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Connie Roth

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Anomalous Physical Aging in Polymer Films

During the past 15 years studies of the physical aging of thin polymer films have frequently observed anomalous aging behavior relative to bulk materials with decreasing film thickness.  The structural relaxation that occurs in non-equilibrium glassy systems leads to a decrease in free volume of the material over logarithmic time scales.  Although these volume contractions are minuscule (<1%), the resulting property changes, termed physical aging, (e.g., mechanical properties and failure modes in particular) can be substantial and often adverse.  As an example, the permeability of gas-separation membranes is exceedingly sensitive to the free volume in the polymer gas-sieving layer.  Decreases in permeability of nearly 50% have been observed as a result of physical aging.
 

Streamlined Ellipsometry Procedure for Characterizing Physical Aging Rates of Thin Polymer Films

      Elizabeth A. Baker, Perla Rittigstein, John M. Torkelson, and Connie B. Roth, J. Polym. Sci., Part B: Polym. Phys. 2009, 47, 2509-2519. [Link]

We have developed a new method of characterizing the physical aging rate of thin polymer films in an efficient manner using ellipsometry.  Ellipsometry measures the change in polarization of light as it is reflected from the sample.  By using Fresnel's equations, it is possible to determine the film thickness and index of refraction of a very thin layer supported on a known substrate (typically silicon).  Using visible light from 400-1000 nm, we are able to measure the film thickness to within a fraction of a nanometer.  In comparison to other methods available for characterizing the structural relaxation of thin films, this method has the advantage that it can be easily applied to different polymers with varying chemical structure.  The structual relaxation of the sample is characterized by measuring the decrease in film thickness and increase in index of refraction resulting from the slow increase in density of the material as a function of time.  We calculate a physical aging rate β from the time-dependent decrease in the film thickness: β = − d(h/h0) / d(log t), i.e., the slope of the data in the figure shown. 
 

Importance of Quench Conditions on the Subsequent Physical Aging Rate of Glassy Polymer Films

      Laura A.G. Gray, Suk W. Yoon, William A. Pahner, James E. Davidheiser, and Connie B. Roth, Macromolecules 2012, 45, 1701-1709. [Link]

For over a decade, research from the gas permeation community has observed faster physical aging rates with decreasing thickness of freestanding films, termed "accelerated aging".  These deviations in the aging rate from bulk behavior occur at film thicknesses of several microns, the largest "confinement" length scale ever reported.  Using our streamlined ellipsometry technique, we have systematically addressed various possible causes of this phenomenon from differences in molecular structure, quench depth below Tg, experimental technique, sample preparation, and stresses on the film.  We have demonstrated that the physical aging of the material is strongly dependent on conditions during the formation of the glassy state.  Although supported films do not display any film thickness dependence to their aging rate at this large micron length scale, films quenched in a freestanding state exhibit a strong thickness dependence.  We suggest that differing quench conditions may impose unintended stresses trapping the glassy films into different states (potential energy minima), dictating the subsequent physical aging rate.  Efforts are underway to investigate the physical aging of glassy polymer films formed under different controlled stress conditions.  [Click here for more information]