CHAPTER SBR/NR BLEND MEMBRANES: MORPHOWGY,...
Transcript of 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
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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
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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
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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
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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
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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
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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
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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.
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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.
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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
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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.
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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
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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
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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.
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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
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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
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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.
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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
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Table 83. Sorptivlty, S (,g/g). , 1 1 I Solvent (Terny;ture( NO / Nib / Nso ( N70 1 Nl(10
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Table 8.4. Permeability P* (=DS) x 10' (crn2/s)
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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)]
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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.
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(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
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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
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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
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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
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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
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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.
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+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
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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.
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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
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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'
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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.
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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
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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.
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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.
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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
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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).
3 . J . S. Chiou and D. R. Paul, J. Appl. Polym, Sci., 32, 2897 (1986).
4 H. Odani, M. Uchikura, Y. Ogino and M. Kurata. J. Membr. Sci., 15, !93 (1983).
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).
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