Scott presents his paper entitled, "Effect of Crosslinking on Gas Transport Properties of High Free Volume Gas Separation Materials" at the 2004 North American Membrane Society (NAMS) meeting. Co-authors Scott Kelman and Benny D. Freeman.
Abstract
High free volume polymer membranes are often very weakly size-sieving and, consequently, can remove large gas or vapor molecules from a gas mixture with smaller molecules. This capability finds application in reverse-selective gas separations such as VOC removal from permanent gas streams and monomer recovery from the exhaust of polymerization reactors. Poly(1-trimethysilyl-1-propyne) (PTMSP) is a stiff chain, high free volume glassy polymer well known for its very high gas permeability [1]. PTMSP also has outstanding vapor/gas selectivity. For example, the n-C4H10/CH4 mixed gas selectivity is 30, which is the highest value ever reported for this gas pair effects [2]. This makes PTMSP an interesting material for vapor/gas separations.
However, gas permeabilities in PTMSP are sensitive to processing history and time [1]. PTMSP undergoes significant physical aging, which is the gradual relaxation of nonequilibrium excess free volume in glassy polymers [3]. PTMSP is also soluble in many organic compounds leading to potential dissolution of the membrane in the process streams where separation properties are of greatest interest. These processes compromise the practical utility of PTMSP. Studies have been performed to slow the aging process in PTMSP. For example, Jia et al. [4] crosslinked PTMSP with bis azides in an effort to stabilize the large excess free volume elements. They found that physical stability of crosslinked PTMSP was achieved at the expense of reduced O2 and N2 permeability.
The effect of crosslinking PTMSP on aging behavior and transport properties of large organic molecules are presented. Crosslinking is successful in maintaining the permeability and vapor/gas selectivity of PTMSP over time. N2, O2, CH4 permeability values were constant over 100 days, while n-butane permeability increased for a crosslinked PTMSP membrane. The chemical resistance of PTMSP is strongly enhanced by crosslinking. For example crosslinked PTMSP is insoluble in common PTMSP solvents such as toluene and cyclohexane. The reaction between the bis azide crosslinker and PTMSP was observed using FTIR and XPS analysis. Crosslinking reduced the FFV of the polymer and therefore permeability decreased. Initial nitrogen permeability in crosslinked PTMSP was a factor of 4 less than pure PTMSP. Nanoparticles such as fumed silica and POSS particles were added to the crosslinked polymer to counteract the decrease in permeability.
[1] K. Nagai, T. Masuda, T. Nakagawa, B. D. Freeman and I. Pinnau, "Poly(1-Trimethylsilyl-1-Propyne) and Related Polymers: Synthesis, Properties and Functions", Progress in Polymer Science, 26 (2001) 721-798.
[2] I. Pinnau and L. G. Toy, "Transport of Organic Vapors through Poly(1-Trimethylsilyl-1-Propyne)", J. Membrane Sci., 116 (1996) 199-209.
[3] L. C. E. Struik, "Physical Aging in Amorphous Polymers and Other Materials", Elsevier, Amsterdam, 1978, pp. 7-9.
[4] J. Jia, Ph.D. Thesis, Michigan State University, 1997.