Structural Relaxation of Poly(methyl methacrylate)

Abstract

The kinetics of structural, relaxation of poly(methyl methacrylate) (PMMA) is studied and it is shown that the activation energy of the process depends on the structural temperature Tf. A new method is proposed allowing the measurement experimentally of how the activation energy decreases with increasing Tf. The dependence of the glass transition temperature Tg on the heating rate q+ is investigated for samples that were previously cooled down at a rate q = 12.5 K/min. It is found that the activation energy for structural relaxation is E = (250 ± 10%) kJ/mol at T=386K. On the other hand, the value of the apparent ”activation energy” obtained from the dependence of the structural temperature Tf on the cooling rate q- is considerably higher. This is an indication that the activation energy depends on Tf. We found that the temperature dependence of the activation energy of PMMA in the vitrification interval is E(T) = 250–670* (386/T-1) kJ/mol.