CHANGES OF THERMODYNAMIC AND DYNAMIC-MECHANICAL PROPERTIES OF POLY(METHYL METHACRYLATE) DUE TO STRUCTURAL RELAXATION - LOW-TEMPERATURE AGING AND MODELING

Structural relaxation of poly(methyl methacrylate) has been studied by differential scanning calorimetric (d.s.c.) and dynamic mechanical te chniques, and the effect of temperature of ageing has been investigate d. Isothermal ageings at temperatures between T-g-30 K and T-g-100 K l ead to d.s.c. pre-peaks and a diminution of the mechanical loss tangen t. The beta relaxation is not affected by ageing and the cc relaxation exhibits a low-temperature tail within the temperature domain of the beta relaxation. A physical model based upon the concept of annihilati on of quasi-point defects through diffusion is used to analyse the res ults. Diffusion is thought to occur by means of hierarchically correla ted movements characterized by a correlation factor b. Simulation of t he calorimetric data is satisfactory and comparison with the dynamic m echanical results underlines a specific mechanism for ageing temperatu res far below T-g.