Mesogenic Ordering-Driven Self-Assembly of Liquid Crystalline Block Copolymers in Solution.

With the development of nanotechnology, the preparation of polymeric nanoparticles with nicely defined structures has been well-developed, and the functionalization and subsequent applications of the resultant nanostructures are becoming increasingly important. Particularly, by introducing mesogenic ordering as the driving force for the solution-state self-assembly of liquid crystalline (LC) block copolymers (BCPs), micellar nanostructures with different morphologies, especially anisotropic morphologies, can be easily prepared. This review summarizes the recent progress in the solution-state self-assembly of LC BCPs and is mostly focused on four main related aspects, including an in-depth understanding of the mesogenic ordering-driven self-assembly, precise assembly methods, utilization of these methods to fabricate hierarchical structures, and the potential applications of these well-defined nanostructures. We hope not only to make a systematic summary of previous studies but also to provide some useful thinking for the future development of this field.

Controllable One-Step Assembly of Uniform Liquid Crystalline Block Copolymer Cylindrical Micelles by a Tailored Nucleation-Growth Process and Their Application as Tougheners.

The fabrication of uniform cylindrical nanoobjects from soft materials has attracted tremendous research attention from both fundamental research and practical application points of view but has also posed outstanding challenges in terms of their preparation. Herein, we report a one-step method to assemble cylindrical micelles (CMs) with highly controllable lengths from a single liquid crystalline block copolymer by an in situ nucleation-growth strategy. By adjusting the assembly conditions, the lengths of the CMs are controlled from hundreds of nanometers to micrometers. Several influencing factors are systematically investigated to comprehensively understand the process. Particularly, the solvent quality is found determinative in either enhancing or suppressing the nucleation process to produce shorter and longer CMs, respectively. Taking advantage of this strategy, the lengths of CMs can be nicely controlled over a wide concentration range of four orders of magnitude. Lastly, CMs are produced on decent scales and applied as additives to dramatically toughen glassy plastic matrix, revealing an unprecedented length-dependent toughening effect.

In situ Nucleation-Growth Strategy for Controllable Heterogeneous Supramolecular Polymerization of Liquid Crystalline Block Copolymers and Their Hierarchical Assembly.

The self-assembly morphologies of subunits are largely governed by thermodynamics, which plays a less important role in dimensional control. Particularly for one-dimensional assemblies from block copolymers (BCPs), the negligible energy difference between short and long ones imposes great challenges in length control. Herein, we report that by incorporating additional polymers to induce in situ nucleation and trigger the subsequent growth, controllable supramolecular polymerization driven by mesogenic ordering effect could be realized from liquid crystalline BCPs. The length of the resultant fibrillar supramolecular polymers (SP) is controlled by tuning the ratio between nucleating and growing components. Depending on the choice of BCPs, the SPs can be homopolymer-like, heterogeneous triblock, and even pentablock copolymer-like. More interestingly, with insoluble BCP as a nucleating component, amphiphilic SPs are fabricated, which can undergo spontaneous hierarchical assembly.

Self-sorting assembly of artificial building blocks.

Self-assembly to build high-level structures, which is ubiquitous in living systems, has captured the imagination of scientists, striving to emulate the intricacy, homogeneity and versatility of the naturally occurring systems, and to pursue a similar level of organization in artificial building blocks. In particular, self-sorting assembly in multicomponent systems, based on the spontaneous recognition and consequent spatial aggregation of the same or interactive building units, is able to realize very complicated assembly behaviours, and usually results in multiple well-ordered products or hierarchical structures in a one-step manner. This highly efficient assembly strategy has attracted tremendous research attention in recent years, and numerous examples have been reported in artificial systems, particularly with supramolecular and polymeric building blocks. In the current review, we summarize the progress in recent years, and classify them into five main categories, based on their working mechanisms or principles. With the review of these strategies, we hope to provide not only some deep insights into this field, but also and more importantly, useful thoughts in the design and fabrication of self-sorting systems in the future.

Supramolecular Hexagonal Platelet Assemblies with Uniform and Precisely-Controlled Dimensions.

Geometric structures are commonly encountered in natural and designed systems. However, the bottom-up fabrication of regular geometric assemblies with precise dimensional control, especially from soft materials, poses an outstanding challenge in contemporary materials science and chemistry. Herein, we present a general method for the preparation of colloidally stable, hexagonal platelets via the formation of crystalline inclusion complexes of tris-o-phenylenedioxycyclotriphosphazene and block copolymers bearing interactive blocks. Dictated by the screw dislocation growth of inclusion complexes, uniform hexagonal platelets with precisely controllable dimensions can be prepared. This supramolecular assembly approach is further utilized to produce concentric hexagonal platelets via stepwise seeded growth from various inclusion complexes.

Monodisperse Cylindrical Micelles of Controlled Length with a Liquid-Crystalline Perfluorinated Core by 1D "Self-Seeding".

Precise control over the morphology and dimensions of block copolymer (BCP) micelles has attracted interest due to the potential of this approach to generate functional nanostructures. Incorporation of liquid crystalline (LC) block can provide additional ways to vary micellar morphologies, but the formation of uniform micelles with controllable dimensions from LC BCPs has not yet been realized. Herein, we report the preparation of monodisperse cylindrical micelles with a LC poly(2-(perfluorooctyl)ethyl methacrylate (PFMA) core via a fragmentation-thermal annealing (F-TA) process, resembling the "self-seeding" process of crystalline BCP micelles. The average length of the cylinders increases with annealing temperature, with a narrow length distribution (Lw /Ln <1.1). We also demonstrate the potential application of the cylinders with LC cores as a cargo-carrier by the successful incorporation of a hydrophobic fluorescent dye tagged with a fluorooctyl group.

Convoluted micellar morphological transitions driven by tailorable mesogenic ordering effect from discotic mesogen-containing block copolymer.

Solution-state self-assemblies of block copolymers to form nanostructures are tremendously attractive for their tailorable morphologies and functionalities. While incorporating moieties with strong ordering effects may introduce highly orientational control over the molecular packing and dictate assembly behaviors, subtle and delicate driving forces can yield slower kinetics to reveal manifold metastable morphologies. Herein, we report the unusually convoluted self-assembly behaviors of a liquid crystalline block copolymer bearing triphenylene discotic mesogens. They undergo unusual multiple morphological transitions spontaneously, driven by their intrinsic subtle liquid crystalline ordering effect. Meanwhile, liquid crystalline orderedness can also be built very quickly by doping the mesogens with small-molecule dopants, and the morphological transitions are dramatically accelerated and various exotic micelles are produced. Surprisingly, with high doping levels, the self-assembly mechanism of this block copolymer is completely changed from intramolecular chain shuffling and rearrangement to nucleation-growth mode, based on which self-seeding experiments can be conducted to produce highly uniform fibrils.

Assembly of Supramolecular Nanoplatelets with Tailorable Geometrical Shapes and Dimensions.

The craving for controllable assembly of geometrical nanostructures from artificial building motifs, which is routinely achieved in naturally occurring systems, has been a perpetual and outstanding challenge in the field of chemistry and materials science. In particular, the assembly of nanostructures with different geometries and controllable dimensions is crucial for their functionalities and is usually achieved with distinct assembling subunits via convoluted assembly strategies. Herein, we report that with the same building subunits of α-cyclodextrin (α-CD)/block copolymer inclusion complex (IC), geometrical nanoplatelets with hexagonal, square, and circular shapes could be produced by simply controlling the solvent conditions via one-step assembly procedure, driven by the crystallization of IC. Interestingly, these nanoplatelets with different shapes shared the same crystalline lattice and could therefore be interconverted to each other by merely tuning the solvent compositions. Moreover, the dimensions of these platelets could be decently controlled by tuning the overall concentrations.

One-pot universal initiation-growth methods from a liquid crystalline block copolymer.

The construction of hierarchical nanostructures with precise morphological and dimensional control has been one of the ultimate goals of contemporary materials science and chemistry, and the emulation of tailor-made nanoscale superstructures realized in the nature, using artificial building blocks, poses outstanding challenges. Herein we report a one-pot strategy to precisely synthesize hierarchical nanostructures through an in-situ initiation-growth process from a liquid crystalline block copolymer. The assembly process, analogous to living chain polymerization, can be triggered by small-molecule, macromolecule or even nanoobject initiators to produce various hierarchical superstructures with highly uniform morphologies and finely tunable dimensions. Because of the high degree of controllability and predictability, this assembly strategy opens the avenue to the design and construction of hierarchical structures with broad utility and accessibility.