Materials ChemistrJa and Physics, 19 (1988) 123 - 130 123

EFFECTS OF TEMPERATURE ON THE FLOW OF COPOLYMER MELTS

A.V. SHENOY

Department of Material Science and Engineering, University of Florida, Gainesville, Fl 32611 (USA)

D.R. SAINI

Polymer Science and Engineering Group, Chemical Engineering Division, National Chemical Laboratory, Pune 411 008 (India)

Received April 16, 1987; accepted July 22, 1987

ABSTRACT

The temperature dependent flow behaviour of copolymers is useful for obtaining the monomolecular melt, which is essential for processing in order to get the defect free copolymer products. Melt flow index is shown to be useful in depicting the temperature dependence of copolymer melts and in estimation of their flow activation energy. The three copolymer systems included are styrene-butadiene-styrene (SBS), ethyl vinyl acetate (EVA) and a liquid crystalline copolyester based on hydroxybenzoic acid-polyethylene terephthalate (HBA-PET) respectively. The observed behaviour indicates a significant degree of retention of physical network structure below the cross-over point in a MFI-temperature-l plot.

INTRODUCTION

A study of the temperature dependence of melt viscosity

helps in elucidating the mechanism of polymer melt flow processes

in relation to the nature and composition of the material.

Recently, Saini and Shenoy [l] have suggested a new method for

the determination of the activation energy for viscous flow of

polymer melts. Their method is based on the melt flow index

(MFI) rather than on the conventional zero shear viscosity. The

advantage lies in the fact that MFI is a very simple rheological

parameter easily determinable on relatively inexpensive equipment;

by contrast, zero shear viscosity is difficult to measure and

needs highly sophisticated, expensive equipment. Further, MFI is

determined under constant shear stress conditions. It is known

0254-0584/88/$3.50 0 Elsevier Sequoia/Printed in The Netherlands

124

that, for non-Newtonian materials, it is beneficial to use the

flow activation energy at a fixed shear stress rather than at a

fixed shear rate due to the constancy of the value. The values

of activation energy for viscous flow using the suggested

technique [l] have been calculated for a number of homopolymers

falling within the olefinic, styrenic, cellulosic and engineering

thermoplastic groups.

In the present work, the method of Saini and Shenoy [l] has

been extended to various copolymers and shown to depict the

anomalous temperature dependence of copolymer melt viscosity

fairly effectively. Similarly, La Mantia [2] has shown the

usefulness of MFI by plotting a melt index-composition curve

for studying the influence of the processing conditions on the

rheological properties of polycarbona.te/polypropylene blend.

DATA ANALYSIS

The melt flow index is defined as the weight of polymer in

grams extruded in ten minutes through a capillary of specific

diameter and length by pressure applied through dead weight as

per ASTM 1238-73. The melt indexer, being an extrusion

rheometer, gives a simple viscometric flow of the polymer and the

expressions for shear stress T and shear rate are given by well

known conventional forms as follows:

(1)

(2)

where piston radius RR = 0.4737 cm, nozzle radius RN = 0.105 cm,

nozzle length .& = 0.8 cm, force F = test load L (Kg) x 9.807 x lo5

dynes, flow rate is Q = MFI/600p , cm3/sec; (J is the density of

the polymer.

Since the geometry of a melt flow indexer is fixed as given

above, expressions for2 and 2 in an MFI apparatus are given

by Shenoy et al. [3,4] as

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T = 9.13 x 10 4L (3)

ri = 1.83 y (4)

As each MFI is taken under fixed load conditions, it is evident

from eqn. (3) that the MFI test is truly a constant shear stress

measurement. Moreover, from cqn. (4) it is clear that for each

value of MFI for a given system at a specific temperature, a

value of shear rate can be obtained at a specific shear stress.

It would then represent a single point on the shear stress versus

shear rate curve at that specific temperature. This fact can be

used to determine the value of MFI from a known shear stress vs -

shear rate curve for a specific polymeric system. MFI values

determined in this way are tabulated in Table I with sources.

Table I. Activation Energy Estimated Based on MFI Method.

MFI T ('C) Activation Activation Source ( 4d energy energy 10 min) below cross- above cross-

(Ref.No.1

over point over point

Joules/ (Joules/ mo1e.K) mo1e.K)

SBS 0.05 110 5 0.34 130 1.2x105 5 1.7 150 5

2.7 170 4.2~10~

5 4.8 190 5 7.8 210 5

EVA 0.02 60 65 2.3~10~

6 0.12 6 0.18 70 6

0.34 0.50 2.37 4.8

;z 100 125

6.7~10~

6,7 7 7

PET-HBA 0.1 275 0.44 285 6.8 296 4.6x105

19.1 305

63.7 315 1.3x105

8 122.5 330 a

126

This method has been used earlier by Rideal and Padget [9].

AST, 1238-73 specifications are such as to obtain MFI values

under conditions of shear stress and shear rate which are ideal

for the use of the

absolute theory of

Y=A exp C-&i)

Arrehenius-type equation,

rate processes derived by

where y is the viscosity at temperature T, R is the gas constant,

based on the

Eyring [lo] as

(5)

A is the frequency term depending on the entropy of activation

for flow, and E is taken to be the energy of activation for

viscous flow.

Equation (5) can be modified easily to get an expression for

the sensitivity of MFI on temperature in contrast to the

conventional viscosity term. The relationship between MFI and7

is obvious from the ratio of eqns. (3) and (4) as

(6)

For each polymeric system, the density 0 is constant, and

the load condition is fixed as per ASTM 1238-73. Thus

'y1 MFI = constant (7)

Saini and Shenoy [l] have given a modified Arrhenius-type eqn.(S)

using the above relations as

MFI = B exp ( +$--I (8)

where gas constant R is 8.3~10~ Joules/mole. OK, T is the

temperature in K and the activation energy for viscous flow can

be calculated from the In MFI versus T -1

plot.

RESULTS AND DISCUSSION

Plots of MFI versus T-l on semi-orgarithmic scales are shown

in Figs. 1-3, which include three copolymer systems, namely,

styrene-butadiene-styrene (SBS), ethyl vinyl acetate (EVA) and

liquid crystalline copolyester, hydroxybenzoic acid -polyethylene

terephthalate (HBA-PET), respectively. The data for these

127

Fig. 1. plot of for SBS.

Semi-logarithmic ElFI versus T-1

I I I I I I I \

STYRENE BUTADIENE STYRENE \

-2 10 1 1 ! I I I I I I

2.2 2-4 2.6

fx#TEMP.,K

lb2 2-5 2-6 2.7 2.8 2-S 3.0 3-l

+x10 TEMP., K

Fig. 2. Semi-logarithmic plot of MFI versus T ml for EVA.

128

lo” I I I 1.5 1.6 1.7 14 1.9

$ x IO TEMP., K

Fig. 3. Semi-logarithmic plot of MFI versus T -I for HBA-PET

copolyester.

systems are given in Table I along with the sources. It can be

seen that in. each of the Figs., the points are laid out in such

a way as to give two distinct straight lines. This indicates

the existence of two separate values of activation energy

accompanied by a sudden change at some characteristic temperature,,

It means that at this temperature a structural (or relaxation)

transition occurs leading to an alteration in the mechanism of

flow.

Copolymers have polymer chains comprised of more than one

type of monomeric building block. The nature of comonomers

and their placement in the chain have a major influence on the

elastomeric behaviour and melt rheology. Except for uniform

random copolymers, all other types of block copolymers show

129

microphase separation. Their melt viscosities are a manifesta-

tion of the existing two-phase structured system, probably a

weaker version of the three-dimensional network which exists at

lower temperatures. AS the temperature is raised, only one of

the domains of the two-phase system melts, but the system is

able to flow as a whole due to the fluidity created by one of

the domains despite the fact that the two domains are not

compatible. However, such a flow involves disruption of the

melted domain and transfer of the segments through a thermody-

namically incompatible unmelted second domain. This requires

additional energy, giving rise to a high value of activation

energy. As the temperature rises, a stage is reached when both

domains become fluid and the additional resistance to flow due

to the presence of the unmeltci Gomain is removed. The

temperature at which this occurs would then become the cross-

over point to the lo\Jer activation energy level as can be seen

in Figs. l-3.

The values of the two activation energies on either side of

the crossover point would bc such that one would be tending to

the activation energy of one phase while the other would tend

to the activation energy of the second phase. For example,

in the case of SBS, between 110-150°C, the activation energy El

has a value of 1.2 x lo5 Joulcs/molc.K which is not too

different from the activation energy for homopolystyrene

(E = 1.0 x 105 Joules/mole.K); whereas in the temperature

range between 150 to 210°C, the activation energy tends to

that of polybutadiene (E = 1.9 x 104 to 3.3 x 104 Joules/mole.K)

and takes a lower value of E2 = 4.2 x 10 4

Joules/mole.K.

It is essential to exercise caution when determining the

activation energy for viscous flow in the case of copolymers.

If the attempt at such determinations is done through few data

points, there is a likelihood of error as the crossover point

would not come out distinctly and the plot of viscosity or MEI

versus T -1 may be mistaken for a curve that cannot be approxi-

mated by a straight line. This was one of the reasons why the

particular systems in Figs. l-3 were specifically chosen as they

provide at least six data points. For other copolymers like

acrylonitrile butadiene styrene (ABS), vinyl chloride - Vinyl

acetate (VCVA), styrene acrylonitrile (SAN), etc. though data

of MEI versus temperature is available [l] it is not sufficient

130

to depict two distinct activation energies and hence was not

included here. Shenoy and Saini [ll] have noted that for copo-

lymers when PIFI values at different temperatures are required,

the modified WLF-type equation cannot be used because of the

existence of two glass-transition temperatures in such systems.

However, they have not suggested an alternative method. We now

have acquired such a method: eqn. (8) can be effectively used

provided the appropriate value of E is inserted, based on

whether the temperature of interest is above or below the

temperature of the crossover.

CONCLUSION

Copolymers on heating show a mel-t rheological transition

temperature, above which the two phase structure is destroyed to

form a complete mixed phase. It is shown that transition

temperature can conveniently be detected by change in the slope

of the plot of Melt Flow Index against temperature -1 . This

technique is easy and has given satisfactory results for the

systems, namely, styrene-butadiene-styrene, ethyl vinyl acetate,

and hydroxybenzoic acid - polyethylene terephthalate.

REFERENCES

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2 F.P. La Mantia, Mater. Chem. Phys., 16 (1987) 115. -

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4 A.V. Shenoy, D.R. Saini and V.M. Nadkarni, J. Appl. Polym.Sci, 27 (1982) 4399. -

5 A. Ghijsels and J. Raadsen, Pure Appl. Chem.,52 (1980) 1359. -

6 J. Lyngaae-Jorgensen and A.L. Borring, Proc. VIIth Intnl. Congr. Rheol., Gothenburg, Sweden, 1977.

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