- -~',~, ~T'~ Poh'mer Vol. 36 No. 12. pp. 2379 2387, 1995 Copyright {" 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0032-3861/95/$10.00 + 0.00 Accelerated physical ageing of thin glassy polymer films: evidence from gas transport measurements P. H. Pfromm* and W. J. Korost Department of Chemical Engineering, The University of Texas at Austin, Austin, TX 78712, USA (Received 1 May 1994; revised 28 October 1994) Single-gas permeabilities, selectivities and activation energies of permeation of thin (~ 0.5/~m), intermediate and thick (~25tlm) amorphous glassy polymer films of a polysulfone (Udel 1700) and a polyimide (6FDA-IPDA) were investigated. The films had identical thermal histories and were solvent-free. Evidence of strongly thickness-dependent physical ageing was discovered. Thin films showed decreasing permeabilities and increasing selectivities that surpassed those of intermediate and thick films. Thin films showed increased activation energies of permeation. These results provide evidence for an accelerated approach of thin films to the densified equilibrium state. (Keywords: thin films; ageing; gas separation) INTRODUCTION Thin membranes (of several hundred to several thousand ~ngstr6ms thickness) made from glassy polymers are important for gas separations by membrane permeation 1 3. Physical ageing of glassy polymers is a well known phenomenon 4 6. It is the purpose of this paper to document physical ageing of thin ( ~ 5000 ,~ = 0.5 ]~m) and thick (~ 25/2m) glassy polymer films far below the glass transition temperature T~. According to our results, ageing apparently proceeds much faster in the thin polymer films with a thickness characteristic of gas separation membranes than in thick films. This paper deals with the influence of this apparent accelerated ageing on the gas transport properties. Although the vast majority of work on ageing of glassy polymers has been carried out on samples that are significantly thicker than the active layer of gas separation membranes, some studies have considered ageing of samples with comparable geometry. Braun and Kovacs 7 have studied the volume recovery of glassy polystyrene samples in the form of moulded sheets (thickness 1 ram) and a fibrous powder (thickness 0.1 to I/2m). This is one of the few examples of a direct comparison of thin and thick samples. Only the data for dilatometry in virtually non-sorbing organic liquids and mercury are of interest here, since highly sorbing liquids may influence the results. Physical ageing was studied down to 20'~C below T~ for about 100h. The authors concluded that no significant differences were found for thin and thick samples. Braun and Kovacs' work differs greatly from the work presented here. In the present case, the temperature was at least 150C below T, and ageing * Present address: Institute of Paper Science and Technology, 500 10th Street NW, Atlanta, GA 30318, USA tTo whom correspondence should be addressed was observed over several thousand hours via gas permeation property probes. The work on sorption in small polymer spheres appears to be the most extensive, with a geometry that is comparable to thin films s lo. For the most part, non-Fickian kinetics and swelling-induced effects were the focus of these studies. Evidence of peculiarities in permeation properties of gas separation membranes with thin skins has been reported11 is for integral-asymmetric structures and composites formed from a wide variety of glassy polymers. The apparent active thickness ranged from as low as a few hundred /~ngstr6ms to several thousand gmgstr6ms. The experimental results can be summarized as follows: (1) Pressure-normalized gas fluxes of thin-skinned composite and integral-asymmetric membranes often decrease significantly with time. (2) Simultaneous with the flux reductions, gas selectivities increase with time and exceed the values for thick films made from the same materials. (3) The activation energies of permeation are often higher for thin-skinned membranes than for thick reference films. The interpretation of these results is complicated by the differences in thermal and mechanical history of the membranes and thick reference films. A strong history dependence is, indeed, one of the intriguing features of glassy polymers. In addition, doubts are not easily removed that residual solvents and mechanical tension may be partly responsible for the observed effects in thin-skinned gas separation membranes. It has been hypothesized that physical ageing of glassy polymers may show a thickness dependence, resulting in accelerated densification of thin polymer films. Alfrey and co-workers 1 ~ suggested a diffusive removal of free volume POLYMER Volume 36 Number 12 1995 2379