Effect of Aging Time on a Glass Transition of Amouphous Polymers at Heating
Artikel i vetenskaplig tidskrift, 2014
The relaxation and consolidation processes accompanying the glass transition are linked in this paper with an induction mechanism introducing a cooperative character of relaxation into those processes. The induction mechanism supplements the proposition underlying the internal energy material clock used in a recently developed thermodynamically consistent non-linear viscoelastic theory modeling thermal and mechanical behavior of amorphous polymers in the glass transition range(1,2). The mechanism is introduced into the constitutive theory framework where it replaces the stretched exponential relaxation functions. The proposed modification is used to describe the dilatometric behavior of PMMA during heating from the glassy state after varying aging times. The modified theoretical approach is found to produce a fairly good description of the peaks in thermal expansivity, both with regard to their intensity and position along the temperature axis. The length of the aging period in the glassy state is found to affect the material time scale while the relaxation functions are unchanged.