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

Abstract

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 °C, confined crystallization of PLLA occurred, forming an unconventional β form along with the α(α′) forms. This process increased microdomain thickness from 27 ± 0.5 to 32 ± 0.5 nm. At higher temperatures, 110–140 °C, the metastable γ form appeared along with the α(α′) and β 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 °C, the metastable β and γ forms transformed into the more stable α form just before melting. We propose that confinement imposed by the covalently linked PI chains, in the presence of preformed α(α′) and β crystals, promoted the formation of these unconventional metastable crystalline forms (β and γ 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.