Specific Heat of Atactic and Isotactic Polypropylene and the Entropy of the Glass

Abstract

The specific heat of atactic and isotactic polypropylene has been measured from 80° to 490°K. The isotactic sample was 64.9% crystalline, as determined by specific volume, and the atactic sample was 2–3% crystalline, as determined by the specific heat measurements. Below the glass temperature, which was found to be 259°K, and above the melting point, which was found to be 447°K, the specific heats are very nearly the same. From the data, the entropy,enthalpy, and free energy referred to absolute zero were computed. The heat of fusion of pure isotactic polypropylene crystal was found to be 45 cal/g. On the basis that the thermodynamic functions for the two materials above the melting point are the same, the residual entropy of the glass at absolute zero is calculated to be 0.62±0.2 eu/mole. The behavior of the thermodynamic functions is such that a lowering of the glass temperature by 53±20°K would produce a glass of zero configurational entropy. A lowering of 150±15°K is necessary to make the enthalpy of the glass equal that of the crystal. It is deduced that underlying the observed glass transition is a second‐order transition in which the entropy of the supercooled liquid is a primary factor.