Interpretation of differences in temperature and pressure dependences of density and concentration fluctuations in amorphous poly(phenylmethyl siloxane)

Differences in the temperature and pressure dependences of the relaxation times of a slow diffusional process and the α structural relaxation pose an interesting problem. This feature, observed by dynamic light scattering in amorphous poly(phenylmethyl siloxane), is related to another basic feature of lack of thermorheological simplicity discovered by Plazek in polystyrene, poly(vinyl acetate), and amorphous polypropylene. A quantitative explanation based on the predictions of a general coupling theory of relaxations has been found. The coupling theory also predicts the Kohlrausch fractional exponential time correlation function exp[−(tτ*)1−n] at long times, as observed by photon correlation spectroscopy, and crossover to an exponential time dependence exp–(t/τ0) at short times, as frequently assumed in Brillouin scattering. An additional relation between τ* and τ0 predicted by the theory is confirmed also by the experimental data.