Mesochiral phases from the self-assembly of chiral block copolymers

Abstract

Herein, we aim to create new phases with chiral sense from the self-assembly of block copolymers (BCPs). The main idea is to create a BCP with a chiral block, yielding BCPs with a helical polymer chain for self-assembly in the bulk; this BCP system is denoted as a chiral BCP (BCP*). With the introduction of chirality effects in BCP self-assembly, a helical phase (H*) with hexagonally packed twisted cylinders can be obtained. By taking advantage of the homochiral evolution from a monomer to a polymer chain and further extending to a hierarchical superstructure, controlled chirality of H* can be achieved; this phase is, therefore, referred to as one of the mesochiral phases from the self-assembly of BCPs*. A methodology to scrutinize chirality transfers at different length scales for BCP* self-assembly was, therefore, developed by using electronic circular dichroism, vibrational circular dichroism, and electron microscopy tomography for the examination of monomer chirality (optical activity of the molecule), polymer chain chirality (intrachain chiral interactions), and hierarchical chirality (twisting of multichain mesodomain attributable to interchain chiral interactions), respectively. The chirality effects and the corresponding homochiral evolution during BCP self-assembly have been proven to be experimentally generic and supported by the tilted chiral lipid bilayer model and orientational self-consistent field theory. Most interestingly, the formed helical cylinder can serve as the stepping stone for the formation of a network phase, namely, a gyroid, via twisting to yield a wide region for the formed network phase. With the easy development of the gyroid phase, a platform technology, referred as templated syntheses, for the fabrication of nanonetwork-structured materials was, therefore, developed. Consequently, various gyroid-structured materials could be fabricated via templated atomic layer deposition, electroplating, electroless plating, sol–gel reaction, and polymerization to yield well-ordered nanonetwork semiconductive, metallic, ceramic, and polymeric materials after template removal. Note that the hidden chirality of the gyroid can be defined by using the dihedral (torsion) angle concept from the simplified building unit with a tripod texture. With the controlled chirality of the helical cylinder and chiral network, it is appealing and promising to fabricate chiral materials by combining the mesochiral self-assembly and platform technology of templated syntheses, yielding well-ordered nanohybrids and nanoporous materials with helical sense. By taking advantage of the deliberate structuring, the fabrication of chiral metamaterials with appealing applications in negative refractive index materials, devices with giant optical activity, and beam splitters can be expected.