Interfacial free energy small crystallites

The effect of small crystallite thickness on the observed melting temperature-composition relation of random copolymers of vinylidene chloride and methyl methacrylate was analyzed by utilizing the Gibbs-Thomson equation.(16) However, to adapt this procedure to copolymers the dependence of both the crystallite thickness and the interfacial free energy aec on copolymer composition needs to be specified. It was possible to explain the observed melting temperature-composition relation for this copolymer by assuming the dependence of these two quantities on composition. 

A variety of real non-equilibrium features can be addressed. These include, among others, the formation of crystallites of small size, folded-chain crystallites, the role of the interfacial free energy, Oec, characteristic of the surface normal to the chain axis, and its dependence on the composition of the copolymer. For convenience the discussion that follows is divided into two categories. In one, the B units are excluded from the lattice in the other, they are allowed to enter. 

The brief review of the basic crystallite structure has centered on the nature of the lamellar habit. There are several points that should be emphasized. The molecular basis for the formation of lamellae can be developed in a natural way, based on the conformational properties of long chains. It is not necessary to invoke any inherent propensity of chains to fold or to arbitrarily draw upon monomeric nucleation theory. The structure of the equilibrium interphase has been quantitatively described. Although the number of chains that return to the crystallite from which they emanate is relatively large, the number that return in adjacent positions is small. These equilibrium considerations will be tempered in general by kinetic factors and by the melt structure at high molecular weights. The amount of adjacent reentry will be further reduced while the residual melt structure will enhance the value of the interfacial free energy. 

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