Thermodynamics of Crystallization in High Polymers. Gutta-Percha

Abstract

The melting temperatures of two of the crystalline modifications of gutta-percha and their mixtures with either methyl oleate or tetradecane as diluent were determined by x-ray diffraction and dilatometric methods. It was found that the melting temperatures of the two forms in the pure polymer were 64 and 74°, respectively. From these studies the heat of fusion per repeating unit of the high melting form was deduced to be 3070 ± 250 cal./mole, while the heat of fusion for the low melting modification was found to be about one-half to three-fourths this value. An analysis indicates the low melting form is a metastable one. The heats and entropies of fusion of each of the forms of gutta-percha are appreciably greater than those of the monomorphic as-hydro-carbon, natural rubber. The difference in the entropies of fusion of the two polymers persists even when allowance is made for the significant contribution of the volume change accompanying fusion.