Structural relaxation near the glass transition: Observing kovacs kinetic phenomenology by mechanical measurements

Abstract

This paper presents the structural relaxation behavior near the glass transition temperature (Tg) of poly (phenylene sulfide) (PPS). Since the test temperature is close to Tg, the specimen is able to reach equilibrium after isothermal or nonisothermal temperature jumps in the experimental time scale; as such, the classic phenomenology of structural relaxation into equilibrium can be pursued by mechanical testing (physical aging) in a temperature range near Tg. In our study, physical aging effects were observed by sequential creep test method using a dynamic mechanical analyzer (DMA). The physical aging of PPS near Tg from mechanical testing clearly showed the similar behavior from the classic volumetric responses found by Kovacs in 1960s. The mechanical aging shift factors manifested all of the three "essential ingredients" in the kinetics of structural relaxation which constitute the most physical phenomenology of glassy-forming materials: intrinsic isotherms; asymmetry of approaching equilibrium; and memory effects. The findings in this study offer a valuable approach to investigate the relationship between the mechanical response and thermodynamic properties (volume, enthalpy, etc.) during structural relaxation of glassy materials. ©2010 Society for Experimental Mechanics Inc.