The melting behavior of n-alkanes up to carbon number 390 and molecular weight fractions of linear polyethylene up to M" = 5600 have been analyzed. Surprisingly, it is found that the melting temperature-molecular weight relation is the same for both types of chain molecules. The results are explained in terms of premelting of the n-alkanes, with the disordering of sequences of end units, so that the structure prior to melting is the same as that of the polymer. Support of this conclusion is given by theoretical considerations and experimental results. The difficulties in extrapolating the present data base to the equilibrium melting temperature of the infinite molecular weight chain are discussed.
Primary nucleation of poly(vinylidene fluoride) (PVF2) γ‐phase crystals from the melt is affected by the presence of an electric field. Crystallization under the electric field is studied by polarized optical microscopy, thermal analysis, and wide‐angle x‐ray diffraction. Preliminary results indicate that the γ‐phase crystal nucleation rate and content are enhanced by the electric field.
A modification of the classical theory of homogeneous nucleation of a crystalline phase is proposed to account for the experimental observations. Electrostatic interaction between the nucleus total polarization and the electric field contributes to the free energy of nucleation to a very large extent at low undercooling. Theoretical predictions indicate that a static electric field will increase the nucleation rate of a polar phase and will decrease the nucleation rate of a nonpolar phase. Confirmation of the former fact is observed experimentally.
The crystallization, melting, and morphology of low molecular weight fractions of linear polyethylene, Mn = 1.6 X 103 to 8.0 X 103, have been studied in detail. In this molecular weight range it is possible under appropriate crystallization conditions, to form extended-chain crystals as well as much smaller size crystallites. When this size change occurs in the same molecular weight fraction, it does so rather abruptly with only a small change in the crystallization temperature. Detailed studies of the course of the isothermal crystallization, utilizing the low-frequency Raman acoustical mode as a major diagnostic tool, along with thermodynamic measurements, have led to an understanding of the crystallization process and of the size differences. The melting temperatures of the extended-chain crystals have been analyzed according to Flory's theory. The problems involved in extrapolating to infinite molecular weight have been put into proper perspective. Transmission electron micrographs for a fraction Ma = 5600 crystallized at different temperatures have also been studied. Roof-shaped lamellae, with a lateral extension of several microns, are observed even for quenched samples. Analysis of the apex angles determines the angle of inclination between the chain axis and the lamellar normal. This tilt angle is found to be dependent on the crystallization conditions.
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