Brandon's presentation at the North American Membrane Society's (NAMS) 2006 Annual Meeting in Chicago, Illinois, "Tracking Physical Aging of Thin Glassy Polymer Films by Ellipsometry". Co-authors Brandon Rowe, Donald R. Paul, and Benny D. Freeman.
Abstract
Current gas separation membranes are typically formed from glassy polymers because of their exceptional permeability-selectivity properties. Glassy polymers are non-equilibrium materials whose properties (e.g., density, permeability, etc.) spontaneously, but usually slowly, evolve over time towards an equilibrium state. This process is known as physical aging. Interestingly, the physical aging rate becomes orders of magnitude more rapid if the thickness of the film is decreased below about one micron[1]. This phenomenon is an intrinsically fascinating scientific issue, and understanding physical aging is valuable for the gas separation industry.
Commercially used membranes for gas separation have a thickness of approximately 0.1 microns or less and may continue to decrease as the technology for making these films develops. Films ranging from 400 nm to 1000 nm have recently been shown to have rapid and thickness dependent aging rates as determined by gas permeability measurements. In addition to studying the gas transport properties of polymer films, physical aging is being examined by tracking the change in density of polymer films over time using high precision ellipsometry. Ellipsometry is a sensitive optical technique for determining properties of surfaces and thin films by measuring the changes in the polarization state of light when it is reflected from a sample. The refractive index of a polymer sample, measured by ellipsometry, is related to the polymer density through the Lorentz-Lorenz equation. Studies show an increase of polymer density with time, which correlates with the reduction in size and/or concentration of free volume elements in the sample. The effect of humidity on ellipsometry measurements has been studied and a direct relationship between refractive index and relative humidity has been found. The influence of humidity on physical aging has also been investigated.
[1] Huang, Y. and D.R. Paul, Physical aging of thin glassy polymer films monitored by gas permeability. Polymer, 2004. 45(25): p. 8377-8393, 2004 November.