Influence of side chains assembly on the structure and transport properties of comb-like polysiloxanes in hydrocarbon separation

Abstract We have synthesized a number of comb-like polysiloxanes with linear, branched, cyclic and silicon-containing substituents; most of them are new and previously not studied polymers. The physicochemical properties of comb-like polysiloxanes have been systematically investigated. Differential-scanning calorimetry and wide-angle X-ray scattering data revealed the side-chain microphase assembly for polymers with linear aliphatic substituents, while the polymers with bulky substituents did not form a microphase. It is shown that the ratio of microphase in the polymer is greater, the closer the values of the thickness of the microphase layer and the length of the cross-link. The effect of the side-chain substituent on the hydrocarbon transport properties of comb-like polysiloxanes was studied. All synthesized polymers are promising as membrane materials for a vital process of hydrocarbon separation. This is associated with an increase in the solubility selectivity of n-butane/methane because the solubility coefficient of methane sharply decreases when long side chains are introduced into the polysiloxane. It was shown for the first time that microphase forming polymers have a significantly higher butane/methane selectivity (23.2–27.5) than polysiloxanes not forming a microphase (selectivity 12.3–20.0). The effect is demonstrated on polysiloxanes with various types of side substituents. It was revealed that for the comb-like polysiloxanes, the diffusivity selectivity and permselectivity are proportional to the fraction of the side-chain microphase in the polymer. With the increase in the hydrocarbon microphase share, the diffusion coefficient of the permanent gas methane is decreasing more rapidly than n-butane, which dissolves well in hydrocarbons and plasticizes polymer. Consequently, the polymers forming the microphase have a higher selectivity C3+/CH4 in the separation of a multicomponent hydrocarbons mixture.