Chapter 5 Mechanical and Morphological Characterisation of...
Transcript of Chapter 5 Mechanical and Morphological Characterisation of...
117
Chapter 5
Mechanical and Morphological Characterisation of GAP and GAP Based Blends
5.1 Introduction
Solid rocket motors have to withstand a wide range of mechanical loads for
successful performance during their mission. The various mechanical loads are
imposed on it during storage, handling and operational phase. The loading
environments include thermal cycling, vibration and ignition pressurisation.1
Quantitative measurements of mechanical characteristics are important as the
propellant is utilized as a material of construction in case bonded configuration. The
different loads may sometimes be experienced in combination. Structural integrity is
one of the most important factors that is to be considered for propellant grain design.
The mechanical properties of solid propellants depend both on intrinsic and extrinsic
variables.2 These intrinsic variables are molecular weight, effect of crosslinking,
branching and crystallinity, plasticisation, quality and quantity of fillers. The external
variables are temperature, time frequency or strain rate, pressure, amplitude of stress
or strain, mode of deformation and nature of surrounding atmosphere.
Composite solid propellants display viscoelastic characteristics as they show
behaviour similar to both viscous liquids in which rate of deformation is proportional
to applied forces and to elastic solids, in which magnitude of deformation is
proportional to applied forces. The polymeric binder is the main contributor to the
viscoelastic nature of solid propellants. The mechanical behaviour is mainly
controlled by the polymer matrix. Fundamental contribution to the field of
thermodynamics of rubber elasticity made by Saunders,3 Mullins4 and Treolar5 forms
the basics of this area of study. The theoretical approaches provide relationship
between the material characteristics and its macroscopic behaviour. Another
118
important contribution in this field was by William et.al6 in the study of
time–temperature superposition method. The mechanical properties of propellant are
strongly dependant on quality of bonding at the interface between solid fillers and the
resin matrix. Propellant dewetting takes place before mechanical failure. In the
dewetted condition, the mechanical strength essentially depends on the binder
characteristics. Higher crosslink density leads to decrease in elongation of the binder
matrix. Increase in strain rates leads to increase in stress at which failure occurs in
the binder matrix. These factors imply that the mechanical integrity of the propellant
system is strongly dependant on strain capability of binder matrix.7, 8 It is important
to correlate mechanical properties as a function of their composition during
propellant development. The mechanical properties are mainly determined by the
crosslink density of the binder matrix.9 An analysis of the interrelation between
modulus, ultimate tensile strength and elongation at break and the specific
deformation energy can be used as basic data for the preparation of propellant
formulations. The mechanical properties or the crosslink density of the binder matrix
can be tailored by quantitative variation of polymer diol, curative and triol.10 In the
case of GAP based high burn rate propellant compositions, it has been reported that
GAP concentration in the range of 33 to 60% has been used.11, 12
5.2 Chemistry of GAP based polyurethane network
Polyurethane based matrix systems find application as engineering materials
due to their good thermal, physical and mechanical properties. Polyurethanes based
on HTPB and isocyanate curatives are widely used as binder for composite solid
propellant application.13 Detailed studies on the use of polyfunctional groups to tailor
the properties of HTPB based polyurethane network have been presented in literature
by many authors.14, 15 The impact of variation of NCO/OH ratio on the properties of
the polymers has been reported.16 Long chain polyurethane network formation
reaction between hydroxyl terminated GAP and polyisocyanate have been reported.17
119
Use of organo metallic catalysts like dibutyl tin dilaurate was found to have strong
influence on the rate of cure reaction.17 The equivalent ratio of NCO and OH groups
and the triol to diol ratio were varied to adjust the crosslink density.10 The use of long
chain diols as binder helps to reduce the problems of cure shrinkage, exotherm due to
curing and non elastomeric properties. The urethane formation reaction can be
represented schematically as shown in scheme 5.1. The most favoured mechanism in
the scheme is the nucleophilic attack of the hydroxylated compound to the isocyanate
group.18, 19
Scheme 5.1 Urethane formation reaction
The urethane link is chain extended by 1, 4-butanediol (BD) and crosslinked by
trimethylol propane (TMP) through OH functional groups and isocyanate. The
urethane reaction leads to crosslinking. The poly condensation reaction between the
polymer functional groups, crosslinking agent and the isocyanate which leads to the
crosslinked network structure is shown schematically in scheme 5.2.
OHHO OHHO HO OH
OH
+ +GAP BD TMP
+OCN NCODiisocyanate
NHCOO NHCOO NHCOOOOCHN OOCHNOOCHN
OOCHN OOCHNOOCHN NHCOONHCOONHCOO
OO
CHN N
HC
OO
NHC
OO O
O
CHN
Scheme 5.2 Poly condensation reaction of GAP diol, BD and TMP with diisocyanate forming polyurethane network
120
5.3 Studies on gumstock properties of crosslinked GAP
The various applications of plastics, elastomres and fibers depend mainly on
their mechanical strength than their chemical characteristics. The various
classification of polymers used for most of the practical applications can be classified
as follows.20
i. Soft, weak which have low modulus, tensile strength with moderate
elongation at break.
ii. Hard, brittle with high modulus and tensile strength with low elongation
iii. Soft, tough with high values of tensile strength, elongation and low modulus
iv. Hard, tough with high tensile strength, elongation and modulus
v. Hard, strong with properties intermediate between hard and brittle and hard
and tough.
The mechanical properties of polyurethane network prepared with GAP and
GAP based blends including that of interpenetrating network structure with GAP and
HTPB were evaluated from the stress-strain curves in terms of tensile strength,
elongation at break and modulus.
5.3.1 Experimental
GAP resin and the crosslinker used for the matrix preparation were dried in
rotary vacuum evaporator to remove moisture and volatiles to the extent of less than
0.1%. Presence of moisture in GAP causes formation of large extent of blowholes
while curing. This is mainly due to the dominating reaction between isocyanate and
moisture compared to the lower reactivity of the secondary hydroxyl functional in
GAP.
121
5.3.1.1 Materials
GAP resin with molecular weight 2000 (by VPO) and hydroxyl value 45 mg
KOH/g was prepared in VSSC as mentioned in section 4.3.5 for the experiments.
The crosslinker used was a mix of 1, 4-butanediol and trimethylol propane in the
weight ratio of 1:2. Curatives used include toluene diisocyanate (TDI), isophorone
diisocyanate (IPDI) and methylene bis (cyclohexyl isocyanate) (MDCI) available
from commercial sources. The catalyst (0.006 g per drop) used was a 10% solution
of DBTDL in toluene.
5.3.1.2 Sample preparation and testing
For preparation of crosslinked GAP samples, first GAP was mixed with
crosslinker and dried at 1000C for 30 minutes in a vacuum flash evaporator. After
cooling the mix, weighed quantity of curative was added and mixed thoroughly. In
this preparation, the stochiometric ratios of curatives and crosslinker were varied to
study the effect on the gumstock properties. After addition and mixing of curatives,
weighed quantity of catalyst was added and mixed. The mix was evacuated to
remove entrapped volatiles. The resin mix was then poured into mould and
evacuated. The initial curing was done under nitrogen blanket for 24 hours at
ambient conditions. This was followed by curing at 600C for 48 hours in air oven.
From the cured slabs, dumbbells were prepared and tested as mentioned in section
2.5.
5.3.1.3 Results and discussion
In this study, effect of concentration of reactive species, curing temperature,
curing time, and effect of different type of curatives on the gumstock properties of
GAP were evaluated.
122
The influence of the NCO/OH ratio (R value) on the gumstock properties of
GAP matrix was investigated experimentally by varying the NCO/OH ratio in the
range of 0.7 to 1.75 for a constant crosslinker content of 5%. This evaluation was
also carried out with different curatives like TDI, IPDI and MDCI. Figures 5.1 to 5.4
shows the variation of tensile strength, elongation, stress at 100% elongation and
shore A hardness with NCO/OH ratio of the crosslinked network with different
curatives. Samples prepared with NCO/OH ratio below 0.9 were found to be of too
low strength to be tested. The increase in tensile strength, stress at 100% elongation
and shore A hardness and reduction in elongation with increase in NCO/OH ratio
could be related to the increase in crosslink density of the network. The results show
that increasing the NCO content improves the tensile strength. Elongation is found to
decrease up to a NCO/OH ratio of 1, beyond which no significant change was
observed.
Figure 5.1 Variation of tensile strength with NCO/OH ratio
Figure 5.2 Variation of elongation
with NCO/OH ratio
123
Tensile strength, stress at 100% elongation and hardness for the three
curatives were found to be in the order of MDCI > IPDI > TDI. The change from
aromatic diisocyanate to aliphatic diisocyanate is found to significantly improve the
mechanical strength. This could be due to the high reactivity of aromatic
diisocyanate leading to side reactions forming more of biuret and allophanate
linkages compared to aliphatic diisocyanate.
The effect of the crosslinker content on crosslinked networks was evaluated
by testing the gum stock properties of the network prepared with varying
concentration of the crosslinker. The crosslinker content in the GAP mix was varied
from 3 to 9% for a NCO/OH ratio of unity. When the crosslinker content was
reduced below 3%, the cohesive strength of the crosslinked slab was affected
severely. The low cohesive strength of the network could be due to the low crosslink
density resulting from the lower functionality (less than 2) of the GAP resin. Hence,
for GAP based networks, to achieve sufficient crosslink density, a minimum
crosslinker content of 3% is necessary for NCO/OH ratio above 0.90. The test results
show that increase in crosslinker content improves the tensile strength, stress at
100% elongation, shore A hardness and reduce the elongation due to increase in
Figure 5.3 Variation of stress at 100% elongation with NCO/OH ratio
Figure 5.4 Variation of hardness with NCO/OH ratio
124
crosslink density of cured network. Figures 5.5 to 5.8 show the variation of the
properties with crosslinker content.
Figure 5.5 Effect of crosslinker content on tensile strength
Figure 5.6 Effect of crosslinker content on elongation
Figure 5.7 Effect of crosslinker content on stress at 100% elongation
Figure 5.8 Effect of crosslinker content on hardness
125
Cure cycle studies were done on GAP binder at ambient condition and at
600C and the properties were compared. The effect of curing time was also evaluated
by curing the samples at extended periods of time. Figures 5.9 to 5.12 and figures
5.13 to 5.16 show the effect of curing time at ambient and at 600C on mechanical
properties respectively. The test results show that optimum mechanical properties
were obtained at ambient curing for 96 hrs. beyond which no significant changes in
mechanical properties are observed.21
Figure 5.9 Effect of ambient curing on tensile strength
Figure 5.10 Effect of ambient curing on elongation
2.52.72.93.13.33.53.73.9
0 100 200 300Curing time (hrs)
Stre
ss a
t !00
% e
long
atio
n (k
sc)
Figure 5.11 Effect of ambient curing on stress at 100% elongation
15
20
25
30
35
0 100 200 300Curing time (hrs)
Sho
re A
har
dnes
s
Figure 5.12 Effect of ambient
curing on hardness
Tens
ile st
reng
th (k
sc)
126
In the case of accelerated curing, curing of GAP is almost complete within 48
hours itself and extended curing does not increase the cross-link density appreciably
as against the drastic change in mechanical properties seen on variation of curing
agent or crosslinker content.
5
6
7
8
9
0 50 100 150 200 250
Curing time (hrs)
Tesi
le s
treng
th (k
sc)
Figure 5.13 Effect of cure time
at 600C on tensile strength
150
170
190
210
230
0 50 100 150 200 250Curing time (hrs)
% E
long
atio
n
Figure 5.14 Effect of cure time
at 600C on elongation
2.5
3
3.5
4
0 50 100 150 200 250
Curing time (hrs)
Stre
ss a
t 100
% e
long
atio
n (k
sc)
Figure 5.15 Effect of cure time at 600C on stress at 100% elongation
05
101520253035
0 50 100 150 200 250Curing time ( hrs)
Sho
re A
har
dnes
s
Figure 5.16 Effect of cure time
at 600C on hardness
127
5.4 Studies on the gumstock properties of GAP-HTPB blends
The use of GAP as an energetic additive or plasticiser with other polymeric
binders have already been reported.17 In this study, crosslinked GAP-HTPB blends
with varying percentage of the binders were prepared. TDI, IPDI and MDCI were
used for preparation of the crosslinked networks. The effect of NCO/OH ratio on
gumstock properties of GAP-HTPB blend prepared with a 50:50 ratio of GAP and
HTPB was also evaluated.
5.4.1 Experimental
GAP and HTPB were found to be immiscible. The two resins were found to
phase separate when kept for a while after mixing. However, crosslinked networks of
GAP-HTPB blends could be prepared without phase separation by increasing the
reaction rate and gelling of the mix by adjusting the catalyst content. The catalyst
concentration was varied between 0.036 to 0.1% in the trials.
5.4.1.1 Materials
GAP, TDI, IPDI, MDCI, crosslinker and catalyst used were obtained from
sources as mentioned in section 5.3.1.1. HTPB resin with molecular weight 2500 (by
VPO) and hydroxyl value of 43 mg KOH/g, produced in VSSC by free radical
polymerisation of butadiene gas was utilised for the study.
5.4.1.2 Sample preparation
GAP-HTPB blends were prepared with GAP content ranging from zero to
100% and the mechanical properties of the crosslinked networks were evaluated. The
crosslinker concentration was maintained at 5% level with R value of 1. For
preparation of the blend, first GAP and HTPB were mixed as per required ratio.
Required amount of crosslinker was added and then the mix was evacuated and dried
at 1000C using rotary vacuum flash evaporator. After cooling, weighed quantity of
128
curative was added and stirred well. After addition of catalyst and thorough mixing,
it was poured into a mould and allowed to cure initially under nitrogen atmosphere at
ambient temperature for 24 hours and then at 600C in an air oven for 48 hours.
During the experiment, care was taken to make sure that the initial gelling of the mix
takes place without any phase separation. From the slab prepared, dumbbells were
cut and tested using Instron testing machine as mentioned under section 2.5.
Crosslinked samples of GAP-HTPB blend with a ratio of 50:50 was also prepared
with varying NCO/OH by the same procedure. In these experiments, the TDI was
used as curative and the NCO/OH ratio was varied from 0.85 to 1.2.
5.4.1.3 Results and discussion
The gum stock properties GAP-HTPB blends prepared with various ratios of
GAP and HTPB and crosslinked with different curatives are shown in Figures 5.17 to
5.20. The test results of the blends show that the tensile strength and stress at 100%
elongation increase with increase in the percentage of HTPB up to 70%, beyond
which a decreasing trend was seen. The tensile strength was increased to 25 ksc and
elongation reduced to 120% by increasing HTPB content to 70%. The results show
higher values for tensile strength and stress at 100% elongation for crosslinked
HTPB than for the crosslinked GAP as expected due to the higher functionality of
HTPB binder (F = 2.3) compared to GAP (F = 1.7). However, GAP–HTPB blends of
50:50 to 30:70 ratios show superior properties over the virgin networks of GAP and
HTPB due to the synergetic effect of interlocking of the two networks resulting in
interpenetrating network structure (IPN).22 The formation of IPN structure was
further studied using optical micrography and scanning electron micrography. The
details of the studies are presented in section 5.6.
129
Figure 5.17 Effect of GAP content on tensile strength of GAP-HTPB blend
Figure 5.18 Effect of GAP content on elongation of GAP-HTPB blend
Figure 5.19 Effect of GAP content on
stress at 100% elongation of GAP-HTPB blend
Figure 5.20 Effect of GAP content on hardness of GAP-HTPB blend
Shor
e A
had
ness
130
The three curing agents show a similar pattern with respect to the gumstock
properties of the blends. MDCI was found to give relatively higher strength
compared to TDI and IPDI as in the case of GAP gum stock properties.
The effect of NCO/OH ratio on the gum stock properties of GAP-HTPB
blend was found to be similar to that of crosslinked GAP formulation. It was
observed that for the GAP-HTPB blend, a minimum NCO/OH ratio of 1 is required
for achieving satisfactory mechanical strength for the cured slab. The test results are
shown in table 5.1.
Table 5.1 Effect of variation of NCO/OH ratio on gumstock properties and hardness of crosslinked GAP-HTPB network
NCO/OH ratio
Tensile strength (ksc)
Elongation (%)
Stress at 100 % elongation (ksc)
Hardness (Shore A)
0.85 2.4 21 - -
0.95 3.4 25 - 35
1.0 19.4 130 17 64
1.1 22.7 97 23 65
1.2 23.2 92 24 68
Non-curing was observed at a NCO/OH ratio of 0.8 even for extended periods
of curing. However, when the NCO/OH ratio was increased above 0.95, tensile
strength, stress at 100% elongation and hardness were found to increase remarkably.
5.5 Effect of plasticiser content on gumstock properties of crosslinked GAP
The mechanical properties of crosslinked GAP with plasticiser content was
determined to evaluate the compatibility of different plasticiser systems with GAP.
Energetic plasticiser systems like trimethylol ethanetrinitrite (TMETN),23
bis(2, 2-dinitropropyl) formal/acetal (BDNPF/A),24 derivatives of glycidyl nitrate viz.
131
GLYN dimer25 have been synthesised for use with GAP. Low molecular weight
GAP26 and GAP polyester having acyl residue of an organic carboxylic acid27 have
also been reported for use as energetic plasticisers. Gumstock properties of GAP
plasticised with BDNPF/A has been reported.28 In this study, the compatibility of
GAP was evaluated with isodecylpelargonate (IDP), dioctyladipate (DOA),
dioctylphthalate (DOP) and paraffin oil. It was observed that, with IDP and paraffin
oil, GAP shows phase separation when kept for a while after mixing. In the case of
paraffin oil, the phase separation was found to be very fast. No phase separation was
found to occur when DOA and DOP were used as plasticisers.
5.5.1 Experimental
Prior to determination of the gum stock properties, miscibility of different
plasticiser systems (DOA, DOP, IDP and paraffin oil) with GAP was studied by
preparing mix of GAP and plasticisers with varying concentration in the range of
0 to 50%. Phase separation between different layers was visually checked after
different intervals.
5.5.1.1 Materials
GAP resin with molecular weight 2000 (by VPO) produced in VSSC was
used for the trials. Toluene diisocyanate, (Bayer, Germany) which is an 80:20
mixture of 2, 4 and 2, 6 isomers was used as the curing agent. Dibutyl tin dilaurate
(DBTDL) in toluene solution was used as cure catalyst. Cross-linking agent used was
a diol-triol mixture (a mixture of 1,4-butanediol and 1,1,1-trimethylolpropane in the
weight ratio of 1:2) and was prepared by mixing 1,4-butanediol and 1,1,1-
trimethylolpropane and the mixture was dried under vacuum for extended periods to
remove volatiles and moisture. DOA, DOP, IDP and paraffin oil were used as
available from commercial sources.
132
5.5.1.2 Sample preparation
Gum stock properties of crosslinked GAP network plasticised with DOA and
DOP were prepared. Plasticiser content was varied from 0 to 20%. In these trails,
TDI was used as curative. GAP was first mixed with plasticiser and then mixed with
other ingredients and crosslinked slabs were prepared as mentioned under 5.3.1.2.
The specimens were tested using Instron testing machine as mentioned in section 2.5.
5.5.1.3 Results and discussion
During the miscibility experiments, it was found that DOA and DOP are
completely miscible in all the proportions used for experimentation. This may be due
to the polar nature of GAP and ester linkage in DOA/DOP. Miscibility is found to
decrease considerably and phase separation is found to occur when plasticiser was
changed to IDP, due to the long hydrocarbon chains of IDP compared to DOA/DOP.
Miscibility was totally absent when paraffin oil was used. This may be due to the
non-polar nature of the hydrocarbon chain.
Plasticiser content was found to strongly influence the gumstock properties. It
was noted that, increasing the plasticiser content from 0 to 20%, reduced the tensile
strength from 7.5 to 4 ksc and increased the elongation from 235 to 300%. Shore A
hardness was found to decrease with increasing plasticiser content. Figures 5.21 to
5.24 show the variation of the properties with palsticiser content.
133
5.6 Effect of plasticiser content on the gumstock properties of GAP-HTPB blend GAP-HTPB blends were prepared without separation and the mechanical
properties were evaluated. For use as propellant binder, the resin is required to be
plasticised for improving the processability. GAP-HTPB blend with 50:50 ratio was
Figure 5.21 Effect of plasticiser on tensile strength of crosslinked GAP
Figure 5.22 Effect of plasticiser on elongation of crosslinked GAP
Figure 5.23 Effect of plasticiser on stress at 100% elongation of
crosslinked GAP
Figure 5.24 Effect of plasticiser on hardness of crosslinked GAP
Shor
e A
had
ness
134
selected for the study. DOA was found to be miscible with the blend. The DOA
content was varied from 0 to 20% in this study.
5.6.1 Experimental
For preparation of the blend, first GAP was mixed with HTPB at weight ratio
of 50:50. Weighed quantity of plasticiser was added and mixed thoroughly.
Crosslinker content of 5% was used in the experiments. TDI was used as the curative
and a NCO/OH ratio of unity was employed. Catalyst used was a 10% solution of
DBTDL in toluene. The curing of the samples and preparation of dumbbells were
done as mentioned in section 5.3.1.
5.6.1.1 Materials
GAP processed in VSSC, with molecular weight 2000 (by VPO) and hydroxyl
value of 45 mg KOH/g was used for the experiments. HTPB with molecular weight
of 2500 (by VPO), produced in VSSC was used. DOA available from commercial
sources was used as such. The crosslinker and curative were used from sources as
mentioned in section 5.3.1.1.
5.6.1.2 Sample preparation
For preparation of the blends, first GAP and HTPB were mixed and then
desired quantity of plasticiser was mixed. Mixing of crosslinker, curative, catalyst
and preparation of slabs were done as explained in section 5.3.1.2.
5.6.1.3 Results and discussion
Crosslinked GAP-HTPB blends were prepared with plasticiser content up to
20% for the evaluation. The effect of plasticiser content in crosslinked GAP-HTPB
blend was found to be similar to that for crosslinked GAP. The tensile strength was
found to decrease from 19.4 to 4.4 ksc for an increase in plasticiser content from 5 to
135
20%. Hardness was also found to decrease from 68 to 25 shore A with increase in
plasticiser content as mentioned above. The elongation was found to increase to
132% for a plasticiser content of 10%. Further increase in plasticiser content was
found to decrease the elongation. Figures 5.25 to 5.28 shows the graphical
representation of the results.
Figure 5.25 Effect of plasticiser on tensile strength of crosslinked blend
Figure 5.26 Effect of plasticiser on elongation of crosslinked blend
Figure 5.27 Effect of plasticiser on stress at 100% elongation of
crosslinked blend
Figure 5.28 Effect of plasticiser on hardness of crosslinked blend
136
5.7 Study on the morphology of GAP-HTPB blends
From the study of the gumstock properties of GAP-HTPB blends, it was
found that when the GAP-HTPB blends were crosslinked without phase separation, it
leads to formation of interpenetrating network structure (IPN). IPN structure forms
when a pair of polymeric networks are synthesised in intimate contact with one
another. When the two networks are totally compatible, one network fully
interpenetrates the other and forms catenated structure without any chemical
crosslinks.29 However, in most cases a certain degree of phase separation in the blend
could lead to substantially pure domains of each network. The physical interlinking
could be localised at phase boundaries and to optimise the IPN structure, the phases
should be as discontinuous as possible.30
GAP and HTPB are immiscible and phase separation occurs when the blend
is uncured and left undisturbed. When GAP-HTPB blend is mechanically agitated
and gelling of the blend is accelerated by tailoring the catalyst content, IPN structure
was found to form. Gumstock properties of the blend evaluated at definite
proportions indicate formation of IPN structure. In order to verify this observation,
morphological study was carried out.
5.7.1 Optical micrographic study of GAP-HTPB blend
The morphology of GAP-HTPB network was investigated with the help of
optical micrography to study the interpenetrating network structure of the blend. The
details of the equipment used for the study is given under section 2.7 in chapter 2.
5.7.2 Scanning electron micrographic study of GAP-HTPB blends
The morphology of crosslinked GAP-HTPB network was also investigated
with scanning electron micrography (SEM) to confirm the observation of IPN
137
structure of the blend with specific concentration of the polymers. The details of the
equipment used are given in section 2.7.
5.7.3 Experimental
The crosslinked GAP-HTPB samples prepared with varying compositions were
subjected to morphological evaluation by optical micrography and scanning electron
micrography.
5.7.3.1 Materials
GAP, TDI, crosslinker and catalyst used for the study were obtained from
sources as mentioned in section 5.3.1.1.
5.7.3.2 Sample preparation
For Optical micrography and SEM evaluation, crosslinked GAP/HTPB
samples were prepared by mixing GAP and HTPB at 30:70 and 50:50 ratios. Virgin
GAP and HTPB samples were also prepared for comparison. 5% crosslinker was
added to the blend and the mix was dried in a rotary vacuum flash evaporator at
1000C for 30 minutes. After drying, TDI was added as curator and mixed. NCO/OH
ratio of unity was followed for the study. Finally catalyst was added and again
mixed. The catalyst concentration was varied (0.036 to 0.1%) to study the effect. The
mix was evacuated to remove entrapped air bubbles and then poured into teflon
coated moulds and cured under nitrogen envelope. The curing was done at room
temperature for 24 hrs followed by at 600C for 48 hrs.
For optical micrographic evaluation, fresh surface of the samples were
prepared by freeze fracturing the specimens as mentioned in section 2.7.1. The
micrographs were taken at a magnification of 600
138
For evaluation under scanning electron micrography, crosslinked GAP-
HTPB samples of size 10 x 10 mm and thickness 2 mm were cut and made
conducting by gold coating using Denton vacuum sputtering unit. For scanning
electron micrography, the equipment used was Philips XL30 Scanning electron
microscope. The details of equipment used are provided in chapter 2. The
micrographs were taken at a magnification of 1460.
5.7.3.3 Results and discussion
The micrographs show the intricate entanglements of both the networks of
HTPB and GAP and was found to be more pronounced when GAP:HTPB ratio was
50:50 and 30:70. The figures 5.29 and 5.30 show the micrographs of the crosslinked
GAP-HTPB blends prepared with GAP and HTPB with a ratio of 50:50 and 30:70
respectively.
When the crosslinking reaction was slower, phase separation was found to
result between the two networks. The extent of phase separation was found to vary
depending on the rate of the gelling of the two networks. Figures 5.31 and 5.32 show
different views of partial phase separation noted when low catalyst concentration
Figure 5.29 Optical micrograph of GAP-HTPB blend with 50:50 ratio
Figure 5.30 Optical micrograph of GAP-HTPB blend with 30:70 ratio
139
(0.036%) was employed for crosslinking reaction for a GAP: HTPB blend with 70:30
and 50:50 ratio respectively.
The scanning electron micrographs (SEM) of GAP-HTPB blends show
entanglement of network featuring typical IPN structure. Figure 5.33 and 5.34 show
the SEM of crosslinked pure GAP and HTPB. Figures 5.35 and 5.36 show the SEM
of GAP-HTPB blends prepared with ratio of 50:50 and 30:70 respectively. The phase
contrasts of SEM images of the blends were compared with that of virgin networks
for establishing the phase morphology. Distinct morphological variation was noticed
in the SEM images of the blends prepared with specific concentration levels of GAP
and HTPB. The superior properties of GAP-HTPB blends over the virgin networks
could well be explained by the greater degree of topological entanglements of both
the networks at specific concentration levels. Phase separation was observed in GAP-
HTPB blend, prepared with low catalyst content (0.036%) due to low rate of
crosslinking reaction. Figure 5.37 show SEM of the blend prepared with 50:50 ratio
indicating phase separation.
Figure 5.31 Optical micrograph of
GAP-HTPB blend (70:30ratio) with phase separation
Figure 5.32 Optical micrograph of GAP-HTPB blend (50:50 ratio) with
local phase separation
140
Figure 5.33 Scanning electron micrograph of crosslinked GAP
Figure 5.34 Scanning electron micrograph of crosslinked HTPB
139 (a)
141
Figure 5.35 Scanning electron micrograph of GAP-HTPB blend with 50:50 ratio
Figure 5.36 Scanning electron micrograph of GAP-HTPB blend with 30:70 ratio
139 (b)
140
Figure 5.37 SEM of GAP-HTPB blend with 50: 50 ratio with phase separation
5.8 Characterisation of crosslinked GAP network structures by swelling and
mechanical methods
For the characterisation of crosslinked network of polymers, determination of
thermodynamic solution properties are important as it forms the basis for formulating
a structure property correlation. The average molecular weight between crosslinks
(Mc) or crosslink density of the network influence all the structural characteristics of
the material. These parameters are generally controlled by adjusting the weight ratio
of curatives, crosslinker, and binder.31 Different methods were utilised for
determination of the crosslink density of the polymer network. These include
swelling method32-34 and mechanical method based on stress strain measurement.5, 35
In this study, the crosslink density of crosslinked GAP network was determined from
swelling data and from Youngs modulus obtained from stress strain curve. For these
141
experiments, the crosslinked network based on GAP was prepared with TDI and
IPDI as curatives. The swelling ratios of GAP-HTPB blends prepared with varying
concentrations of GAP were determined for comparison purpose.
5.9 Swelling study on crosslinked GAP and GAP-HTPB blend
The crosslink density is defined as the moles of effective network chain per
unit volume. Swelling of a polymer by a solvent depends on the crosslink density of
the polymer apart from solubility aspect and temperature. In this study, crosslinked
GAP was swollen with tetrahydrofuran (THF) and toluene for evaluating the
swelling parameters. THF was found to be more effective as a swelling agent for
GAP, based on solubility criteria. In the swelling studies carried out, the swell ratio
of the polymer in the solvent was determined experimentally.
5.9.1 Experimental
The swelling experiments were carried out with THF and toluene.
Crosslinked GAP samples were prepared for the study by reacting GAP and
crosslinking agent with TDI or IPDI in the presence of a catalyst. The swelling
experiments were carried out for crosslinked GAP with different NCO/OH ratios.
The mixing of the materials and curing were done as mentioned in section 5.2.1.
Samples were also prepared with varying crosslinker content for the study. The
different crosslinker contents used were 3, 5, 7 and 9%. The swelling experiments
were carried out in THF and toluene at ambient temperature. Crosslinked GAP
samples of weight in the range of 0.3 to 0.35 g were kept in excess quantity of THF
and toluene for more than 10 hours and samples were taken out at 1 hour interval and
wiped with blotting paper to remove solvent on surface of the sample and weighed.
From the initial weight, final weight and densities of sample and polymer, the
swelling ratio of the sample was determined using equation 5.1.
142
where W1 is initial weight of specimen, W2 is the weight of the specimen
after swelling, ρ1 and ρ2 are the densities of the solvent and polymer respectively.
Volume fraction of the polymer in the swollen gel (V2) is given by equation 5.2.
Swelling experiments were also carried out for determination of sol gel
content of the polymer. The sol gel content was determined by solvent extraction.
Samples weighing 0.9 to 1.0 g prepared with different NCO/OH ratios and
crosslinker contents were used for the experiment. The samples were kept in excess
quantity of THF and toluene for 48 hours. The samples were deswollen in
chloroform and dried under vacuum at 600C. After drying the final weight of the
sample was taken. The sol fraction was calculated using the expression 5.3.
where W1 is initial weight of sample and W2 is the final weight of the sample
after drying.
5.9.1.1 Materials
GAP, crosslinker, TDI, IPDI and catalyst used for the study were same as
those mentioned in section 5.3.1.1. Toluene with purity greater than 99.5% and THF
with purity greater than 99.7% both obtained form commercial sources were used as
solvents.
143
5.9.1.2 Sample preparation
Crosslinked GAP and GAP-HTPB samples were prepared with TDI and IPDI
as mentioned in section 5.3.1. Crosslinked GAP containing different crosslinker
content (from 3 to 9%) was also prepared with TDI as curative.
5.9.1.3 Results and discussion
The swelling ratios of the polymer prepared with different NCO/OH ratios
show a similar pattern of variation in both THF and toluene. However, the swelling
ratios were found to be higher in THF. The higher solubility in THF could be a result
of higher polarity of THF. Figures 5.38 and 5.39 show the variation of swelling ratios
with time for GAP crosslinked with TDI with different NCO/OH ratios in THF and
toluene. Figure 5.40 show similar data generated for GAP crosslinked with IPDI in
THF solvent.
Figure 5.38 Swell ratio of GAP crosslinked with TDI with different NCO/OH ratios in THF
144
Figure 5.39 Swell ratio of GAP crosslinked with TDI with different NCO/OH ratios in toluene
Figure 5.40 Swell ratio of GAP crosslinked with IPDI with different NCO/OH ratios in THF
Figure 5.41 shows variation of swell ratio with time for different crosslinker
content for GAP–TDI system in THF solvent. The data show that the swell ratio
decreases with increasing crosslinker content. The increase in solvent resistance is
due to the higher crosslink density resulting from the increase in crosslinker content.
145
Figure 5.41 Swell ratio of crosslinked GAP-TDI system in THF with varying crosslinker content
The equilibrium swell ratio was found to decrease sharply with increase in
NCO/OH ratio initially in THF medium. No considerable variation was noticed in
toluene medium. Figure 5.42 shows the variation of equilibrium swell ratio with
NCO/OH ratio for GAP-TDI system in THF and toluene.
Figure 5.42 Variation of equilibrium swell ratio of GAP-TDI system with different NCO/OH ratio in toluene and THF
146
Among all the systems evaluated, GAP crosslinked with IPDI was found to
have highest swell ratio. This was found to be due to the lower crosslink density of
the network. The variation in NCO/OH ratio was found to influence the swelling
characteristics more effectively than the variation in crosslinker content. The swell
ratio determined for GAP-HTPB blends with varying GAP content shows low values
even with 10% GAP content in the blend. At 50 and 70% GAP contents, the
equilibrium swell ratio were only close to 50% of crosslinked pure GAP.
Figure 5.43 shows swell ratio of crosslinked GAP-HTPB blends with different GAP
content in THF. The large difference in the equilibrium swell ratio of GAP in THF
could be explained based on the polarity criteria. GAP being a polar system, a more
polar solvent like THF is able to swell the polymer to a higher extent compared to
less polar solvent, toluene. Also moderately hydrogen bonded solvent like THF have
a higher solubility parameter (δ = 19.6 J1/2 cm-3/2) compared to poorly hydrogen
bonded solvent, toluene (δ = 18.2 J1/2 cm-3/2).36 Higher polarity and solubility
parameter associated with THF compared to toluene explains the difference observed
in swelling characteristics.
Figure 5.43 Variation of swell ratio of GAP-HTPB blends prepared with varying concentrations of GAP and HTPB
147
The sol fraction determined for the crosslinked polymer shows that the sol
content decrease with increasing NCO/OH ratio or increase in crosslink density. The
higher sol fraction seen at NCO/OH ratio below 1.2 could be due to the low crosslink
density, allowing solvent to easily swell the polymer chain and dissolve a higher
fraction. Figure 5.44 shows variation of sol content of GAP-TDI system with
NCO/OH ratio in toluene and THF.
Figure 5.44 Variation of sol fraction of crosslinked GAP with different NCO/OH ratios in THF and toluene
The reason for difference in sol fraction in THF and toluene is same as seen
for higher equilibrium ratio in THF. The equilibrium swell ratio was found to be
higher in THF than in toluene for crosslinked GAP. The difference in equilibrium
swell ratio was found to decrease for higher NCO/OH equivalent ratios. The swelling
studies proved that THF is a better solvent compared to toluene for crosslinked GAP
networks.
148
5.10 Evaluation of average molecular weight between crosslinks (Mc) by
solvent swelling and mechanical methods
Based on theory of elasticity by Flory37 relationships have been derived
between the deformation of elastic networks during swelling and length of the
polymer chain segments between crosslinks. This theory have been extended for
derivation of expression for determination of molecular weight between
crosslinks.5, 38 In this study, the average molecular weight between crosslinks (Mc)
was determined using data from swelling experiments and modulus of elasticity.
The equilibrium swell ratio as determined from the swelling experiments
as mentioned in section 5.14. was used for the evaluation. The equilibrium swell
ratio is related to the volume fraction of polymer in swollen gel (V2) and is given by
the equation, 5.2. The relation between V2 and the molecular weight between
crosslink is given by Flory- Rehner equation 39-41 as shown below.
where, V1 is the molar volume of the solvent, ρ is the density of the polymer
network, V2 is the volume fraction of polymer in the swollen gel as given in equation
5.2. χ is the Flory-Huggins polymer solvent interaction parameter. In this
experiment, the value of χ (0.25 at 450C) reported in the literature42 for GAP-THF
system was taken for the evaluation. GAP crosslinked with TDI and IPDI curatives
was subjected for the study. GAP-HTPB blends prepared with different GAP content
were also evaluated.
The Mc values of the crosslinked network were also determined by evaluation
of initial modulus determined from stress strain curve. The shear modulus is related
to the Mc by the following relationship.38, 43, 44
149
Where σ is the tensile stress, G is the shear modulus, λ is the extension ratio
in the elastic range. ρ is the density of the polymer, R is the universal gas constant
and T is the absolute temperature. The shear modulus and initial modulus (modulus
of elasticity or Young’s modulus) are related by the equation
where E is the initial modulus. By measuring the initial modulus from the
stress-strain curve, the Mc values of the polymer can be determined.
5.10.1 Experimental
Crosslinked GAP samples with different curatives were prepared as
mentioned in section 5.3.1. The equilibrium swell ratios and sol content were
determined for samples prepared with different formulations as mentioned in section
5.9. For the initial modulus measurement, sample slabs with different formulations
were prepared as mentioned in section 5.3.1. Stress-strain measurements were done
using Instron Universal Testing Machine as mentioned in section 2.4. A crosshead
speed of 50 mm/minute was employed for testing. GAP-HTPB blends prepared
with varying content of GAP were tested for the study.
5.10.1.1 Materials
Materials used for the study were same as mentioned in section 5.9.1.1.
150
5.10.1 2 Results and discussion
The Mc values were determined by swelling experiments for crosslinked
GAP prepared with different curatives, different NCO/OH ratios and crosslinker
content. It was noted that the Mc values of GAP-IPDI system was always higher than
the Mc values for GAP-TDI system for all NCO/OH ratios. The Mc values of
crosslinked GAP were found to decrease with increase in NCO/OH ratios,
irrespective of the curative used. Figure 5.45 shows variation of Mc with NCO/OH
ratio for GAP-TDI and GAP-IPDI systems.
Figure 5.45 Variation of average molecular weight between crosslinks (Mc) with NCO/OH ratio for GAP crosslinked with TDI and IPDI
The sol fraction was found to increase with increase in Mc values. The
variation of Mc values with sol fraction for different formulations are shown in
figure 5.46.
151
Figure 5. 46 Variation of average molecular weight between crosslinks with sol fraction for GAP crosslinked with TDI, using THF as solvent
Increase in crosslinker content was found to have a negative effect on Mc
values of the crosslinked network as in the case of NCO/OH ratio. Figure 5.47 show
the variation of Mc with crosslinker content.
Figure 5.47 Effect of variation of crosslinker content on the Mc of crosslinked GAP
The Mc values determined from measurement of initial modulus using
equations 5.6 and 5.7 were found to vary considerably from those determined from
152
swelling experiments. Table 5.2 shows comparison of Mc values determined by
swelling and Young modulus for GAP-TDI system.
Table 5.2 Comparison of Mc values determined by mechanical and swelling methods for GAP crosslinked with TDI
NCO/OH ratio
Mechanical method Swelling method
Initial modulus
(ksc)
Mc from initial modulus ( g mol-1)
Crosslink density
(mol m-3)
Mc from swelling (g mol-1)
Sol fraction (%)
0.9 5 19396.3 65.9 11654.6 49.5
1.0 8.9 10896.8 117.5 9760.3 45.5
1.1 10 9698.1 131.9 7668.4 39.3
1.2 12 8081.8 158.4 4668.7 11.9
1.5 25 3879.3 329.9 3675.1 7.9
1.75 48 2020.5 633.5 3366.6 7.8
The difference in the Mc values by the two methods could be due to the basic
difference in the methods of determination. The basic molecular dynamics of the
methods differ considerably. The visco-elastic process in stress-strain measurements
leading to dissipation of energy in the polymer network could affect results which
may be totally absent in the case of swelling experiments. Also the Mc values depend
on the time scale of the method.45 The higher Mc values seen in mechanical methods
could be a result of the network defects leading to fast disentaglement.46 For higher
values of NCO/OH ratios the difference between the Mc values were found to be
lower.
The Mc values determined from the swelling study with THF were correlated
with the gum stock properties of GAP prepared with same NCO/OH ratios. The
crosslinker content was maintained at 5% by weight with respect to binder in the
experiments. The correlative equations could be employed to predict the tensile
153
properties of the crosslinked network system when used as binder for propellant
applications. Tensile strength and elongation were correlated with Mc values. The
correlations were determined by selecting a best fit for the data. Different fits were
attempted using computational software, Microcal Origin Version 5.0 and the best fit
was selected. The relationship between tensile strength and Mc values were best
described by a exponential decay function as given by expression 5.8 and the fit was
found to be as shown in figure 5.48.
Where Mc is in g/mol and σ is the tensile strength in ksc.
Figure 5.48 Correlation between Mc and tensile strength with best fit
A second order polynomial was found to be the best fit for relationship
between Mc values and elongation as shown in figure 5.49. The mathematical
relationship between Mc values and percent elongation was given by expression 5.9.
154
Figure 5.49 Correlation between Mc and % elongation with best fit
Mc= 54314.7 0.7 ε 2_ 441. 2 ε _+ Where ε is the % elongation.
The variation of tensile strength and elongation with Mc as given by the
expressions 5.8 and 5.9 are found to be in agreement with reported literature on
polyurethane networks.14, 46, 47 The decrease in tensile strength and increase in
elongation with increasing Mc values and the plateau observed in the curve at higher
values of Mc agree well with experimental observations. The MC values were
determined from initial modulus for GAP crosslinked with different curatives. The
data show that Mc values are in the order GAP-IPDI > GAP-TDI for crosslinked
samples with NCO/OH ratio of unity and a crosslinker content of 5%. Table 5.3
shows comparison of Mc values for GAP-IPDI and GAP-TDI systems.
(5.9)
155
Table 5.3 Comparison of Mc for GAP crosslinked with IPDI and TDI
Curative Initial modulus (ksc)
Mc from initial modulus (g mol-1)
IPDI 3.5 27709
TDI 8.9 10897
GAP-HTPB blends investigated showed comparatively lower values of Mc
than for pure GAP. GAP-HTPB blends with 50:50 ratio showed Mc values as low as
1329 g/mol for a NCO/OH ratio of 1.2 with TDI as curator. Table 5.4 shows Mc
values determined for GAP-HTPB blends prepared with TDI as curing agent. The
low values of Mc noted for GAP-HTPB blends explain the high mechanical strength
seen for the blends at 50:50 or 30:70 ratios.
Table 5.4 Mc values for GAP-HTPB blends prepared with different formulations
GAP:HTPB ratio
NCO/OH ratio
Initial modulus (ksc)
Mc (g mol-1)
Crosslink density (mol m-3)
50:50 0.95 16.9 4925.7 223.3
50:50 1.0 20.0 4167.2 263.9
50:50 1.2 62.7 1328.6 827.9
90:10 1.0 6.6 14234.9 87.1
70:30 1.0 13.1 6766.9 172.9
10:90 1.0 27 2682.7 356.4
5.11 Conclusion
Based on the evaluation of gumstock properties of GAP, it was noted that the
functionality of the polymer, NCO/OH equivalent ratio, concentration of the
crosslinker, and curing conditions significantly influence the mechanical properties
156
of the crosslinked GAP networks. Due to the difunctional nature of GAP, crosslinker
concentration was found to have more dominant effect on GAP gumstock properties
compared to cure temperature and cure duration. The type of curing agent was also
found to affect the curing and gumstock properties. Aliphatic diisocyanate curing
agents were found to improve the mechanical properties of crosslinked GAP more
readily than aromatic type curing agents. The order of improvement in mechanical
strength for curative is found to be MDCI > IPDI > TDI.
Though GAP and HTPB are immiscible, crosslinked polymer network of
GAP and HTPB could be prepared without phase separation by enhancing the cure
reaction rate over diffusion rate. GAP-HTPB blends with varying GAP content from
0 to 90% could be prepared without total phase separation. GAP-HTPB blends of
50:50 to 30:70 ratios show superior properties over the virgin networks of GAP and
HTPB due to the synergetic effect of interlocking of the two networks. The
formation of IPN structure seems to explain the higher mechanical strength noticed
at specific concentration levels of GAP and HTPB in the blend. The gumstock
properties of GAP-HTPB blends with 50:50 ratio was investigated to find the impact
of different curing agents and NCO/OH ratio. The effect of these variables on the
gumstock properties of GAP-HTPB blends were found to follow the pattern similar
to that of crosslinked virgin GAP network.
The morphology of crosslinked GAP-HTPB networks were investigated with
optical micrography and scanning electron micrography. The studies show greater
degree of topological entanglements of both networks at specific concentration
levels. The local phase separation occurring in the blends with slow rate of curing
was also identified in the morphological studies.
The swelling studies carried out showed that swelling behaviour is strongly
influenced by the NCO/OH ratio. Among THF and toluene, THF was found to be a
157
better solvent for the GAP network. GAP crosslinked with IPDI showed higher swell
ratio compared to GAP crosslinked with TDI. The data showed that the swell ratio
decrease with increasing NCO/OH ratios and also with increase in crosslinker
content. The molecular weight between crosslinks was determined from swelling
data and initial modulus. The Mc values determined by both the methods were found
to be comparatively closer at higher NCO/OH ratios. For GAP-HTPB blends, the
swelling ratios and the Mc values were found to be lower than that for virgin GAP
network. An exponential correlation was found to describe the relationship between
tensile strength and Mc values determined by swelling whereas for elongation, a
second order polynomial was found to be best suited.
5.12 References
1. Kelly, F. N., Propellant Manufacture Hazards and Testing, Boyar, C., Klager, K., (eds.), Advances in Chemistry Series 88, American. Chemical Society, Washington. D. C. (1969), 188.
2. Raman, L., Propellants and Explosive Technology, Krishnan, S., Chakravarthy, S, R., Athithan, S, K., (Ed.), Course Notes on Professional Development Short Term Course, Conducted by IIT (Madras), Chennai, India, Allied Publishers, (1998), 149.
3. Rivlin, R. S., Saunders, D, W., Philosophical Transactions of the Royal Society of London Series, 241, (1948), 835.
4. Mullins, L., Rubber Chemistry and Technology, 42, (1969), 339.
5. Treolar, L. R. G., The Physics of Rubber Elasticity, 6th Ed. Clarendon, Oxford England, U.K., (1975).
6. Williams, M. L., Landel, R. F., Ferry, J. D., J. Am. Chem. Soc., 77, (1955), 3701.
158
7. Oberth, A. E., Bruenner, R. S., Propellant Manufacture Hazards and Testing, Boyars,C., Klager, K., (eds.) Advances in Chemistry Series 88, American Chemical Society, Washington D.C., (1969), 84.
8. Hori, K., Iwama, A., Propellants, Explos., Pyrotech., 10, (1985), 176.
9. Kothandaraman, H., Venkatarao, K., Thanoo, B. C., J. Appl. Polym. Sci., 39, (1990), 943.
10. Deuri, A. S., Bhowmick, A. K., Mater. Chem. Phys., 18, (1987), 35.
11. Oyumi, Y., Kimura, E., Hayakaw, S., Nakashita, G., Kato, K., Propellants, Explos., Pyrotech., 21, (1996), 271.
12. Nichaus, M., Propellants, Explos., Pyrotech., 5, (2000), 236.
13. Manjari, R., Joseph, V. C., Pandurang, L. P., Sriram, T., J. Appl. Polym. Sci., 48, (1993), 271.
14. Manjari, R., Somasundaram, U. I., Joseph, V. C., Sriram, T., J. Appl. Polym. Sci., 48, (1993), 279.
15. Manjari, R., Pandurang, L. P., Somasundaram, U. I., Joseph, V. C., Sriram, T., J. Appl. Polym. Sci., 51, (1994), 435.
16. Gupta, D. C., Deo, S. S., Wast, D. V., Raomore, S. S., Gholap, D. H., J. Appl. Polym. Sci., 55, (1995), 1151.
17. Hunter, R., Manzara, T., 30th International Annual Conference of ICT, Karlsruhe, Germany, (1999), 54/1.
18. Barkent, R. G., Adv. Urethane Sci. Technol., 3, (1974), 1.
19. Vratsanos, M. S., Polymeric Materials Encycl., CRC, Salamone, J., (Ed.), (1996), 6947.
20. Careswell, T. S., Nason, H. K., Sym. on Plastics, Spec. Tech. Publ. No. 59. 22, Philadelphia, Am. Soc. Testing Materials, (1994).
21. Manu, S. K., Varghese, T. L., Joseph, M. A., Shanmugham, K., Mathew, S., 35th International Annual Conference of ICT, Karlsruhe, Germany, (2004), 98/1.
22. Varghese, T. L., Krishnamurthy, V. N., J. Appl. Polym. Sci., 13, (1996), 245.
159
23. Urbanski, T., Chemistry and Technology of Explosives, Pergamon Press, Oxford, Vol.2, (1964), 197.
24. Hammel, E. E., 29th International Annual Conference of Technology, ICT, Jahrerstag, (1998), 136/1
25. Cliff, M., Poly GLYN Binder Studies and PBX Formulation, DSTO-TR-0884, DSTO, Salisbury, S. A, (1999).
26. Dhar, S. S., Asthana, S. N., Shrotri, P. G., Singh, H., 31st International Annual Conference of ICT, Karlsruhe, Germany, (2000), 39/1.
27. Flanagan, J. E., Wilson, E. R., U. S. Patent no. 4938812, (1990).
28. Hunsu, K., Firket, P., Saim, O., J. Appl. Polym. Sci., 80, (2001), 65.
29. Miller, J. R., J. Chem. Soc., 263,(1960), 1311.
30. Hourston, D. J., Zia, Y., J. Appl. Polym. Sci., 28, (1983), 2139.
31. Kothandaraman, H., Venkatarao, K., Thanoo, B. C., Polym. J., 21, (1989), 829.
32. Oikawa, H., Murakami, K., Rubber Chemistry and Technology, 60, (1987), 579.
33. Jain, S. R., Sekkar, V., Krishnamurthy, V. N., J. Appl. Polym. Sci., 48, (1993), 1515.
34. Eroglu, M., S., J. Appl. Polym. Sci., 70, (1998), 1129.
35. Spathis, G. D., J. Appl. Polym. Sci., 43, (1991), 613.
36. Van Krevelen, D. W., Hoftyzer, P. J., Properties of Polymers, 2nd Edition, Elseiver Scientific Publishing Company, Amsterdam, (1976), 144.
37. Flory, P. J., Principles of Polymer Chemistry, Cornell University press, Ithaca, New York, (1953), 464.
38. Eisele, U., Introduction to Polymer Physics, Springer Verlag, Berlin, (1990).
39. Flory, P. J., Rehner, J., J. Chem. Phys., 11, (1943), 521.
40. Bell, J. P., J. Polym. Sci., A-2, 8, (1970), 417.
160
41. Erman, B., Flory, P. J., Macromolecules, 19, (1986), 2342.
42. Erogulu, M. S., Guven, O., Polymer, 39, (1998), 1173.
43. Mooney, M., J. Appl. Phys., 11, (1940), 582.
44. Slade Jr., P. S., Jenkins, L. T., Technics and Methods of Polymer Evaluation, Vol. 2, Thermal Characterisation Techniques, Marcel Dekker Inc., New York, (1970), 130.
45. Sekkar, V., Narayanaswamy, S., Scariah, K. J., Nair, P. R., Sastri, K. S., Ang, G. H., J. Appl. Polym. Sci., 101, (2006), 2986.
46. Erogulu, M. S., J. Appl. Polym. Sci., 70, (1998), 1129.
47. Haska, S. R., Bayramli, E., Pekel, F., Ozkar, S., J. Appl. Polym. Sci., 64, (1997), 2347.