The effect of stretching on the process of structure formation in fiber spinning and film extrusion and in isothermal crystallization of crosslinked polyethylene has been investigated. It has been shown that the stretching forces (jet stretching) in these systems cause the formation of spherulites flattened with respect to the stretching direction. As a rule, the spherulite radius decreases while the degree of flattening increases with the jet stretching. Thermodynamic analysis of the kinetics and morphology of the spherulite growth during stretching enabled us to interpret these effects and to establish relationships connecting the degree of flattening of the spherulites with the characteristics of molecular orientation in the melt. Taking into account the changes in the conformational entropy of the deformed macromolecule (or the network) in the course of nucleation and crystal the deformed macromolecule (or the network) in the course of nucleation and crystal growth it is possible to explain not only the effect of the spherulite flattening but also the formation of tilted crystals and other peculiarities of crystallization taking place with the occurrence of molecular orientation. A general picture is given of the effect of a mechanical field on the morphology of the crystallizing polymers at various degrees of stretching.
General expressions are obtained for the nucleation rate taking into account explicitly the contribution of the free energy change when nucleation follows two different mechanisms, both monomolecular and multimoleuclar, i.e., with folded and extended chains. Nucleation with folded chains is the more probable for flexible random coiled macromolecules. However, upon decreasing the degree of coiling without affecting the origin of the uncoiling, the probability of the formation of multimolecular nuclei increases while that of the formation of monomolecular nuclei decreases. At some critical degree of uncoiling a reverse situation as compared to the initial probabilities is attained. A general thermodynamic and thermokinetic interpretation is given of the influence of the equilibrium or induced rigidity on the morphology of crystallization of polymers from solutions and melts.
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