Crystallization of low and high molecular materials

Abstract

For a better understanding of the crystallization of high molecular materials the paper starts with detailed statements as to the crystallization of low molecular materials. In this way it can be shown that there exist more properties in common than supposed before. This fact becomes evident from the thermodynamical point of view if the polymer is formally considered as a 2-components-system. In this system the stable rotational isomers, viz., the trans- and the gauche-form, are the components. In that case the crystallization process of linear polyethylene (HDPE) can be regarded as a crystallization of the trans-form in a trans-gauche mixture. In the paper the critical nuclei, which build up at the beginning of the crystallization and which have so-called equilibrium shapes, as well as the complete crystals, which have so-called growth shapes influenced by the kinetics of formation, are discussed. Special attention is paid to the lamellar growth because it often occurs in polymers. It is shown that the lamellar growth can be attributed to an activated sorption of foreign substances. In many polymers the role of the foreign substance is played by the gauche-form (autogenous activated sorption). By means of electron micrographs and calorimetric measurements of HDPE samples with a bimodal distribution of the lamellar thicknesses it could be proved that the melting range of polymers is due to a distribution of the lamellar thicknesses and apparently not to a changing conformational partΔs Lk of the melting entropy as supposed by other authors [1]. Values from the literature for the conformational entropyΔs Lk of different polymers confirm in a good approximation thatΔs Lk can be compared to a mean entropy of mixing $$\overline {\Delta s_{Mi} } $$ of the 2-components-model for polymers. This result makes evident that Flory's model for a polymer melt of interpenetrating chain coils can indeed be replaced, in a good approximation, by an ideal mixture of independent trans and gauche units in the sense of the 2-components-model. There follows in conclusion thatΔs Lk is a function of the gauche-trans ratio.