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Articles you may be interested inEffect of high hydrostatic pressure on the dielectric relaxation in a non-crystallizable monohydroxy alcohol in its supercooled liquid and glassy statesThe effects of chain irregularities, treated as randomly introduced copolymeric units and of added monomeric diluents, on the nucleation rate of crystallizing polymeric systems have been calculated. A marked attenuation in the nucleation rate is expected under certain conditions as a consequence of the varying composition of the melt. The decrease in the nucleation rate becomes more pronounced at the lower undercoolings and as the concentration of the noncrystallizing component increases.The Avrami mathematical formulation for the kinetics of phase changes can be generalized to allow for nucleation and growth rates which vary with the amount of material transformed. The resulting isotherms for copolymers are no longer superposable, superposability being characteristic of crystallization under conditions of constant nucleation and growth. Possible error in the interpretation of crystallization kinetics utilizing the simplified version of Avrami's equation is pointed out, as is the extreme sensitivity of the crystallization process to the presence of small amounts of chain irregularities.

The droplet technique was utilized to obtain estimates of the isothermal rate of homogeneous crystal nucleation in highly supercooled melts of unfractionated linear polyethylene. In a typical experiment a cell containing a suspension of several hundred micron‐size spherical droplets of the polymer was placed on a microscope hot stage and quenched from a temperature well above the melting point of polyethylene to the desired crystallization temperature. When viewed between crossed polarizers, the droplets were initially invisible but, upon freezing, they appeared as visible birefringent spheres. The time dependence of the process was followed by time‐lapse photography. Half times (T1/2) for the freezing of those droplets remaining unfrozen by the time the crystallization temperature was attained were thus obtained at a series of undercoolings and related to the desired nucleation rate constant I (nuclei cm.−1 sec.−1) by the equation I = In 2/T1/2/v, where v is the droplet volume. The temperature dependence of I, as obtained by the above procedure, was analyzed in accord with current theories of homogeneous nucleation of chain‐folded polymer crystals. From this analysis, an estimate, 14,960 ergs3 cm.−6 was made of the quantity (%sG2%sGe), where %sG and %sGe are, respectively, the lateral and end‐surface free energies of the crystal.