This work aims to suggest a represented block copolymer (BCP) system, polystyrene-b-poly(L-lactide) (PS-PLLA), for examination of the phase behaviors of semiflexiblecoil BCPs under different segregation strengths. Owing to the chiral polylactide with intrinsic chirality and semiflexible behavior, PS-PLLA can be referred to as a semiflexible-coil chiral block copolymer (BCP*), giving rise to a variety of self-assembled phases due to multiple effects of conformational asymmetry and rod−rod interaction as well as chirality on BCP self-assembly. With increasing segregation strength, conformational asymmetry and chirality effect lead to the formation and the enlarged forming window of the helical phase as PS being the major component. As PLLA becomes the major component, only the lamellar phase can be formed due to the rod−rod interaction. Interestingly, in the strong segregation region, the lamellar phase with out-of-phase undulation due to conformational asymmetry and in-phase undulation due to chirality effect (referred to as undulated lamellar phase) can be found.
Significant enhancement of segment-scale chirality, as measured by vibrational circular dichroism (VCD), is observed in the helical phase (H*) of polylactide-based chiral block copolymers (BCPs*) due to the mesoscale chirality of the microphase-separated domains. Here, we report a weaker, yet meaningful, enhancement on the VCD signal of a double gyroid phase (DG) as compared to a double diamond phase (DD) and disordered phase from the same diblock BCPs*. Residual VCD enhancement indicates a weak degree of chiral symmetry breaking, implying the formation of a chiral double gyroid (DG*) instead of the canonical achiral form. Calculations on the basis of orientational self-consistent field theory, comparing coupling between the segmental-scale preference of an intradomain twist and morphological chirality, show that a transition between DG and DG* takes place above the critical chiral strength, driving a weak volume asymmetry between the two enantiomeric single networks of DG*. The formation of nanostructures with controllable mesoscale chiral asymmetry indicates a pathway for the amplification of optical activity driven by self-assembly.
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