Structure and Hydrogen Bonding of Water in Polyacrylate Gels: Effects of Polymer Hydrophilicity and Water Concentration

Abstract

The ability to tune the hydrophilicity of polyacrylate copolymers by altering their composition makes these materials attractive candidates for membranes used to separate alcohol-water mixtures. The separation behavior of these polyacrylate membranes is governed by a complex interplay of factors such as water and alcohol concentrations, water structure in the membrane, polymer hydrophilicity, and temperature. We use molecular dynamics simulations to investigate the effect of polymer hydrophilicity and water concentration on the structure and dynamics of water molecules in the polymer matrix. Samples of poly(n-butyl acrylate) (PBA), poly(2-hydroxyethyl acrylate) (PHEA), and a 50/50 copolymer of BA and HEA were synthesized in laboratory, and their properties were measured. Model structures of these systems were validated by comparing the simulated values of their volumetric properties with the experimental values. Molecular simulations of polyacrylate gels swollen in water and ethanol mixtures showed that water exhibits very different affinities toward the different (carbonyl, alkoxy, and hydroxyl) functional groups of the polymers. Water molecules are well dispersed in the system at low concentrations and predominantly form hydrogen bonds with the polymer. However, water forms large clusters at high concentrations along with the predominant formation of water-water hydrogen bonds and the acceleration of hydrogen bond dynamics.

Water structure and mobility in acrylamide copolymer glycohydrogels with galactose and siloxane pendant groups

“…It can be seen that the T g values calculated from simulations are higher than the experimental values for GalEAm and DMA linear polymers by 38 and 111 K, respectively. Similar observations have been reported in literature for a wide range of systems, such as asphalt, ionic liquids, linear polyacrylates, and crosslinked epoxies that differ in their molecular weight, connectivity, and interactions. Specifically, T g value differences of 113 K (asphalt), 77 K (ionic liquids), 70–106 K (linear polyacrylates), and 27 K (crosslinked epoxies) were observed and were attributed to significantly faster cooling rates used in simulations compared to experiments which lead to higher T g values in simulations compared to experiments.…”

“…The amount of water at equilibrium in the hydrogel is known to affect its dynamics; therefore, the correlation of bound water fraction to EWC was examined as shown in Figure (the inset shows the average hydrogen bond lifetime, the significance of which is explained in detail in the next section). For the GalEAm hydrogels, bound water appears to decrease with respect to EWC in both experiment and simulation.…”

“…(a,b)] should also be taken into consideration . The structural relaxation of water molecules is dependent on the number of available hydrogen bonding partners in their vicinity …”

“…These observations can be explained in terms of the number of available hydrogen bonding partners near the polymer groups. In previous work, Khare et al has shown that the number of water molecules around hydroxyl and carbonyl groups in polyacrylate gels of different hydrophobicities are different, with water molecules clustering around the hydroxyl groups . The different local environments around these groups lead to varied relaxation dynamics as the orientational and translational relaxation of water occurs by continually breaking the existing hydrogen bonds and forming new ones with the surrounding water/polymer groups.…”

“…Historically, hydrophilic monomers, such as N ‐vinylpyrrolidone, (meth)acrylate derivatives, and (meth)acrylamide derivatives, have been studied through both experimental and modeling approaches in an attempt to understand how water mobility and organization are affected by chemical composition, network architecture, and swelling environment . Despite literature precedent, the relationship between structural water and diffusional processes is still not fully understood, especially in high water content glycohydrogel structures.…”