Membranes for gas separation based on poly(1-trimethylsilyl-1-propyne)- silica nanocomposites

Nanocomposite membranes based on poly(1-trimethylsilyl-1-propyne) (PTMSP) and silica were synthesized by sol-gel copolymerization of tetraethoxysilane (TEOS) with different organoalkoxysilanes in tetrahydrofuran solutions of PTMSP. The influence of the synthesis parameters (type and concentration of organoalkoxysilanes, temperature and time) on the silica conversion and the gas permeation performance of PTMSP-silica nanocomposite membranes was investigated and discussed in this paper. The nanocomposite membranes were characterized by single and mixed gas permeation, thermogravimetric analysis and scanning electron microscopy. The butane permeability and the butane/methane selectivity increased simultaneously when high silica conversion was obtained and the size of particle was in the range 20-40 nm. For the sake of comparison, nanocomposite membranes based on PTMSP were also prepared by dispersing silica particles with different functional groups into the PTMSP casting solution. The addition of fillers to the polymer matrix can be performed up to a higher content of silica (30% silica-filled PTMSP in contrast to 6 wt.% for the in situ-generated silica). In this case, the simultaneous increase in butane permeability and butane/methane selectivity was significantly higher when compared to the nanocomposite membranes prepared by sol-gel process. The addition of fillers with 50% of surface modification with hydrophobic groups (Si-C₈H ₁₇ and Si-C₁₆H₃₃) seems not to lead to a significant increase of the butane/methane selectivity and butane permeability when compared to the silica with hydrophilic surface groups, probably because of the unfavorable polymer/filler interaction, leading to an agglomeration of the long n-alkyl groups at the surface of the polymer. An increase of butane permeability up to six-fold of unfilled polymer was obtained.