Physical model explaining the effect of physical ageing on dynamic mechanical and calorimetric properties of poly(methyl methacrylate)

Using the theory of non-elastic deformation based on a specific model for the mobility in amorphous polymers, an equation was obtained including the real and imaginary parts of the complex modulus and the heat capacity as a function of temperature and ageing time. Thus, the model is based on the calculation of time and temperature evolution of the population of “faults” (corresponding to an excess of enthalpy and entropy) in the structure. We employ the model with only one “characteristic” relaxation time obtained from a concept of hierarchical correlation. The parameters appearing in the equations for G* and c_p are the same in each case and are obtained from classical experiments (mechanical spectroscopy, dilatometry, calorimetry, etc.), thus having a specific physical meaning. Dynamic loss tangent and specific heat of PMMA measured after different thermal histories involving isothermal ageing agrees quite well with the prediction of the model.