THE EFFECT OF THERMAL-DEGRADATION OF POLY (METHYL METHACRYLATE) ON THE GLASS-TRANSITION AND RELAXATION-TIME SPECTRA

Samples of PMMA with different molecular masses exceeding a certain cr itical magnitude were found to show different glass transition tempera tures, although in this region of molecular masses, the glass transiti on temperature was commonly assumed to be independent of molecular mas s. The observed phenomenon was explained in terms of different microst ructures of PMMA samples. It was demonstrated that for PMMA with diffe rent molecular masses, the parameter of the Kohlrausch function, descr ibing the width of relaxation time distribution, was constant at tempe ratures up to 140-degrees-C. At higher temperatures this parameter dec reased; this pattern contradicts the data reported for other polymers. At temperatures above 140-degrees-C, depolymerization of PMMA occurs. Therefore, the reduced Kohlrausch parameter may be attributed to the structural changes resulting from depolymerization, such as increased free volume (due to the higher concentration of chain ends) and change s in microstructure.