Isotactic poly(propylenes) (iPPs) are found to display profound melting kinetics at temperatures well below the apparent melting point. This unexpected behavior has not been observed earlier in semicrystalline polymers. This work presents a comprehensive study of different factors associated with the observed melting kinetics in the two major types of iPPs, Ziegler-Natta and metallocene types. Systematic studies of the temporal variation of crystallinity are also conducted and analyzed to extract the rate laws governing melting kinetics.
Factors that may impact melting kinetics such as superheating, conformational changes in the intercrystalline regions during prolonged crystallizations and the unique cross-hatched semicrystalline lamellar morphology of iPPs are analyzed in detail. Extrapolated melting temperatures at heating rate zero led to values higher than the temperatures at which melting is completed after long time annealing, and hence, rule out superheating effects. Melting kinetics and spin lattice relaxation in the interlamellar region of samples crystallized at different lengths of time were identical or very similar confirming lack of any significant change in entropy in the amorphous regions with prolonged crystallization.
Moreover, a direct correlation is established between the content and type of cross-hatched morphology and the isothermal melting behavior. The lowest Ta at which iPP crystals would melt is obtained as a function of defect content and crystallization temperature for the samples investigated, and its impact on the determination of equilibrium melting temperature of the pure isotactic chain is discussed.
