(57b) Relaxation Characteristics of Thermally-Modified Aromatic Polyimides and Copolymers Designed for Selective Separations

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 (i.e., > 350°C) 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 gas separation performance reported by Park et al [1].

A series of aromatic polyimides (API) was synthesized based on the reaction of ortho-functionalized diamine-dianhydride pairs intentionally selected to foster high permeability; conversion of the API precursors was achieved by either chemical or thermal imidization.  In select cases, the extent of functionalization (and the ultimate degree of thermal rearrangement) was controlled by the introduction of varying amounts of non-functional diamine co-monomer.  API test specimens were cast from solution and then subject to thermal exposure at discrete temperatures in the range of 350°C to 450°C in order to achieve thermal rearrangement to polybenzoxazoles.

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 separation performance.  Further, the contrasting nature of the dynamic mechanical and dielectric probes can be used to more precisely establish the structural origins and mechanisms associated with each relaxation process.

The polymers display thermomechanical relaxation characteristics common to most polyimides [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 in-situ 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 the point of degradation.

Investigation of the sub-glass relaxations reveals a sensitivity to moisture, and shifts in relaxation position and intensity with thermal rearrangement.  In all samples, the detection of the lowest-temperature γ relaxation is contingent upon the presence of ambient moisture in the sample; rigorous drying of the polymer prior to measurement leads to full removal of the γ relaxation response, which is restored with subsequent exposure to ambient conditions.  This behavior can be correlated to the nature of the ortho functional groups, and the apparent disruption of intermolecular (i.e., polymer-polymer) interactions upon adsorption of water molecules [3].  For the β relaxation, increasing extents of TR conversion result in a decrease in viscoelastic relaxation intensity and peak temperature, reflecting motions of a more compact, less cooperative character.

[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.

[3] H. Ohya, V.V. Kudryavtsev, S.I. Semenova.  Polyimide Membranes: Applications, Fabrications and Properties (Chapter 3); Gordon and Breach Science Publishers: Amsterdam, 1996.