Influence of Side-Chain Length on the Gas Permeation Properties of Poly(2-alkylacetylenes)

The pure-gas permeation properties for a systematic series of disubstituted, amorphous, glassy poly(2-alkylacetylenes), i.e., poly(2-hexyne), poly(2-octyne), poly(2-nonyne), poly(2-decyne), and poly(2-undecyne), were determined at 35 °C for H 2, CO 2, O 2, N 2, CH 4, C 2 H 6, C 3 H 8, and n -C 4 H 10 . Gas permeability increased by increasing the (i) side-chain length of the polymers and (ii) size and condensability of the penetrants. For example, the nitrogen and n -butane permeabilities of poly(2-undecyne) were 2- and 10-fold higher than those of poly(2-hexyne). As the fractional free volume decreases with increased side-chain length, the permeability increases. This behavior is opposite to that observed in polyacetylenes containing terminal bulky side-group substituents, such as trimethylsilyl or isopropyl groups. In branched acetylene-based polymers, the gas permeation properties correlate well with their free volume values; as fractional free volume increases, a simultaneous increase in permeability is observed. In linear poly(2-alkylacetylenes), an increase in side-chain length increases the overall chain mobility and increases the chain packing density of the polymers. In mixed-gas permeation experiments, an increase in mixed-gas n -butane permeability and n -butane/methane selectivity was observed with increasing side-chain length of poly(2-alkylacetylenes).