The thermal behaviour of water in hydrogels

Abstract

The melting enthalpy and the absorption enthalpy of water in polymethacrylic acid and copolymers of methacrylic acid and hydroxyethylmethacrylate have been determined. The melting enthalpy of water in hydrogels increased by increasing the equilibration time at −15°C, but was always smaller than that of pure water. The absorption enthalpy of water was negative and decreased by increasing the initial water content of the hydrogels. However, the observed lack in melting enthalpy of water was not fully compensated by the negative change in absorption enthalpy. This led to the conclusion that a part of the water does not freeze during the cooling-heating process. The glass transition temperatures were determined as a function of the water content. Upon hydration, the glass transition temperature decreased in temperature. The amount of water necessary to decrease the glass transition temperature to 0°C was approximately equal to the amount of non-freezable water in the hydrogels. These observations lead to the conclusion that the amount of non-freezable water cannot be explained by different types of water, but is based either on a restriction of the diffusion of water in hydrogels or on a restriction of a further growth of ice crystals after transformation of the hydrogel from rubbery state (high flexibility of the polymer network) to glassy state (low flexibility of the polymer network).