Equilibrium melting temperature of isotactic polypropylene with high tacticity. 2. Determination by optical microscopy

In part 1 of this series, we proposed a new method to determine the correct equilibrium melting temperature (T_m⁰), Effects of the "melting kinetics" and lamellar thickening were omitted from T_m. The correct T_m⁰ of isotactic polypropylene (iPP) ([mmmm] = 99.6%, M_n = 64 × 10³, and M_w/M_n = 2.4) was observed to be 186.1 °C. In this paper, the rigorous Gibbs-Thomson plot was obtained by using the direct correspondence between maximum melting temperature (T_m,max) and maximum lamellar thickness (l_max).T_{m, max} and l_max were observed by means of optical microscope and transmission electron microscope (TEM), respectively. The validity of the Gibbs-Thomson plot obtained by means of a differential scanning calorimeter (DSC) (part 1 of this series) was confirmed by comparing it with the rigorous Gibbs-Thomson plot in this paper. The Hoffman-Weeks plot is widely used as one of the methods to obtain T_m⁰. It was shown that the Hoffman-Weeks plot is correct only when l ∝ 1/ΔT, where ΔT is the degree of supercooling, is satisfied. However, in the case of iPP, the condition is not satisfied, and so the result obtained by the Gibbs-Thomson plot is not equivalent to that obtained by the Hoffman-Weeks plot. The existence of α2′ phase was confirmed again by breakings in slopes of l and T_m against T_c at 159 °C. Furthermore, the broad bimodal distribution of l was caused by the difference between the lamellar thickening growth rate of isolated mother lamellae and the lamellae thickening rate of stacked daughter lamellae.