Abstract
In this study, we aim to examine the chiral effect on self-assembled structures in the bulk by using a diblock copolymer system constituting both achiral and chiral blocks, poly(styrene)-b-poly(L-lactide) (PS-PLLA). In contrast to the specific nanohelical phase in PS-rich PS-PLLA fractions, our results indicated that unique nanostructures were formed in the self-assembly of PLLA-rich PS-PLLA fractions. As evidenced by transmission electron microscopy (TEM) and small angle X-ray scattering (SAXS), a hexagonally packed core-shell cylinder structure was obtained for PS-PLLA with fPLLAv=0.65. Also, tilting experiments presented identical results. The calculated volume fraction from TEM micrographs was consistent with synthetic characterization, which further confirmed the novel core-shell cylinder structure and suggested that PS component forms the shell of the cylinder morphology whereas PLLA presents as core and matrix. Moreover, PS hollow-tube-like morphology can be observed after hydrolyzing PLLA blocks; the hydrolyzed morphology is in line with suggested core-shell cylinder structure phase. Furthermore, the core-shell cylinder morphology can also be observed in blending system whenever the effective volume fraction of PLLA (i.e., the overall content of PLLA blocks and PLLA homopolymer) is around 0.65. Long-time annealing experiments were performed to ascertain the stability of core-shell cylinder morphology. According to these results, core-shell cylinder morphology remains, and 1D SAXS profile also indicates hexagonally packed lattice. The preserved core-shell cylinder structure after reasonably long-time annealing reflects that the formed core-shell structure appears as a stable phase. The competition between crystallization and phase separation in the self-assembly of PLLA-rich block copolymers was also studied by differential scanning calorimeter, TEM and SAXS. These results all indicate the core-shell cylinder morphology is destroyed as crystallization of PLLA blocks occur. In contrast to the self-assembly of achiral block copolymers, polystyrene-b-poly(racemic lactide) (PS-PLA) and ABC triblock copolymers, the formation of core-shell cylinder phase structure explicitly reflects the significant chiral effect on microphase separation of block copolymers. The calculated results of theory of TCLB indicate that formation of tubular morphology was identified as the most stable morphology, and the tubular structure is indeed transformed from single-strand helices by scrolling. Besides, Nandi and Bagchi also proposed the tubular formation is driven by an intrinsic bending force in addition to twisting due to molecular chirality. In conclusion, on the basis of energetic consideration, we suggest that the observed tube-like core-shell cylinder morphology is the consequence of bending origin to induce the scrolling of helical microdomains; demonstrating a possible way to generate unique phase with helical sense for achiral component by introducing chiral effect for microphase separation.