(748a) Structure and Relaxation Characteristics of Thermally-Modified Aromatic Polyimides for Selective Separations

A new class of thermally-modified aromatic polyimides has recently been identified as a promising membrane material for processes requiring high permeability and selectivity in combination with intrinsic thermal and chemical resistance properties [1]. These membranes are based on soluble aromatic polyimides with ortho-positioned functional groups; exposure of the polyimides to thermal rearrangement (TR) at 350°C to 450°C leads to fully-aromatic, insoluble polybenzoxazoles (PBO) with exceptional thermal and chemical resistance characteristics. The thermal conversion step produces fundamental changes in molecular connectivity and conformation that alter chain packing, resulting in a narrow distribution of free volume elements and unique ?hourglass? shaped cavities. It is the distinctive shape and distribution of these cavities that appear to be responsible for the unprecedented gas separation performance reported by Park et al [1].

A series of aromatic polyimides (API) were synthesized based on the reaction of diamine-dianhydride pairs intentionally selected to introduce ortho-positioned functional groups along the polymer backbone; imidization of the API precursors was achieved by two different routes (chemical vs. thermal imidization), leading to the introduction of ?OCOCH₃ and ?OH ortho functional groups, respectively. These polymers were then subject to thermal exposure at discrete temperatures in the range 350°C to 450°C in order to achieve thermal rearrangement to polybenzoxazoles.

In this paper, 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 polymers, their local relaxation environment and corresponding free volume, and the influence of thermal rearrangement on segmental mobility. Further, the contrasting nature of the dynamic mechanical and dielectric probes can be used to more precisely establish those structural elements encompassed by a given relaxation process.

The polymers display thermomechanical relaxation characteristics common to most polyimides (see, for example, characterization of commercial Matrimid® polyimide [2]). Three relaxations are observed with increasing temperature: γ and β sub-glass relaxations, and the glass-rubber (α) relaxation. The ability of the API materials to undergo thermal rearrangement is governed by the glass transition temperature of the as-synthesized polymer, and dynamic mechanical sweeps beyond T_g can be used as a direct and sensitive means to monitor the progress of the TR conversion as a function of time and temperature. For those polymers exposed to conditions consistent with a high degree of thermal rearrangement, nearly complete suppression of the glass transition process is observed, with the resulting PBO structures showing high-modulus, glassy behavior up to 500°C. While the low-temperature γ transition appears to be unaffected by thermal rearrangement, the sub-glass β process is observed to decrease in intensity and shift to lower temperatures, possibly reflecting motions of a more compact, less cooperative character. These results can be correlated with the static properties of the TR polymers (e.g. fractional free volume) and separation characteristics for both gas separations and pervaporation applications.

[1] H. B. Park, C. H. Jung, Y. M. Lee, A. J. Hill, S. J. Pas, S. T. Mudie, E. Van Wagner, B. D. Freeman and D. J. Cookson, Science 318 (2007) 254-258.

[2] A.C. Comer, D.S. Kalika, B.W. Rowe, B.D. Freeman and D.R. Paul, Polymer 50 (2009) 891-897.