A BRILLOUIN-SCATTERING INVESTIGATION OF RELAXATION VERSUS CROSS-LINK DENSITY IN GLASS-FORMING AND GEL-FORMING POLYMERS

A Brillouin scattering study of the relaxational processes above the c alorimetric glass transition temperature T-g (in the 1.2-1.6 T-g tempe rature range) on polyurethane gels is presented. The specificity of th is work is to investigate several samples corresponding to various cro sslink densities. The relaxational process observed by Brillouin scatt ering is compared to low-frequency measurements of the relaxational pr ocesses by other techniques: in contrast to low-frequency determinatio ns, the Brillouin scattering relaxation time exhibits an Arrhenius beh aviour for all the samples; however, the characteristic parameter of t he time distribution width is the same for the two processes. The low- frequency relaxation and the high-frequency Brillouin scattering relax ation seem to merge for temperatures higher than 1.6 T-g and these dat a are discussed on the basis of different models for the glass transit ion. The influence of the crosslink density yields an increase of the mean relaxation time with increasing connectivity and small variations of the apparent activation energy. In contrast, the distribution time width is not composition dependent. Nevertheless, the most striking r esults are that a rescaling to T-g fully accounts for the variations o f the mean relaxation time with the crosslink density and a correspond ence between the crosslink density and the inverse of the temperature is suggested.