An investigation on the non-isothermal reversing crystallization of isotactic polypropylene by Temperature Modulated FT-IR

Modulated temperature FTIR (MTFTIR) analysis allowed the description at molecular level of the reversing and non-reversing tranformations occurring during the isotactic polypropylene non-isothermal melt crystallization. The intensity of regularity absorption bands, assigned to conformation sequences arranged in 31 helix of different length, were acquired in MTFTIR experiments. The data elaboration gave the band intensity total variation as well as its reversing and non-reversing components as a function of mean temperature. The results revealed that a reversing variation of helix concentration occurred from few degrees below the crystallization onset to lower temperature, when the primary transition ceased, and that the extent of the transformation depended on helix length. The comparison of the MTFTIR results with those obtained by WAXD highlighted that the reversing formation-disruption of longer helices was related to the reversing crystallization-melting process. The reversing transformation of shorter helices originated from an ordering-disordering process not affecting the sample crystallinity.