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

Abstract

The spherulitic growth rates of α-phase poly(pivalolactone) (PPVL) in blends with poly(vinylidene fluoride) (PVF 2) were measured by polarized optical microscopy as a function of blend composition and isothermal crystallization temperature T x between 160 and 215°C. The 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 nucleation constants K g(II) and K g(III) and the surface free energy product σσ e were determined directly from the temperature dependence of the spherulitic growth rate data for each blend. The equilibrium melting temperature, the nucleation constants and the product of the fold and lateral surface free energies of PPVL a-phase crystals are observed to decrease with increasing PVF 2 content. The observed depression in equilibrium melting temperature was successfully analysed following the treatment proposed by Nishi and Wang and based on Scott's expression for chemical potentials in a binary polymer mixture to yield a negative interaction parameter ( ξ=−0.13±0.05). The magnitude of this interaction parameter is consistent with that found in earlier studies of poly(vinylidene fluoride)/poly(methyl methacrylate) blends. Finally, the observed decrease in crystal/melt surface free energy product is discussed in the context of a recent model correlating the lateral crystal/melt interfacial free energy with the characteristic ratio of the crystallizable polymer chain. Our analysis suggests that the lateral crystal/melt interface thickness should increase with PVFZ concentration in the blend in order to minimize the demixing of a crystallizable chain as it diffuses into the melt/crystal interface to become physically adsorbed onto the crystal growth front.