MOLECULAR MOTIONS AND VISCOELASTICITY OF AMORPHOUS POLYMERS NEAR T-G

Rotational correlation times tau(c) for rubrene and tetracene are reported near and below T-g in three polymers: polyisobutylene, polystyrene, and Bisphenol A polysulfone. A photobleaching method was used to obtain tau(c) values from 10(-1) to 10(3) s. In each polymer matrix, the orientation autocorrelation function for tetracene (the smaller probe) decays more rapidly and less exponentially than the correlation function for rubrene. tau(c) for a given probe at the T(g)s of the matrices varies more than 3 decades. For the three polymers studied, probe rotation times at T-g showed a systematic decrease with increasing matrix T-g. Viscoelastic relaxation times characteristic of the Rouse modes of the matrix polymers are closely related to probe rotation times and also not constant at T-g. Thus T-g is not an isolocal mobility state for molecular motions on a fixed length scale. On the other hand, the viscoelastic relaxation time associated with the glassy modulus is almost constant at T-g. These results suggest that the characteristic length scale for motions associated with the relaxation of the glassy modulus varies significantly for the three polymers studied. Trends in the KWW beta values which describe probe reorientation support this interpretation.