Glassy polymers are non-equilibrium materials that will spontaneously, but usually slowly, change over time towards an equilibrium state by a process known as physical aging. The physical aging rate becomes orders of magnitude more rapid when film thickness is decreased below one micron, indicating a strong influence of the free surface on aging behavior.1 This phenomenon is an intrinsically fascinating scientific issue, and understanding physical aging has broad impacts in several technologies including gas separation materials, sensors, and microelectronics.

New insight regarding the mechanisms behind physical aging and the role of the free surface can be gained by studying the free volume profile in polymer films during the aging process. Positron annihilation lifetime spectroscopy (PALS) is capable of determining the size and concentration of free volume sites in polymer systems by measuring the lifetime of injected positrons. 2 The coupling of PALS with a variable mono-energetic positron beam source has resulted in a relatively new technique which allows the energy of the incident positron beam, and, therefore, penetration depth, to be controlled.3 This technique allows the free volume properties to be studied as a function of distance from the surface. This research will help provide a better fundamental scientific understanding of why aging rate depends on thickness, particularly at the molecular level.

The effect of physical aging on free volume and its distribution across the thickness of thin (l ~ 450 nm) polysulfone (PSF) films was investigated using variable energy PALS. This study is the first report using variable energy PALS to investigate physical aging in glassy polymers. Previous work has typically been completed using films without well defined thermal histories. The concentration and average size of free volume elements were measured at 18 different energies, probing across the entire thickness of each sample. The data show the average free volume element size is reduced near the film surface (up to 50 nm deep) as compared to the interior of the film. Reduced free volume size near the surface indicates that the near-surface layer aged more rapidly than the film interior. The overall free volume element size decreased with aging, with no significant change in concentration. These results are consistent with accelerated physical aging in thin films tracked by gas permeability measurements. The influence of exposure to high pressure CO2 on thin film free volume properties was also examined.

(1) Huang, Y.; Paul, D. R. Polymer 2004, 45, 8377-8393.

(2) Mallon, P. E. In Positron & Positronium Chemistry; Jean, Y. C.; Mallon, P. E.; Schrader, D. M., Eds.: World Scientific, New Jersey, 2003; pp 253-280.

(3) Jean, Y. C.; Cao, H.; Dai, G. H.; Suzuki, R.; Ohdaira, T.; Kobayashi, Y.; Hirata, K. Applied Surface Science 1997, 116, 251-255.