Thermal behavior of water in the selected starch- and cellulose-based polymeric hydrogels

In a polymer–water matrix, freezable water is depressed due to either porosity confinement or interaction. The aim of the study was to examine water crystallization/melting depression by sub-ambient differential scanning calorimetry. The selected starch- and cellulose-based polymers including pre-gelatinized starch (PS), sodium alginate, sodium starch glycolate, hydroxypropylmethyl cellulose (HPMC), sodium carboxymethyl cellulose, and croscarmellose sodium were employed. The pre-treated with ambient humidity (85–100% relative humidity, at 30.0 ± 0.2 °C for 10 days) and with excess water (hydrogels) samples were subjected to between 25 and −150 °C cooling–heating cycle at 5.00 °C/min rate. The volume fractions of hydrogels were measured by light scattering technique. It was observed that all polymers but PS and HPMC with ambient humidity presented freezable water in two distinct fractions namely bound water where crystallizing/melting temperature was depressed and bulk water. The water transition in samples with various contents exhibited the pattern as a polymer solution, thus rather than confinement, the depression was due to interaction. The volume fraction-melting temperature data derived from endotherms of hydrogels were successfully fitted to Flory's model (r2: 0.934–0.999). The Flory's interaction parameters (χ1) were found to vary between 0.520 and 0.847. In addition, the smaller the value of χ1, the larger melting was depressed, i.e., stronger affinity for water.