MOLECULAR MOTIONS AND VISCOELASTICITY OF AMORPHOUS POLYMERS NEAR T-G

Rotational correlation times tau(c) for rubrene and tetracene are repo rted near and below T-g in three polymers: polyisobutylene, polystyren e, and Bisphenol A polysulfone. A photobleaching method was used to ob tain 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 func tion for rubrene. tau(c) for a given probe at the T(g)s of the matrice s varies more than 3 decades. For the three polymers studied, probe ro tation times at T-g showed a systematic decrease with increasing matri x T-g. Viscoelastic relaxation times characteristic of the Rouse modes of the matrix polymers are closely related to probe rotation times an d 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 al most constant at T-g. These results suggest that the characteristic le ngth scale for motions associated with the relaxation of the glassy mo dulus varies significantly for the three polymers studied. Trends in t he KWW beta values which describe probe reorientation support this int erpretation.