Liquid heat capacity of amorphous poly(vinyl methyl ether)

Abstract

In this paper, the experimental heat capacity of the liquid state above the glass transition 248 K (–25 °C) was linked to the molecular motions and computed as the sum of vibrational, external (anharmonic), and conformational contributions. Most of the liquid heat capacity of poly(vinyl methyl ether) (PVME) arises from the vibrational motion calculated as the group and skeletal vibrational heat capacity contributions. The external contribution to liquid Cp was computed as a function of temperature from experimental data of the thermal expansivity and compressibility of the liquid state. The conformational heat capacity contribution to the total Cp of the amorphous poly(vinyl methyl ether) was established. It was calculated from a fit of the experimental, liquid heat capacity after subtracting the vibrational and external parts to a one-dimensional Ising-type model with two discrete states characterized by parameters linked to stiffness, cooperativity, and degeneracy. The computed and experimental data of Cp at the liquid state showed good agreement, i.e., within a few percent, close to the experimental precision at the temperature region from 250 to 360 K. The proposed approach can be used for a determination of heat capacities of more complex systems such as poly(vinyl methyl ether)-water which is widely used in medical applications.