Unveiling the apparent density of swollen PDMS in hydrocarbons – A distinction between dimensional-based and volume-additivity methods

Abstract

This article reports the swelling-induced effect on the density, mass, area, and thickness of the polydimethylsiloxane (PDMS) membrane in C₆–C₁₂ linear alkanes. Their time dependence (up to 500 min) was determined at 20 °C and atmospheric pressure. From the combination of areal and thickness data, the experimental PDMS volumes of the swollen polymer were calculated, and the time dependence of the apparent density of the swollen membrane (i.e. its mass divided by its real volume) was revealed. The equilibrium apparent density of the swollen PDMS increased with the increasing number of C-atoms of liquid alkanes and liquid density. For higher alkanes (C number > 10), the calculated equilibrium apparent density of the swollen polymer was thus higher than that of the dry polymer. The dimensional-based PDMS volumes of the swollen polymer were compared with those calculated by the volume-additivity rules. For each C₆–C₁₂ linear alkane, the volume-additivity method overestimated the swollen polymer volumes compared to the experimentally determined ones. Such a finding indicates the non-ideal PDMS swelling character, which is not solely additive due to the force interactions between the polymer chains and solvent molecules and moving of polymer chains apart (free volume increases).