Structure and Relaxation Properties of Thermally-Modified Aromatic Polyimides for Selective Membrane Separations

摘要

A new class of thermally-modified aromatic polyimides 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 [1,2]. The membranes are cast from soluble aromatic polyimides containing ortho-positioned functional groups on the diamine moiety; exposure of the polyimides to thermal rearrangement (TR) at temperatures above the glass transition (i.e., > 350°C) leads to the formation of 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 gas separation performance initially reported by Park et al [1].