Computation of liquid heat capacities of polymers

摘要

The heat capacities of liquid polymers have been estimated as the sum of vibrational, external, and conformational contributions. The major part of the total liquid heat capacity originates from vibrational motion and can be calculated as the sum of heat-capacity contributions from group and skeletal vibrations. The external contribution to the liquid heat capacity, C_(ext), is: C_p - C_v =TV #alpha#~2/#beta#, where V is the molar volume; #alpha#, thermal expansivity and #beta3, the compressibility. All need to be known as function of temperature, T. The conformational contribution is calculated using a one-dimensional, Ising model with two discrete, degenerate conformational states. Depending on the stiffness and cooperativity parameters, the conformational heat capacities were calculated for a series of macromolecules. Results are compared to experimental data for the liquid heat capacity of polyethylene, polyprpylene, poly(methyl methacrylate), poly(n-butyl methacrylate), polycrbonate, polystyrene, and polytetrafluoroethylene. Agreement between both, computed and experimental liquid heat capacities is within a few percent.