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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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