Aging relaxation temperature dependence

A unified approach to the glass transition, viscoelastic response and yield behavior of crosslinking systems is presented by extending our statistical mechanical theory of physical aging. We have (1) explained the transition of a WLF dependence to an Arrhenius temperature dependence of the relaxation time in the vicinity of Tg, (2) derived the empirical Nielson equation for Tg, and (3) determined the Chasset and Thirion exponent (m) as a function of cross-link density instead of as a constant reported by others. In addition, the effect of crosslinks on yield stress is analyzed and compared with other kinetic effects — physical aging and strain rate. 

Experimental data are presented to show that the JG relaxation mimics the structural relaxation in its volume-pressure and entropy-temperature dependences, as well as changes in physical aging. These features indicate that the dependences of molecular mobility on volume-pressure and entropy-temperature have entered into the faster JG relaxation long before structural relaxation, suggesting that the JG relaxation must be considered in any complete theory of the glass transition. 

Figure 3.1. Schematic illustration of temperature dependences of the specific volumes of amorphous materials. This figure also illustrates the effects of the nonequilibrium nature of glass structure, which results from kinetic factors. Glass 1 and Glass 2 are specimens of the same polymer, but subjected to different thermal histories. For example, Glass 1 may have been quenched from the melt very rapidly, while Glass 2 may either have been cooled slowly or subjected to volumetric relaxation via annealing ( physical aging ) in the glassy state.

This aspect was addressed in a model developed by Kovacs et al. [1979], which assumed that aging involves a distribution of relaxation times with multiple relaxation processes and that each relaxation time depends on the temperature and the glass structure. The model does not attempt to curve-fit the heat capacity data but, instead, uses a peak shift method [Hutchinson, 1992], which removes the need to assume a particular distribution of relaxation times. 

Water and low temperature (20-35 C) aging influence the dynamic mechanical properties of a poly(amide-imide). At concentrations below 2 weight percent, water contributes to a low temperature relaxation between -120 and -50 C. Above 2 weight percent the water influences the beta transition. The enthalpy of activation for the beta relaxation is dependent upon aging temperature and time. Aging temperatures closer to the beta transition temperature result in higher activation enthalpies for that dispersion. 

As stated in a preceding section, physical aging starts when a polymer is quenched to a temperature below its T, irrespective of the mechanical loading on the material. Therefore, when a sub-Tg polymer is subjected to mechanical load, two processes occur simultaneously (1) physical aging and (2) stress relaxation or creep associated with mechanical load or deformation. Each of these processes has its own unique time and temperature dependence and hence, both effects must be accounted for in the long... 

Novel mechanical tests are being developed and shown to be useful tools to investigate thermal and structural relaxation. These include the modulated-temperature-thermomechanometry technique (mT-TM) which has been recently employed to investigate blends of core cross-linked (CCS) PS and PMMA (Spoljaric et al. 2011) and nanomechanical thermal analysis. By using silicon microcantilever deflection measurements, the latter technique can provide a measure of temperature-dependent thermal stresses and therefore investigate the influence of physical aging (Yun et al. 2011). 

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