Cooperative and Sequential Phase Transitions in it-Poly(propylene oxide)-b-poly(ethylene oxide)-b-it-poly(propylene oxide) Triblock Copolymers
Sequential phase transitions were investigated in detail for a set of newly designed poly(propylene oxide)-b-poly(ethylene oxide)-b-poly(propylene oxide) (PPO–PEO–PPO) triblock copolymers, where the PPO blocks have isotactic, (R)- or (S)-, or atactic configurations. Microphase separation and thermal composition fluctuations were studied near the order–disorder transition temperature (133 ± 1 °C), and the interaction parameters χ(T) were determined. A unique feature of these materials is the cooperative and sequential nature of phase transitions, which leads to the inversion of the crystallization/melting sequence of PEO and isotactic PPO blocks. The PEO blocks crystallize quickly (ca. 50 °C) within a soft, confined lamellar template, which is followed by slow crystallization of the isotactic PPO blocks (ca. 40 °C) under hard confinement. However, in the melting process, PEO crystals melt first (ca. 60 °C) affording soft confinement which enables isotactic PPO crystals to undergo recrystallization and lamellar thickening. This cooperative behavior allows the PPO crystals to survive at higher temperatures (68 °C). The c-axes of PEO and isotactic PPO crystal lattices are both found perpendicular to the lamellar planes. We believe this study provides insight into the fundamental interrelationship between microphase separation and confined crystallization that can occur in semicrystalline materials capable of self-assembly across multiple length scales.