Abstract
Among self-assembled architectures, helical morphology is probably the most fascinating texture in nature. By introducing the chirality into synthetic molecules, helical textures on different length scales can be obtained by self-assembling through interplay of secondary interactions (i.e., non-covalent bonding forces). Our previous studies demonstrated that induced circular dichroism (ICD) of stereoregular vinyl polymers, poly(2-vinyl pyridine)s (P2VPs) can be driven by association of chiral dopant, suggesting the formation of helical polymer chain with exclusive handedness. The ICD via the association with chiral acids strongly depends upon the isotacticity of the P2VPs. In this study, we aim to examine the self-assembled behaviors of isotactic poly(2-vinyl pyridine) (iP2VP) with L- and D-mandelic acids (MA) as chiral dopants. For the iP2VP complexing with the chiral acids in the film state, mirror image of the corresponding ICD indicates that the formation of preferential helical conformation can be achieved by complexation of chiral dopants with pyridine. Most interestingly, bands spherulites (the presence of concentric rings or extinction bands under polarizes light microscope (PLM) observations) can be formed in the film state by crystallization of the iP2VPs with chiral dopants, providing a model system for the examination of the chirality transfer from conformational chirality to hierarchical chirality. Controlled handedness of crystalline lamellae in the banded spherulites can be found in crystallized iP2VP with chiral MA. Owing to the crystalline lamellar twisting, resulting from imbalanced stresses at the opposite folding surfaces, the preferential sense of the lamellar twisting can be determined using polarized light microscopy (PLM). As a result, homochiral evolution from molecular chirality to hierarchical chirality can be found in the crystallized iP2VP with ICD. In contrast to the homochiral evolution in crystallized iP2VP, we are also interested in the chirality transfer of self-assembled block copolymers (BCPs). A series of polystyrene-b-poly(isotactic 2-vinyl pyridine) (PS-iP2VP) with cylinder-forming phase are synthesized for the examination of the homochiral evolution on BCP self-assembly through the ICD. Helix-like transmission electron microscopy (TEM) projections can be observed in the PS-iP2VP complexing with the chiral dopants (i.e., MA). On the basis of small-angle X-ray scattering experiments, those helices can be found to appear as hexagonally packed lattice. As a result, helical phase is identified for the self-assembled PS-iP2VP/R- or S-MA, suggesting the chirality effect on the self-assembly of the PS-iP2VP/R- or S-MA. To further examine the homochiral evolution, the ICD behaviors of the PS-iP2VP driven by the introduced enantiomeric MA are examined by CD, and similar mirror-imaged results can be found. With the examination of the handedness of forming helical phase, a methodology for the examination of the homochiral evolution in the BCPs with ICD is suggested.