Sorption, Diffusion and Side-Chain Melting/Crystallization in Comb-Like Poly(N-Tetradecyl Methyl Siloxane) Facilitated by Condensable Hydrocarbon Gases

Polyalkylsiloxanes are promising membrane materials for hydrocarbon separation. However, the selectivity and permeability of polyalkylsiloxane-based membranes can change significantly in the presence of C2+ hydrocarbons due to their plasticization. To estimate the solubility and diffusivity at various hydrocarbon concentrations, we present for the first time a detailed study of sorption/desorption isotherms and kinetics of C2H6 and n-C4H10 in comb-like semicrystalline (5 °C) and amorphous (25 °C) poly (n-tetradecyl methyl siloxane) (PTDMS) in the pressure range up to 10 and 2 atm, respectively. The sorbed gas concentration reached the values of 33 for ethane and 129 cm3(STP)/cm3 for n-butane (at p/p0 0.37 and 0.91 for C2H6 and n-C4H10).The most important result was obtained for semicrystalline PTDMS. The anomalous hydrocarbons sorption and diffusion behavior of semicrystalline PTDMS was discovered, which is consistently explained by an increase in the volume fraction of amorphous phase of PTDMS as a result of the melting of side-chain crystallites, facilitated by hydrocarbons sorption. The melting of side-chain crystallites begins at hydrocarbons concentration above the critical value of 5 cm3(STP)/cm3. PTDMS goes into a completely amorphous state at a hydrocarbon concentration of 30 cm3(STP)/cm3 and higher. A low value of side-chain crystallization enthalpy and a wide range of investigated sorbate concentrations, as well as temperature conditions close to the side-chain melting point made it possible to observe a first order phase transition in semicrystalline comb-like polysiloxane in hydrocarbon media for the first time.