The effects of molecular orientation on the physical aging behavior of oriented glassy amorphous polymers
The objective of this study was to determine whether molecular orientation has an effect on the rate of physical aging in amorphous glassy polymers. There is already a large body of literature concerning the phenomenon of physical aging, although the vast majority has been directed toward isotropic, unoriented systems. The importance of this study is therefore twofold: first, from a theoretical standpoint, a better understanding of physical aging in oriented systems will help to elucidate the physics of glassy relaxation which is important since the exact mechanism behind physical aging are still unknown. Second, from an engineering standpoint, a knowledge of the orientation aging relationship will help the designer/engineer with product development since many commercially produced plastic items have some degree of orientation present as a result of the processing methods involved.
To measure the aging behavior, samples of bisphenol A polycarbonate and atactic polystyrene were hot drawn (i.e. stretched above Tg) to varying stretch ratios and the degree of orientation quantified using birefringence and the Herman’s orientation function, f. Physical aging rates were determined as a function of fusing volume and linear dilatometry, mechanical creep measurements, DSC, and tensile properties. The molecular state, including the free volume, of the oriented polymers was quantified using Positron Annihilation Lifetime Spectroscopy (PALS), oxygen permeability/diffusion measurements, dynamic mechanical analysis, DSC, and density measurements. The data indicate that physical aging rates are influenced by orientation but the degree varies with the method of testing. Volume relaxation rates were approximately 50% higher for the oriented samples, however, mechanical shift rates determined from the creep data showed a slight decrease with orientation. Further analysis shows that the effective relaxation/retardation times decrease significantly with orientation even though the free volume--as determined by density and PALS measurements--also decreases. This implies a serious deficiency in the free volume theory for molecular mobility. Implications for these findings and possible explanations for this behavior are discussed.