Liquid-Crystalline Polymers: Molecular Engineering, Hierarchical Structures, and Applications.

Liquid-crystalline polymers (LCPs) are a unique class of soft materials that combine liquid crystal and polymer characteristics. This perspective highlights recent advances of LCPs on the aspects of molecular engineering, hierarchical structures, and emerging applications. The strategy of sequence control in polymer synthesis has been introduced to tailor the primary structures of LCPs as well as their phases and orders. By incorporating mesogenic motifs rich in shape, order, and interaction into LCPs, novel bulk and interfacial structures on hierarchical scales are anticipated. The intrinsic features and fascinating properties of LCPs enable them to find potential applications in emerging areas including integrated circuits, lasing, environment, and energy, implying compelling opportunities for LCPs in fundamental science and transformative technologies.

Cholesteric Liquid Crystalline Polyether with Broad Tunable Circularly Polarized Luminescence.

Strong circularly polarized luminescence (CPL) with high purity and broad tunability was achieved in a new type of polyether-based cholesteric liquid crystalline (CLC) copolymers comprising chiral cholesteryl, nematic mesogens, and cross-linkable moieties. The phase boundary diagram of the copolymers was constructed, wherein the CLC phase in a wide composition and temperature window down to room temperature was achieved. Furthermore, reflection colors across the infrared and visible light regions can be continuously tuned by altering composition or temperature, which can be further fixed in the flexible CLC elastomer by photo-cross-linking. Introducing achiral dyes in the CLC thin films can generate strong CPL with distinct handedness and high dissymmetry factors (glum). Particularly, the left-handed full-color CPL is obtained by selective circularly polarized scattering in the spectral region outside the band gap of the CLC thin film, and the right-handed CPL with glum up to -1.05 is achieved within the band gap of the CLC thin film following the selective circularly polarized reflection mechanism. This type of CPL active material is expected to have potential applications in liquid crystal display and photonics.

Chiral Photonic Liquid Crystalline Polyethers with Widely Tunable Helical Superstructures.

Liquid crystalline polymers with tunable structures on the scale of visible wavelength are important in optical technology due to their enhanced mechanical stability, processability, and structural integrity. Herein, we report a series of cholesteric liquid crystalline (CLC) polyethers with a widely tunable pitch length and a broad CLC phase window through a bottom-up structural design. The well-defined multicomponent polyethers were successfully synthesized by utilizing monomer-activated anionic ring-opening polymerization. Through adjustment of the composition of chiral cholesteryl (Ch) and photochromic azobenzene (Az) mesogenic moieties, rich phase behaviors have been discovered, and a phase boundary diagram was constructed consequently, wherein cholesteric helical superstructures in a broad composition range and temperature window straight down to the glassy state at room temperature were achieved. Particularly, the planar oriented helical superstructures can exhibit widely tunable and switchable reflections over the entire visible range across red, green, and blue colors through temperature and light control, which are closely related to the extraordinary flexibility of the polyether backbone. Their thermo-light dual-responsive properties provide an alternative opportunity to fabricate smart and switchable polymeric LC materials for optical applications.

Ion conduction in the comb-branched polyether electrolytes with controlled network structures.

Solid polymer electrolytes (SPEs) based on centipede-like polyethers composed of short ethylene oxide (EO) brushes and allyl functional groups were generated and followed by in situ crosslinking via thiol-ene "click" chemistry. The delicate control of the mesh sizes of the networks was achieved by tuning the composition of the backbone and the length of the bi-functional EO crosslinkers, which was further evaluated by the equilibrium swelling experiments and the Flory-Rehner theory. This type of SPE demonstrates good compatibility with lithium anodes and a high ionic conductivity up to 1.6 × 10-4 S cm-1 at room temperature, 2 orders of magnitude higher than that of the typical linear PEO. The temperature dependence of the ionic conductivity can be described by the Vogel-Tammann-Fulcher (VTF) equation, which shows a systematic variation of the ion conduction behaviors with the network structures. Particularly, the increase of mesh size results in the increase of the conductivity and the decrease in the content of ion pairs, which is verified in the networks based on end-functionalized systems as well. The higher free ion content in the loose network has been attributed to its larger conformational freedom and optimized complexation of the lithium ions. This type of comb-branched polyether with solvent-like oligomer EO brushes also shows the potential to alleviate the compensation effect between the apparent activation energy and the ion carrier contents, which may provide a promising platform to fabricate high performance electrolytes with optimized ionic conductivity.

Statistics and Dynamics of Polymer Melt in Neutral Diblock Copolymer Single-Crystal Platelets.

Polymer single-crystal (SC) platelets of poly(butylene oxide)-b-poly(lactic acid) (PBO-b-PLLA) of a well-defined shape, size, and grafting density have been fabricated and embedded into PBO melt for the study of the statistics and dynamics of the host polymers. The colloidal liquid-crystalline order of SCs above a threshold concentration of ∼2.2 vol % provides a confining environment for the molten PBO. Meanwhile, the peculiar type-A characteristics of PBO allow us to simultaneously probe the dielectric chain dimensions and the hierarchical dynamics of polymers under confinement. We observe negligible changes to the mean-square end-to-end distance of the polymer melt as well as the chain and segment dynamics, even the interlayer distance approaches the length scale comparable to the size of the host polymers. Our results provide direct evidence of the impacts of neutral walls on both the statistics and the dynamics of confined polymer melts, which can be also enlightening for the field of polymer nanocomposites.

Quasi-Living Copolymerization of Aryl Isocyanates and Epoxides.

Nontraditional polyurethane (PU) has been successfully synthesized by anionic copolymerization of some typical aryl isocyanates and epoxides with ammonium halide onium salt (Lewis base) as the initiator and triisobutylaluminum (Lewis acid) as the activator and the synergistic coordinator. In contrast to the traditional step-growth approach, this chain-growth copolymerization can maintain the anionic propagation site and exhibit some living features with a high activity, by which the copolymers synthesized have narrow molecular weight distributions and discrete end groups. The copolymer is primarily constituted by a urethane linkage, and the byproducts of isocyanurate trimer and oxazolidinone can be effectively suppressed as the polymerization proceeds. Density functional theory (DFT) calculations were also performed to support the proposed reaction mechanism.

Sequencing of Side-Chain Liquid Crystalline Copolymers by Matrix-Assisted Laser Desorption/Ionization Tandem Mass Spectrometry.

Polyether based side-chain liquid crystalline (SCLC) copolymers with distinct microstructures were prepared using living anionic polymerization techniques. The composition, end groups, purity, and sequence of the resulting copolymers were elucidated by matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) and tandem mass spectrometry (MS/MS). MALDI-MS analysis confirmed the presence of (CH3)3CO- and -H end groups at the initiating (α) and terminating (ω) chain end, respectively, and allowed determination of the molecular weight distribution and comonomer content of the copolymers. The comonomer positions along the polymer chain were identified by MS/MS, from the fragments formed via C-O and C-C bond cleavages in the polyether backbone. Random and block architectures could readily be distinguished by the contiguous fragment series formed in these reactions. Notably, backbone C-C bond scission was promoted by a radical formed via initial C-O bond cleavage in the mesogenic side chain. This result documents the ability of a properly substituted side chain to induce sequence indicative bond cleavages in the polyether backbone.

Orientation and Morphology Control of the Liquid Crystalline Block Copolymer Thin Film by Liquid Crystalline Solvent.

The critical challenge to engineer the morphological structures in the strongly phase-segregated block copolymer thin film is to overcome the preferential wetting of the blocks at the interface and direct the self-assembly process. Herein, we utilize surface activity and selective solvation of a nematic (N) liquid crystalline (LC) solvent, 5CB, to facilely alter the LC anchoring and the orientation of the nanophase separated structures of the smectic-nematic (S-N) LC block copolymer thin film. For the neat S-N diblock copolymer thin film, the nanostructures are parallel aligned. In contrast, with continuous introduction of 5CB into the system, the orientations of the characteristic nanostructures and the morphologies of the LC thin film can be consequently changed, yielding the perpendicularly oriented lamellar or cylindrical structures with the feature size below 10 nm. The homeotropic alignment of the 5CB nematics near the air interface plays a critical role to induce this unique behavior in the S-N/5CB systems, which offers an opportunity to fine-tune the interfacial structures and the morphological patterning in the block copolymer thin film.

Discovery of hierarchical superstructures in block copolymers by integrating different liquid crystalline interactions.

Hierarchically assembled superstructures of block copolymers are discovered by introduction of the competition of anisotropy attributed from liquid crystalline ordering and nano-phase separation. When the two prototypical fields of liquid crystals (LCs), smectics (S) and nematics (N), are adjoined within the framework of diblock copolymers (S-N) in a variety of precisely tuned compositions, a novel class of superlattices with an interdigitated array of smectic layers between the nearest lamellae or cylinders are observed. This generates unconventional nano-phase separated orthorhombic (Lo) and pseudo-hexagonal (Ho) structures, where the stretched N tethers, projected regularly from the S layers, could transfer the local organization and guide the correlated assembly. This type of superstructure is highly frustrated in reversed cylindrical morphology and absent in S-coil diblock copolymers. The collective interplay of the S and N interactions with nano-phase separation induces the formation of the final complex yet equilibrium structures, providing unprecedented opportunities towards exquisite structural diversity.

An asymmetric A-B-A' metallo-supramolecular triblock copolymer linked by Ni(2+)-bis-terpyridine complexes at one junction.

A metallo-supramolecular triblock copolymer polystyrene-b-polyisoprene-[Ni(2+)]-polystyrene (SI-[Ni(2+)]-S') has been efficiently prepared using a one-pot, two-step procedure, where the blocks are held by bis-terpyridine complexes at the junction of SI-S'. This specific metallo-supramolecular chemistry is demonstrated to be a robust approach to potentially broaden the diversity of block copolymers. The location of the metal-ligand complexes has a profound influence on the phase separation of the triblock copolymer in the bulk, which results in a distinctive phase segregation between the end blocks and leads to an unexpected asymmetry of the triblock copolymer. The metal-ligand complexes are found to be preferentially located on the adjacent spherical domain and form a core-shell structure. The resulting multiphase material exhibits distinct elastomeric properties with significant toughness and creep recovery behavior. This type of triblock copolymer is anticipated to be a novel class of hybrid thermo-plastic elastomeric material with wide tunability and functionality.

Hierarchical Nanostructures and Self-Assemblies in Smectic-Nematic Liquid Crystalline Diblock Copolymers.

Coexistence of smectic and nematic orders in 3D curvaceous bicontineous cubic or hexagonal hierarchical structures is observed in a novel class of nanophase separated, flexible double liquid crystalline (LC) diblock copolymers of different molecular weights (MWs) but similar compositions, obtained via sequential anionic polymerization. The diblock copolymer of higher MW exhibits an exceptional order-order transition (OOT) from lamellae (Lam) to hexagonal-packed cylinder (HPC) upon nematic ordering. In contrast, the polymer with lower MW forms a thermodynamically stable, ordered gyroid structure, interwining with LC defects on nanoscale. Delicate balance of collective LC interactions and geometric frustration dictates this unique behavior, which offers a genuine way to fine-tune 2D and 3D complex structures with sub-10 nm feature sizes.

Dielectric Chain Dynamics of Side-Chain Liquid Crystalline Polymer.

Both chain dynamics and statistics of side-chain liquid crystalline polymer are experimentally explored. Dielectric measurements over a wide range of frequencies and temperature windows demonstrate that the chain dynamics in the liquid crystalline phase (SmA) is retarded and has higher apparent activation energy compared to that in isotropic melt. The molecular weight dependence of the normal mode relaxation time in the isotropic melt conforms to the Zimm model with excluded volume effect, while it shows Rouse behavior in the liquid crystalline phase. The mean squared end-to-end distance of polymer chain in the liquid crystalline phase decreases compared to that in the isotropic melt. The main chain takes a self-avoiding walk with ⟨Re2⟩ ∼ N2ν, ν = 0.54-0.6 in the isotropic melt, whereas a random walk between smectic layers with ν ≅ 0.5, consistent with the results from chain dynamics.

Breaking symmetry toward nonspherical Janus particles based on polyhedral oligomeric silsesquioxanes: molecular design, "click" synthesis, and hierarchical structure.

The design, synthesis, and self-assembly of a series of precisely defined, nonspherical, polyhedral oligomeric silsesquioxane (POSS)-based molecular Janus particles are reported. The synthesis aims to fulfill the "click" philosophy by using thiol-ene chemistry to efficiently install versatile functionalities on one of the POSS cages. In such a way, both the geometrical and chemical symmetries were broken to create the Janus feature. These particles self-organize into hierarchically ordered supramolecular structures in the bulk. For example, the Janus particle with isobutyl groups on one POSS and carboxylic groups on the other self-assembles into a bilayered structure with head-to-head, tail-to-tail arrangements of each particle, which further organize into a three-dimensional orthorhombic lattice. While the ordered structure in the layers was lost upon heating via a first-order transition, the bilayered structure persisted throughout. This study provides a model system of well-defined molecular Janus particles for the general understanding of their self-assembly and hierarchical structure formation in the condensed state.

Assembly, structure and optical response of three-dimensional dynamically tunable multicomponent superlattices.

We report the successful fabrication of optically active three-dimensional (3D) superlattices that incorporate DNA-encoded components, metallic nanoparticles, and molecular chromophores in well-defined positions. A DNA linker with three distinct binding sites serves as an assembly agent and dynamically tunable structural element for the superlattice. Using small angle X-ray scattering we have revealed the organization of particle-chromophore 3D arrays and monitored their reversible contractions and expansions that were modulated by ionic strength changes. As the distance between the molecular chromophores and plasmonic nanoparticles in the superlattice was regulated in situ, we were able to uncover the relationship between experimentally determined structure and optical response of the system. This dynamical tunability of superlattice results in a dramatic optical response: nearly a three times change of emission rate of the chromophore. The evolution of lifetime with structural changes reasonably agrees with the calculations based on a cumulitative coupling of chromophores with metallic nanoparticles in different coordination shells.

Phase behavior of nanoparticles assembled by DNA linkers.

The phase diagram of DNA linker mediated nanoparticle assemblies was experimentally investigated and constructed. Using small angle x-ray scattering we studied the dependence of the internal structure of assembly on two main system parameters: DNA linker length and the number of linkers per particle. The formation of a crystalline bcc phase was observed for a limited range of linker lengths, while the number of linkers per particle controlled the onset of system crystallization. The influence of linkage defects on crystalline structure was also examined.

Temperature-induced reversible morphological changes of polystyrene-block-poly(ethylene oxide) micelles in solution.

Temperature-induced reversible morphological changes of polystyrene-block-poly(ethylene oxide) micelles with degrees of polymerization of 962 for the PS and 227 for the PEO blocks (PS962-b-PEO227) in N,N-dimethylformamide (DMF)/water, in which water is a selective solvent for the PEO block, were observed. For a system with 0.2 wt % copolymer concentration and 4.5 wt % water concentration in DMF/water, the micelle morphology observed in transmission electron microscopy changed from vesicles at room temperature to worm-like cylinders and then to spheres with increasing temperature. Mixed morphologies were also formed in the intermediate temperature regions. Cooling the system back to room temperature regenerated the vesicle morphology, indicating that the morphological changes were reversible. No hysteresis was observed in the morphological changes during heating and cooling. Dynamic light scattering revealed that the hydrodynamic radius of the micelles decreased with increasing temperature. Combined static and dynamic light scattering results supported the change in morphology with temperature. The critical micellization temperatures and critical morphological transition temperatures were determined by turbidity measurements and were found to be dependent on the copolymer and water concentrations in the DMF/water system. The morphological changes were only possible if the water concentration in the DMF/water system was low, or else the mobility of the PS blocks would be severely restricted. The driving force for these morphological changes was understood to be mainly a reduction in the free energy of the corona and a minor reduction in the free energy of the interface. Morphological observations at different time periods of isothermal experiments indicated that in the pathway from one equilibrium morphology to another, large compound micelles formed as an intermediate or metastable stage.

Phase behaviors and supra-molecular structures of a series of symmetrically tapered bisamides.

A series of symmetrically tapered 1,4-bis[3,4,5-tris(alkan-1-yloxy)benzamido] benzene bisamides (CPhBA, where is the number of carbon atoms in the alkyl chains, = 10, 12 and 16), was synthesized in order to investigate the effect of alkyl chain length on supra-molecular ordered structures induced by hydrogen (H)-bonding and micro-phase separation. These bisamides consist of a rigid aromatic bisamide core with three flexible alkyl chains at each end of the core. Major phase transitions and their origins in CPhBA bisamides were studied with differential scanning calorimetry, one-dimensional (1D) wide angle X-ray diffraction (WAXD), infrared spectroscopy, and solid-state carbon-13 nuclear magnetic resonance experiments. The structures of these compounds in different phases were identified using 2D WAXD from oriented samples and were also confirmed by selected area electron diffractions in transmission electron microscopy from stacked single crystals and by computer simulations. All of the CPhBA bisamides in this series formed a highly ordered oblique columnar () phase and a low-ordered oblique columnar () phase, similar to a recent report on C14PhBA. The two main driving forces in the formation of these two supra-molecular columnar structures were identified: One was the H-bond formation between N-H and C[double bond, length as m-dash]O groups, and the other was the micro-phase separation between the bisamide cores and the alkyl chains. With increasing the length of alkyl tails, the isotropization temperature decreased, while the disordering temperature of the alkyl tails increased. The 2D lattice structures perpendicular to the columnar axis also increasingly deviated from the pseudo-hexagonal packing with increasing the alkyl tail length. However, the alkyl tail length did not have a significant influence on the packing along the columnar axis direction. Utilizing polarized optical microscopy, the phase identifications were also supported by the observation of texture changes and molecular arrangements inside of the micro-sized domains.

Onset of tethered chain overcrowding.

We proposed an approach to precisely control the density of tethered chains on solid substrates using PEO-b-PS and PLLA-b-PS. As the crystallization temperature Tx increased, the PEO or PLLA lamellar crystal thickness d(L) increased as well as the reduced tethering density sigma; of the PS chains. The onset of tethered PS chains overcrowding in solution occurs at sigma(*) approximately 3.7-3.8 as evidenced by an abrupt change in the slope between (d(L))(-1) and Tx. This results from the extra surface free energy created by the tethered chain that starts to affect the growth barrier of the crystalline blocks.