Coalescence and split of high-entropy polymer lamellar cocrystals

Abstract The solvent evaporation has been identified able to induce the cocrystallization of dissolved polyvinylidene fluoride (PVDF) homopolymer and poly(vinylidene fluoride trifluoroethylene) (PVDF-TrFE) copolymer solutes in this research. Upon the random incorporation of either trifluoroethylene (TrFE) or vinylidene fluoride (VDF) motif during this selected route of cocrystallization, several ferroelectric crystalline phases grow concurrently. For the development of each crystalline form, there is an appropriate composition range of TrFE motif, instead of a specific composition. When the inclusion of TrFE is beyond a threshold level, the non-polar HT-form crystals were found to solely develop upon slow heating above Curie temperature, and therefore inherits various compositions of TrFE motif. During the stay above Curie temperature, stick-like HT-form lamellar cocrystals preferably associate together and coalesce into coarser crystalline lamellae. This coalescence behavior is deduced to be driven by the secondary crystallization above Curie temperature, presumably enabled by the acceptable level of lattice-packing entropy of HT-form cocrystals. During the subsequent cooling below Curie temperature, the coalesced HT-form lamellar cocrystals reversely split into thinner lamellar cocrystals with various types of ferroelectric lattice packing comprising narrower ranges of TrFE composition. The Curie transition of cocrystals is thus accompanied with the alteration of the stacking dispersion of crystalline lamellae within materials, which associates with the variation of motif composition and therefore the lattice-packing entropy.