(518i) Elucidating the Relationship between the States of Water and Transport Properties of Ions in Swollen Polymer Networks

The transport of ions through polymer matrix is essential in a wide variety of technologies such as microfluidic, batteries, fuel cells, and desalination/water purification. Conventionally, the transport properties of ions are correlated with the total water content in the swollen polymers. However, water in the polymer exists in three states: non-freezable, intermediate water, and free water. While it is intuitive to expect that water molecules at different states would contribute to the ion sorption and diffusion differently, there lacks systematic studies of the relationship between the state of water and ion transport properties. In this study, we prepare three polymer networks: non-charged cross-linked poly(ethylene oxide) from poly(ethylene glycol) diacrylate (PEGDA) and poly(ethylene glycol) methyl ether acrylate (PEGMEA), zwitterionic polymers from sulfobetaine methacrylate (SBMA) and PEGDA, and polyelectrolytes from 2-acrylamido-2-methyl-1-propanesulfonate sodium (AMPS, CX) or [2-(acryloyloxy)ethyl] trimethyl ammonium chloride (AETMAC, AX) and PEGDA. The effect of polymer composition on the state of water was systematically investigated using the Differential Scanning Calorimetry (DSC). The number of non-freezable water molecules was 1.9 per ethylene oxide unit, 17 per SBMA, 15.1 per AMPS, and 9.1 per AETMAC. Increasing the PEGDA content increases the cross-linking density and thus the amount of free water. In contrast to conventional studies based on the total water content, the T_g of the hydrated polymers is correlated with the content of the non-freezable water using the Gordon-Taylor equation, and the salt solubility is correlated with the content of the freezable water. Interestingly, the salt diffusivity is related to the total water content (instead of the freezable water content) using the free volume model. This work is one of the very few elucidating the effect of the states of water on the sorption and diffusion of the water and ions in polymers.