Ultrasonic absorption and dielectric loss in heterophase block copolymers

A styrene–butadiene–styrene block copolymer (SBS) and a plasticized SBS were studied as a function of temperature by an ultrasonic wave propagation technique at 9 MHz. Two absorption maxima were found for each of these polymers, one being attributable to the primary glass transition of the polybutadiene blocks and the other to that of the polystyrene blocks. The SBS was cast from two different solvents, namely benzene and tetrahydrofuran–methyl ethyl ketone. Parallel dielectric loss measurements were also made of the SBS in the frequency range of 50–10 5 Hz. Relaxation temperatures determined from the ultrasonic and dielectric loss maxima over a range of measurement frequencies can be correlated by an Arrhenius‐type equation. The polystyrene loss peak in the ultrasonic data was found to be much weaker than the polybutadiene loss peak. However, these two peaks were of comparable magnitude in dielectric data. This observation was interpreted as being due to the onset of structured–unstructured (heterophase to homogeneous) transitions at sufficiently high temperatures. Ultrasonic data were also compared with low‐frequency dynamic mechanical data (11 Hz) and stress relaxation data (10 2 –10 5 sec) through the use of simple time–temperature superposition principle. Considerable discrepancies were found by using this principle, indicating that the heterophase SBS block copolymer was thermorheologically complex.