Characterization of sorbed water in saponified start–g‐polyacrylonitrile with differential scanning calorimetry

Abstract

SynopsisA saponified starch-polyacrylonitrile graft copolymer containing water was studied using differential scanning calorimetry. The samples had a moisture content in the range of 0-1.05 g water per gram of dry polymer. Molar heat capacity measurements at temperatures higher than 273 K gave partial molar values between those of ice and bulk liquid water. No crystallizable water was detected in samples with water contents lower than 0.44 g/g dry polymer.In samples containing larger amounts of water, both the amount of "bound" uncrystallizable water and the amount of "free" crystallizable water increased linearly. Two melting endotherms were detected, suggesting that the free water could be divided into two components with different melting points (268 K and 274 K, respectively). INTRODUCTIONThe recent developments of a highly water-absorbing polymer at this laboratory1.2 gave reason to study the water structure in hydrophilic polymers. Many investigators have tried to interpret the structure of water in aqueous solutions of polymers as well as adsorbed water or water in swollen polymeric substance^.^-^ Most of the work regarding water structure has been carried out using broad-line nuclear magnetic resonance spectroscopy (NMR)3 and differential scanning calorimetry (DSC)P-9 Broad-line proton-NMR experiments investigate the mobility of protons in various energy states. Using DSC measurements, the heat capacity, the heat of fusion, the melting temperature, and the amount of water that can crystallize can be estimated.Although there are many different hypotheses on the structure of water in polymers, it is generally accepted that water molecules in a polymer matrix behave differently from those in bulk water because of interaction between water and polymer. Schematically there are two opinions; first, that water in hydrophilic polymers can be divided into bound water and free liquid ~a t e r ,~-~ and second, all sorbed water is liq~idlike.'-~ Yasuda et al.3 studied the hydrogel of a polymer of glycerol monomethacrylate (poly-GMA) with NMR and DSC. They suggested that a constant amount of water is strongly associated with the macromolecules and does not crystallize when the temperature is lowered well below the freezing presented DSC data for mucopolysaccharides and cellulose acetate membranes, respectively, which contained multiple endothermic peaks. They suggested different states of water in the vicinity of the polymer segments.On the other hand, on the basis of heat capacity measurements on waterswollen poly[2-(2-hydroxyethoxyl)ethyl methacrylate)] (PHEOEMA), Pouchly et al.' claimed that the structure of water sorbed by the polymer is similar to liquid water and rather labile. After investigations of heat capacities of the water-collagen system, Hoeve8s9 suggested a liquidlike, onephase model consisting of chains of hydrogen-bonded water molecules that diffuse through the interstices preserving their mutual hydrogen bonding to each other.This work has been carried out using DSC on watercontaining hydrolyzed sta...

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