Effect of Different Quench Condition and Stress During Vitrification on the Physical Aging of Glassy Polymer Films

摘要

Since the mid-1990s a number of studies, primarily from the gas permeation community, have observed faster physical aging rates with decreasing film thickness, termed "accelerated aging". These deviations in aging rate from bulk behavior occur at large micron length scales (～4000 nm in thickness) representing one of the largest "confinement" length scales ever reported. These studies contradict the vast majority of the literature on confinement effects, which demonstrate changes in the glass transition temperature (T_g) and physical aging below only 100-200 nm in thickness. We have recently addressed the various differences between these two conflicting bodies of work. After systematically addressing differences in chemical structure (stiff aromatic backbone polymers vs. flexible C-C backbone polymers), experimental technique, and sample preparation factors, we have concluded that the likely cause of the accelerated aging behavior observed at micron length scales results from differences in stress imparted to the films during thermal cooling. We propose that these unintended stresses during vitrification can trap the glassy polymer into different states (potential energy minima) that dictate the subsequent physical aging rate of the material.