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CHAPTER 3
SYNTHESIS AND CHARACTERIZATION OF DICARBOXYLIC ACIDS BASED CHAIN
EXTENDED POLYURETHANES
This chapter deals with the preparation and characterization of different
dicarboxylic acids based chain extended bio polyurethanes (CEPUs). A series of
castor oil (CO) based CEPUs have been prepared using different diisocyanates
(toluene diisocyanate (TDI) and hexamethylene diisocyanate (HDI) and six
dicarboxylic acids such as maleic acid (MA), glutaric acid (GA), citric acid (CA),
phthalic acid (PA), tartaric acid (TA) and itaconic acid (IA) as chain extenders. The
prepared CEPUs have been characterized for spectral studies, physico-mechanical and
chemical resistance. The effect of heat aging on the mechanical behaviours of CEPUs
have been reported. The heat aging properties of PUs also have been reported. The
thermal behaviours of the prepared PUs has been performed using DSC, TGA and
DMA. The thermal degradation behaviors of CEPUs have been established using
TGA thermograms. The microcrystalline parameters such as, lattice strain (g %),
surface weighted crystal size (Ds), number of unit cells (<N>), interplanar distance
(d), crystallite area and percent of crystallinity have been evaluated using wide angle
X-ray spectroscopy (WAXS). The structure-property relationship of CEPUs has been
established using X-ray and mechanical data. The morphological behaviour of CEPUs
has been analyzed using scanning electron microscope (SEM).
3.1 Introduction
The preparation of polymers from renewable sources such as vegetable oil-
based materials is currently receiving increasing attention because of economic and
environmental concerns [1-4]. In order to use these compounds as starting materials
for polyurethane (PU) synthesis, it is necessary to functionalize them to form polyols.
Epoxidation and ring opening reaction with haloacids or alcohols, ozonolysis and
hydration are some of the common methods for functionalization of unsaturated
72
vegetable oils [5-9]. Among vegetable oils, castor oil (CO) represents a promising raw
material due to its low cost, low toxicity and its availability as a renewable
agricultural resource. Its major constituent, recinoleic acid (12 -hydroxy-cis-9-
octadecenoic acid) is a hydroxyl containing fatty acid [10]. So castor oil can be used
directly as a raw material for the preparation of PUs without any further modification
[11-14].
CEPUs have a wide range of industrial applications and they are well known
for their mechanical and barrier properties. The three main components of PUs are;
a long chain polyol, a diisocyanate and a chain extender. These polymers typically
exhibit a two-phase morphology due to the incompatibility of the soft and hard
segments. The excellent mechanical properties of PUs such as high tensile strength
and toughness are primarily due to the two-phase microstructure resulting from this
phase separation [15-18]. The structure and the molecular weight of the macrodiol
significantly influence the phase separation behaviour of PUs and consequently, their
properties. Most previous studies on structure-property relationship of PUs have been
focused on polyether, polyester and polycarbonate (PC) macrodiols [15-20]. There
has been some recent interest on PUs based on chain extenders such as, diamines
[21-22], diols [23], phenolphthalein [24] and carboxylic acid [25-26].
A thorough literature survey reveals that, not much work has been done on the
studies of the naturally occurring polyol (castor oil) based dicarboxylic acids based
CEPUs. PU and modified PUs are extensively used in a variety of applications
[27-28]. Hence, this kind of research investigation gives some input to material
technologists to develop PUs for tailor-made applications.
The objective of this research investigation was the synthesis and
characterization of a series of CEPUs based on castor oil with dicarboxylic acids as
chain extenders. Having this goal in mind six dicarboxylic acids and two
diisocyanates based CEPUs were prepared and characterized. The physico-
mechanical, swelling, optical and thermal properties of the prepared biobased CEPUs
were studied and correlated to chain extenders structure.
73
The microstructural parameters by X-ray and morphological behavior by SEM
for the prepared dicarbocylic acid biobased CEPUs have been briefly described in this
chapter. The results of this chapter can provide some insight and scientific data for
filling the gap in the area of PU technology of renewable resource-based bio PUs.
3.2 Synthesis of dicarboxylic acid based chain extended polyurethanes
A castor oil based bio CEPUs have been prepared using different
diisocyanates (TDI and HDI) and different dicarboxylic acids such as maleic acid
(MA), citric acid (CA), glutaric acid (GA), phthalic acid (PA), tataric acid (TA) and
itaconic acid (IA) as chain extenders as per the procedure reported elsewhere [29].
3.2.1 Formation of pre polyurethane
Castor oil (0.001 mol) was dissolved in about 50 ml of methyl ethyl ketone
(MEK) in a 250 ml three necked round bottomed flask. Diisocyanate (0.0015 mol)
was added drop wise to the flask with constant stirring followed by 2-3 drops of
DBTL. The resultant reaction mixture was purged with oxygen free nitrogen gas to
prepare the isocyanate terminated pre polyurethane polymer. The contents of the flask
were stirred constantly for about 1 h at 60-70 oC. The formation of pre polymer is
shown in Scheme 3.1 (Step 1).
3.2.2 Formation of chain extended PU
After the required isocyanate content was achieved as determined by
dibutylamine titration the prepolymer was made to react with the equal molar ratio
(0.001 mol) of dicarboxylic acid dissolved in MEK [30]. The mixture was stirred for
about 30 min at the same temperature (60-70 oC). Then the mixture was degassed and
poured into a cleaned and releasing agent coated glass mould. The mould was kept at
room temperature for 12 h and in a hot air oven at 70 oC for 8 h. Chemical reaction for
the formation of dicarboxylic acid based CEPU system is shown in Scheme 3.1
(Step 2). The tough and transparent PU sheets thus formed were cooled slowly and
removed from the mould. Similarly different CEPUs were synthesized by changing
dicarboxylic acid. The reactants and the number of modes used for PU synthesis is
given in Table 3.1.
74
Scheme 3.1. Schematic representation of formation of CEPUs
75
Table 3.1. The reactants and the molar ratios used for the synthesis of different CEPUs
Reactants Molecular weight Weight of the reactant (g) No. of moles
Castor oil 933 9.33 0.001
TDI 174 2.61 0.0015
HDI 168 2.52 0.0015
Citric acid 192.13 1.92 0.001
Glutaric acid 132.12 1.32 0.001
Phthalic acid 166.14 1.66 0.001
Maleic acid 116.1 1.16 0.001
Tataric acid 150.08 1.50 0.001
Itaconic acid 130.09 1.30 0.001
3.3 Results and Discussion
A series of CEPUs were synthesized with different dicarboxylic acids like
MA, CA, TA, IA, GA and PA as chain extenders. All CEPUs were obtained as tough
and transparent sheets. It was found to be golden yellow to yellow in color. The
properties of the prepared dicarboxylic acid based CEPUs are briefly explained in the
forthcoming sections.
3.3.1 Fourier transform infrared spectroscopy
Fourier transform infrared spectroscopy (FTIR) spectra of PU and CEPUs are
shown in Figure 3.1. FTIR spectra of both PU and CEPUs are in good agreement IR
data in the literature [31-32]. The IR spectra of CEPUs show characteristic bands at
3216, 2900, 2356, 1760, 1590, 1500, 1320, 1282, 1222, 1100, 860 and 665 cm-1. The
expected and the observed IR data for characteristic groups of CEPUs are given in
Table 3.2 (a) – (b). The spectra of HDI and TDI based CEPUs are substantially
similar to each other. The regions of C=O vibration are in focus because this regions
provides useful information of amide linkage and the mode of hydrogen bonding [33].
Dicarboxylic acid based CEPUs demonstrated a shoulder peak between
1650 -1680 cm-1 which arose from the stretching of amide carbonyl group. The amide
group formed by isocyanate terminated free -NCO groups can react with carboxylic
acid groups of chain extender to form anhydride compound which will further
76
decompose into amide and carbon dioxide at normal temperature as expressed in the
following reaction scheme [34-35];
Scheme 3.2. Formation of amide group by reaction of
carboxylic acid with isocyanate group
The characteristic absorption peaks are observed at 1658 and 1640 cm-1 for
itaconic acid and maleic acid based CEPUs respectively, which are due to C=C group.
The strong absorption band observed around 2925 cm-1 for GA based CEPU, which
indicates the methylene group of GA. The phthalic acid based CEPU spectrum shows
aromatic peak at 1600 cm-1 and ortho substituted benzene at 770 cm-1.
It has been proposed that the relative degree of microphase separation in
CEPUs can be assessed by determining the degree of amide C=O hydrogen bonding
and that an increase in the extent of microphase separation is accompanied by changes
in the absorbance of the amide C=O peak [33]. The absence of the peak at 2220 cm-1
clearly indicates that, the chain extenders reacted completely with free -NCO
terminated groups [36].
All dicarboxylic acid based CEPUs exhibit the carbonyl absorption bands at
an approximately the wave number range 1640- 1690 cm-1, which can be attributed to
the stretching mode of the hydrogen bonded and free carbonyl groups respectively.
CA based PUs exhibit the characteristic absorption bands at 1670 and 1700 cm-1
which are due to >C=O group of urethane linkage and >C=O group of free acid
respectively. The characteristic peak is located at about 3305- 3310 cm-1 in the
spectrum which is the characteristic of hydrogen bonded –NH groups. There are three
main possibilities for hydrogen bond formation i.e., ester-urethane, urethane-urethane
and urethane - amide hydrogen bonding [37]. An attempt was made to assess the
relative contribution to the formation of hydrogen bonding in such systems by the two
acceptors; ester and urethane carbonyls. Seymour et al [38] concluded that all the –
NH groups of urethane linkage are involved in the formation of hydrogen bonding.
The schematic representation for the hydrogen bond formation in CEPU is shown in
Scheme 3.3. From Table 3.2 (b) it can be observed that the absorption bands for both
77
–NH and >C=O stretching frequencies are different for different dicarboxylic acid
based CEPUs. The variation in stretching frequencies from one CEPU to another
CEPU is due to the change in variation in degree of hydrogen bond and chemical
and/or physical interactions of chain extenders.
Scheme 3.3. Schematic representation of hydrogen bond formation between
CEPU networks
Table 3.2 (a). Important band assignments of FT-IR spectra of dicarboxylic acid based CEPUs
Group Expected peaks (cm-1) Observed peaks (cm-1)
C=O 1630-1690 1670 N-H stretching with hydrogen bonding
3200-3400 3346
Aromatic C-H stretching 3000-3100 2880, 2990 & 3010 C=C aromatic ring 1600 & 1450 1600 & 1430 1,4 - substituted phenyl ring 860 840 Free –N=C=O– in pre PU (i.e., PU is isocyanate terminated group)
2356-2264
2290
C=O ( amide) 1650-1700 1670 O || − C − O (ester)
1750-1700 1720 & 1090
Aliphatic diisocyanates 2960 & 1450 2900 & 1400 C = C (in alkenes) 1620-1680 1600
78
Table 3.2( b). The characteristic absorption bands obtained from FTIR spectra for different dicarboxylic acids based CEPUs
Characteristic peaks (cm-1)
Groups Expected peaks (cm-1) PA CA GA IA MA TA
>C=O 1650-1700 1660 1670 1680 1650 1640 1660-NH (stretching) 3300-3400 3352 3332 3326 3342 3310 3320C-H stretch in (i) CH3 or aromatic 2800-3000 2900 2920 2960 2990 2990 2900
(ii) C-H def in aromatic 900-700 790 790 800 800 810 780 (iii) Di substituent para C-H deformation 840-800 850 840 830 850 840 800
(iv) C-C stretching in aromatic ring 1410-1430 1400 1400 1400 1400 1410 1420
(v) Aliphatic -(CH2)3 (a) C-H stretching 2960-2850 2950 2900 2960 2930 2981 2964
(b) C-H def 1470-1430 1460 1450 1448 1452 1435 1446
Figure 3.1. FT-IR spectra of PUs and PA and MA based CEPUs
79
3.3.2 Physico-mechanical properties
The measured physico-mechanical properties such as density, resilience,
surface hardness, tensile strength, percentage elongation at break and tensile modulus
of dicarboxylic acid (PA, TA, IA, MA, CA and GA) based CEPUs are given in
Table 3.3.
3.3.2.1 Density
The density of all CEPUs lie in the range 1.044-1.152 g/cc and 1.036-1.125 g/cc for
TDI and HDI systems respectively. From Table 3.3, it is seen that the density values
of all CEPUs are higher than water, because they are crosslinked. The density of PU
is 1.011 + 0.05 g/cc [39]. There is no systematic variation of density values with
molecular weight of chain extenders. From Table 3.3 it was noticed that lower density
values were observed for HDI based CEPUs as compared to TDI based PU systems.
This is because HDI is an aliphatic chain extender which imparts high soft component
to the PU network. Similar aspects have been noticed for GA based CEPU. The
lower density of GA based CEPU as compared to other CEPUs, is because GA has
more flexible – CH2 – groups and linear structure.
The density of CEPUs has been calculated theoretically which is obtained by
group additive method. It is observed that experimentally obtained values are in good
agreement with the density values calculated from group additive method. From Table
3.3, it was also noticed that the experimentally obtained density values of CEPUs are
slightly lower than theoretically calculated values. This may be due to microvoid
formation between the two phases or poor interfacial adhesion between the polymer
networks.
3.3.2.2 Resilience
This test method covers the determination of impact resilience of dicarboxylic
acids based CEPUs from the vertical rebound of a dropped mass method. The
resilience values (Table 3.3) of all CEPUs lies in the range of 8-19. The resilience
data are found to vary according to the sequence; GA>IA>MA> TA>CA>PA.
Among diisocyanates, higher resilience values were noticed for HDI based PUs and
among chain extenders; GA based CEPUs show higher resilience values. This is due
80
to the presence of more flexible (-CH2-) groups in GA and HDI molecules which
impart soft segment to the PU network.
3.3.2.3 Surface hardness
The dimensional stability of both TDI and HDI based CEPUs have been
measured using shore A and shore D hardness testers and the obtained values have
been given in Table 3.3. The surface hardness values of all CEPUs lie in the range of
63-96 shore A and corresponding shore D values lie in the range 19-57. Higher
surface hardness values were noticed for aromatic diisocyanate based CEPUs as
compared to aliphatic diisocyanate based systems. The variation of dimensional
stability is also significantly dependent on the structure of chain extenders and
secondary forces of attraction (inter and intra molecular forces) exerted by the chain
extenders. From these results it was confirmed that all dicarboxylic acid based CEPUs
are crosslinked. The higher surface hardness value of CO+TDI+PA system is due to
the fact that the presence of rigid aromatic ring enhances the dimensional stability of
the PU.
3.3.2.4 Tensile behavior
The tensile properties such as tensile strength, percentage elongation at break
and tensile modulus has been evaluated by using UTM. Stress verses strain plots for
HDI and TDI based CEPUs are shown in Figures 3.2. From this figure the different
deformation patterns for different CEPUs was observed. The decreasing order of
deformation pattern of dicarboxylic acid based CEPUs are; PA > TA > IA > MA >
CA > GA. The effect of the nature of diisocyanates on the deformation pattern (stress-
strain) for PA based PUs is shown in Figure 3.3. Higher slope of the stress-strain
deformation patterns were noticed for TDI based PUs as compared to HDI based
systems as expected.
The calculated tensile properties from stress-strain curves of all CEPUs are
given in Table 3.3. From the table higher tensile behaviors for CEPUs as compared to
PU (without chain extender) were noticed. Tensile strength falls in the range 7.9-
14.64 MPa and 5.1-10.3 MPa for TDI and HDI based CEPU systems respectively.
81
From Table 3.3 higher tensile strength and tensile modulus for PA based CEPUs and lowest values for GA based CEPUs was noticed. The highest tensile properties for PA based PU system is due to the aromatic nature of (rigid phenyl structure) chain extender, which imparts higher percentage of hard segment and induced crystallinity. Due to higher crystallinity PA based PUs exhibit higher initial modulus and tensile strength, however lower elongation at break. These results are in good agreement with the viscoelastic behavior of samples as evaluated by DMA. The lowest tensile strength and tensile modulus of GA based PUs were due to the presence of three -CH2- groups in its structure.
Among saturated CEPUs, the order of the tensile strength and tensile modulus
is as follows; TA>CA>GA. The higher tensile strength and tensile modulus of TA based PU is due to trans nature of chain extender. In addition to two –COOH groups, it has two substituted –OH groups to carbon atoms. The presence of –COOH and –OH groups enhances the inter and intra hydrogen bond formation and physical interaction between the polymer networks and hence, restrict the molecular mobility in the polymer chains. Even though CA has tricarboxylic acid groups and one -OH group, it shows less tensile strength and tensile modulus as compared to TA because it has two flexible –CH2– groups along the main chain and also because the molecules have stearic hindrance effect. Similarly in the case of GA based system, the three –CH2- groups present in the chemical structure enhances the flexibility of the polymer molecules. Among unsaturated chain extenders IA based CEPUs showed higher tensile strength and tensile modulus as compared to MA based PUs. This is due to IA based system having isomeric structure which restricts the free rotation of the molecules.
From the table it was noticed that higher percentage elongation at break for
both TDI and HDI based CEPUs as compared to corresponding PUs. The sequences of variation in percentage elongation at break of the CEPUs are; GA>IA>MA >CA>TA>PA. The GA based PUs which showed higher percentage elongation at break, due to the presence of three numbers of -CH2- groups. In case of IA based system, out of two carbon atoms one is a substituted carbon atom, which reduces the percentage elongation as compared to GA. In case of MA, the presence of double bond (-CH=CH-) between two carbon atoms, further reduces the percentage elongation. CA based PU shows less percentage elongation property. This was probably due to the presence of free hydroxyl/carbonyl groups in CA, although the
82
reactivity of the hydroxyl and carboxylic acid groups at the tertiary carbon atom should be lower than the other reactive groups in the system. The presence of uneven structure of CA in the system probably led to the formation of decrease in crystallinity and mechanical properties. But it shows higher properties than GA, probably due to the intermolecular and intramolecular hydrogen bond formation between the hydroxyl and acid functional side groups. However, it has lower properties than TA based system. Because of the trans configuration of TA which led to the crosslinking, crystalnity and higher interaction.
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.80
2
4
6
8
10
12
14
16TDI
Stre
ss (M
Pa)
Strain (mm/mm)
PA IA TA MA CA GA PU
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.20
2
4
6
8
10
12HDI
Stre
ss (M
Pa)
Strain (mm/mm)
PA IA TA MA CA GA PU
Figure 3.2. Stress versus strain curves for different
dicarboxylic acid based CEPUs, (a) TDI and (b) HDI
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.40
2
4
6
8
10
12
14
16
Stre
ss (M
Pa)
Strain (mm/mm)
TDI HDI
Figure 3.3. Stress versus strain curves for PA based
CEPUs with different diisocyanates
83
Table 3.3. Physico- mechanical properties of dicarboxylic acid based CEPUs
Density (g/cc) Surface hardness Sample
code Experimental Theoretical
Tensile strength (MPa) + 2 %
% Elongation at break + 2.5 %
Tensile modulus (MPa) + 2.2%
Shore A Shore D Resilience
TPA 1.152 1.168 14.64 112 16.39 96 57 8 TTA 1.138 1.151 11.75 128 13.57 92 54 12 TIA 1.120 1.127 11.57 156 12.07 89 51 15
TMA 1.117 1.123 10.98 147 10.38 85 47 14 TCA 1.109 1.119 9.18 131 7.91 81 35 9 TGA 1.108 1.116 8.92 176 7.15 79 29 17 TPU 1.044 1.066 7.90 118 7.86 80 31 9 HPA 1.125 1.139 10.30 138 8.80 77 37 11 HTA 1.105 1.108 8.72 150 6.70 75 33 13 HIA 1.055 1.089 7.91 193 6.12 73 29 16
HMA 1.045 1.086 7.04 162 4.73 72 26 14 HCA 1.044 1.066 6.23 143 4.06 66 24 12 HGA 1.043 1.062 5.68 206 3.02 63 19 19 HPU 1.036 1.057 5.10 138 3.23 67 20 14
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3.3.3 Effect of heat aging on mechanical properties
The effect of heat aging (90 oC) and its duration (24 and 48h) on the
mechanical properties of dicarboxylic acid based CEPUs have been studied. Figures
3.4 and 3.5 shows the stress-strain profiles for 24 and 48 h heat aged dicarboxylic acid
based CEPU specimens at 90 oC respectively. The trend of stress verses strain curves
of TDI and HDI based CEPUs are almost identical (Figure 3.2). From these stress
verses strain curves the tensile properties such as tensile strength, percentage
elongation at break and tensile modulus of all heat aged CEPUs at 90 oC for 24 and
48h have been calculated (Table 3.4). From the table, it was noticed that after heat
aging there was a significant reduction in tensile behavior for all CEPUs. The
percentage reduction in mechanical properties after heat aging has been given in
Table 3.5.
The effect of duration of thermal treatment on the mechanical property has
also been studied. The effect of duration of heat aging on the nature of stress-strain
curves of MA based CEPUs system at 90 oC is shown in Figure 3.7. The percentage
reduction in tensile strength and modulus for 48 h heat aged specimens was more than
20 %. This may be due to the decrosslinking and weakening of secondary forces
between polymer networks or reduction in degree of interaction between different
networks of PUs after heat aging. Among all chain extenders, the percentage of
reduction in tensile strength and tensile modulus is higher for GA based PUs. This
result indicates that GA based PUs are more sensitive to heat aging. From Figure 3.7,
it was noticed that increase in duration of heat aging, reduces the mechanical
performance. The orders of stress-strain curves are; unexposed > 24h > 48 h.
Among diisocyantes, slightly higher reduction of mechanical properties have
been observed for HDI based PUs. This can be clearly observed from the stress verses
strain plots of maleic acid based CEPU is shown in the Figures 3.6 (a) – (b) for 24 h
and 48 h respectively. This is ascertained to be because HDI is an aliphatic
diisocyanate and possesses more flexible -CH2- groups in the polymer backbone
whereas, TDI is an aromatic diisocyanate, which is more stable towards heat aging.
85
For comparison, a bargraph corresponding to the percentage reduction in tensile strength of PUs before and after heat aging is shown in Figure 3.8. From these bar graphs it can be concluded that a higher degree of reduction in tensile strength was observed for 48 h exposed samples as compared to 24 h heat aged specimens. From these results it can be concluded that carboxylic acid based CEPUs will retain useful properties at lower temperatures. That means, one of the most outstanding properties of dicarboxylic acid based CEPUs are their performance at lower temperatures.
0.0 0.4 0.8 1.2 1.60
2
4
6
8
10
12
14(a)
Stre
ss (M
Pa)
Strain (mm/mm)
PA TA IA MA CA PU GA
0.0 0.4 0.8 1.2 1.6 2.00
2
4
6
8
10(b)
Stre
ss (M
Pa)
Strain (mm/mm)
PA TA IA MA CA GA PU
Figure 3.4. Stress verses strain curves of CEPUs after heat aging at 90 oC for 24 h, (a) TDI and (b) HDI
0.0 0.4 0.8 1.2 1.60
2
4
6
8
10
12 (a)
Stre
ss (M
Pa)
Strain (mm/mm)
PA TA IA MA CA GA PU
0.0 0.4 0.8 1.2 1.6 2.00
2
4
6
8 (b)
Stre
ss (M
Pa)
Strain (mm/mm)
PA TA IA MA CA PU GA
Figure 3.5. Stress verses strain curves of CEPUs after heat aging at
90 oC for 48 h, (a) TDI and (b) HDI
86
Table 3.4. Tensile properties of CEPU samples after heat aging at 90 oC
Tensile strength (MPa) + 2 %
Elongation at break (%) + 2.5 %
Tensile modulus (MPa) + 2.2 % Sample
code *RT 24 h 48 h RT 24 h 48 h RT 24 h 48 h
TPA 14.64 12.92 11.15 112 106 100 16.39 13.17 10.37
TTA 11.57 10.76 8.93 128 112 106 13.57 11.80 9.56
TIA 11.75 10.60 8.32 156 148 131 12.07 9.93 7.39
TMA 10.98 9.95 8.21 147 131 128 10.38 8.73 7.28
TCA 9.18 7.86 5.69 131 128 125 7.91 6.27 5.09
TGA 8.92 7.56 6.98 176 165 156 7.15 6.15 5.51
TPU 7.90 7.50 6.57 118 112 106 7.86 6.65 6.05
HPA 10.30 8.90 7.69 138 125 118 8.80 7.47 6.92
HTA 8.72 7.60 5.97 150 144 125 6.70 5.05 4.55
HIA 7.91 7.04 6.32 193 187 181 6.14 4.97 3.84
HMA 7.04 6.31 5.10 162 156 150 4.75 4.01 3.18
HCA 6.23 5.13 3.98 143 137 110 4.06 3.33 2.55
HGA 5.68 5.05 4.65 206 193 188 3.02 2.43 2.09
HPU 5.13 4.74 4.23 138 125 115 3.23 3.08 2.59
* RT – Room temperature
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.60
2
4
6
8
10 (a)
Stre
ss (M
Pa)
Strain (mm/mm)
TDI HDI
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.60
2
4
6
8 (b)
Stre
ss (M
Pa)
Strain (mm/mm)
TDI HDI
Figure 3.6. Stress verses strain curves for maleic acid based CEPUs with different diisocyanates after, (a) 24 h and (b) 48 h heat aging
87
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.60
2
4
6
8
10
12(a)TDI
Stre
ss (M
Pa)
Strain (mm/mm)
RT 24h 48h
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.80
2
4
6
8(b)HDI
Stre
ss (M
Pa)
Strain (mm/mm)
RT 24h 48h
Figure 3.7. Effect of duration of heat aging on stress - strain curves, of, (a) CO+TDI +MA and (b) CO+HDI +MA CEPUs
Table 3.5. Percentage reduction in mechanical properties of CEPUs
after heat aging for 24 h and 48 h at 90oC
% Reduction in tensile strength
% Reduction in elongation at break
% Reduction in tensile modulus Sample
code 24 h 48 h 24 h 48 h 24 h 48 h
TPA 11.7 23.8 5.4 10.1 19.7 36.7
TTA 10.4 22.8 12.5 17.2 13.0 25.7
TIA 9.8 29.2 5.2 16.0 17.7 38.8
TMA 9.4 34.3 10.8 12.9 16.6 30.0
TCA 16.5 38.1 4.6 14.5 20.7 35.6
TGA 15.2 21.7 6.2 11.4 16.7 23.0
TPU 5.1 16.8 13.5 15.3 15.3 20.0
HPA 13.6 25.3 9.4 14.5 15.1 21.4
HTA 12.8 31.5 4.0 8.7 24.6 32.1
HIA 11.0 20.1 3.1 6.2 18.5 37.0
HMA 10.4 27.6 3.7 7.4 14.7 32.3
HCA 17.6 36.1 4.2 17.5 17.9 37.2
HGA 16.9 37.3 6.3 8.7 19.5 30.8
HPU 7.60 17.6 9.4 16.6 4.6 19.8
88
Figure 3.8. Percentage deviation of tensile strength before and after heat ageing of CEPUs at 90 ºC
3.3.4 Chemical resistance
The percentage change in weight of CEPUs was determined by immersing the
specimen in 100 ml of 10% different chemical reagents or solvents such as NaOH,
H2O2, HCl, CCl4, KMnO4, acetone, acetic acid, benzene and water at 25 ºC for 7 days.
The CEPU specimens exposed to the above chemical reagents at room temperature
were examined for the percentage change in weight and the results are given in Table
3.6. Except samples immersed in KMnO4 solution, there was almost no significant
change in the physical appearance of CEPUs in the chemical reagents under
investigation. This shows that CEPUs are moderately resistant to dilute alkalis and
acids. From the table it was noticed that there was a slight change in weight in HCl,
CH3COOH, H2O2 and water. However, prominent swelling of PUs was noticed in
organic solvents such as carbon tetrachloride and acetone. The maximum percentage
swelling was observed in acetone as compared to the other reagents/solvents [40-41].
This is due to the fact that acetone can penetrate into the core of the PUs with less
resistance, thereby increasing the swellability of the materials. Among all PUs, TDI
based CEPUs show more chemical resistance to almost all chemical reagents.
A pronounced change in weight was noticed for the specimens exposed to
KMnO4 solution. PU degraded in the oxidation media. Based on these observations,
the chemical resistivity depends on the structure and morphology of the PU. That is
both the hydrogen bonds and intermolecular Van der Waals forces of the irregularly
PA TA IA MA CA GA0
2
4
6
8
10
12
14
TDI CEPUs
% R
educ
tion
in te
nsile
stre
ngth
RT 24 h 48 h
PA TA IA MA CA GA0
2
4
6
8
10
HDI CEPUs
% R
educ
tion
in te
nsile
stre
ngth
RT 24 h 48 h
89
oriented macromolecular chains were weakened, which caused the weakening of
partial intermolecular attraction and the slipping of intermolecular chains. As a result,
the degree of physical cross-linking decreased and some chemical bonds in the higher
stress-intensity zone were ruptured. It is likely that the fractured site of the chains is at
the soft part joining the connection of the hard part and the soft part of the
macromolecular chains as the most easily cleaved chemical bonds.
Table 3.6. Change in weight of CEPUs after exposure to different chemical reagents for 7 days
% Change in weight for 7 days at room tempr. for various chemical reagents
Sample code NaOH
(10%) HCl
(10%)
CH3
COOH(10%)
H2O2 (10%)
KMnO4
(10%) Benzene Acetone CCl4 H2O
TPA 12.2 9.5 4.0 4.5 7.28 24.9 17.0 22.6 1.0
TTA 18.5 8.9 4.9 1.5 15 28.7 19.3 24.0 2.0
TIA 12.1 10.4 5.2 3.8 D 27.4 11.5 27.5 0.7
TMA 15.9 12.4 4.7 3.5 D 26.9 19.9 22.0 2.0
TCA 18.0 9.6 6.1 4.3 D 24.7 19.7 22.0 4.8
TGA 12.1 9.2 4.3 1.9 D 25.5 24.8 22.5 2.8
TPU 20.3 11.2 7.2 5.8 D 28.3 15.5 27.5 5.6
HPA D 11.6 5.5 6.0 D 35.4 23.3 32.9 2.7
HTA 18.9 12.6 4.8 2.7 D 36.9 25.6 34.6 3.1
HIA 20.8 9.9 8.5 5.8 D 37.3 21.0 38.2 1.9
HMA D 12.3 5.8 5.0 D 38.7 26.2 39.4 1.2
HCA 18.6 10.4 6.5 2.7 D 37.9 28.6 32.1 3.7
HGA D 13.9 8.5 2.0 D 36.7 27.3 36.8 5.8
HPU 19.6 11.7 7.9 4.2 D 38.5 25.3 29.0 4.8
D* denotes disintegrated in the chemical reagents.
3.3.5 Optical properties
The optical properties of CEPUs were carried out according to the procedure
mentioned in Chapter 2. The results of total percentage transmittance, total diffusion
and haze values of PA, TA, IA, MA, CA and GA based CEPUs are given in Table
3.7. The percentage transmittance and haze values of all CEPUs lies in the range of
90
72.8 - 95.0 and 7.4 - 48.1 respectively. From the table it was found that the percentage
of transmittance of all CEPUs are, > 83.4 %, except CA based CEPUs. This result
clearly indicates that all prepared CEPUs are optically transparent films [41-42]. The
total diffusion of light values of both series of CEPUs lies in the range 6.7-63.4. HDI
based CEPUs have slightly higher total percentage transmittance as compared to TDI
based CEPUs. This can be ascribed to aliphatic diisocyanate and chain extender based
CEPUs being amorphous in nature. The percentage transmittance of CEPU films
depends on the levels of NCO/OH ratios and percentage of hard segment.
Table 3.7. Optical properties of dicarboxylic acids based CEPUs
Sample code Total transmittance (%) Total diffusion (%) Haze TPA 83.4 7.3 10.9 TTA 86.6 55.5 41.2 TIA 88.6 41.2 33.7
TMA 90.4 6.7 7.4 TCA 72.8 19.7 17.2 TGA 87.5 6.6 7.5 HPA 91.7 45.5 40.3 HTA 91.0 7.6 10.4 HIA 92.0 54.5 39.3
HMA 90.4 43.4 35.0 HCA 75.8 31.2 30.9 HGA 95.0 63.4 48.1
3.3.6 Swelling behaviours of CEPUs
The swelling behaviour of CEPUs has been measured by using the mixture of
toluene and methyl acetate to probe the possible application range. The measured
change in weight of CEPUs after immersing in the solvent mixtures for 7 days at
room temperature are given in Table 3.8. The observed percentage swelling of the
crosslinked CEPUs followed the order, CA>TA>GA>IA>MA>PA. The plot of
percentage weight change as functional composition of toluene is shown in Figures
3.9(a)–(b) for TDI and HDI based CEPU systems respectively. The extent of
swellability of the CEPUs depends upon the nature of the chain extenders used and
this could be explained by considering the composition of hard /soft segment ratios.
Among all CEPUs, the maximum swelling was observed in case of CA based CEPU.
91
Table 3.8. Percentage swelling of dicarboxylic acid based CEPUs
Change in weight (%)
Toluene /methyl acetate Sample
code 100/0 75 /25 50/50 25/75 0/100
TPA 98 68 51 41 31
TTA 124 98 71 58 51
TIA 113 82 60 51 41
TMA 101 75 53 49 37
TCA 128 108 81 68 57
TGA 119 89 68 55 48
TPU 56 47 40 30 18
HPA 139 111 90 79 68
HTA 174 151 119 102 97
HIA 156 131 114 91 83
HMA 147 128 98 82 75
HCA 186 163 123 114 106
HGA 167 141 123 96 90
HPU 112 94 79 65 52
0 20 40 60 80 100
20
40
60
80
100
120
140(a)
Wei
ght c
hang
e (%
)
Percentage of methyl acetate
MA GA IA PA TA CA PU
0 20 40 60 80 10040
60
80
100
120
140
160
180(b)
Wei
ght c
hang
e (%
)
Percentage of methyl acetate
MA GA IA PA TA CA PU
Figure 3.9. Effect of toluene composition in toluene / methyl acetate mixture on swelling behavior of CEPUs, (a) TDI and (b) HDI
The observed swelling values are significantly higher in toluene when
compared to methyl acetate. The degree of swelling behavior decreases with increase
in methyl acetate content. This result clearly indicates that the interaction between
92
CEPUs is more in aromatic solvent than in methyl acetate which is as expected.
Hence, toluene has high penetration power than methyl acetate. Among diisocyanates,
HDI based PUs show higher degree of solvent uptake than TDI based systems. This
result indicates that the swellability depends upon the hard and soft segments ratio of
PU. Similarly composition of the solvent mixture also plays a vital role on the
swelling behavior of CEPUs.
3.3.7 Thermoanalytical studies
When the effects of chemical structures and phase structures of linear PUs on
their properties which are important from the viewpoint of materials technology are
considered. It is necessary to pay attention to thermal properties of those plastics.
3.3.7.1 Differential scanning calorimeter
Differential scanning calorimetry (DSC) is a common tool to determine the
changes in the state of organization, like segregation, Tg and Tm of the PU molecule.
The practical use of DSC in analyzing the thermal response of CEPUs with respect to
engineering properties has been illustrated by Goyert and Hespy [43]. The effect
of the nature of chain extenders on DSC thermograms of CEPUs is shown in
Figures 3.10 (a)-(b).
The DSC thermogram reveals that, the low temperature transition is due to Tg
of soft segment domains and high temperature transition is due to the Tm of crystalline
hard segment domains. Table 3.9 summarizes the Tg, Tm and heat of fusion (∆Hf) of
crystalline hard segment domains. Tg is reported for the inflection of the thermal
transition process and Tm is taken at the peak temperature of the endothermic melting
peak.
Three or even four phases could be distinguished in some of the synthesized
CEPUs; hard crystalline phase composed of TDI segments and the cross-linking
compound; intermediate phase – i.e., mixture of TDI-derived hard blocks, cross-
linking agent and oligomerol-derived soft blocks; soft phase composed of polyol-type
93
soft segments; and crystalline phase composed of soft blocks was observed in some
samples. The data obtained from DSC thermograms for CEPUs is in Table 3.9.
The obtained values revealed that glass transition temperatures of all CEPUs
lies below room temperature. PUs with structural segments derived from aliphatic
diisocyanates had lower separation degrees than their equivalent PUs obtained from
aromatic diisocyanates. The microphase separation is generally more prominent in
dicarboxylic acids based CEPU due to polar interactions between amide and urethane
groups in PU chains.
Finally CEPU confirms clearly the micro-phase separation process, their
effects were analyzed from chemical structures present in the CEPUs. Two glassy
temperatures were observed for HDI based CEPUs which are typical for elastomers:
the first one for soft segments which appeared in the range -9 to -23 oC and the second
one could be observed in the range 55 to 64 oC which represented relaxation of hard
segments in CEPUs. Among HDI based CEPUs, HMA and HIA systems shows
different thermal transition behaviour, i.e., two Tg’s and one Tm, whereas remaining
CEPUs show two Tg’s. Tg of TDI based CEPUs lies in the range −20 to −2 oC, DSC
thermograms exhibit second thermal transition, which lies in the range 52 - 101oC.
-50 0 50 100 150-1.0
-0.9
-0.8
-0.7
-0.6
-0.5
-0.4
-0.3(a)
TAGA
PAIA
PU
MA
CA
TDI
Hea
t flo
w (a
u)
Temp (0C)
-50 0 50 100 150-1.0
-0.8
-0.6
-0.4(b)HDI
PAIA
TA
CA
PU
MA
GAHea
t flo
w (a
u)
Temp (0C) Figure 3.10. DSC thermograms of dicarboxylic acid based CEPUs;
(a) TDI and (b) HDI
94
Table 3.9. Thermal transition data obtained from DSC thermograms for CEPUs
Sample code
Tg (soft seg) (°C)
∆T (°C)
Tm (°C)
∆Hf (J/g)
TPA -13 4 65 3.0 TTA -13 4 71 2.5 TIA -2 15 97 0.8
TMA -10 7 101 2.5 TCA -20 0 52 2.9 TGA -19 -2 - - TPU -17 - - -
Tg 1 (°C) Tg 2 (°C) HPA -9 4 60 HTA -12.4 -0.6 59 HIA -20 -7 55
HMA -23 -10 56 HCA -15.0 -2 64 HGA -17 (-8) -4 - HPU -13. - 59
∆T = Tg - Tgº; where, Tgº and Tg are the glass transition temperature of PU (without chain extenders) and CEPU respectively. ∆Hf = The total heat of fusion of the total melting point.
Incorporation of the chain extender groups led to enhancement in thermal
stability of PUs. This is due to increase in the component of hard segment (TDI based
CEPUs), which shifts the Tg to a higher temperature region. The introduction of
dicarboxylic acid groups in the TDI based CEPU matrix led to an increase in
columbic force of attraction between the hard domains, which gave rise to segmental
incompatibility, which in turn led to microphase separation [44]. But there is no
systematic variation in Tg values in case of dicarboxylic acid based CEPUs. The
observed transition temperatures variation may be explained by changing the nature
of chain extender (hard segment), which are disordered in the soft matrix.
The Tg of PUs is different from CEPUs. Two Tg’s were noticed at −13 and
59 oC for HDI based PU in addition to one Tm peak at 130 oC. Furthermore the DSC
scan of TDI based PU showed one Tg at -17 oC and one Tm peak at 150 oC. This can
be attributed to the high hard crystalline domain content in TDI based PUs.
95
TDI based CEPUs exhibit a single melting peak Tm their ∆Hf changing with
change in the structure of the hard segments (chain extender) (Table 3.9). It is noted
that ∆Hf values lie in the range 0.8-3.0 J/g. Accordingly, the higher value of Tm and
∆Hf reflects the larger fraction of hydrogen-bonded carbonyls and the stronger
hydrogen bond occurring in the crystalline hard segment domains. Therefore, these
trends change the average length of hard segments with the fraction of hydrogen-
bonded carbonyls and the strength of hydrogen bonding between urethane >CO and -
NH groups. The FTIR analysis is consistent with those shown in Tm and ∆Hf.
3.3.7.2 Dynamic mechanical analyser
It is well known that the dynamic mechanical analysis (DMA), which is
sensitive to the molecular motion in the polymer, can provide important information
on thermal motions of the hard and soft segments in CEPUs [45]. The plots of storage
modulus, tan δ and loss modulus at 1, 5 and 10 Hz as a function of temperature for
TA based CEPUs is shown in Figure 3.11.
-50 0 50 100 150-0.2
-0.1
0.0
0.1
0.2
0.3 TTA
10
10
5
5
1
11
Temp (0C)
G'(P
a)
0
200
400
600
800
1000
1200
1400
Tan Delta
Figure 3.11. Storage modulus and Tan δ at 1, 5 and 10 Hz for tataric acid based CEPUs
The results of DMA analysis for the determination of the Tg of PU and CEPUs
are demonstrated at frequency 10 Hz, and the thermograms are shown in Figures 3.12
(a) - (c) for storage modulus, tan δ and loss modulus respectively. The storage
96
modulus of the polymer decreases rapidly whereas the loss modulus goes through a
maximum when the polymer is heated through the Tg. The changes in tensile storage
modulus (G′) of CEPUs on heating are shown in Figure 3.12 (a). G′ is a measure of
material stiffness or flexibility. From the figure (a), sudden drop of G′ at the Tg range
can be observed. Figure 3.12 (a) shows that CEPU containing PA has the highest
storage modulus. With the change in type of extenders, the storage modulus varies
due to varying of hard/soft segment ratios, which in turn increases the interaction
between the polymer chains and consequently increases the storage modulus.
TPA based CEPUs show higher storage modulus. There may be two reasons
for the higher Tg; (i) the increment of the molecular weight reduced the number of
free chain ends and thus reduced the free volumes [46-47] and (ii) the introduction of
a rigid aromatic structure into the polymer chain, hinders the chain movement
sterically. From the DMA results it can be concluded that the heat resistance of CEPU
can be improved by chain extending with phthalic acid.
The value for Tg was reported as the location of the primary peak in Tan δ
curve, which fell in the range 2 to 53 oC (Table 3.10) and is comparable to that of PU.
From Figure 3.12 (b) it was noticed that there was variation in Tg with change in
chain extenders. From DMA thermograms it was noticed that the Tg values of CEPUs
are higher than PU (without chain extender). This confinement the effect of chain
extender to PU molecules and the strong interactions such as hydrogen bond between
the urethane groups of PU molecules and the oxygen atoms of the amide [48]. Similar
conclusions can be drawn from DSC results (Table 3.10).
The loss moduli of CEPUs are shown in Figure 3.12 (c). The loss modulus
goes through a maximum when the polymer is heated through the Tg. The higher loss
modulus means the higher ability of the materials to lose energy. The sample GA
based CEPU exhibits the broadest G″ transition peak compared to other dicarboxylic
acid based CEPUs. The broad G″ peak of the GA based CEPU system reflects the
multi-molecular motion of polymer chains. A high value of G″ suggests the greater
mobility of the polymer chains associated with dissipation of energy when the
97
polymer is subjected to deformation [45]. Thus, the CEPUs exhibit high and broad G″
transition peaks which has the ability to absorb energy associated with impact.
Similarly second peak in case of CA based CEPU at low loss modulus is due to the
steric hindrance structure of CA.
-40 0 40 80 120 1600
1000
2000
3000
4000
5000
6000TDI
TGA
TMA
TTA
TCA
TPA
TPU
TIA
G'(P
a)
Temp (0C)
-40 0 40 80 120 1600
1000
2000
3000
4000HDI
HMA
HPA
HTA
HCA
HPU
HGAHIA
G'(P
a)
Temp (0C) Figure 3.12 (a). Storage modulus of dicarboxylic acid based CEPUs for TDI and
HDI series at 10Hz
-40 0 40 80 120 160-0.6
-0.4
-0.2
0.0
0.2
0.4
0.6 TDI
TTATGA
TCA
TPU TMA
TIATPA
Tan
delta
Temp (0C)
-40 0 40 80 120-0.8
-0.6
-0.4
-0.2
0.0
0.2
0.4
0.6 HDI
Temp (0C)
HCA
HGA
HPA
HIAHMA
HPU
HTA
Tan
delta
Figure 3.12(b). Tan δ of dicarboxylic acid based CEPUs for, TDI and HDI series at 10Hz
98
-40 0 40 80 120
0
50
100
150
200
250
300TDI
TGA
TIA
TCA
TPUTMA
TPA
TTA
G" (
Pa)
Temp (0C)
-40 0 40 800
50
100
150
200
250
300HDI
HIA
HTA
HPA
HCAHPU
HGA
HMA
G" (
Pa)
Temp (0C) Figure 3.12 (c). Loss modulus of dicarboxylic acid based CEPUs for TDI and
HDI series at 10Hz
Table 3.10. Transition temperature data obtained from DMA analysis for CEPUs
Sample
code Storage modulus
(G′) MPa Tan δ
Tg (°C) TDI based CEPUs
TPA 29.0 53.1 TTA 19.0 42.0 TIA 23.0 53.4
TMA 19.0 38.0 TCA 2.0 57.0 TGA 6.0 -6.7 TPU -17.0 -4.0
HDI based CEPUs HPA -15 -2.1 HTA -28 -4.0 HIA -20 -8.3
HMA -25 -6.0 HCA -20 -7.0 HGA -25 -12.3 HPU -14 - 15.0
99
All DMA thermograms (G′, G″ and tan δ verses temperature) confirm the
greater phase difference in TDI series whereas, there is only a slight change in Tg for
HDI series. All CEPUs except CA and GA based CEPUs show higher storage
modulus, tan δ and lower loss modulus.
From Table 3.10 it was noticed that the Tg of TDI based CEPUs is above
>38 °C (except TCA), but for HDI based CEPUs, the Tg values are below 0 °C. This
is due to increasing chain flexibility in HDI based CEPUs because of the presence of
the higher soft segment which increases the glassy state modulus [49-50].
3.3.7.3 Thermogravimetric analyser
Typical TGA thermograms and its derivative curves for TDI and HDI based
CEPUs and its corresponding PUs are shown in Figure 3.13. From this figure it was
observed that the, decomposition of the PUs when viewed as a whole is a complex
process to follow [51]. As the change in chain extender changes the onset
decomposition temperature (Ti), is shifted towards different temperatures. After
formation of the amide groups between the CEPUs showed enhanced thermal
stability. The multi-stage decomposition observed for CEPUs is due to the scission of
chemically different segments in the polymer chain. PUs with aromatic diisocyanates
is more stable thermally than those based on aliphatic diisocyanates. But very high
thermal stability is not observed due to the inherent cleavage of the urethane groups.
The thermograms obtained during TGA scans were analyzed to give the
percentage weight loss as a function of temperature. T0 (temperature of onset
decomposition), T10, T20, T50 and Tmax (temperature for 10, 20, 50% and maximum
weight loss) are the main criteria to indicate the thermal stability of the PUs. The
higher the values of T10, T20, T50 and Tmax the higher will be the heat stability of
CEPUs. The relative thermal stability of CEPUs was evaluated by comparing
decomposition temperatures at various percent weight losses and is given in
Table 3.11.
From the table it was noticed that TDI based CEPUs showed higher thermal
stability than HDI based CEPUs. The degradation products obtained from hard
segments would be further converted to produce a stable residue. The presence of
100
aromatic rings in hard segments usually has a stabilizing effect and it reduces the
volume of volatile degradation products released [52]. It is also seen from Table 3.11
that the OI values are very low and almost same for all CEPUs and lies in the range
0.007-0.097. Based upon the mass of carbonaceous char, it is concluded that CEPUs
are not good flame retardant as evidenced by their OI values [41]. This conclusion is
drawn on the basis of lower OI values.
0 100 200 300 400 500 600 7000
20
40
60
80
100TDI
Wei
ght l
oss (
%)
Temperature (0C)
TTA TPU TPA TMA TCA TGA TIA
0 100 200 300 400 500 600 700
0
20
40
60
80
100HDI
Wei
ght l
oss (
%)
Tempertaure (0C)
HPU HTA HCA HIA HGA HPA HMA
Figure 3.13. TGA thermograms of PUs and dicarboxylic acid based CEPUs
Table 3.11. Data obtained from TGA scans for dicarboxylic acid based CEPUs
Transition temperature (oC) ±2 Sample code T0 T10 T20 T50 Tmax
OI
TDI based CEPUs TPA 197 311 326 359 494 0.028 TTA 161 265 297 368 596 0.014 TIA 170 304 322 365 483 0.063
TMA 185 315 333 371 495 0.069 TCA 130 307 334 419 610 0.020 TGA 170 303 326 406 576 0.069 TPU 120 275 298 347 568 0.007
HDI based CEPUs HPA 200 294 313 365 489 0.063
HTA 133 261 301 394 600 0.056 HIA 143 258 285 335 488 0.097
HMA 137 272 297 347 485 0.097 HCA 130 249 283 348 493 0.035 HGA 171 277 303 368 560 0.014 HPU 155 287 304 354 488 0.049
101
0 100 200 300 400 500 600 7000
20
40
60
80
100 TPU
Temp (0C)
Wei
ght l
oss (
%)
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Derv. w
eight (%/ 0C
)
0 100 200 300 400 500 600 7000
20
40
60
80
100HPU
Temp (0C)
Wei
ght l
oss (
%)
0.0
0.2
0.4
0.6
0.8
1.0
Derv. w
eight (%/ 0C
)
0 100 200 300 400 500 600 7000
20
40
60
80
100 TPA
Temp (0C)
Wei
ght l
oss (
%)
0.0
0.2
0.4
0.6
0.8
Derv. w
eight (%/ 0C
)
0 100 200 300 400 500 600 700-0.2
0.0
0.2
0.4
0.6
0.8
1.0
Temp (0C)
wei
ght l
oss
(%)
0
20
40
60
80
100HPA
Derv. w
eight (%/ 0C
)
0 100 200 300 400 500 600 7000
20
40
60
80
100 TIA
Temp (0C)
Wei
ght l
oss (
%)
0.0
0.2
0.4
0.6
0.8
1.0
Derv. w
eight (%/ 0C
)
0 100 200 300 400 500 600 7000
20
40
60
80
100
Temp (0C)
Wei
ght l
oss (
%)
0.0
0.2
0.4
0.6
0.8HIA
Derv. w
eight (%/ 0C
)
Figure 3.14. Typical TGA and its derivative thermograms of PUs and
dicarboxylic acid based CEPUs
102
For the sake of clarity a typical TGA and their derivative curves for PUs and
CEPUs (TPA, TIA, HPA and HIA) are shown in Figure 3.14. From the figure it can
be seen that the TGA thermogram of CEPUs are stable upto 130 °C and completely
degrades around 610 °C. The principal process which takes place during degradation
is depolymerisation, i.e., decomposition of PU to yield its parent substances;
diisocyanates and polyols.
Table 3.12. Data obtained from TGA thermograms for dicarboxylic acid based chain extended PUs
Transition temperature range (oC) ± 2 Samples
code Process Ti T max Tc
Weight loss (%)
TDI based CEPU
TPA
1 2 3
Ash
207 331 432
-
319 364 465
-
331 432 514
-
31.3 46.9 21.1 0.7
TTA
1 2 3
Ash
193 387 500
-
366 466 544
-
387 500 643
-
48.2 32.9 18.1 0.8
TIA
1 2 3
Ash
204 325 393
-
302 358 457
-
325 393 575
-
29.7 27.1 43.0 0.2
TMA
1 2 3
Ash
207 315 416
-
300 345 455
-
315 416 680
-
30.0 45.2 23.4 1.4
TCA
1 2 3
Ash
191 316 414
-
289 352 459
-
316 414 680
-
33.9 42.8 22.8 0.5
TGA
1 2 3
Ash
218 313 428
-
298 345 451
-
313 428 498
-
38.6 45.9 14.1 1.4
TPU 1 2
Ash
216 331
-
310 366
-
331 500
-
38.6 60.5 0.9
103
HDI based CEPUs
HPA
1 2 3
Ash
224 333 405
-
308 360 424
-
333 405 580
-
42.8 28.5 28.3 0.4
HTA
1 2 3
Ash
200 385 492
-
365 465 536
-
385 492 625
-
55.2 27.3 17.3 0.2
HMA 1 2
Ash
237 436
-
358 410
-
400 513
-
61.5 37.4 1.1
HIA 1 2
Ash
226 397
-
329 452
-
397 497
-
62.3 36.8 0.9
HGA
1 2 3
Ash
199 384 499
-
334 471 527
-
384 499 605
-
46.3 40.8 11.9 1.0
HCA
1 2 3
Ash
216 386 510
-
352 479 576
-
386 510 640
-
42.4 44.6 12.7 0.3
HPU 1 2
Ash
254 430
-
344 461
-
430 502
-
77.8 22.1 0.1
From Figure 3.14, a two step thermal degradation processes for PUs was
noticed. This can be attributed to the presence of both soft and hard segments. The first stage degradation occurs in the temperature range 216-430 °C with the weight loss of 38.6%. The weight loss in this step was due to soft segment of PU and the main pyrolysis product may be carbon dioxide [53]. The second stage degradation occurred in the temperature range 331-502°C with the weight loss of 60.5%. This could be due to thermal decomposition of hard segment of PU. In this step weight loss may be due to liberation of HCN, nitriles of aromatic carbon and ethers [53-54]. Pielichowski et al [55-56] noticed a similar trend for the thermal degradation of PU obtained from TDI and different polyols. The percentage of ash content also depends on nature of PUs. Increase in nitrogen content in PU enhanced the thermal stability and yield of higher char.
104
PUs generally has relatively low thermal stability as compared to CEPUs. The TGA curves of CEPUs showed three significant thermal degradation steps in the temperature range 191–397, 313–513 and 405–680 °C for first, second and third steps respectively. The temperature range of decomposition, the percentage weight loss for each thermal degradation step and percentage of residue/ash for all CEPUs are given in Table 3.12.
Three mechanisms of decomposition of urethane bonds have been proposed [57]: (i) RHNCOOR’ RNCO + HOR’
(ii) RHNCOOCH2CH2R’ RNH2 + CO2 + R’CH = CH2 (iii) RHNCOOR’ RHNR’ + CO2
All three reactions may proceed simultaneously. PUs from vegetable-oil-based
polyols with secondary -OHs has been found to start thermal degradation below
300 °C [58]. The degradation in our samples also starts somewhat below 300 °C by
the loss of carbon dioxide from the urethane bond and this process is faster in PUs
from secondary -OHs as in castor oil based PUs.
The weight loss during first step degradation was different for different
CEPUs. The CEPUs show initial weight loss in the temperature range 191-397 °C
with a weight loss in the range 29.7–62.3% and this step is termed as first stage of
thermal degradation process. The weight loss in this step is attributed to the loss of
moisture, linear aliphatic hydrocarbons of castor oil, oligomers, etc. The second step
thermal degradation occurs is in the temperature range 313–513 °C with a weight loss
in the range 27.1–46.9%. The major weight loss which occurred in this step, is
assigned to the thermal degradation of linear and hard component of PU. The weight
loss in the last and final stage occurs in the temperature range 405–640 °C. The
weight loss which occurs in this step of CEPUs, which lies in the range 11.9-43%,
indicating the complete decomposition of crosslinked CEPUs and residual hard
component.
All CEPUs show very low ash content of 0.2-1.4% [59-60]. It was observed
that there was no systematic variation in weight loss with respect to nature of chain
extenders. Generally the CEPUs under observation do not break down in a simple
manner and there is a change in the morphological structure of the CEPUs at each and
every instant of pyrolysis and that affects the rate of decomposition. Also the degree
105
of hydrogen-bond formation between the carbonyl group and -NH (urethane) group or
physical interaction of PUs is different for the different chain extenders.
3.3.8 X-ray profile analysis
To probe the microstructure of PUs and CEPUs, the powder X-ray diffraction
patterns have been recorded. X-ray diffractrograms of PUs and CEPUs are shown in
Figure 3.15. From X -ray diffractograms three peaks were observed for all CEPUs,
two weak peaks at 2θ about 14.3o and 17.6o and another broad and intense peak at 2θ
around 20.5o. The shape and size of peaks in X-ray profiles depends on the nature of
diisocyanate and dicarboxylic acids.
All CEPUs belong to the orthorhombic system and the lattice parameters are;
a = 5.067, b = 5.498 and c = 16.14 Ǻ. It is evident from the Figures 3.15 (a) – (b) that
there is a broadening of peaks arising due to two main factors. According to Warren
[61] these are due to decrease in (i) crystal size <N> and an increase in (ii) strain
(lattice disorder) (g in %) present in the samples.
The peak centered at about 2θ = 17.6o may be ascribed to periodicity parallel
to the polymer chain, while the peak at 2θ = 20.5o may be due to the periodicity
perpendicular to the polymer chain [62]. The intense peak that appeared at around
2θ = 17.6o is a relatively sharp, well-defined peak and the other peaks are also due to
the Bragg-like order of the material associated with paracrystalline disorder [63].
A strong diffraction peak which was observed at about 2θ = 20.5o for CEPUs
(Figure 3.15) is due to the peak that originated from the partially ordered structure
formed by hard segment domain where inter-chain attractions such as hydrogen
bonding and dipole–dipole interaction drew the hard and soft segments together
(Scheme 3.3) [64-65]. However, appearance of diffraction peak at 2θ = 20.5o indicates
the presence of hard segment domain in PU.
The microcrystalline parameters such as number of unit cells <N>, width of
the crystal size distribution (α), the smallest crystal unit (p), lattice disorder (g),
surface weighted crystal size (DS) and the enthalpy (α*) for CEPUs have been
106
calculated from X-ray profiles using exponential distribution function and are given in
Table 3.13(a)-(b) for TDI and HDI based systems respectively. From the table it is
evident that the microcrystalline parameters such as (<N>), p, α and D values are
different for different CEPUs. The different microstructural parameters of CEPUs are
due to different molecular organization, hard to soft segment ratios and morphological
0 20 40 60 80
TDI
MA
CA
TA
GA
PA
PU
TIA
Inte
nsity
(au)
2 θ
0 20 40 60 80
HDI
IAPU
GA
CA
MA
TA
PA
Inte
nsity
(au)
2θ Figure 3.15. X-ray diffraction patterns of TDI and HDI based PUs and CEPUs.
107
behavior. The lattice strain is constant for all CEPUs. The dhkl values of CEPUs lies in
the range 4.06-6.18 Ǻ. This result indicates that the intraplanar distances are different
for different CEPUs because it depends on the chemical structure of chain extenders.
Table 3.13 (a). Microcrystalline parameters for TDI based CEPUs obtained from WAXS studies using exponential distribution function
Sample 2 θ (o) g (%) <N> p dhkl (Ǻ) Ds
(Ǻ) δ (%) α *
14.87 0.1 14.86 13.62 5.96 88.56 3.86
17.65 0.1 16.49 14.83 5.02 82.78 3.96
TPA 20.06 0.1 2.17 2.06 4.46 11.12 5.25
0.406
14.52 0.1 11.04 10.90 6.10 67.34 4.03
17.27 0.2 16.14 16.02 5.13 82.79 3.78 TTA
19.98 0.1 2.48 1.93 4.44 11.01 5.35
0.402
14.34 0.1 13.92 12.93 6.18 86.02 3.57
17.12 0.1 14.90 13.35 5.18 77.18 4.01
TIA 20.32 0.1 2.40 1.91 4.37 10.49 4.59
0.386
14.86 0.1 13.18 11.61 5.96 78.55 4.08
17.64 0.1 16.56 13.34 5.01 82.96 4.54
TMA 20.97 0.1 2.24 2.15 4.23 9.47 4.80
0.407
14.78 0.1 13.11 11.64 5.99 78.53 4.09
17.63 0.1 15.29 13.48 5.03 76.90 4.27
TCA 20.77 0.1 2.30 2.21 4.27 9.82 5.30
0.391
14.77 0.1 12.54 11.75 5.99 75.11 3.76
17.56 0.1 13.93 12.18 5.05 70.35 4.31
TGA 21.32 0.2 2.39 2.27 4.17 9.96 6.23
0.373
14.36 0.1 09.61 08.50 6.17 59.29 3.88
17.16 0.1 11.22 09.78 5.17 58.00 4.16
TPU 20.16 0.2 2.61 2.02 4.40 11.48 5.52
0.334
108
Table 3.13. (b). Microcrystalline parameters for HDI based CEPUs obtained from WAXS studies using exponential distribution function
Sample 2 θ (o) g
(%) <N> p dhkl (Ǻ)
Ds
(Ǻ) δ (%) α *
14.82 0.1 16.41 15.84 5.97 97.96 4.09
17.64 0.1 18.42 18.01 5.03 92.65 4.20
HPA 21.89 0.2 02.42 02.30 4.06 09.82 6.28
0.429
14.79 0.1 14.37 14.29 5.99 86.07 3.32
17.56 0.1 15.05 13.60 5.05 76.00 4.00
HTA 20.38 0.1 02.74 02.61 4.35 11.92 6.45
0.387
14.52 0.1 12.71 11.43 6.10 77.53 3.94
17.28 0.1 13.71 12.05 5.13 70.33 4.18
HIA 20.98 0.1 2.52 01.95 4.23 10.66 5.48
0.370
14.67 0.1 12.26 10.82 6.03 73.92 4.16 0.370
17.45 0.1 13.70 11.89 5.07 69.46 4.19
HMA 20.57 0.2 02.63 02.50 4.31 11.33 7.06
14.62 0.1 10.53 09.47 6.05 63.70 3.97 0.347
17.43 0.1 12.05 10.58 5.10 61.45 4.32
HCA 20.71 0.1 02.88 02.37 4.28 12.33 5.64
14.74 0.1 06.92 06.00 6.01 41.58 3.91
17.30 0.2 10.35 09.04 5.12 52.99 4.12
HGA 20.60 0.2 02.77 02.20 4.31 11.94 5.89
0.643
14.39 0.1 08.04 06.95 6.15 49.45 4.09
17.22 0.1 10.85 09.64 5.15 55.87 4.01
HPU 20.26 0.1 02.96 02.45 4.38 12.96 5.36
0.329
109
We have estimated microstructural parameters by simulating the profile
employing the procedure described earlier and for Bragg reflection at 2θ = 17.9o
[66-69]. For the sake of completeness, we have reproduced in Figure 3.16 the
simulated and experimental profiles for both pure PU and PA based CEPUs ((a) TPA
and (b) HPA)). The percentage of deviation between stimulated and experimental
profiles was less than 15 % in all the samples and for all the reflection. This result
indicates that the model used here is quite reliable. These results are further justified
by the behavior of microstructural quantities such as crystal size or correlation length
(<N>) and lattice strain (g) at microscopic level as given in Table 3.13. The molecular
level interaction via hydrogen bonding between polymer networks leads to the
reorientation which causes higher values of <N> and p.
From <N> and ‘g’ parameters, we can also estimate the enthalpy (α*) of
CEPUs using following relation [70];
α* = <N> ½ g (1)
The enthalpy (α*) value implies physically that the growth of paracrystals in a
particular material is appreciably controlled by the level ‘g’ in the net plane structure.
The estimated values of enthalpy (α*) is also given in Tables 3.13 (a)-(b) and it lies in
the range of 0.329-0.643 for all CEPUs, which is in good agreement with the data
published elsewhere [71]. It is also noticed that after chain extension of PU using
diacids the α* value is reduced. The lower values of α*, implies the phase
stabilization of CEPUs. This conclusion was drawn on the basis of the minimum
value of α* (0.329-0.643), the enthalpy that is a measure of the energy required for
the formation of the net plane structure, and is in agreement with the values reported
by Hosemann [70].
Table 3.13 (a-b) gives the surface weighted crystal size (DS) calculated by
Fourier’s and simulation method for different dicarboxylic acids based CEPUs. The
order of magnitude of the surface weighted crystal size clearly indicates the extent of
crystallinity present in the system. This change in the microstructural parameters is
due to molecular organizational changes in the CEPUs.
110
Figure 3.16. Experimental and stimulated X-ray intensity profiles of
(a) TPU, (b) HPU, (c) TPA and (d) HPA
To plot these results in a common x-y plane, the following equation was used
and fitted the crystal size values of CEPUs were fitted into an ellipsoid with one
Ds (2θ =14.65o) in Ǻ along the X-axis and the other Ds (2θ =17.41) along y-axis.
Here 2θ is the angle between the two (hkl) planes and Ds is the crystal size
corresponding to the particular (hkl) reflection. Figures 3.17 (a)-(c) shows the
comparison of the shape ellipsoid of crystallites of CEPUs.
22
sincos2
+
=
xyNhkl
θθ (2)
According to Hosemann’s model these changes in crystal size values as well
as shape ellipsoids attributes to the interplay between the strains present in the
polymer network and also the number of the unit cells coherently contributing to the
X-ray reflection.
111
Figure 3.17. Variation in crystallite shape ellipsoid of TDI
and HDI based CEPUs with PUs
From Table 3.13 (a-b) it was observed that <N>, p and Ds values for CEPUs
were higher as compared to PUs. This is due to strong secondary forces of interaction
between polymer networks in case of CEPUs than PUs. Furthermore PA and/or TA
based CEPUs have higher values of <N>, p and Ds whereas, GA based systems have
lower values. This result indicates that the values of <N>, p and Ds are structure
D in Angstrom along 2θ = 14.64 deg
D in Angstrom along 2θ = 14.65 deg
D in
Ang
stro
m a
long
2θ
= 17
.43
deg
D in
Ang
stro
m a
long
2θ
= 17
.41
deg
112
sensitive. That means the systems which possesses higher hard/soft segment ratios has
higher values of <N>, p and Ds.
From the table it was noticed that there was higher percentage of crystallinity
for PA and IA based and lower values for CA and GA based CEPUs. However, the
values of crystallite area for the CEPUs lies above the PU. Crystallite area and
percentage of crystallinity for PUs and CEPUs were calculated using X-ray profile
with the help of asymmetric distribution function and the results obtained are given in
Table 3.14. The variation in crystallite area and percentage of crystallinity of CEPUs,
indicates that the aggregation of crystallizable segments and the formation of
crystallites are significantly affected by the chemical nature of the dicaroxylic acid
units. The crystalline form of hard segments depends upon their structure as well as
on the crystallization conditions [72]. All CEPUs displayed the semi-crystalline
nature. This could be due to variations in the structural units of the chain extender
base backbone of the main PU chain.
Table 3.14. Percentage of crystallinity and crystallite area of CEPUs
Name of the PU Total area Crystallite area % of crystallinity TDI based CEPUs
TPA 7165.8 4724.6 68.93 TTA 4733.9 3219.6 66.01 TIA 6945.4 4274.8 61.55
TMA 8142.9 5666.6 65.58 TCA 4955.5 3077.3 62.09 TGA 6351.3 3295.4 64.74 TPU 7153.0 4631.4 51.88
HDI based CEPUs HPA 5301.2 2405.0 45.36 HTA 8157.9 3429.0 42.03 HIA 10674.3 4161.3 38.98
HMA 7495.9 3829.3 41.08 HCA 7094.6 3602.9 40.78 HGA 4812.0 4136.7 38.26 HPU 5035.6 2289.9 35.46
113
From Table 3.14, it was noticed that the crystallite area and percentage of crystallnity of CEPUs lies in the range of 2289-5666 and 35.46-68.93 respectively. Higher values of crystallite area and percentage of crystallnity for TDI based CEPUs than HDI based CEPUs was also noticed. From the table higher percentage of crystallinity for PA and IA based CEPUs and lower values for CA and GA based CEPUs was noticed. However, the values of crystallite area for the CEPUs lies above the PU. The higher values of percentage of crystallinity for CEPUs can be associated to co-operative movements of the molecular chains.
This fact is also realized in tensile strength and modulus, wherein these values also high for the systems having higher values of <N>, p, Ds and percentage of crystallinity. This implies that the polymer network of CEPUs with higher hard components needs more strength (external) or energy to disturb the system. These results justify the insignificant changes in physical properties.
3.3.9 Morphological behavior
SEM has several potential advantages in morphological investigation and has
been extensively applied in the field of polymer, biomaterial and composite.
Additional information on morphology is provided by surface morphology. The
toughening mechanism can also be explained in terms of morphological behavior.
This is because the morphological examination can give interesting information on
the microstructure of PUs.
The SEM of the fractured surface of PU and chain extended (MA and GA)
PUs is shown in Figures 3.18 (a) - (f). The SEM photomicrographs revealed the two-
phase morphology for CEPUs. The microphase separation is generally more
prominent in CEPUs due to polar interaction between amide and urethane groups in
PU networks. The images of SEM reveal the formation of domain phase and the layer
like structure of CEPUs. This is because PUs have both hard and soft components. In
the golden-yellow colored transparent CEPUs, the polymeric chains are interwoven
with one another. The extent of interweaving depends on the nature of the polymer
systems, methods of preparation and the chemical interaction between the chains. The
phase segregation observed is more predominant in CEPUs than PU. The degree of
phase separation varied from one chain extender to another. This is due to the domain
structure that results from the phase segregation of the hard and soft segments in the
114
CEPUs, which is well recognized as the principle of this class of PUs [32, 73]. This
result is supported by the multiple transitions noticed in DSC thermograms.
.
Figure 3.18. SEM photomicrographs of (a) TPU, (b) HPU, (c) TMA, (d) HMA, (e) TGA and (f) HGA
115
3.3.10 Soil degradation
Degradation behavior of all CEPUs was studied by the soil burial method (200
cm3) [74]. The measured percentage of reduction in weight after soil degradation is
given in Table 3.15. The percentage of reduction in weight is less than 10 % (-9.8 to -
1.08 %) in both series of chain extended bio polyurethanes. It was observed that a
slight discoloration at the surface of all PUs occurred which can be caused by fungus,
but it does not indicate a mechanical damage of the material [75].
Table 3.15. Change in weight of the CEPUs after soil degradation
TDI based CEPUs
Weight loss (%)
HDI based CEPUs
Weight loss (%)
TPA -5.44 HPA -7.77 TTA -2.03 HTA -5.14 TIA -1.17 HIA -6.76
TMA -1.67 HMA -2.46 TCA -9.8 HCA -4.51 TGA -4.47 HGA -3.22 TPU -1.08 HPU -1.97
A slight reduction in weight was also noticed in all PUs. More weight loss
occurred (4.5 %) in case of HCA based PUs as compared to other CEPUs. This is
because hard domains of PUs are normally resistant to microorganism attack as
compared to soft domains of PUs.
3.4 Conclusions
Chain extended PUs have been synthesized using dicarboxylic acids as chain
extenders. Six different kinds of chain extenders such as aliphatic (CA, TA and CA),
aromatic (PA), unsaturated (IA and MA) and saturated di- and poly functional groups
(TA and CA) of dicarboxylic acids have been used for the synthesis of CEPUs. The
effect of chemical structure of dicarboxylic acids on the performance of CEPUs such
as mechanical properties, chemical resistivity, swelling behavior, thermal behavior
and morphological behaviors has been systematically investigated. From experimental
results the following conclusions are drawn.
(i) Tough and transparent diacarboxylic acid based CEPUs are obtained. The
percent of transmittance of all CEPUs are greater than 75%.
116
(ii) Castor oil based PU and CEPU were characterized by the spectral techniques
using FTIR. According to the characteristic peaks of C=O (1733 and
1703 cm-1) and N-H (1550 cm-1) the formation of urethane group (-NHCOO-)
was confirmed. The disappearance of N=C=O (2270 cm-1) stretching peaks in
IR spectra indicates that all the residual -NCO groups were consumed by diol
and disappeared after chain extension.
(iii) Tensile behavior data indicates that CEPU exhibits higher tensile strength and
tensile modulus than PU. The increased physico - mechanical properties is due
to the intramolecular and intermolecular hydrogen bonding between the PU
networks. This result would indicate that during the chain extension reaction,
there is significant bond formation rather than any significant increase in
molecular weight, which would result in an increase in mechanical properties.
This study also indicates that the incorporation of chain extender (0.1 molar
ratios) into the PU polymer will increases the material elasticity to a greater
extent. Moreover, the use of maleic acid as the chain extender allows the
insertion of reactive double bonds in the polymer chains. These double bonds
can perform as grafting sites for further derivatization, thus allowing specific
tailoring of the base polymers. It is assumed that the dicarboxylic acids acts as
an additional physical crosslinker, increased modulus of the flexible segment in
the polyurethane matrix, resulting in increased hardness and modulus.
(iv) Higher mechanical and thermal properties were observed for TDI based chain
extended PUs. Among all chain extenders, poor mechanical and thermal
properties were observed for GA based CEPUs because GA is an aliphatic chain
extender.
(v) Heat ageing studies reveal that CEPUs show outstanding performance at low
temperature.
(vi) The chemical resistivity of CEPUs has been measured. The variations in surface
characteristics of test specimens after exposure to different chemical
environments are unaltered. CEPUs are slightly sensitive to alkaline and acid.
Swelling occurred in organic solvents. The CEPUs are degraded in KMnO4
solution.
117
(vii) From TGA analysis it was noticed that all CEPUs are thermally stable. TGA
curves of HDI based CEPUs show two step thermal degradation processes
where as TDI based CEPUs exhibit three step thermal degradation processes.
(viii) The phase behavior of the poly (urethane-amide) has been investigated by
DMA and DSC techniques, probing the structure from the molecular to the
macroscopic levels. All data point to the same conclusions despite the presence
of hard segments in the soft phase, as demonstrated by WAXS and supported by
DSC and DMA experiments. Finally, the addition of PA greatly enhanced phase
separation. For GA based CEPU, urethane hydrogen bonding is greatly reduced
which leads to less phase separation and ultimately to reduced mechanical
stiffness.
(ix) A strong diffraction peak was observed around 2θ = 20.5o for CEPU. The
diffraction peak originated from the partially ordered structure formed at hard
segment domain where inter-chain attractions such as hydrogen bonding and
dipole–dipole interaction drew the hard segments together. As the polymeric
chain is dynamic and flexible, sharp diffraction peak is observed for the PU.
However, appearance of diffraction peak at 2θ = 20.5o partially supports the
presence of hard segment domain in PU. CEPUs are made up of both soft and
hard segments as observed from WAXS studies. Semi-crystalline nature of the
prepared CEPUs was confirmed by X-ray diffraction studies.
(x) The SEM images reveal the formation of domain phase and the layer like
structure of CEPUs. The SEM photomicrographs reveal the two-phase
morphology of the CEPUs due to the presence of hard and soft components.
(xi) By fully understanding the implications of CEPUs chemistry and preparation
techniques, this class of elastomers can be exploited to take full advantages of
their inherent flexibility, durability and strength.
118
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