The phase behavior for chiral block copolymers (BCPs*), poly(styrene)-b-poly(L-lactide) (PS-PLLA) with the composition range from 0.24 to 0.51 poly(L-lactide) volume fraction (fPLLAv), has been examined to study the effect of chirality on the self-assembly of block copolymers (BCPs). Apart from observing the conventional phases, including sphere (S), hexagonally packed cylinder (HC), gyroid (G) and lamellae (L), a unique phase with three-dimensional hexagonally packed PLLA helices in a PS matrix, a helical phase (H*), was discovered from the self-assembly of PS-rich PS-PLLA at 0.32□ fPLLAv □0.36, whereas no such phase was reported in racemic poly(styrene)-b-poly(D,L-lactide) (PS-PLA) BCPs. The occurrence of the H* was found to be molecular- weight dependent. For PS-PLLA with same composition but smaller molecular weight, gyroid was observed instead. Moreover, phase transitions from the H* to the stable HC and G were found after long-time annealing, suggesting that the H* is a long-lived metastable phase. The slow kinetics associated with the H*□ G or H*□ C relaxation were found to be especially severe in highly entangled systems (i.e., high molecular-weight fractions).
In-situ small-angle X-ray scattering (SAXS) was utilized to effectively investigate the interaction difference between PS and PLLA blocks and that of PS and PLA blocks. From the discontinuity in the scattering profile, the order-disorder transition temperature (TODT) for each sample can be measured. The corresponding value of Flory-Huggins interaction parameter between PS and PLLA blocks, χPS-PLLA, can be estimated by assuming (χN)ODT as 10.5, where N represents the degree of polymerization. Consequently, the temperature dependence of the χPS-PLLA can be determined. Accordingly, the χPS-PLLA value was estimated higher than that of racemic PS-PLA (χPS-PLA), indicating that the incompatibility between PS and chiral PLLA blocks is higher than that between PS and racemic PLA blocks. We propose that the formation of this novel phase is attributed to intramolecular chiral effect and intermolecular chiral interaction. The enthalpy penalty of forming larger interface between the two incompatible blocks is compromised by the formation of partially ordered state.
Also, phase behavior of PS/PS-PLLA blends was examined in order to gain a deeper understanding of the chiral effect on BCP self-assembly and the origins for H* formation. Homopolymers with three different molecular weights were blended with lamellae-forming PS-PLLA to prepare binary mixture with fPLLAv~0.34. When the molecular weight of PS exceeds that of the PS block in BCPs*, no phase transition occurred due to localized solubilization as expected. However, with shorter PS chains, both C and H* can be observed under the same fPLLAv, indicating that the solubilization mechanism of PS in PS-PLLA might justify the chiral effect on BCP self-assembly.

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