STUDY OF THE TEMPERATURE-DEPENDENCE OF ISOTHERMAL SPHERULITIC GROWTH-RATE DATA FOR POLY(PIVALOLACTONE) IN BLENDS WITH POLY(VINYLIDENE FLUORIDE) - A LINK BETWEEN COHERENT SECONDARY NUCLEATION THEORY AND MIXING THERMODYNAMICS

The spherulitic growth rates of alpha-phase poly(pivalolactone) (PPVL) in blends with poly(vinylidene fluoride) (PVF2) were measured by pola rized optical microscopy as a function of blend composition and isothe rmal crystallization temperature T(x) between 160 and 215-degrees-C. T he PPVL weight fraction in the blends ranges from 100 to 10 wt%, which constitutes the largest compositional range investigated in any such study. Using the Lauritzen-Hoffman kinetic theory of crystallization, the composition dependent equilibrium melting temperatures T(m), the n ucleation constants K(g(II)) and K(g(III)) and the surface free energy product sigmasigma(e) were determined directly from the temperature d ependence of the spherulitic growth rate data for each blend. The equi librium melting temperature, the nucleation constants and the product of the fold and lateral surface free energies of PPVL alpha-phase crys tals are observed to decrease with increasing PVF2 content. The observ ed depression in equilibrium melting temperature was successfully anal ysed following the treatment proposed by Nishi and Wang and based on S cott's expression for chemical potentials in a binary polymer mixture to yield a negative interaction parameter (chi = -0. 13 +/- 0.05). The magnitude of this interaction parameter is consistent with that found in earlier studies of poly(vinylidene fluoride)/poly(methyl methacryl ate) blends. Finally, the observed decrease in crystal/melt surface fr ee energy product is discussed in the context of a recent model correl ating the lateral crystal/melt interfacial free energy with the charac teristic ratio of the crystallizable polymer chain. Our analysis sugge sts that the lateral crystal/melt interface thickness should increase with PVF2 Concentration in the blend in order to minimize the demixing of a crystallizable chain as it diffuses into the melt/crystal interf ace to become physically adsorbed onto the crystal growth front.