Physical ageing of amorphous polymers. Theoretical analysis and experiments on poly(methyl methacrylate)

Structural relaxation of glassy poly(methyl methacrylate) (PMMA) was studied with several techniques: differential scanning calorimetry (DSC), volumetry by thermomechanical analysis (TMA) and mechanical spectroscopy. A theory is proposed in order to interpret the data in a unique theoretical framework. This theory is developed on the basis of two main physical assumptions: (i) the existence in amorphous polymers of quasipunctual defects whose concentration decreases during structural relaxation, and (ii) the occurrence of hierarchical correlation effects conditioning molecular mobility. As a first approximation a self‐consistent semiquantitative description of the results is obtained, i. e., the experimental results obtained for the different quantities on PMMA can be accounted for employing always the same set of model parameters. However, it seems that a significant difference exists between the relaxation times associated with the different experimental techniques.