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Sequential phase transitions wereinvestigated in detail for a set of newly designed poly(propyleneoxide)-b-poly(ethylene oxide)-b-poly(propyleneoxide) (PPO–PEO–PPO) triblock copolymers, where thePPO blocks have isotactic, (R)- or (S)-, or atactic configurations. Microphase separation and thermalcomposition fluctuations were studied near the order–disordertransition temperature (133 ± 1 °C), and the interactionparameters χ(T) were determined. A unique featureof these materials is the cooperative and sequential nature of phasetransitions, which leads to the inversion of the crystallization/meltingsequence of PEO and isotactic PPO blocks. The PEO blocks crystallizequickly (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 confinementwhich enables isotactic PPO crystals to undergo recrystallizationand lamellar thickening. This cooperative behavior allows the PPOcrystals to survive at higher temperatures (68 °C). The c-axes of PEO and isotactic PPO crystal lattices are bothfound perpendicular to the lamellar planes. We believe this studyprovides insight into the fundamental interrelationship between microphaseseparation and confined crystallization that can occur in semicrystallinematerials capable of self-assembly across multiple length scales. [ABSTRACT FROM AUTHOR]