Abstract
Here, a methodology for the examination of the chirality transfer from molecular chirality to phase chirality is developed. The molecular chirality of the polylactide-containing chiral block copolymers (BCPs*) is examined by circular dichroism (CD), and the handedness of the helical conformation (conformational chirality) is determined from a split-type Cotton effect in vibrational circular dichroism (VCD) spectra. Consequently, the handedness of forming helical phase (H*) is directly visualized from transmission electron microscopy tomography (hierarchical chirality). As found, homochiral evolution from molecular chirality can be achieved by self-assembling the polylactide-containing BCPs* to give helical phase with exclusive handedness. Moreover, we aim to examine universal behavior of chirality transfer in the self-assembly of BCPs* at different level scales. A series of BCPs*, poly(4-vinylpyridine)-b-poly(L-lactide) (P4VP-PLLA), are synthesized for self-assembling, giving a H* with hexagonally packed PLLA helices in P4VP matrix due to the chirality effect on block copolymer (BCP) self-assembly. The formation of the H* is strongly dependent upon solvent evaporation rate from solution casting, suggesting that H* phase is a metastable phase. The results are similar to that from the self-assembly of polystyrene-block-poly(L-lactide) (PS-PLLA) with lower segregation strength. In addition, a new type of chiral block polymer, poly(cyclohexylglycolide) (PCG)-containing BCPs*, is designed and synthesized. Enantiomeric poly(cyclohexylglycolide) (PCG)-containing BCPs* (i.e., poly(benzyl methacrylate)-b-poly(L-cyclohexylglycolide) (PBnMA-PLCG) and poly(benzyl methacrylate)-b-poly(D-cyclohexylglycolide) (PBnMA-PDCG)) are used for self-assembly to examine the corresponding chirality effect. Owing to the steric hindrance from bulky side group, crystallization of PLCG can be significantly suppressed to alleviate its effect on microphase separation. The large steric hindrance gives the chiral PCGs (PLCG and PDCG) high twisting power, resulting from intramolecular chiral interaction, as evidenced by CD and VCD results. By taking advantage of intermolecular chiral interaction, the self-assembly of the PCG-containing BCPs* gives rise to the formation of stable H* as compared to polylactide-containing BCPs* due to its high degree of chirality. To expand the phase behaviors of polylactide-containing BCPs*, the phase behaviors of the binary blends of PS-PLLA BCPs* and PS homopolymer (HS) are found to be strongly dependent on the molecular weight (Mn) of the HS. The H* can be easily formed in the blends with low-Mn HS due to an enhancement of helical steric hindrance. By taking advantage of the forming metastable H* from BCPs*, it is feasible to acquire double gyroid (DG) phase from the self-assembly of PS-PLLA BCPs*; consequently, a wide region for the formation of DG can be found in the phase diagram of the BCPs*, suggesting that H* from the self-assembly of BCPs* can serve as a stepping stone for the formation of the DG due to an easy path for order-order transition from two-dimensional to three-dimensional (network) structure through twisting mechanism. Moreover, the order-order transition from metastable H* to stable DG can be expedited by blending the PS-PLLA with styrene oligomer (S) to fine-tune the morphologies of the blends. Owing to the use of the low-molecular-weight oligomer, the increase of BCP chain mobility in the blends significantly reduces the transformation time for the order-order transition from H* to DG. For the homochiral evolution from the BCP self-assembly, it is intuitive to expect that there is a possibility to acquire the gyroid phase with controlled handedness in the self-assembly of BCPs*. Yet, on the basis of thermodynamic consideration for stable equilibrium state, a DG composed of a cubic matrix and a pair of continuous, interpenetrating but independent, coherent single gyroid (SG) networks (one positive chirality and one negative chirality) is formed from self-assembly to give an achiral structure in 3D space. Even with an alternating DG (aDG) in triblock copolymers, the forming SG networks with distinct constituted components can be positive or negative chirality arbitrarily. As a result, the critical challenge for the controlled chirality of chiral SG network from self-assembly remains, that limits its applications in chiral metamaterials. In this study, we first build up the phase diagram of triblock copolymers composed of a chiral segment, poly(L-lactide) (PLLA) for finding various network morphologies. Starting with a diblock, addition of asymmetric amounts of the chiral block to a symmetric diblock creates competing packing constraints. While the equal-sized diblocks prefer a flat interface, a smaller additional block should favor a finite spontaneous curvature. With the introduction of chiral block that drives twisting of the flat interface, the curvature at the saddle surface becomes significant. Consequently, a transformation from two-domain lamellae to cylinder phase can be found in the isopleths of triblock, reflecting chirality effect on the self-assembly of the chiral triblock copolymers. Accordingly, the chirality effect will lead the helical steric hindrance at the interface of achiral and chiral blocks to give the microdomain with large curvature, giving the potential to create network nanostructure from self-assembly. Furthermore, the network morphology, aDG, can be found in the phase diagram of PI-PS-PLLA with specific volume fraction. To acquire the SG with controlled chirality, it is sound to presume that the network from the self-assembled chiral end block in aDG will possess an exclusive chirality due to the chirality transfer from the chiral entities whereas the network with opposite chirality for the minor achiral block can be formed. Accordingly, triblock copolymers composed of chiral end block, polyisoprene-b-polystyrene-b-poly(L-lactide) (PI-PS-PLLA) and polyisoprene-b-polystyrene-b-poly(D-lactide) (PI-PS-PDLA), are synthesized. In the microscopic level, opposite configurational chirality construct the PI-PS-PLLA and PI-PS-PDLA chains as helical conformations with preferential conformational chirality, respectively, due to intramolecular interaction. In the mesoscopic level, the self-assembly of the PI-PS-PLLA and the PI-PS-PDLA triblock copolymers would give rise to the formation of aDG whereas an alternating double diamond (aDD) is found in polyisoprene-b-polystyrene-b-poly(D,L-lactide) (PI-PS-PLA). By taking advantage of chiral intermolecular interaction for self-assembly, a single gyroid with controlled chirality in an aDG can be obtained in a chiral triblock copolymer composed of a chiral block whereas an aDD with a pair of achiral networks is found in an achiral triblock copolymer, suggesting the evolution of homochiral helical self-assembly. By taking advantage of the degradation characters of constituted block(s), well-defined nanoporous polymers with SG nanochannels can be used as templates for templated syntheses to enrich the applications of the forming nanonetwork morphologies. With the control of the periodicity and chirality of SG, the properties of nanomaterials will give a wide variety in applications such as chiral metamaterials.