Characterization of Thermally-Modified Polyimides for Use in Pervaporations and Gas Seperations

A new class of thermally-modified aromatic polyimides and copolymers has been identified as a viable membrane material for separation processes requiring high permeability and selectivity in combination with intrinsic thermal and chemical resistance properties. The membranes are cast from soluble aromatic polyimides and copolymers containing ortho-positioned functional groups on the diamine moiety; exposure of the polyimides to thermal rearrangement (TR) at temperatures above the glass transition leads to the formation of fully-aromatic polybenzoxazoles (PBO's) with exceptional thermal and chemical resistance. The thermal exposure step produces fundamental changes in molecular connectivity that alter chain packing, resulting in a unique distribution of free volume elements at the angstrom scale. It is the distinctive shape and distribution of these elements that appear to be responsible for the unprecedented separation performance observed in these polymers. In this study, we examine the dynamic relaxation properties of the API polymers as a function of backbone structure and degree of thermal rearrangement. Specifically, dynamic mechanical analysis and broadband dielectric spectroscopy are used to elucidate the sub-glass and glass-rubber relaxation characteristics of the polymers as related to their structural details and thermal exposure history; the information obtained through these methods provides insight as to the relative flexibility of the membranes, their local relaxation environment and corresponding free volume, and the influence of thermal rearrangement on segmental mobility and ultimate membrane separation performance.