RHEOLOGICAL INVESTIGATION OF POLYBUTADIENES HAVING DIFFERENT MICROSTRUCTURES OVER A LARGE TEMPERATURE-RANGE

The rheological behavior of five polybutadienes having different micro structures has been characterized using dynamic mechanical, stress rel axation, and viscosity measurements in a temperature range from just b elow the conventional glass transition to 100 degrees C above it. The data covered a broad enough frequency (time) and temperature range tha t we were able to characterize the responses in both the glassy and te rminal dispersions of the polymers and to address the question of the validity of thermorheological simplicity. Uncritical application of ti me-temperature superposition principles to these data resulted in redu ced viscoelastic responses that cover 12-14 decades in frequency or ti me. Close examination of the data in the glassy and terminal dispersio ns shows that the temperature shift factors required to superpose the data in the two regions are, however, different. Such a deviation from thermorheological simplicity can be analyzed within the framework of the coupling model of Ngai(1-3) that relates the shapes of the dispers ion to the temperature dependence of the viscoelastic spectrum. Compar ison of the polybutadienes shows differences in glass transition tempe rature, shape of the segmental relaxation, and fragility that depend o n microstructure. Increasing the content of vinyl side groups causes a n increase of spectral broadening as well as an increase in fragility- two features which can be related within the coupling model.