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CHAPTER
Viscoelastic Behaviour of Natural Rubber1
Polystyrene Interpenetrating
Polymer Networks
The results of t h ~ s chapter have been communicated to
Polymer
The effect of Mend ratio and initiating system on the dynamic mechanical
properties of natural ~Werlpoiystyrene based interpenetrating polymer networks was
investigated in the temperature range of -80 to 150°C. The studies were carried out at different frquencies viz., 100,50, 10, 1 and 0.1 Hz and their effect on storage and loss modulii were
analysed. In aU cases the tan 5, storage modulus (E') and loss modulus ( E ) showed two
distinct T,s corresponding to NR and PS transitions which indicate that the system is not
miscible in the molecular level. However, a slight inward shift is observed in the IPNs, compared to Tgs of virgin polymers showing certain degree of miscibility or intermixing between the two phases
When the frequency increases from 0.1 to 100 Hz the Tg values showed a positive shift in a1 cases. On comparing the 3 initiating system viz. DCP, BPO and AIBN, the DCP system showed
the highest modulus. Various composite models were compared with the experimental data to predict the dynamic mechanical behaviour.
Attempt has been made to relate the v~welastic behaviour with the morphology of the
interpenetrating polymer networks. The area under the linear loss modulus curve was larger
than that obtained by group contribut~on analysis showing that the damping was influenced by the phase morphology, dual phase continuity and
crosslinking of the phases. The homogeneity of the system has been further analysed based on
Cole-Cole analys~s.
5.1 Introduction
L iterature shows that the combination. of a glassy polymer with another which
is rubbery at room temperature can produce IPNs possessing a range of
properties, from reinforced elastomers to high impact plastics.'.' Miscibility
and phase morphology of IPNs are crucial in many applications. Miscibility can
be determined by dynamic mechanical analysis, by considering the T, values of
virgn polymers and the IPNs. A single T, for the composite is an indication of
miscibility and two separate T,s is an inhcation of immiscible system.' IPNs
have been w~dely used as damping materials with excellent results, as these
materials proved to be the most effecti.ve way to widen the loss peak. Proper
selection of su~table monomer and control of network density may make the loss
peak to reach an optimum. By virtue of their unique method of synthesis, most
IPNs exhlb~t microphase separation arid may result in some molecular level
mixing. Because of this partial miscibility IPNs generally have broad glass
transition. S~nce the damping of mechanical vibration maximum near the glass
transition region, IPNs are expected to damp over a wide range of temperature and
frequency. The measure of loss modulii and tan 6 are important parameters wh~ch
can be duectly correlated to damping
ICrarner6 and co-workers made use of DMA to study the properties of
random polybutadiene rubber networks. Tung el al.' studied the presence of
crossllnking of networks in semi-interpenetrating polyimide networks using
DMTA. schranzX has explained the use of DMA as a powerful tool for the study
of phase transition and low-frequency elastic compliances of crystals. ~ s i e h ' and
coworkers reported the preparation and properties of lPNs of polyurethane and
2-hydroxyethyl methacrylate terminated polyurethane. DMA studies of these
lPNs showed that the compatibility of the component polymers depends on the
crosslink dens~ty of the polyurethane network.
In th~s chatper, the dynamic mechanical properties of the IPNs are reported
in the frequency range of 0.1 to 100 Hz as a function of blend composition,
initiating system and crossldung level. The loss modulus (E"), storage modulus
(E') and tan 6 of the tested samples were plotted as a hnction of temperature. The
samples were analysed in the temperature range of -80 to 150°C The T,s of the
homopolymer and the IPNs were det~mnined. The morphology of IPNs was
studied using scannlng and transmission electron microscopies and is correlated
with structure and properties of IPNs. The area under the loss curve (LA) was
evaluated to study the damping behaviour. The experimental viscoelastic results
were fitted w~th var~ous theoretical models. The possibility of development of
microphase homogeneity is looked into using Cole-Cole analysis. The
intertransit~on he~ght and peak width at half height were determined to study the
extent of phase m~xlng.
5.2 Results and discussion
5.2.1 Efiect of blend ratio, crosslinking, initiating system, temperature and
frequency on viscoelastic properties
The damp~ng peaks of NR and PS phases at 5 frequencies are given in
Figure 5. la and 5 1 b The values of tan 6,, and T, corresponding to NR and PS
transitions at 5 frequencies are tabulated in Table 5.1. The T, of NR changed from
-58.6 to -38OC and T, of PS changed from 104 to 112OC as the frequency
increased from 0 1 to 100 Hz. The tan 6,, also varied with frequency. The
storage modulus values of NR was high at low temperature and decreased after the
trans~tion to rubbery phase around -40°C In the case of PS the storage modulus 1s
appreciable up to the transition temperature (1 10°C) thereafter the values fall.
Pime 5.1. Plor of tan 6 against temperature for (a) natural rubber and (b) polystyrene
Table 5.1. Effect of frequency on tan Gmax and T, of NR and PS.
tan 6,, Frequency (Hz) -
Tg ( O C )
NR PS NR PS
The effect of blend ratio, initiating system and frequency on the dynamic
m e c h c a l behawour of semi-IPNs were studied. In Figure 5.2 the tan 6 and
storage modulus of DoN,o, DoNJo and DUN?,, samples are given.
Figure 5.2. Effect of blend ratio of semi-IPNs on tan 6 and storage modulus
As the rubber content increases the peak height due to NR transition
increases and that due to PS transition decreases (Table 5.2). In DON3, and DONSo
samples the T, values show an inward shift compared to the homopolymers, .
suggesting certam level of phase mixing in the system. The occurrence of graft
functions between the two networks might contribute to h s phase mixing. The
possibility of gr&g is confirmed from the calculation of gel content. In the
sample such an inward shift is not noted. This can be explained based on the
morphology. In the N70 samples the PS domains are embedded in the NR phase
and results m low level of interpenetration and phase mixing between the
polymers. In DONjo and samples the PS chains form a continuous phase
resulting in enhanced interpenetration and phase mixing at the interfaces
(Figure 4. I )
In the case of DoN70 as NR content is 70% the storage modulus shows
lower values than other systems. At 50% of NR a good improvement in modulus
is observed. At 30% of NR, the modulus is still higher. But above 50% of NR
only marginal increase is observed. This can be explained again based on the
morphology of the system. The change from dispersed phase morphology in N70
samples to the co-continuous morphology in NJ0 and N30 samples lead to Pins
increase in modulus above 50% of PS.
In Figure 5 3 a and b, the effect of initiating system on tan 6 and storage
modulus of N 5 , samples is shown I : D ~ N ~ ~ , AoNjO, BONS~) at low and high
temperatures respectively. All the 3 samples show two dlstinct peaks
corresponding to NR and PS phases. The peak height due to NR at low
temperature follows the order: &Njo > BoN50 > DON5,. So the damping properties
are hlghest for A,,N5,, sample and least for DONjo sample. This again shows that
the phase mixlng is efficient in DCP series. The intimate mixing of the two phases
restricts the mobility of the NR phase and reduces the damping behaviour of DCP
system. The storage modulus E' is in the order: DCP > BPO > AlBN series.
lo'.' . ' 0 . 9 a ' a ,
'L 0.b
0 0 0 0 s' . 0 . 3
-39 -39 - I7 L
Temperature OC
(a)
* LC "so 1 .1
Figwe 5.3. Effect of initiating system on tan 6 and storage modulus
Table 5.2. tan S m u and T, values of IPNs
tan 6,,
Sample code Tg value Due to NR phase Due to PS phase transition transition
D I N J ~ -39, 103 0.08 0.88
In F~gure 5.4, the effect of frequency on tan 6 and storage modulus is
shown for DoN30 sample. It is observed that as the frequency increases T, values
show a shift to the positive side. The T, values of NR and PS phases increase with
increase in frequency. This is due to the fact that at low frequency, polymer chain
relaxation is faster and at high frequency polymer chain relaxation is delayed.
Figure 5.4. Effect of frequency on tan 6 and storage modulus of semi-1PNs
In Figure 5 5, the tan 6 and storage modulus vs. temperature plots of D3N30,
D,Nm and D3NTO in whch PS phase is tughly crosslinked (6% DVB) are given.
Figure 5.5. Plot of tan 6 and storage n~odulus as a function of temperature for D ~ N ~ o , D ~ N s o and D3N70 samples
In d l cases two Tgs are observed corresponding to the transitions of NR
and PS phases. For the IPN D3N70r the PS peak is at 1 10°C and is quiet prominent
whtle the peak due to NR is at -3S°C and is very weak. In the N50 sample also two
distinct peaks are observed at 101 and -37OC correspondmg to PS and NR
transitions. In the N,o sample the NR peak at -37.5 is quiet prominent and sharp
while the PS peak 1s at 99OC and is very weak. It is seen that the T, of PS is shifted
to low temperature side and that of NR is shifted to high temperature. From the
figure, it is clear that as the NR content increases from 0 to 100% the NR damping
peak height increases and PS peak height decreases. Also, in the case of all the
IPNs, the T,s siuft mwardly showing some microlevel mixing between the two
phases at the mterface. Therefore, the IPNs have some level of mscibility. The
variation of tan 6,., due to rubber transition with wt % of NR is given in
Figure 5.6.
2 5
2 -
x 1 5 -
; U) C m - 1 -
0 5 -
0 0 20 40 60 80 100
W'T % OF NR
P i e 5.6. Effect of wt % of NR on tan peak height.
With Increase m NR content the tan 6,,, increases. The tan 6 peak height
is a measure of damping. As the rubber content increases damping increases as
the introduction of rubber reduces the brittleness of PS and the system as a whole
has reduced plastic nature. Above 50% NR, the tan 6,,, has increased drastically.
This can be explaned based on morphology of the IPNs. In D3N50 and D3N30
samples both the phases are continuous. The chain flexibility of rubber phase is
restricted by the interpenetration with PS phase and as a result the damping
decreases. As the PS content increases the number of crossllnks increases leading
to a decrease in damping.
The storage modulus decreases in the order N30 > NSo > N70 for the IPNs,
i.e., as the rubber content increases the modulus decreases, for the whole range of
temperature between the T,s of NR and PS. However, above 11 O°C the modulus
value decreases sharply. This is an expected trend as modulus decreases sharply
above the T, of PS phase. Up to the Tg value of PS, i.e., around 1 10°C, the
modulus of the NRPS 1PN is retained due to the glassy nature of PS. Above the
T, of PS phase, PS also loses glassy nature and hence the modulus drops
drastically
The effect of crossl~nking of PS phase on the vibration damping properties
was studied. The NR phase was crosslinked initially and the level of crosslinking
was kept constant. The PS crosslinking was varied by varying the crosslinker
content, i.e , 2, 4, and 6% of DVB. Figure 5.7 shows the effect of temperature on
the storage modulus and tan 6,, of DINSO, D2N50 and D3N5, full-1PN samples. The
tan 6 peak he~ght for PS phase is the maximum for DIN5" sample followed by
DzN5o and mrnirnum for D3N50 sample. This trend can be explained based on
brittleness produced in the sample due to the increased crosslinking of PS phase.
As PS phase gets increasingly crosslinked the sample becomes brittle and the
flexibility of polymer chains gets highly restricted. This results in a decrease in
damping behaviow with an increase of crossl~nks in the plastic phase. However,
there is a slight positive sb~fi in the position of tan 6 peak due to PS as we move
from DIN>" to D,NSo sample, whlch is obvious from the figure. This is associated
with the increase m the extent of crosslinlung of the PS phase.
Figure 5.7. Effect of crosslinking of PS on tan 6 and storage modulus (E') for DCP samples
The storage modulus values are the highest for DzNSo sample and DIN5o
and DIN5, samples have almost same storage modulus values. As we move from
DIN30 to D2N50 sample the modulus values increase due to enhanced phase
mixing. But in D3N50 sample the excessive crosslinking reduces the storage
modulus. A good balance of properties is obtained at 4% of DVB content.
The loss modulus (E") as a function of temperature for DINSO, D2N5u and
DINSO samples are given in Figure 5.8. It follows the same trend as tan 6 peak.
Here also the reason is the increase in brittleness due to excessive crosslinkiny.
The morphology developed as a result of crosslinking of PS phase also contributes
to thls behaviour
In F~gure 5.9, the effect of frequency on tan 6 values for DZN,o sample is
illustrated. It IS observed that as frequency increases T, values show a shift to the
positive side. The T, values of NR and PS phases increase with increase in
frequency. At low frequency, polymer cham relaxation is fast and at high
frequency polymer chain relaxation is delayed. The three dimensional
representation of tan 6 versus temperature at different frequencies is given in
F~gure 5.10. The sh~ft in the transition1 of PS and NR phases as a function of
frequency is very clear fiom Figure 5.10.
10.'
z 1 v . -
I"" -
01 - A O l U z
I,,' - I HI 0 I 8 Z
I W _ ' $0 "I ' W " 1 .iii
@ . 0 6 - to - Lo x ? "' ' - 2
I l t . - C 0 . 0 1
IN. - 8" -
,,,. -, " 0
D.N.
/:!id*, . . n.8, u.3,
D 3 f l . h *a A* '2us*rnltt*' y 8
Ylfl.. * 'I, X
iih, *. I 1 I I 1 I I i I I
Figure 5.9. Effect of frequency on tan B and storage modulus (for D3N50 sample)
*I *, UI 1 0 o M UI bu xo IUI 1211
Ik*runlt)
Ngure 5.8. Effect of crosslinking of PS on loss modulus ( E )
Figure 5.10. 3-Dimensional representati~on of effect of frequency on T, and tan 6 for full-IPN (DzN3o)
When seml and full-1PNs are c:ompared, it was found that the tan ti,,,,
values for semi-IPNs are comparable to that of full-IPNs. The presence of graft
sites in the semi-1PNs leads to better phase mixing and limited chain mobility. In
the DON,, and DON, systems the presence of grafting together with co-continuity
restricts the damping behaviour. In DON.,,, system the PS domains embedded in the
NR matrix enhances the damping behaviour and the tan 6,,, is correspondingly
high. The ~ncreased flexibility of the system due to the linearity of the PS phase
makes the semi-1PNs capable of showing high damping properties. Also it is
noticeable that In the case of semi-IPNs the onset temperature of transition for
rubber phase as well as plastic phase is lower compared to full-1PNs The
transition occurs more easily in semi-IPIVs because of low chain entanglement and
interpenetration. The linear PS chains in the semi-1PNs undergo transition easily
compared to the crosslinked PS in full-IPNs. The low entanglement density of
semi-IPNs compared to the crosslmk density of full-IPNs also contibutes to the
elasticity and resultant damping behaviour of semi-IPNs. The entanglement
density of semi-1PNs and crosslmk density of full-IPNs are given in Table 5.2.
The PS crossl~nks in full-IPNs will make the NR networks tighter and compact.
When the storage modulus of IPNs are considered it can be seen that below
the rubber transition temperature all the systems have good modulus values as
both the NR and PS phases are in glassy state. At around -40°C, the NR becomes
elastic in nature and the modulus exhibited is predominantly due to glassy PS
phase. At around 100°C PS also loses glassy nature and the modulus of the
system drops. However, NRPS IPNs, in general show good modulus values in
the temperature range of -70 to 100°C. This suggests that these IPNs can be
effectively used m this temperature range:.
The tan 6 peak height usually gives an indication of phase continuity of the
system, the phase exhibiting the hgher peak representing the more continuous
phase. The height of the region between the tan 6 peaks (inter transition height) is
a measure of the phase mixing. The inter transition height at 3 0 ' ~ was determined
for this system and the values are given in Table 5.3. The inter transition height
values is indicative of phase mixing at the interface with respect to composition,
crosslinker level and ~nitiated system for semi- and full-1PNs. The peak width at
half height for the NR and PS transitions for the IPNs was measured and the
values are glven in Table 5.3. In almost all cases the peak width is higher for the
full-IPNs than semi-IPNs. This clearly shows the existence of better
interpenetration in full-IPNs.
The integral of the loss modulus versus temperature curve is computed in
order to develop a relationslp between the extent of damping and the contribution
for each group towards the damping performance. The area under the linear loss
modulus versus temperature curve (LA) can be evaluated by five different
methods: tan&= 0.03, straght line, height x width, integral area and constant E . "
In t h ~ s system the integral method is made use of The area under the linear loss
modulus versus temperature can be derived kom the phenomenological approach
as reported by Sperling el al."
where Ec;' and ER' are the storage moduli in the glassy and rubbery states and TG
and TR are the glassy and rubbery temperatures just below and above the glass
transition, respectively, E, is the activation energy of the relaxation process, and
R, the gas constant.
Table 5.3. Values of peak width at half height and inter transition height
Peak width ai: half height ( O C )
Sample code Intertransition NR PS height
In order to determine the theoretical values, the group contribution analysis
is made use of. It is based on the assumption that the structural groups in the
repeating units provide a weight fraction additive contribution to the total loss
area. The basic equation for the group contribution analysis of LA is1'.12
where M, 1s the molecular weight of the ith group of the repeating unit, M is the
molecular welght of the whole mer, G,, ?he molar loss constant for the I& group
and n represents the number of moities in the mer. Equation (5.3) provides a
predictive method for LA values via the structure of the polymer
The theoretical and experimental values of LA for various IPNs are given
in Table 5 4 The experimental values are larger than those obtained by group
wntnbution analysis
Table 5.4. Experimental and theoretical values of loss area (LA)
Experimental LA x 10 lo 1Pa.K Theoretical LA x 10 9 ~ a . ~ Sample wde
NR PS Total NR PS Total
DoN3o 2.67 6.67 9.34 6.76 1.53 8 39
The intermixing of the phases is therefore due to crosslinking of the
phases. The higher values of LA indicates enhanced interaction between the
component polymers and enhanced damping. The experimental value of LA is in
fact influenced by the morphology, cn>sslink density, interaction between the
polymer components and phase continuity. There is no possibility for any specific
interaction between the NR and PS phases. The dual phase continuity and nano-
structured morphology contribute to the high LA values. Fay el al, had reported
that in mbberlplastic IPNs, with dual phase continuity and the increase in plastic
phase increase the LA I' A similar trend j~as observed in the case of NRPS 1PNs
also. In D,N,, sample the LA values are lower than expected. This can be
attributed to the decrease in phase mixing, because of the agglomeration of PS
phase at high crossl~nking level.
The apparent acbvation energy for NR and PS phase glass transitions has
been calculated from the dynamic mechanical dataI4-l6 using the Arrhenius
equation,
{ -AH f = 6, exp
where f is the frequency, fo is the frequency when T approaches infinity and T is
the temperature correspondmg to the maximum of loss modulus E" curve. AH was
calculated from the slopes of these plots and is reported in Table 5.5.
Table 5.5. Activation enera of NR and PS glass transitions
Activation energy -AH (kJ/rnol) Sample code
NR PS
It may be noted that the activation energy of NR transition is higher than
that of the PS transition in all the IPNa. The activation energy of semi-1PN is
lower than that of full-IPNs.
5.2,2 Modelling of viscoelastic behaviour
Various composite models such ar the parallel model, the series model, the
Halpin-Tsai equation, Coran's equation, Kerner equation, Davies model and the
Takayanagi model were made use of to predict the dynamic mechanical behaviour
of the IPNs The detruls of the equations are given in Section 4.7.
The phase cont~nuity and phase inversion in polymeric systems can be
stuhed by comparing the modulus values with theoretical models. But the fit of
the experimental data with the models is not satisfactory in most systems. In the
case of NRPS IPNs the NR network is initially formed and is crosslinked in all
cases. Therefore, the NR phase tend to form the continuous phase, ruling out the
possibility of phase inversion. In the semi-1PNs the PS phase may be dispersed or
continuous dependmy on blend ratio. In full-IPNs, as both the phases are
crosslinked and the system tends to be co-continuous.
In F~gure 5 1 1 the theoretical and expenmental values of the storage
modulus (E') for DCP series are compared in the case of semi-IPNs, I e , DO
senes, below 50% PS the serles model comes very close to expenmental value
The expenmental value of N,o system 1s higher than all the theoretical calculation
This may be because of the co-contmuous morphology above 50% of PS
In the full-IPNs, i .e, Dl, D, and 13, series the experimental values comes
closer to the theoretical ones for the Davies equation, whch explains dual phase
continuity. So it may be concluded that all the full-IPNs have a co-continuous
morpholog y
Figure 5.11. Comparison of experimental values of storage modulus with theoretical models
5.2.3 Cole-Cole analysis
Cole-Cole plots were drawn to study the variation of loss modulus E as a
fimction of storage modulus E'. It was reported that homogeneous polymeric
system shows a semi-circular diagram, whilst two phase systems show two
mokfied sem-c1rc1es.l' These curves are usually circular areas at low frequency
and hgh temperature and linear at hgh frequency and low temperature The
Cole-Cole plots of DON,, and D2Nm samples at a frequency of 50 Hz are glven in
Flgure 5 12
The plot shows a behaviour different from homogeneous systems T h ~ s
shows that microlevel homogeneity is absent in the system. Though there is some
level of intermixing between the components in the system, microlevel phase
separation is present. This was clear from. the morphology studies also.
Figure 5.12. Cole-Cole plots of D o N ~ o and D2N30
5.3 References
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