The Effect of Water on Thermal Stresses in Polymer Composites

The fundamentals of the thermodynamic theory of mixtures and continuum thermo-chemistry are reviewed for a mixture of water and polymer. A specific mixture which is mechanically elastic, where the strain states remain infinitesimal and where water concentration gradients are possible is considered. An expression for the partial pressure of water in the mixture is obtained from Gibbs relation based on certain assumptions regarding the thermodynamic state of the water in the mixture. Along with a simple diffusion equation, this partial pressure expression may be used to simulate the thermostructural behavior of polymer composite materials due to water in the free volumes of the polymer. These equations are applied to a specific polymer composite material during isothermal heating conditions. The thermal stresses obtained by the application of the theory are compared to measured results to verify the accuracy of the approach.