Thermal, Crystallization and Structural Studiesshodhganga.inflibnet.ac.in/bitstream/10603/221/15/15...Thermal, Crystallization and Structural Studies 91 4.1 Introduction Most of the

Thermal, Crystallization and Structural Studies

Table of Contents

Introduction

Experimental details

Results and discuss'ion

4.3.1 Modulated difierential scanning

calorimern (MDSC) studies

4.3.1 (i) Melting Ixhavior

4.3.1 (ii) Cqstallizatior~ behavior

4.3.2 X-ray diffraction analysis

4.3.3 Raman studies

Conclusions

Bibliography

Index

91

93

94

94

95

98

102

111

114

I l t

12C

Thermal, Crystallization and Structural Studies 91

4.1 Introduction

Most of the products manufactured from polymers are made by either injection

moulding or extrusion technique. The properties of injection-molded and extruded

products made from polymer composites are governed by their morphology In

case of semicrystalline polymers the morphology is influenced [4.1] by their

crystallization behavior. The morphological features such as degree of crystallinity,

crystallite size, interfacial interactions, domain shape and size are affected by the

relative rates of crystallization of polymer. All these properties in turn are

dependent on the processing conditions and inherent polymer characteristics such

as miscibility, melt flow and crystallisability of the polymer in the composite.

Knowledge of the crystallization behavior of the thermoplastic blend is therefore

necessary for effective manipulation of properties and for specifying the moulding

conditions. ' f ie thermal and crystallization behavior ofthe constituents of polymer

composites are influenced by their relative amounts, chemical compatibility and

dispersion level. Other factors such as the relative crystallisability, diffusion of

the non- crystallizing component and kinetic features associated with the

conditions of processing also influence the phase morphology [4.2]. Therefore,

a scientific understanding of the crystallization behavior of the polymers is

necessary for optimizing processing conditions and properties.

The thermal behavior of polymeric materials is highly different from that of low

molecular weight substances due to special structure of macromolecules. Thcy

do not melt sharply, ie.. the intermolecular forces are not overcome by molecular

motion at a definite temperature. When a blend is cooled gradually from ambient

conditions, the various modes of molecular motions within it begin to be

successively frozen.

LLDPE is a semicrystalline material. Semicrystalline polymers have a range of

properties depending [4.3] on the amount of crystallinity. These properties can

be further enhanced through the use of fillers. Semicrystalline polymers exhibit

92 Chapter 4

short range as well as long range ordering of macromolecular chains. Both types

of ordering contribute towards the degree of crystallinity of the polymer. Several

recent articles deal with the crystallization behavior of various polyblends [4.4 -

4.161. In this study the investigations are aimed at studying the miscibility of two

phases (matrix and Filler) by determining the effect of composition on degree of

crystallinity and structure property relationships.

The crystallization behavior in polymer composites essentially d~ffers from that

in neat polymer systems. A detailed investigation on the fusion behavior and

crystallization kinetics of this polymer composite is essential to understand the

effect of addition of superconductor on the crystalline structure formed by the

polymer. The key to successful blending of polymers with superconductors of

higher strength is to achieve good polymer-superconductor interaction.

Understanding the crystallization and thermal behavior of these polymer-

superconductor composites is relevant to their processing and product

development. A detailed investigation on the fusion behavior and crystallization

kinetics ofthis polymer composite is essential to understand the effect of addition

of superconductor on the crystallinity.

Modulated Differential Scanning Calorimetry (MDSC) analysis can be used

[4.17 - 4.191 to study the mechanism of the melting and recrystallization of

polymers. The crystallization behaviour has been investigated in terms of the

size of crystallites [4.20 - 4.221 and the crystallization temperature in MDSC

scans [4.?3 - 4.261.

X-ray diffraction (XRD) is a powerful non-destructive technique for characterizing

various forms of materials. It provides information on structures, phases, preferred

crystal orientations (texture) and other structural parameters such as average grain

size, crystallinity, strain and crystal defects. The Bragg's law is the cornerstone * of X-ray diffraction analysis. In the powder technique the crystal sample is in a

finely powdered form, in which small crystals are oriented in every possible

Thermal, Crystallization and Sructural Studies 93

d~rection. U'hen the X-ral penetrates ihrvugh the ~xiteri;~ I. a n u t n k of pirtieles

can be expected to bc oriented in such u \\;I)- as tu lilltill rhc Brapg condition for

reflection from even possible inteylanar spacing 14.271.

X-ray diEactometry method could be success full^ used for the de temh~t ion of

the atomic structure for crystalline solids, since crystals have an accurate periodic

texme and play a role of difractio~l gating for X-rays [4.28]. The advent of

hgh resolution photon detectors and on-line con~puting has replaced the age-old

photograplc technique.

Raman spectroscopy has been used as a prohe of the atomic structure of

su~rconductor-polynler composites. Compared to other techques, it offers a

nondestructive and wnvenient means for structural studies. R ~ n ~ i t t spectroscopy

is known to be a powerful tool for the study of the lociil structure and bonding

states of semiconductors due to its ability to probe the synunetry changes in the

material on polymerization.

In this chapter the then~ial, crystallizc~tion and structural inves!igation of the

sipercond~ictoripolymer composites are cnmcd out. l l e effect of filler addition

on the ciystallinih of these coniposites has hen analyzed b\ mnodulated dierential

s ~ ~ n n i n g calorimetry (MDSC)and X-my ditfraction studies (XRD). The structural

investigation of the supercondwtor/pul~rncr composites are also made by using

XRD and Raman studies. For deternunation of the structure of materials, X-ray

diffraction is the obvious first choice. But an attempt llas been made to use

Raman spectroscopy also, since i t is an extremely powedi~l method to understand

the structural aspects of these composites due to its nbilig. to prohe the symmetr).

changes in the campsites on polynerization.

4.2 Experimental details

s The MDSC studies are conducted using n TA MDSC 29 10 calorimeter equipped

with a refrigerated cooling system with the baseline correction applied using an

94 Chapter 4

empty pan. Theheatkc rate is kept constant at 10°C pa~ninute . DSC is recorded

for both heating and cooling cycles. Endothermic changes like melting are

recorded during heatins cycle, and exothermic changes like crystallization are

recorded during cool i~~g. Thc melting point (T,) as well as tenlperature of

crystall~zation [rJ and orr responding endothem~ic ( A [-I;) and exotlemic ( A IIJ

changes in heat capaclly is recorded. All meclsurctnents are nude on samples

that are given the same thermal history to ensure meanixghl comparisons between

the samples. The MIISC measurements are performed at IIJC-DAE, Indore.

Please see chapter 2 for a detailed account of this technique.

Powder X-ray difikaction is used to idnltifi cnst;~lline phase. The X-rav ditfraction

pattern of most of the samples are taken at IIJC-DAE, Indore on R i p k u X-ray

diffractometer from Japan with CuKn radiation (7. = 1.5406AU). The

difiactogam is rccordcd ul terms of 20 in the range of 10 to 60' and in the range

of 20 to 60'. ,The full width at half rnasin~um ( F U ; I w for pure YHCO peak

and for composite peaks are obtained by XXKD hi operating voltage of 40kV

and filament current of 3ChnA are ilscd. I)elm'-Scherrer metllod is used for X-

ray analysis.

The Raman n ~ e a s w e n ~ ~ ~ ~ t s are done on a micro Raman Spectrometer from Jobin

Yvon Horihra LABRAM-HR using a He-Nc laser source (6331x11) and the spectra

are recorded in the wn\' numlxr mnge from 100-50Oc111-~. These measurements

are also done at IUC-I IAE, indore. Detailed description of the technique hiis

already k e n given in ciiapter 2 section 7.1 0 .

4.3 Results and Discussion

4.3.1 Modulated differential scanning calorimetry (MDSC) studies

The technique of nlodulnted dif'ferentisl scanning calorhnetty (MDSC) is used

for im'estigating the thcmil and crvsialllz~tion hekiviors. The thennal behavior * and crystallizatio~~ rate of three diffcrent compositions ofpnl3mer-superconductor

Thermal. Crystallization and Structural Studies 95

composites are reported. The sample compositions used for this study are LLDPE,

loo%, Y- 123/LLDPE, 50% and Y-123/LLDPE, 20%.

4.31 i ) Melting behavior

DSC thermograms of virgin LLDPE and its composites withsuperconductor (YBCO)

in the regons of melting recorded during the heating cycle are shown in figure 4.1(A) - (C). These are usedto determine anumbaofparameters significant in melting behavior.

These include the temperature of onset ofmelting, the melting peak temperature, melting

temperature range (widthofthe melting peak) and heat of fusion. The various parameters

in the melting process are tabulated in Table 4.1.

Table 4.1 : Thermal behavior (melting) of LLDPE virgin, 50% LLDPE composite and 20% LLDPE composite

It can be seen from the table that, the equilibrium melting temperature of LLDPE

depends on the blend composition. Melting temperatures of crystalline polymers

can be related [4.29] to the size and perfection of their crystal units. LLDPE

shows a typical endothermic peak at 122°C. The peak melting temperatures in

LLDPE,100%, Y-123/LLDPE,50% and Y-123/LLDPE,20% compositions are 122,

122 and 123°C respectively and the melting endothems also start at a lower

temperature in these composites. This is probably due [4.30] to the larger

cqstallite size and narrower size distribution of LLDPE in these compositions.

The LLDPE melting range is also considerably smaller for low superconductor

Sample Composition

LLDPE, 100%

Y-123LLDPE, 50%

Y-123LLDPE, 20%

Hr caYg

378

315

296

Onset OC

107

106

105

Peak OC

122

122

123

Pbak width OC

24

27

30

content composites, compared to LLDPE which also suggests a narrower crystallite

distribution. This conclusion is M e r supported by the considerable decrease in the

heat of crystallization of LLDPE in these composites. These features could be explained

as follows. During the process of melting, the amorphous content will be able to

selectively dissolve a certain amount of the more defective LLDPE molecules (i.e., of

lower molecular weight). Therefore, duringpressure moulding and the successive

rapid crystallization this segregation between high molecular weight LLDPE and the

new superconductorlLLDPE phase containing defective LLDPE chains will still be

retained. The low molecular weight &action can crystallize faster than high molecular

weight [4.3 11. This will result in more perfect crystals (high Tm, the melting peak

temperature and also Tc, the peak crystallization temperature) and narrower distributions

of lamellae or crystallite dimension (lower width at halfheights of Tm and Tc peaks) for

the composites than pure LLDPE. It can be seen from the figure 4.1 (A) - (C) that the

20% LLDPE sample has a wide melting peak. In the case of 50% LLDPE, a second

peak begins to appear that is much smaller in area. The 100% LLDPE composite has

a narrower melting peak and a lower temperature peak begins to appear which is

smaller in area. It would seem [4.32] that the 20% sample has a distribution ofcrystal

sizes and this is the origin of the wider, shallower peak.

Temperature (OC)


110 120 80 90 lm 130 I60

Temperature (%)

Temperature ( O C ~

Figure 4.1 : DSC scans of (A) LLDPE virgin, (B) 500.6 I,L.DPE composite and (C) 20% II,LDPE compsite in heating mode.

In the case of LLDPE in the composites, the crystallization from the molten phase

takes place in the presence of solidified Y-123 particles which may act [4.33] as

heterogeneous nuclei. It can be seen that the heat of fusion of 50% LLDPE is

comparable to that of 20% LLDPE whereas it is higher for 100% LLDPE

compositions (Table 4.1). Fibwre 4.2 shows the variation of heat of fusion with

LLDPE volume Fraction.

98 Chapter 4

Volume fraction of LLDPE(%)

Figure 4.2 :A plot of enthalpy released as a function of volume fraction of LLDPE

4.3.1 (ii) Cystallizatiort behavior

The DSC thennograms of virgin LLDPE an3 its composites with superconductor

in the rebions of crystallization, recorded during the cooling scan are depicted in

figure 4.3(A) - (C). The clystallization behavior of the composites is compared

to their behavior in the virgin polymer phase. The crystallization of LLDPE is

studied over a temperature range of 90 to 115OC.

Temperature (OC)


Temperatun, (OC)

Temperature (OC)

Figure 4.3 : DSC scans of (A) LLDPE virgin, (B) 50% LLDPE composite and (C) 20% LLDPE composite in cooling mode

The crystallization of virgin LLDPE exhibits two peaks indicating two different

crystallization processes probably [4.33] corresponding to homogeneous and

heterogeneous nucleation. In the case of iOoh LLDPE composite, a very small

peak along with the main crystalline peak also appears. Rut in the case of 20%

LLDPE, only a single peak is seen. This indicates that the overall amount of

crystallinity is increasing in the composites as the superconductor content

decreases.

The parameters sign~ficant in the crystallization behavior such as the temperature

of onset of crystallization (Tco), which is the temperature where the thennogram

initially departs from the baseline on the high temperature side of the exotherm,

the peak temperature of the crystallization esotherm (TJ, and the heat of

crystallization ( A I lc) are shown in Table 2. The de_mee of supercooling, AT, is

calculated as the difference between the melting peak temperature and the

temperature of onset of crysta!lization in the cooling scan.

Table 4.2 : Thermal behavior (crystallization) of LLDPE virgin,

50% LLDPE composite and 20"0 LLDPE composite

From Table 4.2 i t is clear that the addition of superconductor suppresses

crystallinity in the composites. Also the heat ofcrystallization,AHc iscomparable

to that of 20% LLDPE in the case of 50% LLDPE, while it is higher for 100%

LLDPE composites. The results presented here show that having increased surface

area for crystal nucleation, i.e., adding Y-123 particles is not enough to enhance I

crystallinity. Addition of increasing concentration of superconductor to LLDPE


resulted in a decrease in Tm of LLDPE A decrease in Tm in the composites

clearly indicates [4.34] that the inclusion of a superconducting phase in LLDPE

results in delayed nucleation. The differcncc in To> and Tc values of LLDPE in

the composites were lower than that for pure LLDPE. Such a decrease can be

attributed [4.35] to an increase in the rate of crystallization. Beck and Ledbetter

[4.35] suggested that the small the differcncc between the onset and peak

temperature (Tc,;Tc), the faster the overall crystallization rate. The onset

temperature indicates the beginning of the crystallization process, while the

maximum of the exothermic peak indicates [4.36] the occurrence of the spherulite

impingement. The free expansion of spherulites occurs between the onset and

peak temperatures. The increase in the peak crystallization temperature ofLLDPE

in the lower superconductor content composites relative to virgin LLDPE

suggested [4.37,4.38] that the crystal growth of LLDPE in the composite would

takes place at a higher temperature leading to larger crystallite size, improved

crystal perfection and narrower crystallite size distribution. These conclusions

are supported by the melting characterization of the composites.

Integration of thermal analysis data is a commonly used technique for the

estimation of crystallinity if the latent heat of fusion of a perfect crystal is known.

For LLDPE it is generally accepted [4.39] that a value for the latent heat of fusion

in the neighborhood of 292 Jig is appropriate. The heat of fusion value depends

on the crystallinity of the material. The crystallization fraction of the polymer

and composites is determined from the DSC results using the relation,

where AH, is the heat of fusion, AH, i s the theoretical heat of fusion of 100%

crystalline LLDPE and m,is the mass fraction of the superconductor. The values

of the crystallinity degrees of LLDPE in the composites thus evaluated are

presented in Table 4.3. Table 4.3 contains To,, the melting temperature and X, the

crystalline fraction of polymer and composites.

102 Chapter 4

Table 4.3 : Crystalline fractions of LLDPE virgin, LLDPE, 50% composite and LLDPE, 2096 compostte.

From Table 4.3 it can be seen that the addition of superconductor causes a large

drop in overall crystallinity of LLDPE in the composites. This may be due to the

fact that limited crystallization could be expected if the spacing between the

superconductor particles in the bulk is small. Incomplete crystallization leads to

decrease in AH and hence crystallinity.

Sample composition

LLDPE,100%

Y-123lLLDPE. 5Vh

Y-123/LLDPE, 20%

4.3.2 X-ray diffraction analysis

As mentioned earlier in this chapter, powder X-ray diffraction is used here to

T, "C

122

122

123

identify crystalline phase [4.40, 4.411 and to see whether reaction between

superconductor and pol~mer occur or not. The sample compositions used for

this study are Y-123,100%, Y-123/LLDPE,20%. Y - ~ ~ ~ I L ~ D P E , S O % ,

Y-123/LLDPE, 65%, and LLDPE,IOO%. Fibwres 4.4 (A) to (C) show the X-ray

diffraction patterns for Y-123, Y-123LLDPE,65?6 composite and LLDPE pellet

used in this study.

Crystalline fraction, X %

0 772

0 224

0 033

2-Theta angle


2-Theta angle

u . - c €4 3

50

m - 40 sir- 2-Theta angle

Figure 4.4 : X-ray diffraction patterns of (A) Y-123 prepared in this study,

(B) Y- 123iLLDPE,65% (C) LLDPE pellet.

The XRD pattern shown for the YBCO in the above figwe indicates that it consists

only of the single phase orthorhombic material whose pattern was described by

Beyers et al. [4.42]. It has a broad peak at about 28 - 32.8", which is its

D characteristic peak. The XRD pattern of LLDPE reveals high intensity peaks,

which correspond to the c~ystalline regions and the low intensity peaks, correspond

104 Chapter 4

to the amorphous regions. The distribution of the scattering intensity for a

composite represents a superposition of contribution from each component. The

LLDPE crystallized or amorphous structural state seems [4.43,4.44] to depend

on its fraction in the composite. The XRD pattern of the pure LLDPE showed

two sharp peaks at 21.4' (110 plane) and 23.6' (200 plane). These peaks are

Sound to decrease with decreasing proportion of the polymer and increasing

proportion of the YDCO in the composites

Both crystallized LLDPE and Y-123 phases are clearly identified on each phase

of the composite samples. For the composites, the positions of the Bragg

diffraction peaks do not shift significantly with respect to those of pure LLDPE.

This clearly indicates that the crystal structure of LLDPE remains unchanged

upon addition of different volume % of superconductor, i.e., addition of

superconductor did not produce an effect on the crystalline structure oftheUDPE

in any of the composites, indicating [4.45] the occurrence of no molecular level

interaction.

The X-ray diffractogram when compared with the data available in the Joint

Committee on Powder diffraction Standards (JCPDS) file [4.461 confirms both

crystallized LLDPE and Y- 123 phases. Both powder and composite samples have

the single phase orthorhombic structure and no shift in the peak positions. It is

already mentioned that there is no structural change in superconducting YBCO

compound due to polymer addition. However in the composite samples. additional

peaks are observed at 20 = 21.5' and 23.9" apart from those of pure YBCO. The

intensities of these additional peaks are found to increase with the increase in

levels of polymer addition. 'She presence of these extra peaks in the diffraction

pattern indicates the formation of a second phase in the system. There is no

evidence of any additional peaks other than those of YBCO and LI.DPE in the

XRD patterns indicating that there is no detectable reaction (within the precision ' of XRD) between the two materials. The XRD patterns of the composites show

that the orthorhombic structure is preserved throughout the entire LLDPE range.

This ~ndicates that the oxygen content is stable and not affected by LLDPE.


The appearance of the sharp peaks both i n the pure YBCO as well as in the

composites may indicate some degree of crystallinity in the composites. As

mentioned, the basic peaks of Y-123 are intact in :he composite, indicating that

the material had not changed during the preparation of the Y-I23lLLDPE

composite, nor were the relative peak psitions sh~hed significantly due to the

presence of the polymer. No additional peaks occur which might indicate 14.471

a third phase introduced in the composites. The change in the relative intensity

ofthe pealis in composites is due to crystal alignlent during preparation processes.

All these factors confirmed that YBCO does not react with the polymer.

As the superconductor content in the composite increases, the intensity of the

crystalline diffraction peaks decreases and there is an enhancement of the diffuse

amorphous scattering. These variations in the peak heights could be due [4.45]

to the variation of the mean spherulite sizc or their distribution, deformation at

the spherulite boundaries or any long-range order induced in the structure by the

dispersion of superconductor domains in the 1.L.DPE matrix.

The XRD patterns of Yt3COfl.I.DPIJ cornpmlte samples contalnlng 50 and 80%

YBCO are shown in figure 4 5

106 Chapter 4

Figure 4.5 : The XRD peak protiles of YBCOILLDPE composite samples contalnlng 50 and 80% YBCO

It can be seen that the width at half height of the XRD peaks also increased upon

the addition of superconductor. This also indicates the increase in amorphous

nature ofthe composites as [4.43] compared to the virgin LLDPE and this can be

considered as the consequence of a grain size decrease. Furthermore, the d-

values (Table 4.4) corresponding to different planes obtained from XRD studies

also increases with the initial addition of superconductor, which is due to [4.48]

the migration of superconductor particles in the intraspherulitic structure of

LLDPE. The XRD parameters of superconductorlpolymer composites are shown

in 'Table 4.4.

Table 4.4 : XRD parameters of superconductor/polymer composites.

Sample composition

L

LLDPE, I W/o

Y-123'LLDPE,50%

Y-123LLDPE,20°b

Reflections

110 200

ze ( O C )

21 4

21 3

21 44

20 rc) 23 6

23 5

23 7

d ( " ~ )

4 13

4 14

4 13

d r ~ )

3 733

3 74

3 73


The degree of crystallinity of the composites are calculated from the diffraction

patterns using the following equation [4.49],

where Ic and I,, represents the integrated intensity 'orresponding to the crystalline

and amorphous phases respectively. There is a decrease in crystallinity with the

addition of the superconductor. This can be attributed to [4.44] the migration of

the amorphous content in to the crystalline phase of LLDPE, reducing its crystalline

domains. Thus the progressive addition of superconductor results in continuous

decrease of crystallinity. 'I-he c~ystallinity values thus calculated are shown in

Table 4.5. Crystallinity measurements froni DSC data as mentioned earlier are

also tabulated in the same table.

Table 4.5 : Crystallinity of LLDPE in superconductoriLLDPE composites

As can be seen, the degree of crystallinity of [.LDPE in different composites

evaluated from XRD data is higher than that determined by the DSC. This is in

agreement with the observation [4.50] that the value ofcrystallinity (X<) depends

very much on the method of preparation of thc samples and the technique of

Sample Composition

measurement. Such difference in c~s ta l l in i ty values evaluated by DSC as

compared to XRD has been in confinnat~or\ \ \ ~ t h the earl~cr reported [4 511 values

Degree of crystallint). (X,)%

DSC XRD -- 81.6

1 26.5

8.2

LLDPE, 100%

Y-123lL.L.DPE. 50%

Y-I?3;'LLDPE, 20%

77 2

22.4

-7.3

108 Chapter 4

Fip re 4.6 (A) - (C) represents the crystallization region of interest in expanded

fomi in the XRD pattern for the Y-123 powder. Y-1231650;oLLDPE composite

and LLDPE pellet.

2-Theta angle

0 1 . A l9 2 20 8 22 4 24 0 25 6

2-Theta angle


2-Theta angle

Figure 4.6 : X-ray diffract~on patterns of (A) Y-123, (B) Y-123165YoLLDPE

composite, (C) LLDPE pellet in the regton between 20 - 20" and 25".

The Y-123 pattern (figure 4.6(A)) shows one sharp peak at 23" and another broad

peak at 23.6". Both of these peaks are readily observed in diffraction patterns

appearing in the literature [4.52, 4.531. The broad peak could be due to an

amorphous layer at grain boundaries. XRD pattern of the composite material

(figure 4.6(B)) shows two additional peaks in the region between 20 = 20" and

25". The position of all other peaks appears to be unchanged in the presence of

the polymer. Both powder and disc patterns of LLDPE are examined in this

study. In the XRD pattern oFLLDPE powder prior to melting and pressing, one

dominant fairly sharp peak at 21.4" and a weak broad peak at 23.6' occur. In the

XRD pattern of LLDPE pellet (figure(4.6C')) after melting and pressing two

dominant, fairly sharp peaks at 21.4" and 23.6" and a weak, broad third peak

centred around 19.5" occur. The two dominant peaks and the small broad peak

can be attributed to be arising from crystalline and amorphous regions in LLDPE.

110 Chapter 4

It is clear from the relative areas under these peaks that the polymer sample

produced in disc form after solidification from the melt becomes highly crystalline.

With respect to the pattern for the compjsite material, it is seen that peaks arising

from the LI,DPE are merely superin~posed on the pattern for the Y-123

superconductor. The spectra from all composite samples revealed a systematic

increase in the area under the 2 1.4" crystalline LLDPE relative to the area under

23" Y-123 peaks as the percentage of I I I IPE increased. The second crystalline

LLDPE peak also appeared to increase with increasing percentage of polymer

but situation is complicated in this case by the presence of the underlying diffuse

peak of Y-123 around 24" which is presumably [4.40] decreasing in intensity in

such spectra due to decreasing amounts of Y-123 in the composites samples of

higher LIDPE fraction. It seems that the process of recrystallisation ofthe polymer

upon cooling from the melt under pressure could be influenced by the presence

of Y-123 particles. Since the developmerit of crystallinity in a polymer involves

two stages, nucleation and growth [4.34], some of the Y-123 particles may be

able to act as nucleation centres that enable polymer chains to grow and be ordered

on their surfaces. Limited crystallisation could be expected during such growth

if the spacing between nuclei (Y-123 particles) in the bulk is small. Therefore, it

is reasonable to suggest that the percentage of crystallinity in the composite

samples decreases with increasing volume of the superconductor. Thus the

composites may be regarded as essentially a four phase system with two distinct

cystalline regions (Y-123 c~ystallites and LLDPE crystallites), a third region

(which is amorphous) being due to both Y-123 and LLDPE, and in light of the

data from Table 5.1, chapter 5 regarding voidal regions, a fourth phase being

essentially air.

The average crystallite size of the pure YBCO and composites are calculated by

using the Schemer's formula [4.54],

K A D=- a cos 0 !4.3)

Thermal, Crystallization and Structural Studies ill

where D is crystallite size of particle, I = 1.54 AU being the X-ray wavelength of

CuKa and K the shape factor, whlch can be assigned a value of 0.89 if the shape

unknown, cos0 the cosine of the Bragg angle and P the full width at half height

of angle of diffraction in radians. An average crystallite size of about 18.4nm

could be obtained by considering the above equation for the characteristic peak

of YBCO. The enlargement of the haif height width of the peaks is the consequence

[4.4 1 ] of a grain size decrease.

4.3.3 Raman studies

Figure 4.7(A) - (E) shows typical Raman spectra of pure YBCO, YBCOLLDPE

composites containing 80,60 and 30% YBCO, and pure LLDPE respectively.

Figure E

112 Chapter 4

8000 100 "i 200 300 400 500

Raman shift (cm-')

Figure D

2600 , 100 200 300 400 500

Raman shift (cm")

Figure C


Rarnan shift (crn-I)

Figure B

200 300 400

Raman shift (cm~ ' )

Figure A

Figure 4.7 (A) - (E) : The Raman spectra of pure YBCO, YBCOILLDPE composite samples conta~n~ng 80,60 and 30% YBCO

and pure LLDPE

114 Chapter 4

As that could be seen the Increase of superconductor content has induced lots of

changes in the Raman spectrum of the composites. One strong band and a weak

band are observed in the pure YBCO Raman spectrum. The Raman spectra of

the composite materials also show two bands, one strong and the other weak.

The appearance of sharp band both in the pure YBCO as well as in the composites

may indicate the presence of characteristic structural units in the composites. It

can be seen that as the superconductor volume percentage increases, the bands

becomes sharper in the composite samples.

The main band of pure Y BCO is at 2 12cm-' and the second band is at 340crn'l.

The intensity of the first band is l J2OAU and for the second is 1300AU. The

main band of 80% YBCO composite sample is at 204.928cm-I and the second

band is at 344.194cm". In the case of 60% YBCO composite sample, the strong

band is at 206.591cm-' and the weak band is at 344cm-'. This data shows that for

the superconductoripolymer composite samples also. the positions of the two

bands do not shift significantly with respect to those of pure YBCO. This clearly

indicates that the crystal structure of YBCO remains unchanged upon addition of

LLDPE. Thus, comparison of the Raman spectra of the YBCO and the composites

confirmed [4.55] that YRCO does not react with the polymer. For the 80%

superconductor composite sample. the intensity of the first band is 3500AU and

for the second is 3100AU. In the case of 60% composite sample, the intensity of

the first band decreases to 3790AIJ and that of the second band reduces to 3240AU.

The change in the relative intensity of the bands in composites may be [4.47] due

to crystal alignment during preparation processes.

4.4 Conclusions

Modulated differential scanning calorimetry (MDSC) studies on the LLDPE-

YBCO composite samples shows that the degree of crystallinity ofthe composites 6

decreases with increasing volume fraction of the superconductor. The physical

interactions occur in the melt state and reflect a change in crystallinity upon


cooling from the melt. Although there may be interaction between the polymer

chains and Y-123 particles during processing in the mclt state, these interactions

are not strong or do not persist upon cooling from the melt. The temperature of

onset of LLDPE melting progressively decreased and the melting range increases

with increasing amount of Y-123 particles. It is found that addition of

superconductor decreases the rate of nucleation of polytner and hence a reduction

in the crystallinity.

The XRD patterns of the pure LLDPE showed two sharp peaks and these peaks

are found to decrease with decreasing proportion of the polymer and increasing

proportion of the YBCO in the composites. The relative peak positions are not

shifted significantly due to the presence ofthe supcrconductor. Thus, comparison

of the XRD patterns of the polymer and the composites confirmed that YRCO

does not react with the polymer. The results obtained from XRD measurements

show that the superconductor and polymer remain as two separate phases i n the

composites. The average crystallite size of the samples is studied using XRD.

The enlargement of the half height width of the peaks is the consequence of a

grain size decrease. Therefore, it seems reasonable to suggest that the percentage

of crystallinity in the composite samples decreases with increasing volume of the

superconductor.

A strong band and a weak band are observed in the Raman spectra of pure YBCO

and YBCOLLDPE composite samples. Although the positions of these two bands

in the parent superconductor are preserved in the composite samples, there is a

gradual variation in their intensities. This may be due to crystal alignment during

preparation processes. It is found that as the superconductor volume percentage

in the composites increases, the intensity also increases. No evidence of chemical

interaction between YBCO and LLDPE is observed and the two components

remain segregated. This shows that pressing during processing of the samples . does not result any change in the structure of YRCO.

116 Chapter 4

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120 Chapter 4

4.6 Index

crystallization kinetics

thcmlal behavior

crystallization rate

crystallization fraction

spherulite size

Riaman spectrunl

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