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_m⁰) of isotactic polypropylene (iPP) with high tacticity ([mmmm] = 99.6%) was studied. Four fractionated iPPs with M_n = 23 × 10³, 64 × 10³ 94 × 10³, and 263 × 10³ were used. T_m⁰ 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 T_m during T_m measurement at high temperature was also eliminated by observing thick lamellae formed at high crystallization temperatures (T_c = 148-166°C). With increase of M, T_m⁰ increased significantly. The empirical equation, T_m⁰ = 199.5 + 23.6 × log M - 2.0 × (log M)² (°C), was obtained. The molecular weight dependence of the α2-α2′ transition was observed. The transition temperature (T_α2-α2′) also increased with increase of M. The ΔT dependence of lamellar thickness was concluded to be controlled by that of lamellar thickening.