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

The rheological behavior of five polybutadienes having different microstructures has been characterized using dynamic mechanical, stress relaxation, and viscosity measurements in a temperature range from just below the conventional glass transition to 100 degrees C above it. The data covered a broad enough frequency (time) and temperature range that we were able to characterize the responses in both the glassy and terminal dispersions of the polymers and to address the question of the validity of thermorheological simplicity. Uncritical application of time-temperature superposition principles to these data resulted in reduced viscoelastic responses that cover 12-14 decades in frequency or time. Close examination of the data in the glassy and terminal dispersions 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 dispersion to the temperature dependence of the viscoelastic spectrum. Comparison of the polybutadienes shows differences in glass transition temperature, shape of the segmental relaxation, and fragility that depend on microstructure. Increasing the content of vinyl side groups causes an increase of spectral broadening as well as an increase in fragility-two features which can be related within the coupling model.