Melt Memory in Propene–Butene Isotactic Copolymers: The Case of Defects Hosted in the Crystals and of Isodimorphism in Olefin Copolymers

Self-nucleation (SN) and melt memory in random isotactic propene/butene copolymers (iPcoB) have been analyzed. These copolymers crystallize in the whole range of comonomer composition, from 0 to 100 mol % of butene, because of the cocrystallization of propene and butene in the crystal lattices of isotactic polypropylene (iPP) and polybutene (iPB). Thanks to the continuous structural transition between the iPP and iPB crystalline lattices with composition, iPcoB copolymers provide an ideal polyolefin-based model system to elucidate how melt memory develops in polyolefins due to the presence of constitutional defects and is erased when these irregularities do not act as defects but are hosted in the crystal lattice due to cocrystallization of comonomers. SN experiments reveal that copolymers with either low (lower than 35 mol %) or high (higher than 60 mol %) butene content display a pronounced melt memory with well-developed Domain IIa, whose width and memory effect increase with increasing comonomer content. This is consistent with the mechanism of partitioning of crystallizable sequences with topological constraints left in the melt during crystallization imposed by the partial incorporation of comonomers in the crystals. In contrast, the melt memory effect almost completely disappears in the intermediate-composition range (≈40–60 mol % butene), where the characteristic Domain IIa is suppressed and the melt behaves as if it had lost any structural constraints or residual order. This unexpected disappearance of melt memory has been interpreted in terms of the unique cocrystallization ability of propene and butene comonomeric units. At these intermediate compositions, both comonomer units are incorporated within a common crystalline lattice in statistically similar proportions, giving rise to a structurally uniform crystal phase. Under these conditions, the distinction between regular and defective sequences vanishes; crystallization proceeds without selective segregation of crystallizable sequences, and the melt does not retain residual memory of the crystals. These results have been supported by successive self-nucleation and annealing experiments that have demonstrated the structural homogeneity of copolymers in the intermediate compositional range.