Degree of comonomer inclusion into lamella crystal for propylene/olefin copolymers

Copolymers of propylene with ethylene, butene-1, hexene-1, and octene-1 were synthesized with isospecific metallocene catalyst system. Crystalline structure of the melt-crystallized samples was investigated from the viewpoint of the comonomer location in a solid-state of the copolymers, using the fuming nitric acid (FNA) etching technique followed by 13C NMR spectrum measurement. It was found that the copolymers were oxidatively degraded with FNA at 50 °C and the residue after etching was corresponding to the lamella crystal of the original state. 13C NMR spectrum of the residue revealed that all kinds of comonomer used here were located not only in the amorphous phase but also in the crystalline core. Among comonomers, butene-1 is favorably incorporated in the crystalline phase with the expansion of the crystal lattice, and the amount of comonomer in the crystal is comparable with that in the amorphous phase. Ethylene, hexene-1 and octene-1 were also found to be included into the crystal, but the degree of inclusion in the crystal was much less than that of butene-1. Considering both the continuous increase of the weight loss and the saturation of the molecular weight decrease of the residue during an etching time-course, a plausible mechanism for the FNA degradation of the propylene copolymers was proposed.