Long time response of aging glassy polymers

Abstract

Abstract:Aging amorphous polymeric materials undergo free volume relaxation, which causes slowing down of the relaxation dynamics as a function of time.The resulting time dependency poses difficulties in predicting their long time physical behavior. In this work, we apply effective time domain approach to the experimental data on aging amorphous polymers and demonstrate that it enables prediction of long time behavior over the extraordinary time scales. We demonstrate that, unlike the conventional methods, the proposed effective time domain approach can account for physical aging that occurs over the duration of the experiments. Furthermore, this procedure successfully describes timetemperature superposition and time -stress superposition. It can also allow incorporation of varying dependences of relaxation time on aging time as well as complicated but known deformation history in the same experiments. This work strongly suggests that the effective time domain approach can act as an important tool to analyze the long time physical behavior of aging amorphous polymeric materials. | P a g e I Introduction:Glassy materials such as amorphous polymers (Hodge 1995), colloidal glasses and gels (Fielding et al. 2000), spin glasses (Rodriguez et al. 2003) are usually time dependent materials owing to thermodynamically out of equilibrium state they are arrested in. Typically specific volume (and specific entropy/enthalpy) associated with an amorphous polymer is in excess compared to its equilibrium value (Donth 2001, Larson 1999, Struik 1978.Inherent tendency of any material to achieve equilibrium initiates structural rearrangement in the amorphous polymeric compounds so as to cause relaxation of specific volume (accompanied by decrease in specific entropy/enthalpy) as a function of time (Callen 1985, Struik 1978. This phenomenon is addressed in the literature as physical aging (Hodge 1995). In polymeric glasses, while taking the material closer to the equilibrium state, physical aging manifests itself by causing enhancement in relaxation time and stiffness as a function of time (Struik 1978). This time dependency continuously changes the material response to the external stimuli such as application of deformation field (stress field or strain field), electric field, etc.Consequently, a priory prediction of the long time behavior of the response function becomes a challenging task. In this work we employ the so called effective time domain approach, which allows direct prediction of long time material response of amorphous polymers by carrying out tests over the practical timescales. | P a g eThe amorphous polymeric materials when quenched rapidly from the molten state undergo glass transition (McKenna 1994). Physical aging initiates in the material subsequent thermal quench (Hodge 1995). The time elapsed since thermal quench is typically known as the aging time or the waiting time.If the material is deformed during this annealing period, response functions (such as creep compliance and relaxation modulus, depending upon ...

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