Physical aging effects

The three polymers studied display remarkable physical aging effects [24], with a strong increase of modulus and changes in the location and intensity of the glass transition. This phenomenon is exemplified in Fig. 17 where the changes of dynamic moduli in a PDEB sample aged for 14 months are very apparent. 

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 . 

Royal, J. S., and Torkelson, J. M., Physical ageing effects on molecular-scale polymer relaxations monitored with mobility-sensitive flnorescent molecules. Macromolecules, 26, 5331-5335 (1993). 

The Absence of Physical Aging Effects in the Surface Region of Glassy Polymers... 

This part is divided in the following way. After Introduction (Section-1) and Experiments (Section-2), experimental evidence to prove the absence of physical aging effects on the relaxation of RIB is presented in section-3. In section-4 a phenomenological model based on a generic energy barriers distribution is presented. In section-5 extensive experimental results are analyzed based on the model. The model provides quantitative interpretations that agree very well with all the reported experimental results, which in turn further confirms the absence of physical aging effects. And in section-VI we summarize the main conclusions. 

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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