Molecular Weight and Temperature Dependence of the Melt Viscosity of Poly(ethylene terephthalate)

Abstract

The melt viscosity measurements of poly (ethylene terephthalate) were carried out within the ranges of the molecular weight 9.21 x 1O^2- < M< 1.48 x 105 and the temperature from 265 to 285° C. In order to obtain the molecular weight of polymer running through the viscometer the melton polymer was quenched immediately after each viscosity measurement and the intrinsic viscosities of the quenched samples were measured.
Thus obtained data were examined employing the current emperical equations. In the logarithmic plot of the viscosity against weight average molecular weight, data were approximated by two straight lines with slopes of 3.4 in the high molecular weight range and of 1.7, in the low molecular weight range. The critical chain length calculated from the molecular weight at the intersection of the two straight lines was 211, which is simmilar to the value reported for polyesters.
The parameters in the Berry-Fox emperical equations were discussed. In the logarithmic plot of viscosity against X, which is a function of chain length, mean square radius of gyra- tion, molecular weight, volumetric fraction and specific volume of the polymer, data were approximated by straight lines with the slope of 3.4 in the range of high values of X and of 1.7 in the range of low X. The latter value of the slope became to unity when data were converted to those at iso-free volume state.
Polymers prepared under more oxidative conditions tend to give slightly lower viscosities. Although a definite evidence is lucking, it is considered that the difference in viscosity is attributed to the small degree of branching generated during heating of the polymers.