The role of free volume, hydrogen bonding, and crosslinks on physical aging in polymers of intrinsic microporosity (PIMs)

Abstract

Physical aging is a slow structural relaxation process characteristic of glassy polymers that results in reduced membrane permeabilities. In this study, PIM-1, the archetypal polymer of intrinsic microporosity (PIM), was post-synthetically modified to introduce components that are known to influence physical aging, such as hydrogen bonds and crosslinks. The effects of physical aging were monitored by permeation and sorption experiments, and structural changes were examined by positron annihilation lifetime spectroscopy (PALS) and other characterization techniques. The results suggest that higher initial fractional free volume is the primary factor contributing to higher rates of physical aging and that the introduction of hydrogen bonds and crosslinks reduces the initial free volume of PIM-1. In contrast, structural factors such as hydrogen bonds and crosslinks were the key factors in determining how permselectivity changed with physical aging. This study provides useful structure–property correlations and design principles related to free volume, hydrogen bonds, and crosslinks on physical aging behavior of microporous polymer membranes.