TRACER DIFFUSION AT THE GLASS-TRANSITION

Diffusion coefficients D of photoreactive dye molecules having different sizes and flexibilities were measured in the glass formers polystyrene (PS), poly(ethylstyrene) (PES), poly(methyl methacrylate) (PMMA), poly(ethyl methacrylate) (ΡΕMA), polycarbonate (PC), and tri-α-naphthylbenzene (TNB). The D values were determined by forced Rayleigh scattering from the diffusive decay of holographic gratings in a range ~10-17-10-7 cm2 s-1. At temperatures above the glass transition Tg, the D values could be fitted to the WLF equation of the matrix after introducing a coupling that depends upon tracer solute and matrix properties. At Tg, the D values of the same solute molecule in different glasses vary from 6.3 X 10-12 cm2 s-1 in PC to 7.9 X 1(T16 cm2 s-1 in TNB. Above and below Tg, the temperature dependence of D is between that of the α- and the β-processes of mechanical relaxation. Physical aging of D was observed below Tg and could be quantitatively related with volume retardation. © 1990, American Chemical Society. All rights reserved.