Interactions of small permeants in polymeric matrices: A low-temperature differential scanning calorimetry study

This chapter discusses the interaction between small permeant states in polymer-based membranes. The permeants, in the vapor and liquid phases outside the membranes, are sorbed into different membrane systems, which include hydrophilic, hydrophobic and organic/inorganic composite films. The permeants and membranes are characterized using low temperature differential scanning calorimetry (DSC). The permeant state can be classified into freezable, intermediate, or non-freezable bound states. We use three systems to represent these various permeant-polymer interactions. In the weak interaction system of water and organic solvents in poly(dimethyl siloxane) (PDMS), the sorbed permeant exists in both freezable and non-freezable states. The fusion temperature and enthalpy of the freezable permeant resembles those of the bulk, pure solvent. At low sorption levels, the permeant molecules are present in the non-freezable bound state and the swollen PDMS film behaves like a pseudo-homogeneous system. Beyond a critical sorption level, all of the excessive permeant molecules in the PDMS are in the freezable state and the films behave like a phase-separated solution. In medium interaction systems, such as water in polyvinyl alcohol (PVA), the water distributes into freezable (with freezing at 0°C), intermediate (with sub-zero freezing temperatures), and non-freezable bound water. However, the incorporation of fumed silica (FS) nano-particles into the PVA results in a complex matrix in which FS generates dilution, shielding and suppression of hydrogen bond formation, as well as a shift in the freezable and non-freezable water equilibrium. In the strong interaction system of water in perflurosulfonic acid (Nafion®), no free water is detected. Only non-freezable bound water and intermediate water with subzero fusion temperatures are found. Mathematical models are proposed to express the permeant state distribution in these three systems. The interpretation of the corresponding plots of the solvent content of each state versus total solvent uptake is elucidated. These various micro-interactions may have different impacts on the transport properties of polymer-based membranes, hydrogels, and solid electrolytes used in separation, gas barriers, controlled drug release, fuel cells, and other applications.