Crystallization Behavior of Block Copolymers of Isotactic Polypropylene and Linear Low-Density Polyethylene

The crystallization behavior and the overall kinetics of the crystallization from the melt of samples of diblock copolymers (BCPs) composed of semicrystalline blocks of isotactic polypropylene (iPP) and linear low-density polyethylene (LLDPE), synthesized with an isospecific living organometallic catalyst, have been studied. Samples with different lengths of the two crystallizable blocks and different 1-octene concentrations in the LLDPE block have been analyzed. In almost all samples, the iPP block crystallizes first upon cooling from the melt, producing a bundle-like crystal morphology according to the moderate stereoregularity of the iPP dictated by the catalyst, and the LLDPE block crystallizes at a slightly lower temperature without altering the crystal morphology. All samples have been isothermally crystallized from the melt at different temperatures, and the effect of the covalently linked LLDPE block on the crystallization kinetics of iPP has been analyzed. For almost all samples, only the iPP blocks crystallize during the isothermal crystallization at the chosen crystallization temperatures. The LLDPE block remains in the melt and crystallizes after cooling to room temperature. Only in the samples with the lowest octene fraction of 1.1 mol % and shortest iPP block (w(iPP) = 15 wt %) does the LLDPE block crystallize coincidently with the iPP block in the isothermal step. The analysis of the kinetic data has shown that the covalently linked LLDPE block reduces the overall crystallization rate of the iPP block, with respect to the iPP homopolymer, and the crystallization rate decreases with increasing the length of the LLDPE block. This slowdown of the crystallization rate has been attributed to a dilution exerted by the linked molten LLDPE block. The Avrami indices of all BCPs are between 2 and 3. They are slightly lower than those of the iPP homopolymer, suggesting the occurrence of dimensionality restrictions imposed by confinement into the phase-separated microdomain morphology, confirming the hypothesis of the formation of a phase-separated structure in the melt of these iPP-b-LLDPE, which is partially retained upon crystallization. This work contributes to sheding light on the crystallization mechanism in semicrystalline BCPs, crucial to elucidating and upgrading the performances of these innovative copolymers when employed as compatibilizers in polymer blends.