THE MECHANICAL SPECTRA OF BETA-RELAXATION AND SPONTANEOUS DENSIFICATION EFFECTS IN AN AMORPHOUS POLYMER

The dynamic mechanical spectra of shear modulus of poly(methyl methacrylate) have been measured at several temperatures over the frequency range 10(-4)-1 Hz in order to study localized diffusion of chain segments which appears as beta-relaxation. The shape of the spectra of both the real and imaginary components has been analyzed. It is described by a stretched exponential decay function with exponent of 0.18 and it shows nearly 50% change in the modulus over this frequency range. This exponent and the rate of relaxation are remarkably similar to those observed by dielectric methods. A procedure for obtaining the exponent of the decay function and the relaxation strength of the beta-process has been outlined. The strength of the beta-relaxation, or equivalently the number of molecular segments undergoing a thermally activated localized diffusion, decreases on structural relaxation during the isothermal ageing, and the magnitude of the modulus increases. Qualitatively speaking, these effects seem comparable to the effects of an increase in density that normally occurs with decrease in temperature or increase in pressure, and demonstrate that isothermal ageing causes collapse of "soft sites" in a rigid amorphous matrix.