Estimation of the configurational heat capacity of polyisobutylene, isobutane and 2,2,4-isomethylpentane above the glass transition temperature

In this study, the absolute values for the configurational heat capacity of polyisobutylene (PIB), isobutane (which corresponds to the monomer of PIB), and 2,2,4-isomethylpentane (which corresponds to the dimer of PIB) were estimated based on the assumption that skeletal and group vibration modes do not change before and after the glass transition. The configurational heat capacities evaluated for these three substances decreased with increasing temperature. This temperature dependence of the configurational heat capacity is the same as that previously observed for molecular glass, which has a simple molecular structure. Additionally, the configurational heat capacities for the three substances were found to be different at the same temperature. This suggests that interactions with the surroundings that determine the configuration and orientation of the molecules differ as to the molecular weight changes. We estimated the configurational heat capacity (C-config) of polyisobutylene (PIB), isobutane and 2,2,4-isomethylpentane above the glass transition temperature by reproducing the heat capacity in the solid state using molecular vibration analysis. This is the first time to estimate the C-config of polymer. The C-config decreased with increasing temperature for the three substances in this study. This is a similar pattern as the C-config of molecular glass from a simple molecular structure. Considering that the configuration entropy becomes a constant value at the high temperature limit because the molecules constituting the substance can take any configuration and orientation, it is considered reasonable that the C-config approaches zero at the high temperature limit.

Automated summary

This study estimated the configurational heat capacity of polyisobutylene, isobutane, and 2,2,4-isomethylpentane above the glass transition temperature, finding that their configurational heat capacities decrease with increasing temperature, and that the configurational heat capacities for the three substances are different at the same temperature.