Hierarchical nanostructures of liquid crystal and polymer composites (LCPCs) have recently drawn a lot of research interest because of their potential applications in nanotemplates, nanolithography, information security and etc. The introduction of liquid crystal (LC) moieties enables block copolymers (BCs) with “stimulus-responsive” properties, and makes it possible to realize the fabrication and manipulation of nanostructures in a large area. However, there are still some challenges in the study of the hierarchical nanostructures in LCPC materials.
Firstly, the development of nanoscience demands more complicated and subtler nano-device, which makes it a hot but challenging task to fabricate hierarchical nanostructures in LCPCs. Secondly, it is very hard to control these nanostructures rapidly, reversely and accurately. Professor Haifeng Yu’s group in the Department of Materials Science and Engineering, College of Engineering has recently proposed possible solutions.
Based on their previous research, Professor Yu’s group designed a series of urethane linkage containing liquid-crystalline block copolymers (LCBCs) with azobenzene mesogens in side chain. Urethane linkage could form strong hydrogen bonds with both the dispersed phase and the continuous phase on microphase-separation of the LCBCs. The supramolecular interactions and the interference with the two blocks of the LCBCs significantly affected the crystallization of the component in the dispersed phase when the LCBC was annealed from its isotropic phase to room temperature, resulting in a polymer domain where the amorphous and crystallized regions coexisted in the separated nanocylinders. As a result, the fabrication of sub-10 nm hierarchical nanostructures has been realized based on the hydrogen bonding in the LCBCs themselves without any other dopants. (Figure 1) The above work has been published in Macromolecular Rapid Communications.
Figure 1. Hydrogen bonds induced hierarchical nanostructures in liquid crystal and polymer composites.
Another strategy to fabricate hierarchical nanostructures in LCPCs in a reliable and simple way is doping. Here, dopants serve as the hydrogen bond donors and liquid-crystalline polymers serve as the hydrogen bond acceptors. The selected dopants could interact with both blocks of the LCBC through hydrogen bonding. The chirality of the dopants could also be transferred though the supramolecular interactions and furtherly influence the self-assembly of the liquid-crystalline polymers. Chirality would be firstly transferred to the minor phase of the LCBC because of their good compatibility, inducing helical nanostructures. Optimizing several factors such as thermal treatment condition and contents of dopants could lead to further chirality transfer and generate aggregation chirality in the major phase.
In addition, LC properties have great impact on the chirality induced supramolecular self-assembly in this system. Since self-organization of the major phase of LCBCs could direct self-assembly of both the non-mesogenic block and the mesogenic block by supramolecular cooperative motion (SMCM), photoinduced phase transition led to the breaking of the aggregation chirality.
Furthermore, the occurrence of phase transition in the major phase would change the free volume and induce the reorganization of the segments. Thus, in-plane arrangement nanostructures turned into out-of-plane orientation. Then the following thermal treatment could regenerate the aggregation chirality. This provides a simple but elegant method for construction and non-contact manipulation of complicated hierarchical nanostructures (Figure 2). This work has been published in Angew. Chem. Int. Ed..
Figure 2. An illustration for the transfer, photoinduced breaking, and regeneration of the aggregation chirality.
The 1st author of the above work is Shuai Huang (PhD student, College of Engineering). The corresponding author is Professor Haifeng Yu. The research is financially supported by the National Natural Science Foundation of China.