a constant force is continuously applied on a component, causing it to deform

gradually.

When loading is more than momentary, creep data must be considered for

purposes of material selection and design. The creep modulus represents the

modulus of a material at a specific stress level and temperature over a specified

period of time. By substituting the time, temperature, load dependent creep or

apparent modulus for the instantaneous modulus in appropriate design equations,

creep can be predicted.

1.2 Scope and Objective of the Study

From the above introduction, it can be noted that creep behaviour of polyurethanes

is an important consideration for their industrial application. While the creep

behaviour of steel and alloy has been studied by many researchers, the study on

creep behaviour of polyurethanes has been relatively limited. The scope of the

present study involved discussion on the time-dependent deformation mechanism

and strain components associated with creep processes as well as the mechanism

of creep. Creep measurements involved variable load constant-temperature.

Others characterizations also have been carried out by using Dynamic Mechanical

Analysis (DMA), Differential Scanning Calorimetry (DSC), X-Ray Diffraction (XRD),

and Scanning Electron Microscopy (SEM).



1.3 Dissertation Layout

The dissertation encompasses a detailed study of the mechanical properties of

polyurethanes with the focus on creep deformation. It is organized into five

chapters which include introduction, literature review, experimental methods,

results and discussion, and conclusion. The followings give brief description of

each chapter.

Chapter one is an introduction of this project. It gives an overview of polyurethanes

and shows the importance of creep study.

Literature review of the project is discussed in chapter two. This chapter includes a

general review on history and development of polyurethanes, outline background

information about the material properties and theory applied to the creep behaviour

of polyurethanes. An in-depth analysis of the mechanical properties of the

polymers – polyurethanes is also presented in this chapter.

Chapter three focuses on experimental methods, where the experimental

apparatus for the static creep test and the tensile test, DMA, DSC, XRD and SEM

are discussed. This chapter discusses the experimental procedures for the

specimen preparation and testing.

Chapter four presents a discussion of the experimental results from the static creep

test, tensile test, DMA, DSC, XRD and SEM.



Finally, chapter five concludes the research findings and highlights the problems

encountered during the research study. This chapter also includes the features of

the limitations as well as the weaknesses of the static creep test system and

recommendations for future work.

Chapter 2: Literature Review


CHAPTER 2: LITERATURE REVIEW

2.1 Description of Polyurethane

2.1.1 History and Development

The foundation of the polyurethane (PU) industry was laid in 1937 with the

discovery of the polyaddition reaction between diisocyanate and diols to form PU

by Otto Bayer in Germany [1].

PU is produced by exothermic reaction of molecules containing two or more

isocyanate groups with polyol molecules containing two or more hydroxyl groups

[1,2]. Catalyst was included in the polyol blend to control the rate at which the liquid

mixture reacts to become solid. A key factor in the preparation of polyurethane is

the reactivity of the isocyanates. Aromatic diisocyanates are more reactive than

aliphatic diisocyanate [3].

During World War II, polyurethane coatings were used in the manufacturing of

mustard gas resistant garments, high-gloss airplane finishes, chemical and

corrosion resistant coatings. By the 40’s, polyurethanes could be found in coatings

and adhesives, elastomers and rigid foams for the first commercial applications.

With the development of a low-cost polyether polyol in the 50’s, flexible foams had



opened the door to the upholstery and automotive applications. PU is playing a

vital role in many industries - from aerospace to marine and automotive to medical.

A global market for polyurethanes is currently expanding at a very fast rate. As

demand for products grows, major suppliers of polyurethane raw materials are

currently setting up new world-scale production facilities to establish a strong

technical service and a long term supply base for their customers throughout the

entire region. This will also bring opportunities to develop new products and

applications for polyurethanes, which is without doubt one of the most versatile

materials.

Nowadays, Chinese PU market is the largest market followed by the Indian PU

market. The total Chinese consumption of PU products rose from 2.104 million

tones in 2003, up to 2.59 million tones in 2004 and 3 million tones in 2005 [4]. The

rapid growth is due to the quick growth of PU coatings, thermoplastic polyurethane

(TPU), spandex and synthetic leather resins [4].

2.1.2 Properties and Applications

In order to produce a consistent end product, the ingredient chemicals have to be

mixed in precise ratio and maintain the appropriate processing temperature. In

addition, good formulation selection, appropriate processing parameters and mould

geometry are needed to obtain an optimum PU end product.



PU can be manufactured in wide range of grades in densities from 6 to 1220 kg/m3

and polymer stiffness from flexible elastomers to rigid or hard plastics. PU appears

in variety forms, making them the most versatile of any family of plastic materials

[5]. Figure 2.1 illustrates the broad range of PU with reference to density and

stiffness.

Plastic materials tend to have high modulus and tensile strength properties, but low

elongation whereas rubber materials tend to have high elongation but low modulus

and tensile strength. However, PU elastomer gives high modulus, high tensile

strength and high elongation. PU elastomers can be formulated to cover a wide

hardness range, from 30 Shore A to 85 Shore D. Most rubbers fall between 30 and

80 on the A scale and structural plastics begin at 55 on the D scale. Beside that,

PU also can elongate up to 800% and returns to its' original dimension without a

significant loss of memory.

PU is the materials of choice for application requiring unique physical properties

such as cut and tear resistance, resistance to abrasive wear [6, 7] and high load-

bearing capacity. High load-bearing capacity, coupled with the superior abrasion

resistance, is a very important advantage in applications like press-on tires, caster

wheels, industrial rolls, and press brake or punch press die forming pads.

Rigid PU foam is one of the most effective practical thermal insulation materials,

used in applications ranging from domestic refrigerators to large industrial



buildings. Comfortable, durable mattresses, automotive and domestic seating are

manufactured from flexible foam. Items such as shoe soles, sports equipment, and

car bumpers are produced from different forms of PU elastomers [8].

Highly demanding medical applications use biocompatible PU for artificial joints

and implant coatings because of their excellent biocompatibility and mechanical

properties[9, 10]. PU coatings protect floors and bridges from damage and

corrosion whereas adhesives are used in the construction of items as small as an

electronic circuit board and as large as an aircraft. Advanced glass and carbon

fibre reinforced composites are being evaluated in the automotive and aerospace

industries.


Figure 2.1: Applications of polyurethane with different range of density and stiffness [11].




2.2 Mechanical Properties of Polymeric Materials

The mechanical properties of polymeric materials are critical because all service

conditions and applications of the end products involve mechanical loading. The

materials selection and the design of product for a variety of applications are

usually based on mechanical properties such as elongation, modulus, tensile

strength, abrasion resistance, load-bearing capacity and impact strength.

In polymeric materials, mechanical behaviour is dominated by viscoelastic

phenomena. For a viscoelastic material, the mechanical response of the polymeric

material will be intermediate between an ideal elastic and viscous liquid [12]. If a

weight is suspended from a polymeric material, the polymeric material will deform

quickly to a strain predicted by the stress-strain modulus. Then, it continues to

deform slowly with time indefinitely until rupture or yielding causes failure. This is

due to molecular rearrangement in the solid induced by the stress. The

phenomenon of deformation under a constant load with time is called creep. It is a

manifestation of viscoelastic material [13].

Polymers are viscoelastic at all temperatures. So, when considering the strain

induced in service, it is always required to take into account not only the stress, but

the time for which it is applied. The viscoelastic properties are also highly

temperature-dependent. In practical applications, polymers are seldom subjected

to a single, steady deformation without the presence of other adverse factors such

Chapter 2: Literature Review as environment condition. So, the reported mechanical properties of polymers are

not suitable to be used in real application since they are generated under standard

test conditions.

2.2.1 Stress-Strain Behaviour

Figure 2.2 shows the stress-strain behaviour of polymer, which is measured at a

constant rate of loading. The curves obtained from other loading conditions, such

as compression or shear, are quite similar in appearance except that the

magnitude and the extent to which the curve is followed are different.


Stress(σ)

Figure 2.2: A typical Stress-Strain Diagram [14].

Tensile modulus

Yield C

B

A

Break

0

XElongation at failure

D

Ultimate strength

Strain(ε)



The initial portion o B is linear. For an

he deformation between BC is associated with a straightening out of a coiled

Figure 2.3. These deformations do not

f the stress-strain curve between points A and

elastic material, the stress is proportional to the strain which follows the Hooke’s

Law. Point B is the proportional limit at which the actual curve deviates from the

straight linear [15]. The behaviour of polymer below this point is elastic in nature

and therefore the deformation is recoverable. Up to point A, the deformation is

small and is associated with bending or stretching of the inter-atomic bonds

between atoms of the polymer molecules. This type of deformation is nearly

instantaneous and no permanent displacement of the molecules relative to each

other.

T

portion of the molecular chains if loaded in tension. There is no intermolecular

slippage occurring and the deformation is recoverable ultimately but not

instantaneously. Hence, this is analogous to that of a non-linear spring. The

extension that occurs beyond the point C is called yield point or elastic limit, which

the material is not recoverable. The yield point is the first point on the stress-strain

curve at which an increase in strain occurs without the increase in stress [15].

Beyond this point extensions occur by displacement of molecules with respect

each other, as in Newtonian flow of liquid. This displaced molecules have no

tendency to slip back, therefore this deformation is permanent.

Three types of deformations are shown in

occur separately but are superimposed on each other. The bonding and the

Chapter 2: Literature Review stretching of the interatomic bonds are almost instantaneous. But, the molecular

uncoiling is relatively slow. Molecular slippage effects are the slowest of all three

deformations [16].

The deformations can be further explained by using a mechanical model combines

ooke’s law for an ideal elastic solid provides a relationship between stress and

a spring and a dashpot, known as the Maxwell Model. The spring is perfectly

elastic or Hookean behaviour. The deformations that occur because of the bending

and stretching of the interatomic bonds are analogous to the spring extension. The

nonlinear spring is similar to the deformation of uncoiling of portions of molecular

chains. Whereas the extension in the dashpot which is not recoverable and

correspond to the permanent set can represents the result of intermolecular

slippage.

H

strain for tensile deformation as [17]:

εσ E= (2.1)

where the proportionality factor, E, is c

alled the tensile modulus or Young’s

modulus. As shown in Figure 2.2, only the initial portion of the plot follows Hookean

behaviour. Conversely, the strain and the stress are related by the tensile

compliance, D, defined as:

σε D= (2.2)


Chapter 2: Literature Review Therefore, the compliance is the reciprocal of the modulus [17]:


E

D 1= (2.3)

The stress-strain curve also can serves as a basic for classification of polymeric

materials in terms of their relative softness, brittleness, hardness and toughness.

Figure 2.4 illustrates stress-strain curves for several types of polymeric materials

and the characteristic features of stress-strain curves are listed in Table 2.1. The

area under the stress-strain curve is considered as the toughness of the polymeric

material.

(a)

(b)

(c)

Figure 2.3: Deformation in polymer material [14]. (a) Stretching of polymer molecules. (b)

Straightening out of coiled molecular chain. (c) Intermolecular slippage



Table 2.1: Characteristic Features of

Properties [

polymer

stress strength

t

break

Stress-Strain Curves as It Relates to Polymer

15].

Description of Modulus Yield Ultimate Elongation a

Soft Low

L Mo , weak

Soft, tough

Weak, brittle

Hard, brittle

Hard, strong

Hard, tough

Low

Low

High

High

High

Low

Low

None

None

High

High

ow

Moderate

Low

Moderate

High

High

derate

High

Low

Low

Moderate

High



Figure 2.4: Classification of polymeric materials on the basic stress-strain diagram [14], [15]. (a) Hard and brittle (b) Weak and

brittle (c) Hard and strong (d) Hard and tough (e) Soft and weak (f) Soft and tough

0 0 0

0 0 0

Strain(ε)

Stress(σ) Stress(σ)

Stress(σ) Stress(σ) Stress(σ)

Strain(ε) Strain(ε)

Strain(ε) Strain(ε) Strain(ε)

(a) (b) (c)

(d) (e)

Stress(σ)

(f)



2.2.2 Creep Properties

The long-term stress-strain behaviour of polymers is generally more important than

short-term properties where the product is expected to sustain a stress or strain in

service. Not like most of the metals which only exhibit creep at higher

temperatures, polymeric materials possess significant creep even at room

temperature.

Polymers are used in applications that demand high performance and extreme

reliability. Many components, conventionally made from metals, are now made

from polymers. Thorough studies on the behaviour of polymer under long-term load

and varying temperatures are needed.

When a polymer is subjected to a constant load, it deforms quickly to a strain

roughly predicted by its stress-strain modulus, and then continues to deform slowly

with time-dependant response until rupture or yielding causes failure, sometimes

described as the primary, secondary and tertiary phase of creep [15]. This

phenomenon of deformation under constant load over time is called creep. The

secondary phase is dominant and is essentially combined with the primary phase

in engineering models for engineering applications. The tertiary phase is important

as an indication of initial rupture [18]. At high stresses, the rupture occurs sooner

than at lower stresses. However, at low enough stresses failure may never occur

[19]. All polymers creep to certain extent which its degree depends upon several

Chapter 2: Literature Review factors, such as amount of load, loading time, temperature, types of plastic, the

component geometry and fabrication method [20]. Figure 2.5 showed the effect of

stresses on creep curves.

As mentioned previously, the long-term properties are more useful, the short-term

stress-strain data is of little practical value in actual designing the part, since such

data does not take into account the effect of long-term loading on plastics. Creep

behaviour varies considerably among types of plastics; however, under proper

stress and temperature conditions, all plastics will exhibit a characteristic type of

creep behaviour. One such generalized creep curve is shown in Figure 2.6.


Log time

σ1 σ2

σ3

σ4

σ5

σ1 >σ2> σ3> σ4> σ5

Stra

in p

erce

nt

Figure 2.5: Tensile creep curve with various stresses [21].



Figure 2.6: Characteristic Creep Curve with linear time scale [22].

The response of the specimen loaded by σ0 at time t=0 can be divided into an

elastic and a plastic part as

),()( 0

00 T

TE p σεσ

ε += (2.4)

Where E(T) is the modulus of elasticity. The creep strain in Figure 2.6 can then be

expressed according to

(2.5) kc tt αεεε 0)( −=

Where k <1 in the primary, k=1 in the secondary and k>1 in the tertiary creep

stage. These terms correspond to a decreasing, constant and increasing strain

rate, respectively, and were introduced by Betten [22] and Andrade [23].

A

B

C

D

II

ε0

III

Time

Rupture

E

Stra

in p

erce

nt

I



The result of equations (2.4) and (2.5) from the creep test justified a classification

of material behaviour in three disciplines: elasticity, plasticity and creep mechanics.

The creep curve in Figure 2.6 shows that there is typically an almost instantaneous

elastic strain AB followed by a time-dependent strain, which divided into three

stages: primary or transient creep, BC (stage I), secondary or steady-state creep,

CD (stage II) and tertiary or accelerated creep, DE (stage III) [14]. The initial strain,

AB represents the instantaneous elastic deformation. In primary stage, the strain

occurs rapidly but at a decreasing rate. The dislocation microstructure develops to

reduce strain rate at this stage. It is essentially similar in mechanism to retarded

elasticity and as such, is recoverable if the stress is removed.

The straight portion of the curve (CD) is characterized by a constant rate of creep.

The strain rate during this state is commonly referred to as the creep rate. It

determines the useful life of the material. The equilibrium of deformation and

recovery mechanisms is established to maintain this constant rate of creep. This

process is also called “cold flow”. It is non-recoverable because it is essentially

viscous in character. The stage III is marked by an increase in creep rate as an

increase in the true stress due to the cross section reduces (necking) and leading

to failure or creep rupture typically logarithmic curve [15].

If the applied load is released before the creep rupture occurs, an immediate

elastic recovery, substantially equal to elastic deformation followed by a period of



slow recovery is observed. The material in most cases does not recover to the

original shape and permanent set remains. The magnitude of the permanent set

depends upon length of the time, amount of stress applied, and temperature [15].

The total mechanical response of a specimen includes perfect elasticity (related

with the instantaneous strain in Figure 2.7), a coupling of elastic and viscous

components, called anelasticity and a total irreversible flow [24]. The creep

behaviour of a polymer can be represented by an appropriate combination of

mechanical models of Maxwell and Voight element. Although there are no discrete

molecular structures which behave like the individual elements of the models, but

they aid in understanding the response of polymer materials.

In the creep test, the creep values are obtained by applying constant load to test

specimen rapidly in tension, compression, or flexure and measuring the

deformation as a function of time at a constant temperature. These terms are most

commonly referred to as tensile creep, compressive creep, and flexural creep.

The creep test is more common and probably simpler since dead weight loading

can be used with multiple creep stations. The uniaxial tensile test is considered

most useful for producing accurate and consistent results that can be easily

interpreted [25]. Creep data is usually obtained for a number of different stresses,

as creep modulus will only independent of stress over limited ranges. It may also

be important to obtain data as a function of temperature. Commonly, isochronous

Chapter 2: Literature Review stress-strain curves are derived from the creep curves at different stress levels as a

useful way of displaying the information.

Tensile creep measurements are made by applying the constant load to a tensile

test specimen and measuring its extension as a function of time. The extension

measurement can be carried out in several different ways. The simplest way is to

make two gauge marks on the tensile specimen and measure the distance

between the marks at specified time intervals. The percent creep strain is

determined by dividing the extension by initial gauge length and multiplying by 100.

For a linear viscoelastic material, the effect of an applying stress, σ to the strain,

ε(t) can be divided into three parts by assuming linearity [12]:

(I) ε , an essentially instantaneous response, similar to that of an elastic solid; 1

(II) ε (t), which tends to a constant value as t tends to ∞; and 2

(III) ε (t) which is linear in time. 3

So, a time-dependent creep compliance, J(t) can be defined as [12]:

)()()()()()( 321

321 tJtJJttttJ ++=++==

σσσσε εεε (2.6)

The creep percentage is plotted against time to obtain a tensile creep curve as

illustrated in Figure 2.7. The J3(t) corresponds to flow and will be assumed as zero.

The term J1 corresponds to a response that is faster than can be observed

experimentally, rather than an instantaneous one. The strain ε1 can be called the

unrelaxed response, in contrast to the relaxed response observed at long time


Chapter 2: Literature Review ε(∞). J and J1 2(t) are not usually considered separately in what follows, so that J(t)

is equals to J +J (t) [12]. 1 2


ε

ε1

ε3 (t)

t

ε1

ε2 (t)

ε (t)

0 0

Figure 2.7: Creep of a viscoelastic under a constant stress [12].

The tensile stress values are also determined at specified time intervals to facilitate

plotting a stress-rupture curve. The more accurate measurements require the use

of a strain gauge, which is capable of measuring and amplifying small changes in

length with time and directly plotting them on a chart paper. The test is also carried

out at different stress levels and temperatures to study their effects on tensile

properties.



2.2.3 Interpretation and Application of Creep Data

Although the most common method to display the interdependence of stress, strain

and time is the earlier creep curve as illustrated in Figure 2.5. But, it is not

completely satisfied. The creep data presented in terms of strain is not convenient

to be used in design or for the purpose of comparing materials, especially when

different stress levels are used for different materials.

There are other methods which may be useful in particular applications and readily

useable. Isometric graph can be obtained by taking a constant strain section

through the creep curves and re-plotting the data as stress versus time, as shown

in Figure 2.8. Isometric curve indicates the relaxation of stress in the material when

the strain is kept constant. This isometric curve is often used as a good

approximation of stress relaxation in a polymer since stress relaxation has less

experimental procedure than creep testing. In addition, if the stress (vertical axis in

isometric curve) is divided by the strain, ε’, then a graph of modulus against time

can be obtained as shown in Figure 2.9. This is a good illustration of the time-

dependent variation of modulus.

Isochronous curve is another preferred method because it is less time-consuming

and requires less specimen preparation than creep testing. This isochronous curve

can be obtained by taking a constant time section through the creep curves and

plotting stress versus strain as shown in Figure 2.10. Isochronous test method is

quite similar to the conventional incremental loading test. The differences are the



presence of creep is recognized and the “memory”, which the material has for its

stress history is overcame by the recovery periods. Quite often isochronous data is

presented on log-log scales. The use of log-log scale will usually give a straight-line

graph and the slope indicates the linearity of the material. The slope will be 45° if it

is perfectly linear whereas non-linear material has the slope less than 45°.


Figure 2.8: Construction of isometric graphs from creep curve [21].




Figure 2.9: Creep modulus versus time which derived from isometric curve [21]

.

0

Modulus

Strain = ε'

Log t


Figure 2.10: Construction of isochronous curve from creep curve [21].




2.3 Polymer Characterization

2.3.1 Dynamic Mechanical Analysis (DMA)

DMA is becoming more and more commonly seen in the analytical laboratory to

study the deformation and flow of materials, namely rheology. DMA is more

sensitive than DSC and easily reveals the thermal transition of both soft and hard

segment phases [26, 27]. DMA can be simply described as applying an oscillating

force to a sample and analyzing the material’s response to that force. Figure 2.11

shows the Diamond DMA operation principle and system structure.

The force motor applies the oscillatory force to the sample and the deformation is

detected by using the linear differential transformer. The relevant signal for

viscoelasticity is assessed from the relationship between stress and strain. The

length deformation caused by creep and thermal expansion of the sample is

compensated through the stepping motor.

DMA measures the modulus and damping properties of materials as they deform

under dynamic stress [28]. The modulus is dependent on the temperature and the

applied stress [29]. The modulus indicates how well a material will work in specific

application in the real world. In DMA, a complex modulus (E*), an elastic modulus

(E’), and an imaginary modulus (E”) are calculated from the material response to

the sine wave which show the ability of the material to store energy, lose energy

and the ratio of these effects (tan delta).


WorkStationDMA Module CPU

Differential Transformer

SteppingMotor

Positional Control

Stress Control

DisplacementSignal

Heater Power

Temperature Signal

Probe

Sample

Furnace

Cover

Force Generator

Oscillation/Phase Angle Calculation CircuitPlate

Spring

Figure 2.11: DMA structure.


Chapter 2: Literature Review Figure 2.12 shows an idealized temperature scan of a polymer. Starting at low

temperature the modulus decreases as the molecules gain more free volume,

resulting in more molecular motion. This shows main curve as divided into six

regions which correspond to local motions (1), bond bending and stretching (2),

movements in the side chain or adjacent atoms in the main chain (3), the region of

Tg (4), coordinated movements in the amorphous portion of the chain (5), and the

melting region (6) [29].


(6) Tm-melting

Figure 2.12: An ideal DMA scan showing the types of transition [29].

(4)

(5) Rubbery Plateau E’ /

Pa

Temperature/ K

Tγ(1)

(2)

(3) Tα or Tg

Tβ

Beta transitions are often related to the toughness.

Rubbery plateau is related to the molecular weight between cross-links or entanglements.

For thermosets, no Tm occurs.

In purely crystalline materials, no Tg occurs.

Tg is related to the molecular mass up to a limiting value.

(2) Bend and stretch

(3) Side groups

(4) Gradual main chain

(5) Large scale chain

(6) Chain slippage

(1) Local motion

Chapter 2: Literature Review The Tg is very dependent on the degree of polymerization up to a value known as

the critical T or the critical molecular weight. The Tg g represents a major transition

for many polymers, as physical properties change drastically as the material goes

from hard glassy to rubbery state. The Tg value can be obtained from the peak or

onset of the tan delta curve, the onset of the storage modulus drops, and the onset

or the peak of the loss modulus curve [29]. The values obtained from these

methods can differ up to 25°C from each other on the same run. In additional, a 10-

20°C difference from the DSC is also seen in many materials.

Many methods on dynamic mechanical measurements have been described [29,

30]. In this study, the samples have been scanned at different temperatures with

constant frequency to detect the transitions in materials.

2.3.2 Differential Scanning Calorimetry (DSC)

DSC is a technique which records the difference in temperature between a sample

and a reference material against either time or temperature as the two specimens

are subjected to identical temperature conditions in an environment heated or

cooled at a controlled rate [31].

If unit mass of a substance is heated at constant pressure in such a way that the

rate of supply of energy, Q is dQ/dt and the rate of rise of temperature, T is dT/dt,

the specific heat of the substance at constant pressure C is given by [12]: p



dtdTdtdQdTdQC p /

// == (2.7)

There are two pans in DSC, one is the sample pan where the sample is put and the

other one is the reference pan which will be left empty as shown in Figure 2.13.

The instrument allows the common temperature of the two pans to be changed at a

constant rate dT/dt, while the two pans are ideally maintained at the same

temperature by a feedback loop. Extra energy Q must be supplied to the sample

pan at a rate dQ/dt to maintain its temperature to be the same as the reference

pan. This rate is registered by the instrument and plotted either against

temperature, T or against time, t.


Figure 2.13: A schematic diagram of a DSC apparatus [12].

It is generally possible to assume that, away from any transition in the sample, the

sample and the sample pan are at the same temperature and that the sample and

Reference Pan

Sample Sample

Pan

Individual Heaters

Computer to monitor the temperature and regulate

heat flow Temperature

Sensor

Chapter 2: Literature Review reference pan are identical. It then follows that dQ/dt = mCp, where m is the mass

of the sample and C is the specific heat per unit mass [12]. p

Figure 2.14 shows the DSC trace that often seen for polymers. On warming, at a

certain temperature called the glass transition temperature, Tg, an amorphous

polymer starts to soften. When the polymer is cooled below this temperature, it

becomes hard and brittle, like glass.

Glass transition may occur as the temperature of an amorphous solid is increased.

The heat flow shifts upward suddenly at Tg as shown in Figure 2.14 at a constant

heat rate. This is due to the sample undergoing a change in heat capacity; no

formal phase change occurs.

Above the Tg, polymer chains have a lot of mobility [12]. If the polymer is linear, it

will flow, although the viscosity may be very high. When they reach the right

temperature, the molecules may obtain enough freedom of motion to move into a

very ordered arrangement, which we call crystalline form. This is known as

crystallization temperature, Tc. When polymers fall into crystalline form, they give

off heat. It is an exothermic process and results in a peak in the DSC curve as can

be seen in Figure 2.14. If the polymer is cross-linked, then it never flows and never

really becomes a liquid, but remains a soft amorphous solid like rubber.


Chapter 2: Literature Review When the polymer is keep heating up after Tc, the temperature will continue to

increase until it reaches its melting temperature, Tm; then the temperature remains

constant until it has completely melted. The melting process results in an

endothermic peak in the DSC curve as can be seen in Figure 2.14.


Figure 2.14: A schematic DSC curve demonstrating the appearance of several common

features seen for polymers [12].

2.3.3 X-Ray Diffraction (XRD)

An X-ray diffraction illuminates a sample of material with x-rays of known

wavelength, moving the sample and detector in order to measure the intensity of

Melting

TmTg Tc

Cold Crystallisation

Temperature

Heat Flow

5

Chapter 2: Literature Review the diffracted radiation as a function of beam and sample orientation. From the

resulting intensity versus angle plot much can be inferred about the structure of the

material. When an X-ray monochromatic beam hits on a sample, two processes

can be observed [31]:

i) If the sample has a periodic structure (crystalline regions) the X-ray is

scattered coherently. This process occurs without change of wavelength and

without loss of phase relationship between the incident and scattered rays.

This process is called a diffraction X-ray effect and is measured by wide-

angle X-ray diffraction.

ii) If the sample has an unperiodic structure which posses different electronic

densities (crystalline and amorphous regions), the X-ray is scattered

incoherently (Compton Scattering). This inelastic scattering process occurs

with change of wavelength and change of phase relationship between the

incident and the scattered rays. This process is called a diffuse X-ray

diffraction effect and is measured by small-angle X-ray scattering.

Diffracted waves from different atoms can interfere with each other and the

resultant intensity distribution is strongly modulated by this interaction. If the atoms

are arranged in a periodic order, as in crystals, the diffracted wave will consist of

sharp interference maxima with the same symmetry as in the distribution of atoms.


Chapter 2: Literature Review Measuring the diffraction pattern therefore allows us to deduce the distribution of

atoms in a material.

The peaks in the x-ray diffraction pattern are directly related to the atomic

distances. Figure 2.15 shows the geometry of the diffraction of X-rays in a crystal.

Crystals diffract X-rays coherently according to Bragg’s Law, giving sharp peaks,

while amorphous materials scatter X-rays incoherently giving a diffuse halo [32].

For a set of lattice plane with an inter-plane distance of d (hkl) , the condition for a

diffraction to occur can be simply written as [31, 33]:

nλ = 2 d sin θ (2.8) (hkl)

which is known as the Bragg’s Law. In the equation, n is an integer 0, 1, 2, 3, …,

called the order, λ is the X-ray wavelength, d (hkl) is the distance between adjacent

planes in the crystal, and θ is one-half the angle of deviation of the diffracted rays

from the incident X-rays.




λ

Figure 2.15: Geometry of the diffraction of X-rays in a crystal [31].

X-ray diffraction permits an accurate description of the statistical distribution of the

atoms showing disordered structure, if the substance is homogeneous, the forces

of interaction are isotropic and the substance is monatomic or made up of

molecules of known structure whose orientation is completely random [34]. If the

internal structure of the molecule is unknown and complex, the interpretation of the

pattern become difficult because intramolecular interference adds to the

intermolecular interference and it is impossible to separate the two effects.

The degree of crystallinity can be determined if the crystalline and amorphous

scattering in the diffraction pattern can be separated from each other. The degree

d (hkl)2θθθ

Diffracted X-rays Incident X-rays

Atoms

(hkl)

(hkl)

d (hkl)

(hkl)

8

Chapter 2: Literature Review of crystalline, cχ is equal to the ratio of the integrated crystalline scattering , both

crystalline and amorphous, and is given by [31]:

∫

∫∞

∞

=

0

2

0

2

)(

)(

dssIS

dssIS c

cχ (2.9)

Where S is the magnitude of the reciprocal lattice vector and is given by:

S= (2 sin θ)/ λ (2.10)

θ is one-half the angle of deviation of the diffracted rays from the incident X-rays,

λ is the X-ray wavelength, I(s) is the intensity of coherent X-ray scatter from a

specimen (both crystalline and amorphous), and Ic(s) is the intensity of coherent X-

ray scatter from the crystalline region.

Beside that, the Scherrer equation also is the basis of the XRD method for the

determination of the crystallinity [35]. The Scherrer equation in its general form is

written as [33, 35- 37]:

(2.11) θβ

λcosKL =

Where L is the mean dimension of the crystallites composing the powder, K equals

to 0.9 is the Scherrer constant approximately equal to unity, λ is the radiation

wavelength, θ is the angular position of the peak of interest and β is the full angular

width at half-maximum intensity of the reflection in radians.


Chapter 2: Literature Review 2.3.4 Scanning Electron Microscopy (SEM)

SEM is a microscope that uses electrons instead of light to form an image. Figure

2.16 shows a schematic diagram of a typical SEM and how it works.

The micrographs obtained from the SEM provide a lot of important information of a

sample such as the morphology, the surface features, composition and the

crystallographic information of the sample. The shape and size of the particles

making up the sample and the direct relation between these structures and

materials properties such as ductility, strength and reactivity then can be evaluated.

How the atoms are arranged in the object also has direct relation to the material

properties like conductivity, electrical properties and its strength.

A beam of electrons is produced at the top of the microscope by an electron gun.

The electron beams follows a vertical path through the microscope, which is held

within a vacuum. The electron beam travels through electromagnetic fields and

lenses, which focus the beam down toward the sample.

Once the beam hits the sample, electrons and X-rays are ejected from the sample

as shown in Figure 2.17. These X-ray, backscattered electrons and secondary

electrons will be collected by the detector and then converted into the signal. This

produces the final image.



Figure 2.16: A schematic diagram of a SEM apparatus [38].

Magnification is controlled by adjusting the dimensions of the raster on the

specimen and the display screen. Scan time for examination and recording are

important. Longer scan time give better signal-to-noise characteristics than the

short scan time.




Sample

Primary Backscattered

Secondary Electrons

Incident Beam X-rays

Auger Electrons

Figure 2.17: Secondary signals that are generated during electron beam irradiation [38].

The SEM has many advantages over traditional microscopes. It has a large depth

of field, which allows more of a specimen to be in focus at one time and it also has

much higher resolution.

2

Chapter 3: Experimental Methods


CHAPTER 3: EXPERIMENTAL METHODS

3.1 Materials Description

The polyurethanes were synthesized from poly(tetramethylene oxide) (PTMO),

2,4-Tolylene Diisocyanate (TDI), Isophorone Diisocyanate (IPDI), Hexamethylene

Diisocyanate (HDI), and 1,4- Butanediol (BDO) by 2-step polyaddition reaction.

Chemicals used in the synthesis of polyurethanes are described in Table 3.1.

PTMO of molecular masses 1000 g/mol and 2000 g/mol were purchased from

Aldrich. The polyol was dried at 75-80°C under reduced pressure (vacuum) for 2

hours before use. TDI from Fluka, and IPDI from Aldrich were used. BDO from

Aldrich was dried at 75-80°C under reduced pressure for 2 hours before use.

3.2 Sample Preparation

Polyol and BDO were demoistured under reduced pressure. Demoistured polyol

was then reacted with diisocyanates viz. TDI, HDI or IPDI in the temperature range

of 75 to 80°C. The mixture was stirred continuously for half an hour to one hour to

get the pre-polymer with terminal NCO groups. The pre-polymer was further

reacted with BDO and the mixture was stirred continuously for 15 minutes. The

reaction mixture was poured into a glass die, which was coated with a mould-

releasing agent. The die was kept in a vacuum oven for degassing at 55°C for 48

hours for TDI base polyurethane and 10 days for IPDI-based polyurethane to



obtain the solid cured sheets, which were subjected to characterization after proper

conditioning at room temperature. The ratio of diisocyanate/ polyol/ chain extender

was used according to the compositions as described in Table 3.2.

Polyurethane elastomers are block copolymers which consist of soft segments and

hard segments [39]. The hard segments are made from diisocyanete by addition of

a chain extender like butanediol [40- 42]. The soft segments consist of the long

flexible polyether or polyester chains which interconnect two hard segments [39].

The soft segment provides elastomeric character for the polymer, while hard

segment provides dimensional stability [43] and particularly affects the modulus,

hardness and tear strength [44] .



Table 3.1: The materials used in synthesizing PUs.

Materials Chemical Structure Molecular Weight (g/mol) Source

[ ] HOCHCHCHCHOH −− 142222Poly(tetramethylene oxide)

(PTMO)

1000 and 2000 Aldrich, USA

2,4-Tolylene Diisocyanate

(TDI)

174.17 Fluka, Germany

Isophorone Diisocyanate

(IPDI)

222.29 Aldrich, USA

Hexamethylene Diisocyanate

(HDI)

168.2 Fluka, Germany

1,4- Butanediol (BDO) 90.0 Aldrich, USA


Table 3.2: Compositions of the prepared PU samples.

Composition

Samples Diisocyanate

(3.2 Mol)

Polyol

(1.0 Mol)

Chain Extender

(2.0 Mol)

BDO

BDO

BDO

BDO

PU1-TDI

PU2-TDI

PU1-IPDI

PU1-HDI

TDI

TDI

IPDI

HDI

PTMO (Mw=1000)

PTMO (Mw=2000)

PTMO (Mw=1000)

w=1000) PTMO (M



3.3 Conventional Static Creep Tests

3.3.1 Testing Apparatus

A measurement set-up system as in Figure 3.1 was used for the conventional

creep testing. The apparatus consists of extension measurement, sample holder,

weight and weight holder. This measurement set-up system was made according

to ASTM D2990 requirements [45].


Sample Frame

Ruler

Figure 3.1 Creep testing system set up.

Chapter 3: Experimental Methods The specimen is attached by pins to two grips, one of which is suspended from a

stable frame and the end of the other grip is then loaded by hanging to the weight.

The grip surfaces are deeply serrated with a coarse of single-cut file, serrations

about 3/32 in. apart and about 1/16 in. deep to prevent slippage of the specimen. A

ruler is attached to the apparatus to measure the extension of the specimen. An

electronic balance is used to measure the weights applied to the specimen.

3.3.2 Specimen Preparation

The prepared polyurethane sheets were stamped out using a punch die and

hydraulic system into dog-bone shape or also known as dumbbell shape. See

Figure 3.2.


Figure 3.2 Dumbbell shape cutter with ASTM D638 standard.

Chapter 3: Experimental Methods These dumbbell shape specimens were dimensioned according to ASTM D638

Type IV requirements as shown in Figure 3.3 [45]. Thickness measurements were

made on the individual specimens and actual thickness were found to range from

1.400 mm to 1.800 mm. The specimens were stored under ambient temperature

and humidity conditions; there was no other conditioning of the specimens before

testing.

Figure 3.3 Dimension of ASTM D638 Type IV [45].




3.3.3 Static Creep Test Procedure

The apparatus which was used to conduct the creep test was shown in the

previous section. The procedures to do the creep test are as following.

1. Five pieces of specimens with approximate same thickness from the range

1.40 mm to 1.80 mm were tested at each stress level.

2. The width and thickness of each specimen were measured at both end and

at the centre of the effective gauge length before the test. The values

measured were averaged for calculations. All measurements are made in SI

unit.

3. The effective gauge length and clamp area were measured and marked so

that the measurement can be made correctly and all the specimens are

clamped in approximately same length. See Figure 3.4.

4. Test specimen was clamped properly by the grip to minimize eccentric

loading of the specimen. The grips had been tightened evenly and firmly to

the degree necessary to prevent slippage of the specimen during the test

but not to the point where the specimen would be crushed.

5. The load cell was weighed and recorded. The stress that will be applied to

the specimen was calculated based on the cross-sectional area of the

specimen.

6. The load cell was applied to the specimen quickly but very gently.

7. Test temperature was maintained at room temperature. The temperature

and relative humidity were recorded by using thermometer.


8. The strain readings were taken by following the interval thereafter: 5s, 10s,

15s, 20s, 25s, 30s and 60min, then every one minute to 1 hour; every 24

hour to 500 hour; and then every 48 hour to 1000 hour.

Figure 3.4 Diagram showing gripping surface and effective gauge length for creep test.

3.4 Tensile Tests

3.4.1 Testing Apparatus and Test Procedure

Tensile test were performed by using an Instron model 4443 testing machine as

shown in Figure 3.5 on four different types of polyurethanes. It is a constant-rate-

of-crosshead movement tensile machine. A controlled-velocity drive mechanism is

used.




The Instron Testing System consists a load frame into which a specimen of the

test material is mounted and a control console. The load frame applies a tensile

load to the specimen, while the control console provides calibration, test setup and

test operating controls.

The load frame provides a rigid mounting structure capable of applying a load to a

specimen by means of a moving crosshead. It has an essentially stationary

member carrying one grip and a movable member carrying another grip. Self-

aligning grips employed for holding the test specimen between the two members

prevent alignment problems. A load cell mounted in the frame provides accurate

measurement of the force applied to the specimens.

The control console comprises the main control panel for the system. It has

provision for an IEEE-488 General Purpose Instrument Bus which allows remote

supervisory control of test procedures through a programmable computer.

The samples were tested in the speed of 500 mm/min. The tests were conducted

under ambient temperature and humidity condition. Five pieces of specimens with

approximate same thickness were tested for each sample.

Chapter 3: Experimental Methods 3.4.2 Specimen Preparation

The specimens were prepared according to ASTM D638 Type IV requirements as

stated in Section 3.3.2. The specimens were stored under ambient temperature

and humidity conditions; there was no other conditioning of the specimens before

testing.

Figure 3.5 Instron 4443 Tensile Testing Machine.




3.5 Dynamic Mechanical Analysis (DMA)

3.5.1 Testing Apparatus

DMA is an analyzer which can be use to characterize the material in term of its

behaviour and to study the performance of the material under condition as close as

possible to real.

The Perkin Elmer Diamond DMA was used in this study to determine the glass

transition temperature, Tg by monitor the dynamical mechanical response as a

function of temperature. DMA was also used to determine the damping factor (tan

δ), the dynamic storage modulus (E’) and loss modulus (E”) of PU. Upon passing

through the glass transition, E’ changes up to three orders of magnitude, E” and

tan δ have maxima value. Tg is taken to be the temperature corresponding to one

of these maxima. But usually, they have slightly different. Figure 3.6 shows the

picture of the Perkin Elmer Diamond DMA.

The DMA experiments were carried out by using the tensile mode at a fixed

frequency of 1Hz. Storage modulus, loss modulus and tan delta were determined

during heating over the temperature range from -100 °C to 150 °C at a heating rate

of 2 °C /min.


Figure 3.6: Perkin Elmer Diamond DMA.

3.5.2 Specimen Preparation

The fabricated polyurethane sheets were stamped out using a punch die and

hydraulic system into a rectangular cross section bar as shown in Figure 3.7.

Thickness measurements were made on the individual specimens and actual

thickness were found to range from 1.55 to 2.00mm. The effective gauge length is

20.0mm. The specimens were stored under ambient temperature and humidity

conditions; there was no other conditioning of the specimens before testing.




Figure 3.7 Dynamic mechanical analysis specimen geometry.

3.6 Other Characterization Methods

3.6.1 Differential Scanning Calorimetry (DSC)

Differential Scanning Calorimetry experiments were performed by using a

METTLER TOLEDO DSC822e as shown in Figure 3.8 equipped with an intra-

cooler which allows furnace cool to -60°. The system was automated with a sample

robot. STAR e software was used to connect application between instrument and

computer system. The resulting endotherm data was also analyzed using STAR e

software.

Chapter 3: Experimental Methods The samples were weighted around 10.0 to 13.0 mg for each experiment in a

hermetically sealed atmosphere. The self-generated atmosphere was obtained by

piercing a 50 μm hole in the aluminium lid of a sealed crucible.

The samples were measured with the following temperature program:


(i) Heating the sample from -60 oC to 150 o oC at 5 C/min

(ii) Cooling from 150 oC to -60 oC at 5 oC/min

(iii) Heating from -60 oC to 200 o oC at 5 C/min (the actual measurement)

Figure 3.8: Mettler Toledo DSC822e

The DSC temperature program ran dynamically as shown in Figure 3.9 under

nitrogen condition with a flow rate of 50 ml/min. The first heating was done to

Chapter 3: Experimental Methods eliminate or equalize the thermal history of the samples because the first heating

will show the effects that related to the different thermal history of the sample.

Therefore, the second heating was used for the evaluation purpose.


Figure 3.9: The DSC temperature program used for all samples during the experiment.

3.6.2 X-ray Diffraction (XRD)

XRD experiments were carried out on Siemens D5000 x-ray diffractometer as

shown in Figure 3.10. All experiments were carried out on samples of approximate

1.50 mm thickness at room temperature. Table 3.3 shows the details of the XRD

Diffractometer (Siemens D5000).

-60

0

150

Time/min

Temperature, oC

Chapter 3: Experimental Methods The radiation emanating from the line focus of the X-ray tube is diffracted at the

sample and recorded by the detector. The sample rotates at a constant angular

velocity such that the angle of incidence of primary beam changes whilst the

detector rotates at double angular velocity around the sample. The diffraction angle

is thus always equal to twice the glancing angle.

The detector and the connected measuring electronic measure the intensity of the

reflected radiation; the angular position of the reflections is displayed at the

controller.


Figure 3.10: Siemens D5000 x-ray diffractometer.



Table 3.3: The specifications of XRD Diffractometer -Siemens D5000.

Source Copper (Cu) Kα

Source wavelength 1.54056 Å

Power supply Voltage: 20-40kV Current: 20-40mA

Step mode Continuous scan

Range size 0-120°

Step size 0.1°

Step time 1.0s

Glancing angle 2.0°

Scanning mode Detector scan

3.6.3 Scanning Electron Microscopy (SEM)

Morphology study was carried out using a Leica Stereoscan 420 instrument as

shown in Figure 3.11. The samples were mounted on a double-sided adhesive and

then stick on a disc. The surface of the sample must be electrically conductive. A

very thin metallic surface coating such as gold must be sputtered to avoid build-up

charge.

The surface of the sample is scanned with an electron beam and the reflected

beam of electrons is collected. The image that appears on the screen, which may

be photographed and represents the surface features of the specimen.



Figure 3.11: Leica Stereoscan 420 Scanning Electron Microscopy.

Chapter 4: Results and Discussions


CHAPTER 4: RESULTS AND DISCUSSIONS

4.1 Static Creep Test Results

In this study, creep test has been conducted over a range of stresses to fully

characterize the creep behaviour of PUs. It can be used directly to determine the

time at which a given strain is reached under various stresses. The test was carried

out at room temperature and humidity for 1.0x107 seconds. The percent creep

strains were plotted as a function of linear and logarithmic time to obtain a tensile

creep curve.

Figure 4.1 to Figure 4.4 show the creep curves for all four PUs. This may be useful

information since the failure criterion is usually expressed in the terms of a limiting

strain and thus, knowing the stress, the materials durability can be determined. In

order to predict the response of a certain polymer to a very long period like years or

tens of years, these curves must be extrapolated over a wide time interval.

As the stress increases, the deformation of sample will increase too, as should be

expected from all the samples. The effect of stresses to the creep behaviour of

PU1-TDI is illustrated in Figure 4.1. The elastic strain of PU1-TDI increased with

the increase of applied stress from 2 MPa to 8 MPa. The strain increased more

rapidly in the first few hours followed by a reduce rate of creep later in the



experiment. The creep strain approached a steady state after 29 hours for the

applied stress of 8 MPa and prolong to 300 hours for the applied stress of 2 MPa.

Approximately 550% to 582% strain was achieved by sample PU1-TDI at 1.0x107

seconds.

For PU2-TDI, approximately 410% to 525% strain was achieved at 1.0x107

seconds. PU2-TDI showed a lower creep strain compare to the PU1-TDI. Creep

strains accumulate at a decreasing rate and no third stage creep is observed for

both PU1-TDI and PU2-TDI. No failure occurred for both samples until the end of

the creep test as show in Figure 4.1 and 4.2. From the creep test, it can be seen

that the molecular weight affects the mechanical properties of PUs [27].

PU1-IPDI showed the highest creep deformation among the four samples. PU1-

IPDI failed in 1.62x104 seconds for an applied stress of 2 MPa after it went through

a shortest steady stage compare to PU1-TDI, PU2-TDI and PU1-HDI. For the

applied stresses of 4 MPa, 6 MPa and 8 MPa, PU1-IPDI failed in a more brittle

manner within 1.8x103 seconds. No obvious steady-state creep was found for

these three several of applied stresses on PU1-IPDI. The creep curves in Figure

4.3 show that PU1-IPDI is a soft and weak material.

As PU1-TDI and PU2-TDI, PU1-HDI showed a high creep strain rate at the start of

the creep and decreased with time. It exhibited a longer primary stage which took



about 7.0x106 seconds to proceed to steady stage. PU1-HDI showed the highest

creep strain among four samples, approaching approximately 550% to 635% of

strain at 1.0x107 seconds. No third stage creep is observed for PU1-HDI during the

creep test.

The isochronous method allows the plotting stress-strain curve of PU1-TDI as

plotted in Figure 4.5, which permits the strain interpolation for different stress levels

at a specific time of loading pertinent to particular application. These are obtained

by plotting constant time lines on creep curves from Figure 4.1 and reading off the

combination of stresses required to produce a specified strain. It also can be used

to evaluate the deformational behaviour of a sample because the data presentation

is similar to the conventional tensile test data. Figure 4.5 reports the isochronous

curves for time 100s, 1000s, 10000s and 100000s and proved the non-linearity

strain of the materials.

The Figure 4.6 shows the isometric curve of PU1-TDI at strain equivalent to 545%.

This isometric curve is obtained by plotting constant strain lines on creep curves at

various stresses and constant temperature. This isometric stress-time curve

enables the time required to reach a particular strain to be read off for a specified

stress and temperature. This data can be used as an approximation of stress

relaxation since stress relaxation test is a less common experimental procedure

than creep testing. As can be seen from the Figure 4.6, the stress needed for the



PU1-TDI to reach the strain of 545% at 1x104 s is 4 MPa.

For the purpose of comparison among the materials, the plot of creep modulus

versus time not only realistically ranks materials but also provides modulus values

for use in many design equations. Besides, creep modulus can be calculated by

dividing the initial stress by the strain at certain time to estimate the stress

relaxation. Figure 4.7 to 4.10 show the logarithmic creep modulus versus time at

various stress levels for all four samples. The creep modulus is directly affected by

the increase in the level of stress. The creep modulus being highest for the highest

applied stress.



(a)

0.0 2.0x106 4.0x106 6.0x106 8.0x106 1.0x107500

510

520

530

540

550

560

570

580

590

600

Stra

in/ %

Time/ sec

2MPa 4MPa 6MPa 8MPa

(b)

100 101 102 103 104 105 106 107500

510

520

530

540

550

560

570

580

590

600

Stra

in/ %

Time/ sec

2MPa 4MPa 6MPa 8MPa

Figure 4.1: Creep strain of PU1-TDI as a function of (a) linear and (b) logarithmic time at various stresses.



(a)

0.0 2.0x106 4.0x106 6.0x106 8.0x106 1.0x107340

360

380

400

420

440

460

480

500

520

540

Stra

in/%

T ime / sec

2MPa 4MPa 6MPa 8MPa

(b)

100 101 102 103 104 105 106 107340

360

380

400

420

440

460

480

500

520

540

Stra

in/%

Time / sec

2MPa 4MPa 6MPa 8MPa

Figure 4.2: Creep strain of PU2-TDI as a function of (a) linear and (b) logarithmic time at

various stresses.



(a)

0.0 5.0x103 1.0x104 1.5x104 2.0x104480

490

500

510

520

530

540

550

560

570

580

590

600

Stra

in/ %

Time/ sec

2MPa 4MPa 6MPa 8MPa

(b)

100 101 102 103 104 105 106 107480

490

500

510

520

530

540

550

560

570

580

590

600

Stra

in/ %

Time/ sec

2MPa 4MPa 6MPa 8MPa

Figure 4.3: Creep strain of PU1-IPDI as a function of (a) linear and (b) logarithmic time at

various stresses.

Chapter 4: Results and Discussion

s


Figure 4.4: Creep strain of PU1-HDI as a function of (a) linear and (b) logarithmic time at various stresses.

(b)

(a)

0.0 2.0x106 4.0x106 6.0x106 8.0x106 1.0x107480

500

520

540

560

580

600

620

640

660

Stra

in/ %

Time/

sec

100 101 102 103 104 105 106 107480

500

520

540

560

580

600

620

640

660

Stra

in/ %

Time/ sec

2MPa

2MPa

4MPa

4MPa 6MPa 8MPa

6MPa 8MPa


0

1

2

3

4

5

6

7

8

9

510 520 530 540 550 560 570 580Strain /%

Stre

ss/ M

Pa

1E2sec 1E3sec 1E4sec 1E5sec

Figure 4.5: Isochronous curve of PU1-TDI obtained from the data of Figure 4.1.


s


100 101 102 103 104 105 1060

1

2

3

4

5

6

7

8

9

10

Stre

ss/ M

Pa

Time/ sec

Figure 4.6: Isometric curve of PU1-TDI at strain = 545%.



10 0 10 1 10 2 10 3 10 4 10 5 10 6 10 71 0 3

1 0 4

Cre

ep M

odul

us/ M

Pa

T im e / s e c

2 M P a 4 M P a 6 M P a 8 M P a

Figure 4.7: Effect of stress on creep modulus PU1-TDI.

100 10 1 10 2 10 3 10 4 10 5 10 6 10 7103

104

Cre

ep M

odul

us/ M

Ps

T im e / se c

2 M P a 4 M P a 6 M P a 8 M P a

Figure 4.8: Effect of stress on creep modulus for PU2-TDI.



10 0 10 1 10 2 10 3 10 4 10 510 3

10 4

Cre

ep M

odul

us/ M

Pa

T im e / s e c

2 M P a 4 M P a 6 M P a 8 M P a

Figure 4.9: Effect of stress on creep modulus PU1-IPDI.

100 101 102 103 104 105 106 107103

104

Cre

ep M

odul

us/ M

Pa

T im e / se c

2 M P a 4 M P a 6 M P a 8 M P a

Figure 4.10: Effect of stress on creep modulus PU1-HDI.




4.2 Tensile Test Results

Tensile properties of two different molecular weight (Mw) of soft segment in

polyurethanes (Mw=1000 g/mol and 2000 g/mol) were calculated and shown in

Table 4.1. Table 4.2 shows the three different diisocyanates that were used to

compare the effect of molecular structure on tensile properties.

Chain extender plays an important role in the mechanical properties which can lead

to high elongation. 1,4-Butanediol is the chain extender that had been added to all

the four samples. All the PUs exhibited more than 700% elongation at a stretching

rate of 500 mm/min.

Necking phenomena was not observed for all the PUs at this stretching rate. The

tensile properties of PU were compositions dependent [40, 46]. As it can be seen

from Table 4.1, an improvement in mechanical properties was observed upon

reducing the glycol molecular weight at constant hard segment content. The

decrease of molecular weight of PTMO causes an observed increase in tensile

strength, elastic modulus and ultimate elongation. Tensile strength of PU1-TDI and

PU2-TDI were 25.59 MPa and 18.09 MPa respectively as listed in Table 4.1.

Figure 4.11 shows the stress-strain curve of PU1-TDI and PU2-TDI. It can be

observed that PU1-TDI with lower molecular weight of polyol has a higher tensile

strength. As the increase of molecular weight of polyol, more softness and flexible

chain was obtained. This result is analogous to that reported by Bajsic and his co-



workers [40]. For PU1-TDI and PU2-TDI, the strains at maximum load were 847.88

and 856.91% respectively. When the molecular weight of PTMO increases, the

block length of the soft segment will increase, but the length of hard segment does

not vary. A higher modulus and a weaker elongation at failure were observed when

the polyol molecular weight decreases. It has been observed by many authors [47,

48] that the molecular weight of polyol will influence the mechanical properties of

PUs.

From Figure 4.12 and Table 4.2, TDI-based polyurethane exhibits the best tensile

strength. Being an aromatic isocyanate, TDI is more reactive than the aliphatic

isocyanate namely HDI which results from delocalization of negative charge on –

NCO by its aromatic structure [3]. The mechanical properties of the elastomers are

significantly dependant on the crosslink density of the elastomeric networks [49]. A

higher extent of reaction on the –NCO group will result a higher magnitude for

crosslink density in TDI-based polyurethane. Besides that, higher rigidity of TDI-

based polyurethane was due to the direct association of diisocyanate group with

the phenyl ring.

IPDI-based polyurethane has the lowest tensile strength compared to the TDI and

HDI-based polyurethanes. IPDI-based polyurethane has a cyclic structure which is

lack of delocalization of the negative charge on –NCO group compared to TDI-

based polyurethane. Beside that, IPDI polymer without catalyst also showed a low

reactivity.


s


HDI-based polyurethane showed a lower tensile strength compared to TDI-based

polyurethane due to the lack of rigidity in its backbone chain. HDI-based

polyurethane showed the highest modulus properties compared to the former

diisocyanates. Modulus of HDI-based polyurethane is 855% higher than IPDI-

based polyurethane and 1640% higher than TDI-based polyurethane. This

substantially high value is due to the presence of hydrogen bonding between two

polymeric chains. This is facilitated by the even number of methylene groups

present in diisocyanate as well as in the chain extender. The methylene sequence

of HDI together with that of butanediol forms a tight crystalline structure, hence it

shows the highest modulus properties [50].

Chapter 4: Results and Discussions Table 4.1 Tensile properties of two different molecular weight of soft segments in polyurethanes. Samples Molecular

Weight

Tensile Strength

Auto. (MPa)

Strain at Max.

Load (%)

Modulus

(ManYoung)

Modulus

(AuntYoung)

Load at Auto.

Break (kN) 1 2

(MPa) (MPa)

PU1-TDI 1000 25.59 847.88 2.54 8.21 0.23

PU2-TDI 2000 18.09 856.91 2.27 5.84 0.15

Table 4.2 Tensile properties of three different diisocyanates structure in polyurethanes. Samples Tensile Strength

Auto. (MPa)

Strain at Max.

Load (%)

Modulus

(ManYoung) (MPa)

Modulus

(AuntYoung) (MPa)

Load at Auto.

Break (kN)

PU1-TDI 25.59 847.88 2.54 8.21 0.15

PU1-HDI 23.08 1109.09 41.71 43.50 0.09

PU1-IPDI 21.80 736.67 4.88 7.43 0.24

1 Modulus(ManYoung)- the ratio of stress to corresponding strain below the proportional limit of a material. It is also known as elastic

modulus or Young Modulus. 2 Modulus(AuntYoung)- the ratio of stress to corresponding strain beyond the proportional limit of a material before rupture.



0 200 400 600 8000

5

10

15

20

25

Stre

ss (M

Pa)

S tra in (% )

(a ) P U 1 -T D I (b ) P U 2 -T D I

(a )

(b )

Figure 4.11: Stress-strain curve of two different molecular weight of soft segments in polyurethanes.


s


0 200 400 600 800 10000

5

10

15

20

25

S

tress

(MP

a)

S tra in (% )

(a ) P U 1 -H D I (b ) P U 1 -IP D I (c ) P U 1 -T D I

(b )

(c )

(a )

Figure 4.12: Stress-strain curve of three different diisocyanates structure in polyurethanes.




4.3 Dynamic Mechanical Analysis (DMA) Test Results

The dynamic properties of PUs were illustrated in Figure 4.13 to Figure 4.16. The

temperature associated with the peak of tan delta (δ) reflects the glass transition

temperature (Tg), and its height and shape provide information about the degree of

order and freedom of molecular mobility of soft segment. In addition, crystallization

behaviour of the polymers can be described by the shape of the tan (δ) peak [51,

52].

The plots of tan (δ) versus temperature were shown in Figures 4.13 and 4.14.

Figure 4.13 shows the effect of soft segment chain length on its thermal transition,

whereas Figure 4.14 shows the effect of the hard segment structures. The

temperature at which tan (δ) attains maximum is determined as Tg. It is difficult to

measure exact Tg, since the peaks of the samples were not sharp, however the

roughly Tg of corresponding PUs could be estimated from the tan (δ) peak. Table

4.3 lists the Tg values determined from the peak of tan (δ) curves from dynamic

mechanical analysis (DMA) test.

The Tg of PU2-TDI is lower than of PU1-TDI. This indicates that the Tg shifted to a

lower temperature as the molecular weight of the soft segment is increased. This is

partially due to the better phase separation in the PTMO(2000)-based PU and the

solubilisation of the hard segments in the soft segments is lower than

PTMO(1000)-based PU. A greater degree of phase separation allowed by the

Chapter 4: Results and Discussions longer soft segment lengths resulting in a lower Tg value as reported by many

researchers [40, 48, 53].

Table 4.3: DMA Data.


Samples ( T taken at peak of tan (δ) 0.10C ) ±g

PU1-TDI + 6.5

PU2-TDI -26.4

PU1-IPDI - 2.0

PU1-HDI -58.8

PU1-TDI and PU1-IPDI show T close to each other. PU1-TDI shows higher Tg g

because of higher rigidity compare to that imparted by PU1-IPDI and is in

agreement to that reported by Pandya and his co-workers [50]. Moreover, PU1-

IPDI offers no crosslinking and imparts higher flexibility [50]. PU1-HDI shows the

lowest Tg is due to the maximum flexibility imparted by methylene sequence which

also found in agreement to that reported by Pandya and his co-workers [50]. The

increasing crystalline domain probably restricts the solubilization of the hard

segments in the soft segments and causes the T value to decrease [51]. The Tg g

values for all the PUs are substantially below the pure PTMO homopolymer [54] of

89°C. This indicates that some phase mixing of hard and soft segments occurs in

these polymers [55, 56].



Figure 4.13 shows that the tan (δ) peak shifted to lower temperature as the

molecular weight of the soft segment is increased. Figure 4.13 and 4.14 show that

tan (δ) peak shifted to lower temperature and its amplitude becomes smaller

according to the following order: PU1-TDI > PU1-IPDI > PU2-TDI > PU1-HDI. This

phenomenon is attributed to the decrease of degree of freedom for segmental

mobility of the polymer. For PU1-TDI sample, the glass transition occurs within a

narrow temperature range whereas for others samples, it occurs over a much

broader tan (δ) peak. Tan (δ) value of PU1-TDI is the highest among four samples

indicate greater mobility of the polymer chains, while PU1-HDI shows the lowest

tan (δ).

Storage modulus (E’) as a function of temperature is shown in Figure 4.15. Below

the glass transition temperature, Tg, all the samples exhibit similar storage modulus

of approximately 3 GPa except for PU1-TDI. PU1-TDI shows the highest storage

modulus among the four samples.

From Figure 4.15, it can be seen that once the Tg was reached, the modulus

begins to decrease rapidly (inflection point). The inflection point corresponds to

maximum in tan (δ) versus temperature graph as it can be seen from Figure 4.13

and Figure 4.14. The modulus for samples with lower crosslink density and

crystallinity decreased at a lower temperature and at a faster rate than the samples

with higher crosslink density and crystallinity [57].


s


Beside, the Tg value can be obtained from the peak of the loss modulus curves

change with temperature as shown in Figure 4.16. However, the Tg value for PU1-

IPDI could not be identified as there is no peak was seen.



-100 -50 0 50 100

0.0

0.2

0.4

0.6

0.8

1.0

T e m p e ra tu re ( 0C )

tan

delta

P U 1 -T D I P U 2 -T D I

Figure 4.13: Loss factor (tan delta) versus temperature of two various molecular weights of soft segment in polyurethanes.



-100 -50 0 50 100

0 .0

0 .2

0 .4

0 .6

0 .8

1 .0

tan

delta


P U 1 -T D I P U 1 -IP D I P U 1 -H D I

Figure 4.14: Loss factor (tan delta) versus temperature of three different diisocyanates structure in polyurethanes.



-100 -75 -50 -25 0 25 50 75 100 125 150 1751

10

100

1000

T e m p e ra tu re (0C )

E' (M

Pa)

P U 1 -T D I P U 2 -T D I P U 1 -IP D I P U 1 -H D I

Figure 4.15: Storage modulus as a function of temperature.

s


-7 5 -5 0 -2 5 0 2 5 5 0 7 5 1 0 0 1 2 5 1 5 0

1

1 0

1 0 0

P U 1 -T D I P U 2 -T D I P U 1 -IP D I P U 1 -H D I


E" (

MP

a)

Figure 4.16: Dependence of Loss modulus on temperature.




4.4 DSC Results

Thermal transitions of the PUs were measured by DSC. In order to avoid the

influence of different thermal history of the samples, all the samples were heated

from -60°C to 150°C, cooled to -60°C and then reheated to 200°C at 5°C/min.

Figure 4.17 to 4.20 illustrate DSC scans for reheated samples and the data were

summarized in Table 4.4. When reheating the samples, PU1-TDI and PU1-HDI

showed a sharp exothermic peak at 101°C and 105 °C respectively. PU2-TDI

showed two sharp exothermic peaks at 88°C and 104°C whereas PU1-IPDI

showed a small peak at 121°C.

From Figure 4.21, PU2-HDI showed an endothermic peak at the temperature

around 155°C. Without proper compound data, it would be difficult to assign the

observed melting endotherm unambiguously. No significant endothermic peak

appeared for PU1-TDI, PU2-TDI and PU1-IPDI which indicated the absence of

crystallinity in these materials.

The DSC and DMA data presented in Table 4.4 shows that the Tg values obtained

from DSC are in good agreement with those obtained from DMA. The Tg values

obtained from DSC and DMA differ somewhat may be attributed to the frequency

dependence of the transition phenomenon [19, 43, 58]

Chapter 4: Results and Discussions Segmented polyurethane elastomers with rubbery soft-sgments are microphase

separated and the degree of microphase separation was usually evaluated by

means of the Tg values [59, 60]. When there are hard segments disperse in the soft

segments, the Tg value will increase. From the DMA and DSC results, PU1-TDI

shows the highest Tg value. In keeping with these observations it is expected that

there would be more hard-soft segment mixing in PU1-TDI sample.

Table 4.4: DSC and DMA Thermal Transitions of polyurethanes.

Samples T from DSC Results T from DMA Results g g

(T ± 0.01°C) (T ± 0.1°C) g g

PU1-TDI 6.81 6.5

PU2-TDI -25.80 -26.4

PU1-IPDI 1.88 -2.0

PU1-HDI -50.31 -58.8



Method: D-60-200_3step/5/N2=50ml -60.0-150.0°C 5.00°C/min N2 50.0 ml/min 150.0--60.0°C -5.00°C/min N2 50.0 ml/min -60.0-200.0°C 5.00°C/min N2 50.0 ml/min

Glass TransitionOnset 5.90 °CMidpoint 6.81 °CEndpoint 7.63 °C

]3[UU, 10.9200 mg

mW

-3

-2

-1

0

min

°C-60 -40 -20 0 20 40 60 80 100 120 140 160 180

0 5 10 15 20 25 30 35 40 45 50

êxo

SW 8.00


eRTASLab: METTLER Figure 4.17: DSC thermogram of PU1-TDI

Glass TransitionOnset -35.74 °CMidpoint -25.80 °CEndpoint -27.45 °C

]3[PU2-TDI(J)PU2-TDI(J), 9.5500 mg

mW

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

min

°C-60 -40 -20 0 20 40 60 80 100 120 140 160 180

0 5 10 15 20 25 30 35 40 45 50

êxo PU2-TDI( J) 10.04.2008 12:24:36

SW 8.00eRTASLab: METTLER Figure 4.18: DSC thermogram of PU2-TDI


s


Glass TransitionOnset 1.79 °CMidpoint 1.88 °CEndpoint 1.85 °C

Method: D-60+200/3step/5/N2=50ml -60.0-150.0°C 5.00°C/min N2 50.0 ml/min 150.0--60.0°C -5.00°C/min N2 50.0 ml/min -60.0-200.0°C 5.00°C/min N2 50.0 ml/min

]3[A2A2, 10.2600 mg

mW

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

min

°C120 140 160 180

35 40 45 50

SW 8.00

-60 -40 -20 0 20 40 60 80 100

0 5 10 15 20 25 30

êxo

SLab: METTLER eRTA

êxo

Method: D-60-200_3step/5/N2=50ml -60.0-150.0°C 5.00°C/min N2 50.0 ml/min 150.0--60.0°C -5.00°C/min N2 50.0 ml/min -60.0-200.0°C 5.00°C/min N2 50.0 ml/min

Glass TransitionOnset -53.36 °CMidpoint -50.31 °CEndpoint -50.57 °C

Figure 4.19: DSC thermogram of PU1-IPDI

]3[PU1-HDI(H)PU1-HDI(H), 9.4900 mg

mW

-3

-2

-1

0

1

min

°C-60 -40 -20 0 20 40 60 80 100 120 140 160 180

0 5 10 15 20 25 30 35 40 45 50

SW 8.00SLab: METTLERFigure 4.20: DSC thermogram of PU1-HDI

eRTA

s


-0.35

-0.3

-0.25

-0.2

-0.15

-0.1

-0.05

0

0.05

0.1

0.15

-75 -25 25 75 125 175

Temperature (°C)

Hea

t Flo

w (W

/g)

PU1-TDI PU2-TDI

PU1-IPDI PU1-HDI

Figure 4.21: DSC thermograms of 4 various polyurethanes.




4.5 XRD Results

The X-ray diffraction patterns of the PUs are shown in Figure 4.22 to Figure 4.25.

All the PUs showed no explicit diffraction peak but a broad scattering peak with a

maximum at around 20° of 2θ and a halo spreading over 30° to 60° of 2θ.

Additionally, PU1-HDI exhibited a peak at 24° of 2θ and a sharper upper half peak

height area which indicates that this sample has much bigger crystal particles. It

means that the degree of crystallinity of the PU1-HDI is the highest among all Pus.

Table 4.5 showed the Scherrer length, L of all four samples. PU1-HDI showed the

highest of Scherrer length, whereas PU2-TDI showed the lowest value of Scherrer

length. This study showed that the crystillinity depends on the structure of

diisocyanates and it also agreed with that reported by Zia that the crystallinity

decreased from aliphatic to aromatic characters of diisocyanates used in the PU

[61]. The methylene sequence of HDI together with that of butanediol forms a tight

crystalline structure in the HDI-based polyurethane; hence it shows the highest

degree of crystallinity. Due to this, PU1-HDI also shows the highest modulus

properties among the four samples.

Beside that, in case of aliphatic diisocyanate such as PU1-HDI, the chain flexibility

showed maximum degree of freedom for hard segment to get organized crystalline

form. This degree of freedom decrease to cyclo-aliphatic based PU (PU1-IPDI) and

the main effect was observed in aromatic PU (PU1-TDI and PU2-TDI).


0 10 20 30 40 50 60 70 80 90

0

50

100

150

200

2θ

Inte

nsity

(Cps

)

Figure 4.22: XRD pattern for PU1-TDI.


0 10 20 30 40 50 60 70 80 90

0

50

100

150

200

2θ

Inte

nsity

(Cps

)

Figure 4.23: XRD pattern for PU2-TDI.


0 10 20 30 40 50 60 70 80 90

0

50

100

150

200

2θ

Inte

nsity

(Cps

)

Figure 4.24: XRD pattern for PU1-IPDI.

0 10 20 30 40 50 60 70 80 90

0

50

100

150

200

2θ

Inte

nsity

(Cps

)

Figure 4.25: XRD pattern for PU1-HDI.




Table 4.5: Scherrer length calculated from XRD results.

Samples Scherrer Length, L (Å)

PU1-TDI 9.75 ± 0.01

PU2-TDI 9.04 ± 0.01

PU1-IPDI 10.25 ± 0.01

PU1-HDI 11.83 ± 0.01

4.6 SEM Results

Figure 4.26 to 4.29 show the SEM results for all the four samples with

magnification of 500. It is seen from the figure that the PU1-TDI and PU2-TDI show

almost the same morphology, but PU2-TDI shows more amounts of particles with

bigger size compared to PU1-TDI. From the Figure 4.28, PU1-IPDI shows the most

amounts of particles with the biggest size.

The morphology of the PU1-TDI and PU2-TDI are relatively homogenous compare

with other samples, indicating a relatively dense architecture in the inner structure.

This implies that a strong intermolecular interaction has occurred between the hard

segments and soft segment for PU1-TDI and PU2-TDI, leading to a good

miscibility.

Chapter 4: Results and Discussions PU1-HDI shows a very different morphology among the four samples, which can

be seen from Figure 4.29. It shows that PU1-HDI exhibits multiphase

morphologies. From the DMA and DSC results, PU1-HDI showed the lowest Tg

value and indicated that the degree of microphase separation of PU1-HDI is the

highest. It has been observed by many authors that better physical properties are

achieved when microphase segregation between soft and hard segment increases

[43, 62]. It is obvious from tensile test and creep test that PU1-HDI showed the

highest modulus properties which is also agree with the work done by other

researchers as mentioned above.

Figure 4.26: SEM micrograph of PU1-TDI (Mag.=500x).



Figure 4.27: SEM micrograph of PU2-TDI (Mag.=500x).

Figure 4.28: SEM micrograph of PU1-IPDI (Mag.=500x).



Figure 4.29: SEM micrograph of PU1-HDI (Mag.=500x).


Chapter 5: Conclusion and Recommendations


CHAPTER 5: CONCLUSION AND RECOMMENDATIONS

In the present work, the creep behaviour of four types PU samples has been

studied. The three different isocyanates which are TDI, IPDI and HDI were selected

to synthesis three types of polyurethanes. PTMO of molecular masses 1000

mg/mol and 2000 mg/mol were selected to study their effect on mechanical

properties.

Polyurethane elastomers are block copolymers which consist of polyol sequences

(soft segments) and diisocyanate sequences (hard segments). The compositions

and the conditions of processing are the main factors which influence the chemical

structure and their mechanical properties.

The study on mechanical properties reveals that PU1-TDI has the highest tensile

strength. Its tensile strength is 25.59 MPa whereas PU1-HDI is 23.08 MPa, PU1-

IPDI is 21.80 MPa and PU2-TDI is 18.09 MPa. PU2-TDI showed a lower tensile

strength compare to PU1-TDI due to the difference of the molecular weight of

polyol. The increase of molecular weight of polyol causes an observed decrease in

tensile strength and elastic modulus and leads to increase in strain at maximum.

HDI-based polyurethane showed the highest modulus properties (41.71 MPa) due

to the presence of hydrogen bonding between polymeric chains. The methylene



sequence of HDI together with that of butanediol forms a tight crystalline structure.

As the XRD results also indicated that PU1-HDI showed the highest degree of

crystallinity. From the Scherrer length calculation, it can be said that the crystallinity

decreased from aliphatic to aromatic characters of the diisocyanates used in

synthesis PUs.

The Tg values obtained from the DMA are in good agreement with those obtained

from DSC. The DMA and DSC results showed the effect of soft segment chain

length and the effect of hard segment structure respectively. The Tg shifted to a

lower temperature as the molecular weight of the soft segment is increased. A

greater degree of phase separation allowed by the longer segment lengths results

in greater phase purity and the lower Tg observed.

PU1-TDI shows higher Tg because of higher rigidity compared to that imparted by

PU1-IPDI. Moreover, PU1-IPDI offers no crosslinking and imparts higher flexibility.

PU1-HDI showed the lowest Tg is obviously due to maximum flexibility imparted by

methylene.

SEM analysis shows that PU1-HDI exhibits multiphase morphologies whereas the

morphologies of the PU1-TDI and PU2-TDI are relatively homogenous. From the

DMA and DSC results, PU1-HDI shows the lowest Tg value and indicated that the

degree of microphase separation of PU1-HDI is the highest. It is obvious from

tensile test and creep test that PU1-HDI shows the highest modulus properties. It



can be said that better physical properties are achieved when microphase

segregation between soft and hard segment increases.

Polyurethanes can be fabricated to a range of toughness and softness values to

meet various applications by varying the amount or the types of diisocyanate,

polyol and chain extender. As can be seen from the tensile stress-strain graph in

chapter four, all four types of polyurethanes show different range of toughness and

softness.

From the long-term creep test, it can be observed that all four types of

polyurethanes showed the viscoelastic behaviour, as such, exhibit time dependent

strain. The non-linearity strain of the PU also had been proved by the isochronous

curves in Figure 4.5. The results obtained from the creep test showed that PU1-

HDI exhibited the highest creep strain among the four samples. PU1-IPDI shows

the highest creep deformation among the four samples. PU1-IPDI failed within

1.8x103 seconds for the applied stresses of 2 MPa, 4 MPa, 6 MPa and 8 MPa. No

failure occurred for PU1-TDI, PU2-TDI and PU1-HDI until the end of the creep test.

As can be concluded that, PU1-IPDI exhibited the poorest mechanical properties

among the four samples.

The mechanical properties of PUs depend upon several factors like molecular

weight, types of polyol and diisocyanate, effect of NCO/OH ratio, synthesis

conditions and thermal history. The used of different polyols, NCO/OH ratio and



chain extender should be considered in an attempt to improve the mechanical

properties.

In the addition of the mentioned mechanical properties test, there are certain other

areas of testing and theoretical analysis that can be performed on the

polyurethanes to determine their properties and evaluate their structural behaviour

over time. Creep experiment can be run by varying the temperature with constant

load to study the effect of the temperature on creep. Then, the creep data can be

analyzed in terms of time-temperature superposition formalism to prove the

equivalence of time and temperature.

In the creep test, the cross sectional area of the sample decreases as the

elongation increases. So, the true stress applied to the sample increases with the

creep time. The load during the test should be decreased with the increase of the

elongation of the specimen to minimize this error.

For further research, it is suggested that the effect of different NCO:OH ratio can

also be investigated. Other characterization techniques such as Atomic Force

Microscopy (AFM), Fourier Transform Infrared Spectroscopy (FTIR), and Nuclear

Magnetic Resonance (NMR) can also be carried out in order to study the surface

morphology and structural of the prepared samples.

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