Crystallization Kinetics of Crystalline–Crystalline and Crystalline–Amorphous Block Copolymers of Linear Polyethylene and Isotactic Polypropylene

Crystalline–amorphous diblock copolymers (BCPs) comprising crystalline blocks of isotactic polypropylene (iPP) or polyethylene (PE) linked to amorphous blocks of random ethylene–propylene copolymers (EPR) (iPP-b-EPR and PE-b-EPR) of different block lengths and ethylene concentrations in EPR blocks, and crystalline–crystalline BCPs composed by iPP and PE blocks (iPP-b-PE) have been synthesized with different living catalysts. The effects of the presence of a linked EPR rubbery block of varying composition on the crystallization behaviors and kinetics of PE and iPP, and of crystalline PE or iPP block on the crystallization kinetics of linked iPP or PE, respectively, have been analyzed. All samples have been isothermally crystallized from melting at different temperatures, and the crystallization kinetics have been analyzed. In iPP-b-PE BCPs, the PE block crystallizes first from the melt during nonisothermal cooling or isothermal crystallization. The iPP block crystallizes after PE and nucleates over the PE crystals. In both iPP-b-EPR and PE-b-EPR BCPs, the linked amorphous EPR block slows down the crystallization of iPP and PE blocks with respect to their respective homopolymers. Furthermore, in iPP-b-EPR samples, a higher concentration of propylene in the EPR phase results in a more significant slowdown of the crystallization kinetics of iPP due to a higher solubility between the blocks. Analogously, in PE-b-EPR copolymers, the increase in the length of the EPR block results in a more pronounced slowdown of the crystallization rate of PE with respect to the PE homopolymer due to the significant dilution exerted by the long EPR block. All samples of iPP-b-PE copolymers show crystallization rates lower than that of the PE homopolymer but faster than that of the iPP homopolymer. In isothermal crystallization experiments, the iPP blocks do not crystallize, not even at low crystallization temperatures, but crystallize upon successive cooling, nucleating over the PE crystals formed in the isothermal step. Therefore, the linked iPP, which remains in the melt during the isothermal crystallization, slows down the crystallization kinetics of PE, in contrast to what happens in a sample of iPP/PE blend, where the crystallization kinetics of PE is not affected by the presence of the phase-separated iPP.