chapter sbr/nr blend membranes: morphowgy,...

CHAPTER 8

SBR/NR BLEND MEMBRANES: MORPHOWGY, MISCIBILITY,

AND TRANSPORT BEHAVIOUR

The results of this chapter have been communicated for publication in European Polymer Journal

Prepared by BeeHive Digital Concepts for Mahatma Gandhi University

T he scientific and cornmercial progress in the area of polymer blends during

the past decade has been tremendous and was driven by the realisations that

by blending, new materials can be developed and it can be implemented more

rapidly and economically. The question of whether two polymers are miscible is of

paramount importance as far as its various properties are concerned. The

permeation properhes of polymer blends mainly depend on the miscibility between

two polymers. The blends may be heterogeneous or homogenous. In homogeneous

blends, the permeability is affected by the interaction between the component

po~ymers'.~ while for heterogeneous blends interfacial phenomena and the rubbery

or glassy nature of the phases are importar~t.~

Cabasso el al.' studied the diffusion of benzene-cyclohexane mixtures

through polymer blends composed of poiy(phosphonates) and acetyl cellulose. The

blends were found to be selectively absorbing benzene from benzene-cyclohexane

mixtures. Shchori and ~ a ~ u r - ~ r o d z i n s k i ~ have described the permselective

properties of blends of poly(viny1 pyrrolidone) (PVP) and a crown ether copolymer.

The eight membered crown ether ring strongly absorbs sodium salts from aqueous

solutions. The introduction of hydrophilic PVP into the blend structure significantly

increased the water permeability of the resulting blends. Aminabhavi and phayde7

studied the transport properties of polymer blends consisting of ethylene-propylene

copolymer and isotactic polypropylene in haloalkanes. Aminabhavi and coworkers8


have investigated the sorption of aliphatic esters through tetra fluroethylenel

propylene copolymeric membranes. The results show that the diffusion coefficients,

permeation coefficients and kinetic rate constants decrease with increase in the size

of esters. Recently,in this laboratory Johnson and Thomas have investigated on the

transport behaviour of NR/E:NR blends in n- alkane^.^

It is seen in the previous chapter that the blend morphology and composit

ion are two important factors that decide the hansport behaviour in blends. SO

the objective of the present study is a detailed investigation of morphology,

transport properties and dynamic mechanical and mechanical behaviour of styrene-

butadiene rubberlnatural rubber blends. Efforts have been made to correlate the

transport with miscibility and morphology of the blend. Attempts have been made

to predict the permeation and mechanical behaviours using existing theoretical

models.

8.1. Resub and discussion

8 . Processing characteristics

The elastographs of the mixes are given in Figure 8.1 and the processing

characteristics in Table 8.1. The minimum torque in the elastograph is presented as

minimum viscosity value (ML ) and is a measure of the extent of mastication. The

low value of ML indicates that low viscosity is expected for blend with high NR

content. The slight change in ML value in Nlw might be due to the change in

mixing time. The maximum torque in the elastograph (MH) is an index of

crosslinking density and these values indicate that the maximum crosslinking

density is possessed by No and the minimum by NIOO . The blend composition

possesses intermediate values. The induction time (t,) is the time taken to start

vulcanisation process. The ti values decrease with increase in NR content in

SBRINR blends. The Nloo and N70 take the minimum time to initiate vulcanisation


among all the blend compositions. The scorch time ( t ~ ) is the time taken for

minimum torque value to increase by two units. The scorch time values in

Table 8.1 indicate that pure SBR and SBRNR blends with high SBR content

exhibit better scorch safety. 'With increasing NR content in the blend scorch safety

decreases. This behaviour is associated with the high unsaturation of natural

rubber. Pure SBR compo~ind shows maximum optimun~ cure time. The tsci

decreased with increase of NR content in the blend.

0 1 0 2 4 6 8 10 12

TIME (min)

Figure 8.1. Elastographs of the mixes


The CRI values are also given in Table 8.1. SBR has the lowest CRI value

and it increases with increase of NR content in the blend composition. This trend

is also due to the high unsaturation of natural rubber.

8.1.2 Morphology of blends

Table 8.1. Processing characterist~cs.

The morphology of heterogeneous polymer blends depends on blend

composition, viscosity of individual components and processing history. Danesi

and porter" have shown that for the same processing history, the composition ratio

and melt viscosity differences for the components determine the morphology.

Generally, the least viscous phase was observed to form the continuous phase over

a large composition range. The SEM photographs presented in Figure 8.2 show the

morphology of SBR/NR blends. Figure 8 2 a is the mosphology of Njo blend, in

which NR is dispersed as domains in the continuous SBR matrlx. The two phase

morphology significantly influences the permeation properties. Figure 8 2 b and

8 . 2 ~ represent the morphology of N5o and N70 blends, respectively. In both N5o and

N70. NR becomes continilous and the system exhibits a co-continuous morphology.

The morphology studies reveal that SBR/NR blends are heterogeneous in nature.

CRI (min")

11.71 -

15.24

tro (m:s)

17.02

13.09

t2 (m:s)

8.48

6.53

Blend ratio (dNm) (dNm)

N3o 0.79 7.23

tl (m:s)

7.48

5.59


transition occurs in the case of partially compatible systems. In the case of

compatible and partially compatible blends, the Tgs are sh~fied to higher or lower

temperatures as a function of composition. The variation of tan 6 with temperatures

of the SBR. NR and SBRiNR blends is shown in Figure 8 3 . The tan 6 curve of

NR shows a peak at -51°C due to the a-transition arising from the segmental

motion. This corresponds to the glass transition temperature (T,) of natural rubber.

The styrene-butadiene rubber shows the glass transition temperature, at -39'C, in

the tan S-temperature curve. Natural rubber shows the highest damping properties.

The blends show two tan 6 peaks around -54 and -35OC wh~ch correspond to the

T,s of natural rubber and :styrene-butadiene rubber respectively. The presence of

two separate peaks corresponding to the Tgs of NR and SBR indicate that the

blends are not compatible.

Figure 8.3. Variation of tan 6 with temperature of SBRiNR blends at fi-equency o!' 01HZ


The vanatlon of tan l i , as a functlon of NR content IS shown in Flyure 8 4

It can be seen that the tan 6,,, balue due to NR phase Increases as the SBR content

decreases, I e , the damplng, Increases as the NR content Increases The damp~ng

due to SBR phase decreases wlth Increase In NR content because of the lower

concentration of SBR phase. The variation in tan 6,, can be related to the

morphology of the blend The tan a,,, due to NR phase Increases sharply after

50% of NR because of the hlgher contribution of tan 6,, from continuous NR

phase. But tan 6m, of SBR decreases as the NR content increases and the decrease

is much sharper when the NR content is 50% or more, where NR forms a

continuous phase. The variation of storage modulus E' of blends as a function of

temperature is shown in Figure 8.5. All curves show three distinct regions .i.e., a

glassy region, a transition region and rubbery region. The temperature region

between -70 to -58OC is the glassy region and between -57 to -4I0C is the

transition region. The rubbery region range from -40 to 19'C.

0 0.2 0.4 0.6 0 8 J

1 VOLUME FRACTION OF NR

Figure 8.4. Variation of tan 6,, as a hnction of NR content


Figure 8.5. Variation of storage modulus (E') of SBRNR blends as function of temperature at a frequency of 0.1 Hz.

The storage modulus is found to decrease w ~ t h nse in temperature due to the

decrease in st~ffness of the sample At low temperature region, N I W exhib~ts the

highest modulus and NO the lowest At h ~ g h temperature region N ~ o has the

maximum modulus and NJO has the mlnlmum Interestmgly all the curves

crossover at a temperature of -60°C

The variation of l'oss modulus (E") w ~ t h temperatures (Figure 8.6) also

shows the same trend as that of tan 6, i.e., the curves show maxima corresponding

to the glass transition temperature of N R and SRR. The loss modulus decreases

with an increase in the NK content Thus dynamic mechanical analysis established

the two phase nature of SBR/NR blends.


NIO

Nso I

Figure 8.6. Variation of loss modulus as (E") of SBRI'NR blends as a fbction of temperature at a frequency of 0.1 Hz

The influence of frequency on storage modulus, loss modulus and tan 6 are

shown in Figures 8.7, 8.8 and 8.9, respectively. Storage modulus increased with

increas~ng frequency from 0.1 to 50 Hz whereas it decreased with increasing

temperature (Figure 8.7). Loss modulus decreased ~nltially w ~ t h increase of

frequency and after passing through the transition region an increase was observed.

At low temperature region the tan 6 values decrease with increase of frequency and

just reverse occurred at high temperature region. It was also found that the tan 6,,,

shift to high temperature re;:con at h~gher frequencies.


Figure 8.7. Variation of storage modulus (E') of Nso blend with temperature as a hnction of fr~cquency

Figure 8.8. Variation of loss modulus ( E ) of N5, blend with temperature as a function of fiequency


Figure 8.9. Variation of tan 6 with temperature of Nsci blend as a function of frequency

The three dimensional graphs showing the variation of tan 6 with frequency

and temperature of NO, N ~ , J and NIOO are given in Figures 8.10, 8.1 1 and 8.12.

respectively. It is interesting to note that the glass transition temperature is

progress~vely shlfted to h ~ g h temperature regton w ~ t h tncreaslng frequency

F~yure 8 13 shows the Cole-Cole plot of N50 blend where the loss modulus (E")

data are plotted as a funct~on of the storage modulus (El) It is also reported1' that

homogeneous polymeric systems show a semi-circle diagram while heterophase

systems show two modified semicircles. In this case, the blends show a behaviour

different from homogeneou:j system, due to the presence of two components which

are immiscible.


Figure 8.10. Three dimensional graph showing the variation of tan 6 with frequency and temperature of N,, sample

Figure 8.11. Three dimensional graph showing the variation of tan E with frequer!cy and temperature of Nj, , sample


Figure 8.12. Three dimensional graph showing the variation of tan 6 with frequency and temperature of N,,,,, sample

log E'

Figure 8.13. Cole-Cole plot of N5" sample


8.1.4 Transport properties

(a) EfJect ~fh lend composition

The sorption behaviour of SBR/NR blends in hexane 1s displayed in

Figure 8.14. SBR, NR and NRISBR blends show almost similar sorption behaviour

even though the corresponding maximum uptake is different. The initial portion of

the sorption curves are sigmoitial in shape. The N I W shows the maximum solvent

uptake and No the minimum. The maximum solvent uptake increases with increase

in volume fraction of NR. It is established that the permeability of heterogeneous

rubber-rubber blends is intermediate between that of the components.'2 The

observed solvent uptake is in accordance with this observation. The observed

transport behav~our of SBRINR blends can be explained on the basis of

morphology, chain flexibility and degree of crosslinking of the blends. The

dispersedlmatrix morphology of N,o blend (Flgure 8 2a) offers a tortuous path for

the penetrant and hence the net uptake is low Due to the co-cont~nuous

morphology of NSO and N ~ o blends (F~gures 8 2b and 8 2c) the passage of the

penetrant becomes easier and hence the uptake is high. With the increasing volume

fraction o f NR in the blend, the chain flexibility increases due to the low glass

transition temperature of NR (q of NR -51°C and that of SBR -39°C) and hence

the solvent uptake. The degree of crosslinking (n l ) values for NO, N ~ o , Nso, N70

and Nlw,~amples are respectively 1.984 x lo4, 1.33 x lo-', 1.25 x lo3, 1.09 x lo4

and 0.743 x 10" molicc. As the degree of crosslinking increases the swelling

decreases. From No to N ~ M , the degree of crosslinking decreases and hence the

observed sorption behaviour. It is revealed from Figure 8.15 that the variation in

thickness has the same order ar; that of swelling, ie . , ht % increase in the order

No<N,o <Nso<N70<N~oo.


JTIME (rnin)

Figure 8.14. Mole percent hexane uptake of SBRNR blends

Figure 8.15. Variation of change in thickness (ht %) with square root of time


Influence of penetrant size on the sorption behav~iour of N ~ o blend is shown

in Figure 8.16. The maximurn solvent uptake Increases with increasing penetrant

size from pentane to heptane and decreases for octane. This behaviour can be

explaned based on the solubility parameter difference between the polymer and

penetrant. Smolders el a/. l3 and Belfort el a/. l 4 reported that the differences in

solubility parameter between the polymer and the penetrant have a role in deciding

the sorption behaviour of the penetrant in the polymer membrane. It is found that as

the solubility parameter of polymer and solvent becomes close to one another, the

solubility of the latter in the polymer becomes high. Figure 8.17 shows the variation

of maximum solvent uptake: with solubility parameter difference between the

polymer and solvent. Though this value is lowest for octane, the lowest solvent

uptake for octane is owing to its larger slze and increased number of conformations

compared to pentane, hexane and heptane. However, for the other three solvents,

the uptake is in accordance wrth the solubility parameter difference.

JTIME (rnin)

Figure 8.16. Mole percent uptake of N s ~ in //-alkanes


3 HEPTANE 2.5 - 4 OCTANE

2 -

ix - - 1.5- 8 0

4

1 -

oi_i_i_l-u~- ! 1 I 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

SOLUBlUlY PARAMETER DIFFERENCE (calicm3)"

Figure8.17. Variation of' maximum solvent uptake with solubility parameter difference between polymer and solvent.

(b) Dqfusivity, sorptivity and permeability

The calculated values of D*, S and P are given in Tables 8.2-8.4. The

values of the blends are found to be intermediate between those of the components.

The variation of D* value with volume fraction of NR is given in Figure 8.18.

There is a sharp increase in the values of D*, S and P after 30 wt O/b of NR. This

sharp increase is due to the co-continuous morphology observed in N50 and N70

blends. It was found that I3* values decrease with increasing penetrant size. The

S values are highest in heptane owlng to its closer solubility parameter difference.

The P values decrease gradually from hexane to octane. The P values are

comparably low in pentane due to the low sorptlon coefficient.


Table 8.2. Difisivity (El* x lo5) cm2/s

N70

0.622

0.620

0.919

1 276

132

0.550

0.593

0.675

0.877

0.43

0.595

0.740

0.910

No

0.472

0.595

0.703

0.749

0.763

0.435

0 495

0.583

0.971

0.313

0.338

0.520

0.772

Solvent

n-Pentane

n-Hexane

n-Heptane

??-Octane

Temperature ("C)

25

25

40

50

60

25

40

50

60

25

40

50

Nso

0.882

0.802

1.08

1.36

I .56 -

0.758

0.8 17

0.813

1.08

0.652

0.618

1.014

1.30

N 70

110

0.926

1.09

1.94

2 14

1.156

1.267

1.20

1.40

0.848

1.012

1.30

1.66

NIOO

1.82

1.75

2.74

3.30

3.56 1 1

1.57

1.83

1.93

2.58

1.25

1.45

1.96

2.32


Table 83. Sorptivlty, S (,g/g). , 1 1 I Solvent (Terny;ture( NO / Nib / Nso ( N70 1 Nl(10


Table 8.4. Permeability P* (=DS) x 10' (crn2/s)


I

1.5 - VI . N

5 - .7

X

b

I P

0.5kk- 0 0 0 2 VOLUME 0 4 FRACTION 0 6 OF NR 0 8 1

Figure 8.18. Variation of diffusion coefficient with volume fraction o f M .

Composite models :such as parallel model, serles model, Maxwell model

and Robeson's model have been applied to the SBRINR system to predict the

permeability properties of these blends. In the parallel model, the highest bound

permeability is given by the equationi

where P is the permeabil~ty of the blend, Pi and P2 are the permeabilities of

components 1 and 2, and 41 and 4 2 are the volume fractions of components 1 and 2,

respectively. The parallel model is applicable to systems in which the components

are arranged parallel to on,e another The lowest bound series model is found in

models in which the components are arranged in series The equation for this case I

IS

According to Maxwell equation1

I' = Pm [Pd + 2Pm- 24d (Pm- P:)'P<i + 2Pm + +d(Prn- Pd)]


where the subscripts m and d refer to the continuous matrix phase and dispersed

phase respectively. Robeson extended Maxwell's analysis by assuming that at

intermed~ate concentrations, both phases contribute to continuous and

discontinuous characteristics. The resulting Robeson equation is'

%here X, represents the fraction of the composition in which component 1 IS the

continuous phase and Xb corresponds to a continuous phase of component 2 The

descr~pt~on of such a co-contlnulty 1s l~mited by the restriction that

It can be seen that a composition range in whlch the permeabil~ty data are described

by equation with X , = Xb can be taken as an indication of phase inversion.

Figure 8.19 shows the experimental and theoretical curves of permeability as a

function of volume fraction of NR. It can be seen from the figure that the

experimental data at different: volume fractions of NR (say 0.3,0.5, 0.7) are close to

series, Robeson and Maxwell (NR continuous) models, receptively.

0 0 2 0 4 0 6 0 8 1

VOLUME FRACTION GF NR

Figure 8.19. Comparison o'f experimental permeability with theoretical predictions.


(e) Effect of temperature

D~ffus~on experiments were conducted at four different temperatures. v iz .

as 25, 40, 50 and 60°C. The effect of temperature on diffusion process in No, Nso

and N l w membranes is clezly shown in Figures 8.20-8.22, respectively.

Figure 8.20.

JTIME (inn)

Mole percent octane uptake of No at different temperatures

Figure 8.21. Mole percent octane uptake of N ~ O at different temperatures


Figure 8.22. Mole percent octane uptake of NI,,, at different temperatures

Figure8.20 shows that in No, the sorption increases with increase of

temperature. Similar behaviour was also observed in other solvents. But in Nro

(Figure 8.21) the maximum solvent uptake increases up to 40°C and then slightly

decreases. In Ntw, the Q, first decreases with increase of temperature and then it

gradually increases (Figure8.22). The maximum solvent uptake at higher

temperature is lower than that at 25°C. It was found that with increasing volume

fraction of natural rubber, the maximum solvent uptake decreases. This can be

explained by the heat of sorption values calculated using the Van't Hoff relation.

The enthalpy of sorption is a composite parameter involving both Henry's law and

Langmu~r type sorption. Henry's law requires both the formation of a site and the

dissolution of the species into that site; this involves an endothermic contribution to

the sorption. However, the Langmuir mode involves the s&ption by a hole-filling

mechanism and thus yields exothermic heat. The estimated values of

thermodynamic parameters are glven in Table 8.5. The correlation coefficients in


the determination of AH, and AS are close to 0 99 The AH, kalues are positive for

No suggesting a Henry's type sorption indicating the endothermic heat of sorption

It is clear from the AH, values that the sorptlon changes from Henry's type to

Langmuir type with increases in volume fraction of NR

Diffisivity, sorptivity and permeability increase with increase of

temperature irrespective of the solvents used (Tables 8.2-8.4). Using the Arrhenius

relationship, the activation energy for diffusion ED and that for permeation Ep were

calculated and are given in Table 8.5. The values are maximum for SBR (No) and

min~mum for NR. The values decrease with increase in volume fraction of NR. It

is interesting to note that the AH, values estimated from EP-EI) and Van't Hoff

relation are almost same.

Table 8.5. Thermodynm~ic and activation parameters (octane)

(4 Network choracterkotion

AH, (kl mol-I)

The investigation of the swelling equilibrium can help to elucidate the

structure of the SBR, NR and SBR'NR polymer networks. M,(aff) and M,(ph)

were calculated using the equations (6.8 and 6.9) and are compared with

experimentally determined M, (Table 8.6). It is seen that M, values are close to

&(@. This suggests that in the highly swollen state, the chains in the SBR, NR

and the blends deform affinely.

p E p (kllmol) 29.6

NSO

18.37

N3o

20.2

9.04

N70

15.52

1.37

Nl00

14.65

1 1 6 -0.32 -0.378


fe) Conrparkon with theory

21.4

Table 8.6. Swelling equil~brium properties in n-octane

Figures 8.23-8.26 represent the comparison of experimental diffusion

results with that of theoretical predictions of NO, N ~ o , N s ~ and NIW respectively.

M, \dues -

n ML(aff)

W P ~ )

The No and N3o membranes exhibit an almost Fickian behaviour. Slight deviation

was observed for NSO and N l ~ ~ samples

No

2520

2348

1174

m - THEORETICAL +EXPERIMENTAL

Figure 8.23. Comparison of experimental diffusion results of No with that of theoretical predictions

Nw

3774

3519

1759

Nso

3996

3729

1864

N70

4563

4261

2130

N loo

6725

6288

3144


Figure 8.24.

JTIME ( min )

Comparison of experimental diffusion results of N 3 ~ theoretical predictions

J TIME ( min )

with that

Figure 8.25. Comparison of experimental diffusion results of N5o with that of theoretical predictions


Figure 8.26. Comparison of experimental difhsion results of Nloo with that of theoretical predictions

If) Sorption (S)-desorption (D)-resorption (RS)-redesorption (RU)

Sorption, desorption, resorption and redesorption curves for NO, N,o, N ~ o ,

N70 and Nloo are given in Figures 8.27-8.31, respectively. For desorption

experiments, all the samples have the desorption equilibrium greater than sorption

equilibrium. Among different samples, Nloo has the highest desorption equilibrium

and NO the lowest. Blend compositions have the intermediate values. This

observation might be due to the leaching out of unreacted compounding ingredients

from the NR matrix. This is in accordance with the observation that as the volume

fraction of NR increases, the desorption equilibrium increases. The resorption

experiment shows a higher equilibrium uptake in all the cases, more predominantly

for NlW. This is due to the increased free volume generated by the leaching out of

additives during sorption-desorption experiments. The redesorption equilibrium is

almost similar to resorption equilibr~um indicating no further leaching out of

additives.


+RESORPTION + REDESORPTION

0,8 t "

J TIME (mi")

Figure 8.27. Sorption-desorption-resorption-redesorption of No in ,/-octane

+SORPTION + DESORPTION

-RESORPTION +REDESORPTION 1.2 "I=

Figure 8.28. Sorption-desorption-resorption-redesorptio of Nio in tr-octane


Figure 8.31. Sorption-desorption-resorption-redesorpton of Nloo in n-octane

8.1.5 Mechanical properties

The stress-strain curves of the unswollen, swollen and deswollen samples of

SBIL?.IR blends are illustrated in Figures 8.32, 8.33 and 8.34, respectively, The

nature of deformation of the blends under an applied load can be understood from

the stress strain curves. The deformation curves of homopolymers and their blends

are sim~lar. It can be understood from Figure 8.32 that at low strain level, NO has

the rnaxlmum stress and N ~ W has the minimum. The blend compositions have

intermediate values. At high strain levels, the stress required to deform the sample

increased with increase in NR content. This is mainly due to the strain induced

crystallisation behaviour of h'R.


STRAIN (%)

Figure 8.32. Stress-strain curves of unswollen SBRI NR blends.

STRAIN (%)

Figure 8.33. Stress-strain curves of swollerl SBR/NR blends in 11-hexane


STRAIN (Oh)

Figure 8.34. Stre:ss-strain curves of deswollen SBRINR blends.

Stress-strain curves of samples after reaching equilibrium saturation in n-

hexane reveals that there is only slight difference in the nature of stress-strain

behaviour even after reaching equilibrium. The stress at low straln level (<10(1%)

follows the same pattern as that in the case of unswolle~l samples. With increasing

NR content the stress-strain curve loses its typical elastomeric behaviour. It also

observed that the maximum stress increases with increase of NR content up to

50% and then decreases. This is due to the lack of strain induced crystallisation

behaviour in swollen samples particularly samples with high NR content. The

presence of solvents in the swollen samples restricts the mobility of the polymeric

chains and therefore the orientation is difficult. The stress-strain behaviour of'


deswollen samples are similar to those of unswollen samples. But there is an

overall increase in magnitude of the maximum stress value. This improvement in

properties is mainly attributed to the increased interchain interaction after sorption-

desorption process.

Mechanical properties of homopolymers and blends are given in Table 8.7.

As expected, tensile strength and elongation at break Increase from No to NIW The

mechan~cal strength of SBR Increases upon blendlng it wlth NR Thts ts detinttely

associated wlth the stran Induced crystall~sat~on of NR The decrease in Young's

modulus values from No to Nlm tndicates that the rnttlal stretchmy of SBR and

blend hav~ng hlgher SBR content requlres hlgher stress Slrn~larly the Secant

modulus values also decreased with increase in NR content at 100 to 300%

elongations

In the swollen state, ,there 1s an overall reduction in the magnitude of these

properties. In the equilibrii~m swollen state the rubber-solvent interaction is

maximum and rubber-rubber interaction is minimum and there is a total change in

the confonnat~on of polymer segments and cham entanglements Thls glves rrse to

the abrupt decrease of tensile properties of swollen samples In swollen samples,

the tensile strength values Increased wtth NR content up to 50 wt % and thereafter

decl~nes Thrs 1s due to the. lack of straln Induced crystall~sat~on after extenstve

swell~ng Thls 1s more predominant, when the NR content is hlgh The mechan~cal

properties of deswollen samples showed an improvement compared to unswollen

samples. This might be due to the increase in interchain interaction after a

sorpt~on-desorption process.


Table 8.7. Mechanical properties of SBRNR blends

Secant modulus

Model f i rMng

Mechanical behaviour of the blends has been modelled by using various

composite models such as the parallel, the series, the Kemer and the Kunori. The

parallel model (highest-upper bound model) is given by the equationI5


where M is any mechanical property of the blend and MI and MZ are the

mechanical properties of the components 1 and 2, and 41 and $2 are the volunle

fractions of the components 1 and 2, respectively. The lowest lower bound series

model is found in models in which the components are arranged in series with the

applied stress. The equation for this case is"

Kunori el al.16 reported that when a strong adhesive force exists between the blend

components, the dispersed phase will contribute to the strength of the blend and the

equation is

Considering two possible fracture paths in a blend, equation (8.8) can be modified

as follows depending on whether the fracture is through the interface or through

the matrix. When the fracture is through the interface

G ~ = G m ( ~ - 4 d 2 . 3 ) + ~ ~ d 2 , 3 (8.9)

when the fracture is through the matrix

(Jb= ~ r n ( l + d ) + ( ~ & d

where oh, om and od are the properties of the blend, matrix phase and dispersed

phase respectively and Qd is the volume fraction of the dispersed phase.

Equation (8.10) is same as the parallel model equation. Another important model

for perfect adhesion is that of Kemer." According to this

where E, E, and Ed are the respective properties of the blend, matrix phase and

dispersed phase; +d and 41, are the volume fractions of dispersed and continuous

phase respectively; and v, is the Poisson's ratio of the continuous phase.


F~gure 8 35 shows the theorettcal and exper~mental curves of the tensile

strength values of SBRMR blends In NU and N,u, the expertmental values are

close to that of Kunori model Therefore it can be concluded that the fracture

propagates through the interface rather than through the matrlx But in N7(,. the

experimental value is close to that of parallel model, Therefore In N ~ o , the appl~ed

stress distributes equally in two phases

+PARALLEL +SERIES * KERNER

0 0 0.3 0.5 0.7 1

VOLUME FRACTION OF NR

Figure 8.35. Comparison of experimental tensile strength with theoretical values as hnction of volume fraction of NR.

8.1.6 Comparison of degree of crosslinking estimated from swelling, stress-strain and dynamic mechanical analyses

Degree of crosslinking can be measured from swelling, stress-strain and

dynamic mechanical analysis.


One can use the storage modulus data for dete~mining degree of

crosslinking (n,) from DMA and this has been done using the following re~a t ion . '~

where E' is the storage modulus estimated from the plateau region of E' vs

temperature curve. R is the universal gas constant T, fie absolute temperature.

The degree of crosslinking determined from swelling, stress-strain and

dynamic mechanical analyses is glven in Table 8.8.

They are complementary to each other. Degree of crosslinking calculated

from different methods suggest that the maximum number of crosslirtks per uriit

volume is possessed by the NO sample and the lowest by Nloo sample. The blends

have intermediate values.

Table 8.8. Degree of crosslinking x 10ho l / cc

Sample n I

1.98

N3o -s+ 1.33

1.25

N7o 1.09

N~oo 0.74

n , - From s~elling measurements n2 - From stress-strain mrasurerncnts IL- From d ? W c mechanical analysis.

n2 n4 -1 1.83 I 0 9 4 1.81

1 .80

1.09

0.92

0.79 1

074;-7 0.72

0.7 1


8.2. References

I H B. Hopfenberg and D R. Paul, I'olymer Htetrd., (Eds.. D . R. Paul and S. Newman), Academic Press, New York, 1978, Ch. 10.

2 D. R. Paul, J. hfembr. Scr.. 18, 75 (1984).

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5 I Cabasso, J Jayur-Grodzinski and D Vofsi, J. Appl Po/,~,m. Scl.. 18, 2117 ( 1 974)

6. E. Shchori and J . Jagur-(lirodzinki, J. Appl. f 'ol~m. So . , 20. 773 (1976).

7 T. M Aminabhavi and H.T.S. Phayde, .I Appl. Polym. Sci., 57, 1491 (1995).

8. T. M. Aminabhavi and H. T. S . Phayde, t;rrr. f'olynl. J.. 32, 1 1 17 (1 996).

9. T Johnson and S. Thomas, .J. Marer. Sci. (Submitted)

10. C. S. Danesi and R. S. Porter, l-'o/ymer, 19, 448 (1978).

1 I . C. Wisme, G. Maria ;and P. Monge, t,'tir. J'o/ym. J., 21, 479 (1985).

12. A. K. Bhownick and H. L. Stephens, Hatibook of l.,'lasomer.s. Marcel Dekketr, Inc.. 1988

1.3 P E. Froething, D. M. Koenhen, A. Bantjes and C. A. Smolders, Polymer, 17, 835 (1976).

14 Y M Lee, D. Bourgeois and G. Belfort, Member. Sci., 44, 161 (1 989).

15. S George, L. Prasnnakumar, P. Koshy. K. T. Varughese and S. Thomas, Muter. L,rtt., 26, 51 (1996).

16 T. Kunori and P. H. Geil. J. Macromol. Sci. I'hyr: B. 18(1), 135 (1980).

17. E. H. Kemer, I'roc. 1'hy.s. SIX. .. 696, 808 (1956).

18. R. Hagen, L. Salmen and B. Stenberg, .J. IJo/ym. Sci. Part H l'olym. Phys., 34, 1997 (1996).


chapter sbr/nr blend membranes: morphowgy,...

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