Gas transport in poly(silylpropynes): the chemical structure point of view

Abstract

Poly[l-(trimethylsilyl)propyne] (PTMSP) has the highest gas permeability of any known polymer.1 Understanding the structural origins of the exceedingly high gas permeability of PTMSP should facilitate the design of other highly gas permeable glassy polymers. Results of a systematic investigation of a homologous series of closely related polymers prompt us to offer a structure/property rationale to explain the high gas permeability of PTMSP. The following polymers were synthesized: poly[l-(n-alkyldimethylsilyl)propyne];A-E),poly [l-(isopropyldimethylsilyl)propyne];F),poly [l-(phenyldimethylsilyl)propyne];G),and poly[l-[p-(trimethylsilyl)phenyl]propyne]; H). Polymer conformation is herein discussed in relation to the steric requirement for main-chain conjugation, dynamic mechanical spectra, and a proposed mechanism of polymerization. The critical surface tension of pendant groups was used to qualitatively rationalize the intermolecular interactions for poly(silylpropynes). Gas permeability (P) was measured. For polymers A-E, as the chain length of the n-alkyl group decreases, the gas permeability increases. The large differences in the oxygen permeability coefficient, P(02), for poly[l-(isopropyldimethylsilyl)propyne] (F) and poly[l-(npropyldimethylsilybpropyne] (C) as well as those between polymer G and polymer H can be explained on the basis of the subtle but important London forces of side-chain association. Our conclusion is that the extraordinarily high gas permeability of PTMSP is a consequence of large excess free volume within the polymer matrix which is itself ascribed to (1) the rigid and irregular main-chain conformation and (2) the low interchain attraction force of the side chains which shield the main chain.

The upper bound revisited

“…At equivalent pore diameter, there will be some variation in the permeability value of the fast gas. However, at large pore diameter, the permeability values are in reasonable proximity of each other as noted for PTMSP where variation in permeability range from 6500 barrers for He to 33,000 barrers for CO 2 [47]. This is at least a consequence that the solubility selectivities are the reverse of the diffusion selectivities for the gas pairs shown except in the case of O 2 /N 2 and CO 2 /CH 4 .…”

Side-chain engineering of ladder-structured polysilsesquioxane membranes for gas separations

“…For instance, when the isopropyl-based poly(dialkylacetylenes) of poly(4-methyl-2-pentyne) (PMP) was substituted by longer alkyl groups (i.e., CH 2 (i-C 3 H 7 ) or C 2 H 4 (i-C 3 H 7 )), both FFV and gas permeability were reduced compared to those of pristine PMP [37]. It is noteworthy that the high polymer chain packing with linear side chain length results from enhanced flexibility of alkyl spacer group [37][38][39].…”

Predictive calculation of hydrogen and helium solubility in glassy and rubbery polymers

“…In many industrial processes, hydrogen needs to be recovered from purge gas streams or have its composition adjusted for syngas feeds [3]. Similarly, helium can be recovered from natural gas [4,5,6]. The issue of hydrogen sorption in polymers is of interest for the optimization of energetic devices, such as fuel cells and batteries [7].…”