Physical 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.

One of the first studies of PNC physical aging was published by Lee andLichtenhan [1998] for epoxy containing w = 0 to 9wt% of polyhedral oligomeric silsesquiox-ane (POSS). The presence of POSS increased Tg and the relaxation time thus, the nanoflller slowed down the molecular dynamics. For amorphous polymers at Tpstructural cluster model. The cluster volume fraction depends on temperature ... 

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... 

A systematic study of the relaxation of rubbing induced birefringence in PS has been conducted. Extensive and clear experimental evidence have been foimd that show the absence of the physical aging effects in the relaxation of RIB, and the relaxation of RIB involves very small length scales. The RIB relaxation is then modeled by a relaxation times distribution function that depends only on temperature but not on thermal or strain history. An individual birefringence elements model has been proposed and a systematic way has been devised to extract the parameters in the model from specifically designed experiments, namely the Temperature Lag measurements and the Continuous Curve measurements. The results predicted by the model agree well with experiments. 

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