Double-crystalline PLLA-b-PVDF-b-PLLA triblock copolymers: preparation and crystallization

Double-crystalline poly(L-lactide)-block-poly(vinylidene fluoride)-block-poly(L-lactide) (PLLA-b-PVDF-b-PLLA) triblock copolymers were successfully synthesized through ring opening polymerization of L-lactide and benzoyl peroxide initiated polymerization of vinylidene fluoride, followed by copper(I)-catalyzed azide-alkyne coupling of the functionalized PLLA and PVDF. Three triblock copolymers with different block ratios were prepared via this synthetic approach. The block copolymers were miscible in the melt, and an alternating crystalline lamellar nanostructure was formed upon crystallization from the homogeneous melt. Crystallization behavior of the PLLA component depends strongly on the block composition. The crystallization temperature of the lower temperature crystallizing PLLA block increased considerably with respect to its parent homopolymer for rather symmetric block copolymers, indicating a strong nucleation effect, while on the other hand asymmetric block copolymers with low PLLA content demonstrated a large decrease of crystallization temperature, due to a fractionated crystallization process. A confined crystallization mechanism for the PLLA blocks was suggested, indicated by the low degree of crystallization compared to the respective homopolymers, and confirmed by microstructure analysis performed during isothermal crystallization. Contrary to PLLA, crystallization of the higher temperature crystallizing PVDF component within the block copolymer was not influenced by the block composition and similar crystallization behavior was observed with respect to PVDF homopolymers.