Molecular weight dependence of equilibrium melting temperature and lamellar thickening of isotactic polypropylene with high tacticity

Molecular weight (M) dependence of the equilibrium melting temperature (T-m(o)) of isotactic polypropylene (iPP) with high tacticity ([mmmm] = 99.6%) was studied. Four fractionated iPPs with M-n = 23 x 10(3), 64 x 10(3), 94 x 10(3), and 263 x 10(3) were used. T-m(o) was obtained by using an improved method based on the Gibbs-Thomson plot proposed in previous papers. The effect of melting kinetics on melting temperature (T-m) was eliminated by observing isothermal melting of spherulites. The effect of lamellar thickening on Tm during T, measurement at high temperature was also eliminated by observing thick lamellae formed at high crystallization temperatures (T-c = 148 similar to 166 degreesC). With increase of M, T-m(o) increased significantly. The empirical equation, T-m(o) = 119.5 + 23.6 x log M - 2.0 x (log M)(2) (degreesC), was obtained. The molecular weight dependence of the alpha2-alpha2' transition was observed. The transition temperature (Talpha2-alpha2') also increased with increase of M. The DeltaT dependence of lamellar thickness was concluded to be controlled by that of lamellar thickening.