Revisiting the Mechanism of the Meso-to-α Transition of Isotactic Polypropylene and Ethylene–Propylene Random Copolymers

In this work, we have conducted in situ simultaneous small- and wide-angle X-ray scattering and Raman spectroscopy experiments to investigate the fundamental differences in the mechanism of the mesomorphic to alpha phase transition of the isotactic polypropylene homopolymer and the random ethylene-propylene copolymer. Via quantitative analysis of the results coming from the three techniques, we found that in the homopolymer, chain interlock and chain extension occur during the transition. However, these processes are not necessary for the transition to occur. Indeed, the presence of randomly distributed ethylene co-units hinders the chain interlock process in the early stages of the phase transition (T > 60 degrees C) and suppresses the chain elongation process at the later stages (T > 90 degrees C). Consequently, the mesomorphic to a-phase transition in the random copolymer occurs with inclusion of the ethylene co-units inside the crystal lattice, causing increased lateral interchain distance and larger crystalline sizes. Our results show how differences exist in the way solid phase transitions occur at the molecular scale when comonomers are included into the macromolecular chains, leading to a better understanding of the thermal behavior of semi-crystalline polymers.