Physical ageing temperature

Table 2 Glass transition temperatures Tg, physical aging temperatures prior to corona charging Tage, and aging periods /age of the investigated PPE/PS blend compositions in wt%...

The effect of physical aging on the crystallization state and water vapor sorption behavior of amorphous non-solvated trehalose was studied [91]. It was found that annealing the amorphous substance at temperatures below the glass transition temperature caused nucleation in the sample that served to decrease the onset temperature of crystallization upon subsequent heating. Physical aging caused a decrease in the rate and extent of water vapor adsorption at low relative humidities, but water sorption could serve to remove the effects of physical aging due to a volume expansion that took place in conjunction with the adsorption process. 

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. 

The investigation of the ft transition of PMMA ([1], Sect. 8.1.3) has revealed that physical ageing has an influence on the dynamic mechanical response in the ft-a crossover temperature region. As a consequence, it is worth checking whether it leads to some changes in the plastic deformation behaviour [32]. 

Fig. 13 Stress-strain compression curves of physically aged PMMA obtained at a strain rate of 2 x 10-3 s-1 at various temperatures (From [32])...

Regarding the effect of physical ageing, the associated densification hinders the intramolecular ft-a cooperativity (as discussed in [1], Sect. 8.1.3). This makes the conformational changes more difficult and, so, require a larger stress. The resulting increase of ay corresponds to the situation encountered at a temperature shifted downward by 20 °C. The physical ageing densification does not influence the plastic flow stress. The reason is that the plastic flow happens in a material that has been strained, and it is known that strain induces a disappearance of the physical ageing effects, this is called rejuvenation . 

It is worth pointing out that hindering of the ft-a cooperativity due to physical ageing leads to quite a different temperature dependence of nSSA, as shown in Fig. 27, compared to the case of quenched PMMA shown in Fig. 24. Instead of a plateau, a large increase of nSSA is observed in the temperature range 30-80 °C. 

Fig. 40 Temperature dependence of nSSA of quenched and physically aged PMMA and various CMIMx copolymers...

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