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42456-AC7
Self-Assembled Rod-Coil Diblock Copolymers with Tunable Rods
Block copolymer (BCP) self assembly offers a simple and controllable means to obtain nanostructures with typical orders on the scale of ~5-100 nm. Liquid crystals (LCs) are one class of soft materials that undergo self organization at 1-10 nm. Incorporating LCs as one of the blocks in a BCP results in LCBCPs that possess both structural hierarchy and functionality. One unique type of LCBCP is the rod-coil BCP (RCBCP), in which one block adopts a rigid rod-like conformation. Several factors affect the thermodynamic behavior of these systems, including χ (Flory Huggins parameter), N (degree of polymerization), f (volume fraction of each block), the order parameter of the macromolecular mesogen, and the area per junction of the rod (Arod) and the coil (Acoil). Novel self assembling behaviors with complex phase structures different from the coil-coil systems have been observed. In general, at symmetric volume fractions, lamellar morphology is prevalent. At asymmetric volume fractions, morphologies with highly curved interfaces such as sphere (S) or gyroid (G) have only been observed in low molecular mass rod-coil systems. As the molecular weight of the rod increases, the rigid rod becomes incompatible with the curved inter-material dividing surface (IMDS) dictated by S or G phases; there thus exists competition between liquid crystallinity of the rods and BCP self assembly. Most of the reported work showed that liquid crystallinity dominated the phase behavior and the S or G phase was often degenerated into structures with planar IMDS such as zig-zag, arrow-head, mushroom, perforated lamellae, etc. This is because the interaction between the rods is relatively strong and LC ordering dominates the ordering process. Thus, we hypothesize that in a RCBCP system with relatively weak LC interactions, BCP self assembly could dominate the overall phase structure.
In order to test this hypothesis, we designed a core-shell RCBCP using mesogen-jacketed LC polymers (MJLCP). Side-attaching LC mesogens directly along the polymer backbone leads to MJLCP systems within which the strong interactions between the mesogens and backbone force the backbone to adopt an extended chain conformation and the polymer chains arrange in the form of macromolecular columns. Further linking MJLCPs with coil chains leads to MJ-RCBCPs. Compared to other types of rod-forming macromolecules such as polypeptides, LC/conjugated oligomers, and poly (hexyl isocyanate) (PHIC) and it derivatives, using MJLCP as the rod to form RCBCP is advantageous because the length, diameter and the surface chemistry of the macromolecular rods can be readily controlled. The molculear weight of the rod dictates the rod length and the mesogen structure determines the rod diameter and surface chemistry. By using relatively long soft tails in the molecular design, a core-shell rod can be obtained with an aromatic core and an aliphatic shell. In this system, the shell decouples the strong rod-rod interactions and influences the competition between liquid crystallinity of the rods and BCP self assembly, which in turn, leads to a variety of new hirearchical structures. The unique role of the shell in the RCBCP structure formation is two fold: first, in the symmetric BCPs, both BCP self assembly and LC ordering dictate planar IMDS. In this case, the shell enhances the LC ordering and novel columnar-hexagonal-in-lamellar (ΦH-in-L) hierarchical nanostructure was observed. Second, in the asymmetric BCPs, since BCP self assembly and LC ordering dictate different types of IMDS (curved vs. planar), the shell decreased rod-rod interaction and BCP self assembly became the dominant factor. Consequently, LC symmetry breaks (from ΦH to ΦN) to compromise with the stronger BCP self assembly process.
Poly[styrene-block-{3,5-bis[(4'-((4''-tetradecanoylbenzoyl)oxy)benzoyl)oxy]styrene}] (PS-b-PTBOS) where the mesogen is a bent-core LC (BCLC), was chosen as the model RCBCP system. In the present case, the five-ring mesogen ensures a rigid core of the rod with a relatively large diameter while the 14-C tails of the mesogen render a relatively thick “shell”. Contrary to the conventional rod-coil systems where LC ordering always dictates the phase structure, in the core-shell rod-coil system, BCP self assembly could be the dominant factor in RCBCP structure formation. Novel ΦH-in-L, ΦN-in-PL, ΦN-in-S structures were observed in the symmetric, coil-rich and rod-rich BCPs, respectively. The ΦH symmetry was broken in the asymmetric BCPs, due to the lateral chain repulsion in the perforated layers in the coil-rich sample, and the curved IMDS in the S BCP structure in the rod-rich sample. The The unique core-shell RCBCP also opens a new avenue for the hierarchical structure design of soft matter.
