ANALYSIS OF CREEP AND PHYSICAL AGING IN GLASSY-POLYMERS

Static and dynamic techniques were employed to determine tensile creep compliances over wide ranges of time (t = 10(-8)-10(6) s) at 23-degrees-C for the amorphous glassy polymers poly(methyl methacrylate), poly(vinyl chloride) and polycarbonate. Creep data were similarly obtained for semicrystalline poly(butylene terephthalate) that contains a glassy amorphous phase at room temperature. For each polymer, measurements were made on samples of different age, specified by the elapsed time, t(e), between cooling from temperatures above the glass transition temperature, T(g), and the start of the creep. Different empirical functions have been used to model both short-term (t less-than-or-equal-to 0.2t(e)) and long-term (t > 0.2t(e)) data in terms of compliance contributions from overlapping secondary (beta) and glass-rubber (alpha) processes. It is concluded that physical aging produces a decrease in magnitude of the beta-process and an increase in retardation time for the alpha-process, with negligible change in width of the distributions. The range of validity of functions used to model the alpha-region is discussed and comments are made on the structural significance of the results.