Vliv doby stárnutí na objemovou a entalpickou odezvu v oblasti Tg u PMMA při jeho ohřevu

Abstract

Objemová a entalpická odezva PMMA v průběhu ohřevu přes Tg region je měřena jako funkce doby relaxace pod Tg. Modelování je založeno na nedávno vyvinutém termodynamickém modelu postaveném na nelineární viskoelastické teorii (Caruthers JM, et al. Polymer 2004;45:4577-4597; Adolf DB, et al. Polymer 2004;45:4599-4621). Tento model byl doplněn o operativní model. Tyto odezvy jsou dobře popsány co do vlastní polohy i velikosti.

In this paper we model the volumetric and enthalpic response of a-PMMA during heating at constant rate through the glass transition region following an aging period of varying length. The modeling is based on a recently developed thermodynamically consistent non-linear viscoelastic theory of thermal and mechanical behavior of glassy polymers in the glass transition range (Caruthers JM, et al. Polymer 2004;45:4577-4597

Adolf DB, et al. Polymer 2004;45:4599-4621). The original model is modified by replacing the stretched exponential by a relaxation function based on a cooperative model where the elementary events tend to occur in clusters due to mutual induction reminiscent of the mechanism underlying B-E statistics. The modified approach is found to produce good agreement with the observed temperature dependence of specific volume and enthalpy. Also the peaks in thermal expansivity and heat capacity are well described, both with regard to their intensity and position along the temperature axis.