Rheological Studies of Amorphous Polystyrene Above T _g

Abstract

Studies of the relationship of macroscopic properties to internal structure have been at the forefront of polymer science for many years. We are now in a position to use rheological studies as a method of elucidating internal structure and internal structural distortion mechanisms. This paper will use this approach to determine the characteristics of transitions above the glass transition temperature, T _g , in amorphous polymers with the objective of understanding the structure of these polymers and the mechanisms involved in the transitions.

It has been demonstrated that a transition, T _ll , can be observed well above T _g by many different rheological measurements. These include: a significant change in the elastic modulus over a narrow temperature range, a large change in the recoverable strain characteristics of the melt at the temperature of the transition, an abrupt change of the temperature dependence of the yield stress, an abrupt change in the temperature dependence of the steady state flow stress, and a very great change of the stress relaxation characteristics at the transition temperature.

Of all of these rheological measurements the stress relaxation behavior seems to permit the greatest opportunity for understanding the structural mechanisms involved in the transition. Data will be presented on the relationship between the temperature of the transition and the rate of application of strain prior to measurement of the stress relaxation behavior. This will show that T _ll is rate dependent and extrapolation to zero rate of straining gives an intrinsic T _ll that is well above T _g . Data will also be presented that indicates T _ll is much more easily observed if a large strain magnitude is applied before stress relaxation behavior is studied. This does not mean that T _ll is associated only with large strains.

New results will be presented based on stress relaxation at temperatures below T _ll that indicate that as the test temperature is increased the longer relaxation times disappear as the transition region is approached, then with further increase in test temperature a new set of much longer relaxation times appear and then disappear at the upper end of the transition temperature range.

A model based on temporary network rubber elasticity is presented that may explain this rather unexpected behavior.