Chapter 3

Morphology, Dynamic Mechanical and Mechanical Properties of Latex Blends

Summary: Chapter 3 outlines the morphology, dynamic mechanical and

mechanical properties ot' natural rubber (NR), carboxylated styrene butadiene

rubber and their blends with special reference to blend ratio. Morphology of the

blends has been analysed from scanning electron micrographs (SEM). These

blends system exhibited heterogeneous phase morphology, which indicated the

immiscibility of two phases. The loss tangent curves of NRIXSBR blend system

exhibited two transition peaks corresponding to the glass transition temperature

(T,) of the individual components. The tensile strength of NRIXSBR blend system

is found to be lower than the virgin polymers owing to the immiscibility of two

phases. The variation in crosslink density with blend ratio has been estimated by

different methods such as equilibrium swelling and stress- strain measurements.

A par1 ?/the resulrs ofthi.! chnprt,r /iove heen submitted ro Jonrnid ofNofarrrl Ruhher Research. .&lrrlcr.v~irr and anofher parr hal,e hccri ptrhlished in Jorrr,ml of,lpp/ied I'c,(vnter Science

92 Chapter 3

3.1 Introduction

The blending of polymers has provided an easy and efficient way to generate new

types of high performance materials'? Latex stage blending of polymers will

result in a finer scale of dispersion and there is no contamination by solvents.

Thomas and c o - ~ o r k e r s ~ ' ~ have investigated the mechanical, rheological,

viscoelastic, and thermal properties of NRI SBR and NR/ XSBR latex blends with

reference to various parameters such as blend ratio, shear rate etc.

The control of the phase morphology during blend processing is a key issue for the

production of new materials with improved properties. The shape, size and spatial

distribution of the phases depend on the viscosity of the phases, interfacial

properties, blend composition and processing conditions9. The impact of

morphology on the end use application of polymer blends has been reported by 15-17 . many researchers'"''. Thomas and co-workers mvestigated the dependence of

phase morphology on the properties of several polymer blends. The authors found

that the mechanical and dynamic mechanical properties of polymer blends could

only be explained in terms of their morphology.

The importance of dynamic mechanical analysis (DMA) in understanding the

behaviour of rubber blends is widely accepted. DMA is a reliable technique for

evaluating the dynamic glass transitions and rubber plateaus unambiguously'x.

Many researchersI9'" have carried out both theoretical and experimental

approaches to these properties of blends.

In the present chapter the morphology. dynamic mechanical and mechanical

properties of NRIXSBR latex blends have been analysed and discussed with

special reference to blend ratio.

Morphology, dynamic mecltonienl .... 93

3.2 Results and discussion

3.2.1 Morphology of blends

The morphology of blends has obtained from SEM of cryogenically fractured

surfaces. It is unequivocally established that the morphology is the principal

deciding factor of ultimate properties of a heterogeneous blend. In the present

case, NR is non-polar and XSBR is polar in behaviour. They are highly

immiscible owing to the polarity difference and exhibit independent

characteristics. The establishment of blend morphology is usually determined by

factors related to material parameters such as blend ratio, viscosity of the phases,

interfacial tension, reactive functionalities and the presence of ingredients and the

processing conditionsz2. Many researchers studied the effect of mixing time on the

morphology of melt-mixed elastomer b ~ e n d s ~ ' - ~ ~ . Thomas and c o - w ~ r k e r s ~ ~ ~ ~ ~

extensively investigated the effect of blend ratio, viscosity difference between the

phases and the presence of reactive functionalities in the morphology of various

polymer blends.

The SEM micrographs given in Figure 3.1 demonstrate the phase morphology of

fractured surfaces of NWXSBR blends. One can distinguish two types of

morphology from the figure: (a) dispersed droplet type morphology in 70130

blends (Figure 3. [(a)) where XSBR forms the dispersed phase in continuous NR

matrix and in 30/70 blends (Figure 3.1(c)) NR dispersed in the continuous XSBR

matrix and (b) a co-continuous phase qtructure in 50150 blend (Figure 3.l(b)).

(c)

Figure 3.1 SEM of (a) N,,, (b) NS0 and (c) Njo

The average size of the domains present in N7* and N3u polyrner blends arc given

in Table 3.1. The number average ( fi,,) and weight average ( 6,,) diameter of

dispersed pl~ases are calculated using the t.elationship,

wherc n, is the number o f parlicles with diameter- D,.

Morphology, dynamic mechanicnl.. .. 95

The domain size ofNzo blend has been found to be higher than that of N,,, system.

The domain size largely depends on the viscosity of the continuous phase. Usually

the less viscous component (XSBR) forms smaller dispersed particles in more

viscous matrix (NR) due to relatively restricted difTusion effects on coalescence of

particles. As the concentration of dispersed phase increases particle size increases

and beyond a particular limit of composition both the components form a bi-

continuous phase structure (co-continuous) as evident from SEM micrograph of

NSO blend.

Table 3.1 Domain diameter of dispersed phases in N,o and N,, blends

3.2.2 Dynamic mechanical properties

Properties such as storage modulus (E'), loss modulus (E") and damping (tan 6 ) of

unvulcanised NRIXSBR blends have been evaluated over a wide range of

temperatures and frequencies. Investigations for the temperature- dependence of

viscoelasticity have considerable practical importance and provide evidence

toward a molecular interpretation of viscoelastic behavior as the material changes

from a glassy to a rubbely state. NR latex possesses higher damping values than

XSBR latex. The blends exhibited lesser damping than the individual components

at both phases. Hence it will he useful for the manufacture of low damping

materials. The NRIXSBR blends are immiscible and show two T,s corresponding

to NR and XSBR phases.

The tan 6 versus temperature curves of NW XSBR blends is shown in Figure 3.2.

These curves show two peaks typical of two-phase systems; each peak is

characteristic of the glass transition of each of the component'". which indicates

96 Chapter 3

that the system is immiscible. The T, values are taken from the corresponding

temperature of the peak value of tan 6 for each phase. The transition temperatures

of virgin components are -52 and 23°C respectively. Compared to SBR, XSBR

shows higher T, due to its polarity and self-crosslinking nature. The T, of NR

phase in Nm blend is -59°C at 1 Hz, while that of XSBR phase is 15°C. In blends

the difference in T, is higher due to the immiscibility of two phases. The T,'s of

blends at different frequencies are given in Table 3.2. In all cases the tan IS values

and T,'s increases with frequency.

Figure 3.2 Damping curves of latex blends

The peak areas ofthe tan 6 curves are given in Table 3.3. There is a drastic change

in the peak area of NR phase on blending as compared to that of the XSBR phase.

This is an indication of the extent of mobility of the macromolecular chain

segments at the transition temperature3'. Any restriction in the main chain mobility

in the polymer is expected to decrease the area under the curve. The drastic

decrease in peak area is due to the self- curing behaviour of XSBR latex, which

form networks within the system. Thus the molecular mobility of the polymer

chain segments is restricted at the Tg

Morphology, dynamic mechanical .... - -- 97

Table 3.2 Damping and glass transition values of latex blends

T, from E" peak darnping euwe temperature (OC)

Sample

98 Chapter 3

Table 3.3 Peak area from damping curves

Peak area (em2) Sample

Figure 3.3 shows the tan S,,, values o f NRI XSBR blends with weight % o f NR.

From the curve it i s observed that the tan S,,, values o f NR phase decreases with

increase in XSBR content in blends. NR shows highest damping value and the

decrease is sharper when the XSBR content i s 50 % or more. This can he

explained in temis o f the high segmental mobility o f NR as compared to XSBR. A

sharp decrease in damping value in Nso blend i s due to its co-continuous

morphology. For N7" blend it shows a minimum, and then it decreases for NS0 and

Njg blend with increase in XSBR content. That is, the tan S,,, values o f XSBR

phase increase with decrease in NR content.

Morphology, dynamic mechanical.. .. -- -. -- - 99

Figure 3.3 Variation in damping values with blend ratio

The E' for various blends over a wide range of temperature is shown in Figure 3.4.

The two steps seen in the modulus- temperature curve are characteristic of

immiscible two- phase systems. The two regions in the E' curves shown for blend

system is corresponding to the T, of the two components. The E' of the blends

decreases with increase in N R content. Compared to other blends NSO blend

exhibit highest E' due to the co-continuous nature of the two phases. The value of

E' has been found to decrease with rise in temperature owing to the decrease in

stiffness of the sample

The main chain mobility in the polymer can be understood from the area under the

E" vs temperature curve". The E of NR/ XSBR unvulcanised latex blends are

shown in Figure 3.5. As might be expected, the E" increases sharply up to the

transition zone until they attain maxima and then decreases with temperature. This

curve shows the same trend as that of tan 6 versus temperature curve. The E" peak

temperature is more or less same as that of the T, obtained from the tan 6 versus

temperature curve. The sharp loss peaks indicate that the system components are

immiscible. The T, obtained from E" curves of blends at different frequencies are

shown in Table 3.2.

100 Chapter 3

Figure 3.4 Storage modulus vs temperature curves of latex blends

- 2 o x l O " ~ . , , . 8 , . 8 . % . , . I -80 6 0 -40 -10 0 20 40 60 Xll

Temperature ((Ic)

Figure 3.5 Loss modulus vs temperature curves of latex blends

3.2.3 Time-temperature superposition

The two experimental variables available i n the measurement o f viscoelastic

properties are time and temperaturex~i2. From the master curve, one can easily

understand the complete modulus- time behaviour o f a polymeric material. The

construction o f master curve is based on the time-temperature correspondence

principle. The data collected at one temperature can be superimposed upon data

taken at a different temperature by horizontal shiftsi5.

Morphology, dynamic mechanical.. .. 101

For constructing a master curve one has to pick one teniperature. suppose T, as the

reference temperature. The shift factor is a function of temperature and hence

designated as ar. All other experimental curves. each at a particular temperature

are then shifted horizontally, i.e, along the time axis until it overlaps the curve

with reference temperature T,. The shift factor characterises the rate of the

relaxation mechanism at temperature T, in comparison with the rate at a higher

temperature T,,,. From this the log ar values for all temperatures can be

determined.

The viscoelastic properties at a given frequency fare quantitatively equivalent to

those of an experiment carried out over a time t= 1/(2nt). Figure 3.6 shows the log

E' vs log t curves of NSo blends at different temperatures. The temperature 273K,

was taken as a reference temperature in NSo blends. The master curve was

constructed by plotting log (ETdT) vs log (t/ar) (Figure 3.7), where E is the

storage modulus at a particular temperature, To is the reference temperature on the

Kelvin scale and T is the temperature of the experiment. The master curve depicts

the change in modulus with time over a wide range.

Figure 3.6 log t vs log E' curves of N50 blend

Chapter 3 102 --

Figure 3.7 Master cuwe of Nso blend

3.2.4 Theoretical modeling of dynamic meehanieal properties

The theoretical modeling of polymer blends will help in designing materials for

engineering applications. The objective of the comparison of theoretical and

experimental values is to understand and predict the mechanical properties and

morphology of the

The applicability of Kerner and Halpin-'I'sai models has been discussed in the case

of NIU XSBR latex blends. These models can be used to predict the viscoelastic

behaviour of rubber- rubber b ~ e n d s ~ ~ . ' ~ .

Kerner 39 equation is given by,

where Eb is the modulus of the blend, Em is the ~nodulus of the matrix, E d is the

modulus of the dispersed phase, 4 1 ~ is the volume fraction of the dispersed phase,

@,, is the volume fraction of the matrix, and v,,, is the Poissoll ratio, for rubber zone

the value of Poisson ratio is usually considered as 0.5.

Morphology, dynamic mechanical ....

According to Halpin-~sai~' model,

where,

A, = 0.66, M is the modulus and I$ is the volume fraction, the subscripts 1 and 2

denote the components 1 and 2 respectively.

Figure 3.8 gives the comparison of theoretical and experimental values of N311 and

NjO latex blends. The theoretical values obtained are lower than the experimental

values at low temperature while at higher temperature there is a good agreement

between experimental and theoretical values. Due to the higher storage modulus of

XSBR, the N,o blend shows higher values compared to N,' blend. As the XSBR

content increases the system shows more elastic behavior because of the self-

curing nature of XSBR. The morphology of the blends (Figure 3.1 (a &c)) also

supports the theoretical modeling since the system is considered as a phase

dispersed in a continuous matrix. In both NjO and Nm blends one phase is

dispersed in another continuous matrix. At low temperature Halpin-Tsai model fits

more with the experimental curve of N30 and N,o than Kerner. But at higher

temperature both models matches well with the experimental results.

104 Chapter 3

L , I 4 4 . a D O 20 40 M

rarcpraueh

Figure 3.8 Theoretical modeling of N,, and NTO blend

3.2.5 Mechanical properties

Figure 3.9 gives the stress- strain behaviour of latex blends. It can be seen that as

the weight percentage of NR increases the strain induced crystallization behaviour

also increases. The deformation behaviour of various blends under an applied load

is obtained from stress- strain measurements. NR shows strain induced

crystallisation nature, which is the reason for better mechanical properties than

other synthetic rubbers. The strain- induced crystallisation of NR has been

extensively studied by X-ray diffraction technique since 1940s"-'~. However, the

vulcanized synthetic rubbers are considerably weaker than the NR because of the

lack of crystallisation under stretching at room temperature. Recently, Toki et a/.

4446 found that under high strains (>600 %) a very large fraction of unoriented

amorphous (- 75 wt%) phase still remained in the stretched sample in addition to

-20 wt% strain- induced crystals along with -5 wt% oriented amorphous chains.

According to their investigations due to the non-homogeneous distribution of

crosslinked network only a small fraction of crosslinked chains are responsible for

the entire mechanical performance of the NR. The non-homogeneous network

topology is due to the presence of chemically reactive center, unsaturated bond

and the average chain length between two ad.jacent reactive points. The ~nolcculcs

Morphology. dynamic mechanical.. ..

o f small chain length between the densely packed networks can he oriented and

form crystallites where as the molecules o f much longer chain lengths would

remain in the random coil state under stretching. In conventionally vulcanized

rubber the network formed is composed o f molecules with a broad distribution o f

chain lengths between the network points and hence i t cannot create a

homogeneous network distribution. From Figure 3.9 i t is clear that the strain-

induced crystallisation is predominant i n N R and NTO blend, in which XSBR

dispersed in the N R continuous matrix. As the weight percentage o f XSBR

increases the crystallisation behaviour decreases.

Figure 3.9 Stress- strain curves of latex blends

The variation in tensile strength with blend ratio is shown in Figure 3.10. I t i s

obvious from the figure that the blend system shows negative deviation from the

additivity line. Since N R i s non-polar and XSBR is polar, their blends are

thermodynamically immiscible due to the lack o f favourable interaction between

the components at the interface due to the polarity difference. Therefore, the

blends exhibit inferior mechanical properties to its individual component.

However, N 7 ~ blend has tensile strength higher than XSBR due to the nature of

continuous N R matrix. The i~iferior properties o f N5,, and N;,, blends a!-e due to the

106 Chapter 3

difference in affinity of curing agents by two phases. This can be further explained

by the fact that since NR has higher degree of unsaturation than XSBR the NR

phase gets overcured. The uneven distribution of networks within the phases

worsens the tensile properties. The reduced strain induced clystallisation with the

increasing concentration of XSBR, the uneven distribution of curing agents and

poor interfacial adhesion between the polar XSBR and non-polar NR phases

account for the reduced mechanical properties of blend system.

b , 8 . 8 ' 1

0 20 40 60 80 IW

Weight %. of NK

Figure3.10 Variation in tensile strength with blend ratio

Figure 3.1 1 displays the elongation at break and modulus at 300 % elongation as

a function of blend ratio. The modulus of the blend decreases and the values of

elongation at break increase with increase in the weight % of NR. It is evident

from the figure that as the weight percentage of NR increases the elongation at

break of blends also increases. The values are intermediate between the virgin

polymers. The N7,, blend shows abrupt increase in the value due to the continuous

nature of NR phase. Figure 3.12 presents the tear strength values of latex blends. It

is found that the tear strength increases gradually as a function of concentration of

NR latex. It exhibits a negative deviation from the additivity line.

Morpholngv, dynamic mechanical .... 107 .

0 M 40 M 80 IM

Weight %of NR

Figure 3.1 1 Variation in elongation at break and modulus with blend ratio

Figure 3.12 Variation in tear strength with blend ratio

3.2.6 Crosslink density

Crosslinking o f polymers can be carried out to improve a wide variety of

properties including strengh, heat resistance and recovery from deformation.

Generally, in blend systems, uneven distribution o f crosslinks call be observed due

to the difference in affinity o f the two phases towards curatives. I ' l ie ~nigration of

vulcanising agents depends on the blend ratio, degree o f chemical reactive center

108 Chapter 3

(unsaturation or double bond), polarity etc. Even rather a small change in relative

polarity of the rubbers in a blend will affect the crosslink distribution4'. A plot of

crosslink density values of NRIXSBR latex blends obtained from equilibrium

swelling method and stress- strain measurements is shown in Figure 3.13. The

values obtained from stress- strain measurements are higher since it contains a

contribution due to the chain entanglements. Due to the self-crosslinking nature of

XSBR, it possesses higher crosslink density values than NR. Crosslink density

values of blends determined by both methods are lower than virgin polymers. This

is associated with the uneven distribution of curing agents in the two phases

resulting in the formation of nonhomogeneous networks. The solvent resistance

properties of blends are obtained from the swelling ratio. The values are given in

Table 3.4 and it is observed that blends are less resistant to solvent due to the non-

uniform migration of vulcanizing agents.

t from stress-stram

Figure 3.13 Variation in crosslink density with blend ratio

Morphology, dynamic mechczniral.. .. - - 109

Table 3.4 Swell ratio values of latex blends

Sample Swell ratio

3.2.7 Theoretical modeling of mechanical properties

The theoretical correlation of experimentally determined mechanical properties is

relevant for predicting the nature of the blend systems. Various composite models

have been applied to relate the experimental results with theoretical predictions.

The structure and properties of the interface can be understood from these models.

Different models have been applied for predicting the mechanical property of

NRIXSBR latex blends. These include parallel, series and Kunori models. The

upper bound parallel model is given by the rule of mixtures",

M = MI$, + M2$2

where M is the mechanical property of blend

M I is the mechanical property of component 1

M2 is the mechanical property of component 2

$, is the volume ti-action of component I

is the volume fraction of component 2

This equation is suitable for models in which the components are connected

parallel to each other and the applied stress elongates each component by the same

amount. In the lowel. bound model the components are perpendicular to the

applied force. The equation is given by,

110 -- Chapter .Z

Nielsen4%nd Kunori and ~ e i l ' ~ stated that tensile failure of a blend is due to the

lack of interfacial adhesion. According to them, the tensile strength of a blend in

the absence of adhesion between components may be written as,

where a, is the tensile strength of blend

5 , is the tensile strength of the matrix and

Ad represents the area of fraction occupied by the dispersed phase in

transverse cross section

Kunori and ~ e i 1 ' ~ assumed that if there is a strong adhesive force between blend

components, the dispersed phase would also contribute to the strength of the

blend. Based on this they modified equation (3.8),

If the fracture propagates mainly through the interface, equation (3.9) becomes,

If the fmcture propagates through the matrix, then the equation (3.9) may written as,

From Figure 3.14 if is clear that N,Q blend fits more to parallel and Kunori-2

models. This indicates that the two phases are imlniscible and the fracture

propagates through the matrix. The morphology studies disclose that in N ~ Q blend

NR is dispersed in the continuous XSBR matrix. Due to the co-continuous

morphology of NSQ blend it deviates from all theoretical predictions. The N,,, blend

Morphology, dynnmic mechanicrrl.. .. -- 1 I I

agrees well with theoretical values of series model, in which XSBR is dispersed in

the continuous NR matrix.

Figure 3.14 Theoretical modeling of modulus of blends

3.3 Conclusion

.:. NWXSBR latex blends exhibited heterogeneous morphology. The 50150

blend system exhibited co-conlinuous morphology.

.:+ Damping curves of NWXSBR blend showed two transition peaks

corresponding to individual components, indicating the immiscibility of two

components.

.:* Damping decreased with increase in concentration of XSBR.

.:. Tensile and tear strength of NWXSBR latex blends exhibited negative

deviation from the additivity line due to the immiscibility of two phases.

The modulus of the blend system increased with increase in concentration of

112 Chapter 3

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