Cross-linked polydecylmethylsiloxane membranes for the separation of volatile organic compounds: effect of cross-linker amount

Membrane-based recovery of volatile organic compound (VOC) vapors is a promising separation process both from the perspective of reducing environmental impact and for resource conservation. Silicone rubber-based membranes are traditionally used for VOC separation from gaseous streams. In this work, highly selective polydecylmethylsiloxane (C10)-based membranes were studied, with a focus on the effect of cross-linking degree — varied by changing the 1-decene/1,7-octadiene (OD) ratio — on transport and separation properties. The influence of cross-linking on the sorption behavior of C10 was evaluated. Based on spectroscopic ellipsometry data, Flory–Huggins interaction parameters (χ₀, χ₁), Henry’s solubility constants (S₀), and swelling coefficients of thin films of cross-linked C10 in saturated VOC vapors (n-octane, iso-octane, toluene, butyl acetate) were determined. The highest sorption values were observed for iso-octane, which correlates with earlier findings for polydimethylsiloxanes. However, no direct correlation was found between sorption properties of C10 and VOC vapor transport through the membranes. The most suitable parameter for predicting changes in VOC vapor permeability across the series of cross-linked C10 samples was found to be χ₁ — the interaction parameter reflecting changes in solubility with increasing sorbate concentration. In binary VOC/N₂ separation, both permeability and selectivity decreased with increasing cross-linking degree, especially for iso-octane, likely due to reduced free volume in the polymer network. Membranes with lower cross-linking density showed the smallest (4–12%) relative change in nitrogen permeability (change in permeability coefficient when transitioning from pure gases to vapor-gas mixtures), which may be related to structural changes in the membrane at high OD concentration.