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

1. K. Frisch. D. Klempner and S. E. Migdal, J Polp. .%. Polym. Chem. Edn., 12, 885 (1974).

3. 0. Olabiii, L. M. Robeson and M. T. Shaw (Eds.), Polymer-Polymer MiscibiJ@, Academic Press, London, 1979.

4. Gingmin Chen, Hamhua Ge, Dongzhon~g Chen, Xiandong He and Xuchai Yu, J &p/. Polym. Sd. 54.1191 (1994).

5. D. Hourston and F. Schafer, Polm. Adv. Technol., 7,273 (1996)

6. S. Ndoni. A Vorup and Kramer, Maoomohcuks, 31,3353 (1998)

H. C. Lou, P. S. Ho and B. Tung, J. &pL P o p . S u , 70,261 (1998).

W. Schranz, Phase Transition, 64,103 (1997).

T. T. Hsieh, K. H. Hsieh, G. P. Simon, I,. Xu and H. D. Hsu, J Polym. Res.-Taiwan, 5, 153 (1998).

J. J. Fay. D. A Thomas and L. H. Sperling, J /.pl Polym. Sd., 43, 1617 (1991).

M. C. 0. Chang, D. A Thomas and L. H. Sperling, J &pl Polp. Sci., 34,409 (1987).

M. C. 0. Chang, D. A. Thomas and I-. H. Sperling, J Polp. Sci. Po&. Php. B, 26,1627 (1987)

J. J. Fay. C. J. Murphy, D. A. Thomas, and L. H. Sperling, Polp. Eng Sd., 31, 1731 (1991).

S. Haviliak and S. Negami, Poher , 8,161 (1967).

J. D. Jerry. L4scoelasticPropr6'es of Polymers, 3rd Edn., Wiley, N. Y., 1980.

D. G. Fesko and N. W. Tschoegel, J. Polp. Scj., C35,51 (1971).

L. Ibarra. M. Macicis and E. Palma, J Appl Polym. Sci, 57,831 (1995).

K. S. Cole and R. H. Cole, J Chem. mr., 9,341 (1941).