CALORIMETRIC STUDIES OF DISSOLUTION OF HYDROXYPROPYL METHYLCELLULOSE E5 (HPMC-E5) IN WATER

The interaction of water with polymers is important because it directl y or indirectly affects various properties such as rheological and tra nsport properties. In this study, the hydration/dissolution of a hydro philic polymer was investigated by calorimetric techniques. Hydroxypro pyl methylcellulose E5 (Methocel, HPMC E5) was used as the model polym er. A water-insoluble, hydrophobic polymer, ethyl cellulose E4 (Ethoce l, EC E4), was selected for comparison. The number of moles of non-fre ezing and freezing water were determined by differential scanning calo rimetry. Moles of non-freezing water per polymer repeat unit were 6.2 +/- 1.3 and 1.6 +/- 0.3 for HPMC E5 and EC E4, respectively. The hydra tion/dissolution of HPMC E5 was exothermic, with a total heat of disso lution of -24.1 +/- 1.1 cal/g (n = 4). The heat of hydration of water- insoluble EC E4 was -2.6 cal/g (n = 2). The specific heat capacity val ue of HPMC E5 increased due to the addition of water. The heat of solu tion and an increase in specific heat capacity values were associated mainly with the addition of tightly bound water to the polymer. The di ssolution of HPMC E5 in water is believed to have endothermic and exot hermic components. In summary, the study provided insight into the dis solution of a representative hydrophilic polymer in water. The heat of solution and the specific heat capacity values were mainly dependent on the addition of tightly bound water.