The length of cooperativity at the glass transition in poly(vinyl acetate) from the modeling of the structural relaxation process

Abstract

The structural relaxation process of poly(vinyl acetate) has been studied by differential scanning calorimetry. The sample was subjected to different thermal treatments including an isothermal annealing at temperature T a for a time t a. The heat capacity c p( T) was measured during a heating scan. Thus, the experimental results consist of a series of c p( T) curves determined after thermal histories with different values of T a and t a. The experimental results were compared with the prediction of a multiparameter phenomenological model described in references [15,16]. This model follows the evolution of the configurational entropy of the sample during the whole thermal history. The parameters of the model were calculated by simultaneous fits to five c p( T) curves, corresponding to different thermal histories (the parameters of the model have the character of material parameters, independent of the thermal history). This allows the determination of the temperature dependence of the relaxation times and the β parameter of the Kohlrausch-Williams-Watts equation. From these parameters and an estimate of the internal rotational barrier obtained by molecular mechanics calculations, the size of the smallest cooperative rearranging region in the Adam-Gibbs theory was calculated. This result is compared with the length of cooperativity calculated from the temperature fluctuation theory proposed by Donth.