Formation of Metastable Structures during Confined Crystallization of Poly(<sc>l</sc>-lactide) in Poly(isoprene-b-<sc>l</sc>-lactide) Diblock Copolymer and Their Influence on Phase-Separated Morphology

Well-defined poly(isoprene)-b-poly(l-lactide) (PI-b-PLLA) diblock copolymers were synthesized by ring-opening polymerization of l-lactide using as macroinitiator hydroxy-terminated polyisoprene (PI-OH), synthesized by anionic polymerization using high-vacuum techniques. The resulting copolymers exhibited strong melt-state microphase segregation, with their nanostructures retained upon rapid melt quenching. Upon heating between 70 and 100 degrees C, confined crystallization of PLLA occurred, forming an unconventional beta form along with the alpha(alpha ') forms. This process increased microdomain thickness from 27 +/- 0.5 to 32 +/- 0.5 nm. At higher temperatures, 110-140 degrees C, the metastable gamma form appeared along with the alpha(alpha ') and beta forms, achieving maximum crystallinity. During this stage, breakout crystallization overwrote the existing microphase-separated lamellar morphology due to the increased crystallinity. Notably, between 130 and 150 degrees C, the metastable beta and gamma forms transformed into the more stable alpha form just before melting. We propose that confinement imposed by the covalently linked PI chains, in the presence of preformed alpha(alpha ') and beta crystals, promoted the formation of these unconventional metastable crystalline forms (beta and gamma forms). These insights are crucial for understanding the competitive self-organization processes in semicrystalline block copolymers and the constrained crystallization behaviors within phase-separated structures.