D.s.c. investigation of the states of water in poly(vinyl alcohol) membranes

D.s.c. analysis shows that heat absorbed in the heating process of water in poly(vinyl alcohol) membranes is higher than that evolved in the cooling process. A remarkable discrepancy is found between these heats when the water retention, H, of the membrane is lower than 0.5 g H₂O/g swollen membrane. The amounts of freezable water in the membranes were estimated by use of the relation observed between the temperature and the enthalpy of the crystallization of super-cooled water. The amounts estimated from the heats absorbed or evolved agree within 2% when H was above 0.6. When H is below 0.5, however, the ratio of these two amounts was greater than 2. This difference is explained by the heat capacity of the water below 250 K increasing anomalously compared with that of bulk water.