Phase Separation and Crystallization in Monodisperse Block Copolymers of Linear Low-Density Polyethylene and Isotactic Polypropylene

Samples of block copolymers (BCP) constituted by semicrystalline blocks of linear low-density polyethylene (LLDPE) and isotactic polypropylene (iPP) of different block lengths (iPP-b-LLDPE) have been prepared by living polymerization using the hafnium-based catalyst that provides high steric control and isotactic propagation of propene units. The LLDPE blocks are random ethylene/1-octene copolymers with 1-octene concentrations between 1 and 3 mol %. The crystallization behavior and the morphologies in the melt and after the crystallization of iPP-b-LLDPE are presented. The iPP block melts at 135 °C, according to the moderate isotacticty, whereas the LLDPE blocks melt at lower temperatures, between 101 and 113 °C, depending on the 1-octene concentration. Both blocks crystallize from the melt and wide-angle diffraction and small-angle scattering profiles acquired with synchrotron radiation during cooling have demonstrated that the iPP block crystallizes first upon cooling, except for the sample with the lowest iPP molecular mass, for which PE and iPP crystallize almost simultaneously. The small-angle scattering recorded during cooling and crystallization of iPP and LLDPE blocks shows the emergence of a single broad scattering peak ascribed to the formation of stacks of crystalline lamellae of indistinguishable iPP and LLDPE with values of the long period of 13–14 nm. Electron microscopy (TEM) analysis allowed us to image the microphase-separated microdomain structure present in the melt and frozen or preserved at room temperature after crystallization. The TEM analysis has demonstrated that phase-separation of the dissimilar blocks occurs in the melt. The formed microdomain structure is preserved after crystallization of both blocks, and pass-through crystallization occurs with iPP and PE lamellae crossing through the different microdomains without overwhelming the microphase-separated structure.