Influence of Average Free Volume Element Size on the Transport of Gases Through Polymers With Equivalent Total Free Volu
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Influence of Average Free Volume Element Size on the Transport of Gases Through Polymers With Equivalent Total Free Volumes Broderick R. Wilks, Won J. Chung, Peter J. Ludovice, School of Chemical Engineering, Georgia Institute of Technology, Atlanta, Georgia, 30332 Mary E. Rezac, Department of Chemical Engineering, Kansas State University, Manhattan, Kansas 66506 Pavla Meakin and Anita J. Hill, CSIRO Manufacturing and Infrastructure Technology, Private Bag 33 South Clayton MDC 3169, Victoria Australia and School of Chemistry, Monash University, Clayton, Australia. ABSTRACT The permeability coefficients of gases through glassy polymers have been correlated with the fractional free volume (FFV) of the polymers. In general, polymers with high fractional free volumes have high permeabilities while those with low FFV have low permeabilities. This observation is valid for many, but not all materials. This study evaluates the impact of the average size of a free volume element on the permeability of gases through the polymer. Evaluation of the influence of average free volume element size is only possible by employing model systems in which the chemistry and the total free volume are essentially equivalent. In this study, two stereochemical forms of a methyl-substituted polynorbornene were employed. The isomers are chemically equivalent, with similar total free volumes (0.181 versus 0.188). The average defect size was probed using positron annihilation lifetime spectroscopy. The ortho-positronium lifetimes were measured and it was determined that the difference between the two isomers was approximately 10% with the lower-FFV isomer having the larger average lifetime. For simplicity, the two isomers will be termed Pd and Ni (in reference to the catalysts used in their preparation). The Pd isomer has a slightly lower FFV, but larger average defect size. The Ni isomer has a higher FFV, but smaller average defect size. Transport evaluation indicates that the Pd-isomer has gas permeabilities two to three times those of the Ni-isomer. In depth analysis indicates that the increase in permeability is a result of an increase in both the diffusivity of gases through the polymer and gas solubility. INTRODUCTION As the number of possible polymer chemistries that can be produced is essentially infinite, there is considerable interest in applying combinatorial chemistry methods to efficiently test structure-property hypotheses as well as to speed the development of new polymers [1]. Experimental research attempting to correlate the chemical structure of a polymer with its ultimate properties provides a check to the predictions of combinatorial chemistry models. Predictive models relating polymer structure to gas transport properties, specifically gas permeability and ideal permselectivity are prevalent in the field of polymeric gas separation membranes; however, quantitative predictions are still in the development stage [2, 3]. Even though precise predictions are not possible, a few important correlations have appeared. Lee su
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