Fractionated crystallization and morphology in binary blends of isotactic PP and HDPE with cycloolefin copolymers

Crystallization of binary blends of semi-crystalline polyolefins (i-polypropylene and HD-polyethylenes) and amorphous cycloolefin copolymers (COC) was followed by Differential Scanning Calorimetry, (DSC). Blend morphologies were investigated by Scanning Electronic Microscopy (SEM). The thermal behavior and morphology of iPP- COC blends are characteristic of immiscible blends. The two crystallization exotherms observed on cooling from the melt were explained in terms of fractionated crystallization. This phenomenon occurs when iPP inclusions are finely dispersed in the COC matrix so that their number is much greater than the number of heterogeneities originally present in the bulk polymer, resulting in the suppression of the heterogeneous nucleation. A quite different behavior is observed in PE-COC blends. DSC and DMTA results show a significant decrease of both PE Tin and COC Tg, observations usually associated to some polymer miscibility in the amorphous phase. The crystallization mode and the resulting morphologies also appear to be largely dependent on the molecular weights and polydispersity indices of the dispersed phase.