Conformational heat capacity of interacting systems of polymer and water

Conformational contributions to the heat capacity of the interacting system of polymer-water have been studied on the example of starch with and without low concentrations of water. This work continues a series of thermal analyses of biological materials. The heat capacity has been linked to its vibrational, external, and conformational contributions. The conformational part is evaluated from a fit to a one-dimensional Ising model for two discrete states, characterized by parameters linked to stiffness, cooperativity, degeneracy, and various interactions. The changes in heat capacity within the glass-transition region are interpreted, thus, as contributions of different conformational heat capacities arising from the changes in interaction of the carbohydrate chains with water. The vibrational contribution was calculated as the heat capacity arising from group and skeletal vibrations. The external contribution was computed as a function of temperature from experimental data of the mobile state of the thermal expansivity and compressibility. The stiffness, cooperativity, and degeneracy parameters are discussed in terms of monolayers and clusters of water about the starch. The calculated and experimental heat capacities of the starch-water system are compared from 8 to 490 K, the range of measurement. The system is shown to have a glass transition of at least three steps which begin at about 250 K and are not completed at 500 K, where decomposition starts. Water plasticizes the steps of the transition differently. The proposed approach should also be usable for a more realistic description of heat capacities of other biological materials, such as the cellulose-water or protein-water systems.