Liquid Crystalline Copolymers Research Articles

Ionic nanoporous membranes from self-assembled liquid crystalline brush-like imidazolium triblock copolymers.

There is a need to generate mechanically and thermally robust ionic nanoporous membranes for separation and fuel cell applications. Herein, we report a general approach to the preparation of ionic nanoporous membranes through custom synthesis, self-assembly, and subsequent chemical manipulations of ionic brush block copolymers. We synthesized polynorbornene-based triblock copolymers containing imidazolium cations balanced by counter anions in the central block, side-chain liquid crystalline units, and sidechain polylactide end blocks. This unique platform comprises: (1) imidazolium/bis(trifluoromethanesulfonyl)imide (TFSI) as the middle block, which has an excellent ion-exchange ability, (2) cyanobiphenyl liquid crystalline end block, a sterically hindered hydrophobic segment, which is chemically stable and immune to hydroxide attack, (3) polylactide brush-like units on the other end block that is easily etched under mild alkaline conditions and (4) a polynorbornene backbone, a lightly crosslinked system that offers mechanical robustness. These membranes retain their morphology before and after backbone crosslinking as well as etching of polylactide sidechains. The ion exchange performance and dimensional stability of these membranes were investigated by water uptake capability and swelling ratio. Moreover, the length of the carbon spacer in the imidazolium/TFSI central block moiety endowed the membrane with improved ionic conductivity. The ionic nanoporous materials are unusual due to their singular thermal, mechanical, alkaline stability and ion transport properties. Applications of these materials include electrochemical actuators, solid-state ionic nanochannel biosensors, and ion-conducting membranes.

Birefringence and Orientation Direction Control of Photoalignable Liquid Crystalline Copolymer Films Based on In Situ Modification of Mesogenic Groups.

Controlling the birefringence of optical films is imperative to fabricate thin birefringent optical devices. Here, new photoalignable liquid crystalline copolymers (PLCPs) with 4-formylphneyl benzoate (PA) and cinnamic acid (CA) side groups are synthesized, which attain high photoreactivity and controllability of the orientation direction. Additionally, the exposed films are treated with 2-aminofluorene (AF) for an in situ condensation of PA with AF to form a Schiff base with a high inherent birefringence. Birefringence of the photoaligned film can be controlled up to 0.45 without disturbing the photoinduced orientation structure.

Photo-Durable Molecularly Oriented Liquid Crystalline Copolymer Film based on Photoalignment of N-benzylideneaniline.

Copolymer films of photoalignable liquid crystalline (LC) copolymethacrylates comprised of a phenyl benzoate mesogen connected with N-benzylideneaniline end moiety (NBA2) and benzoic acid (BA) side groups exhibit a photoinduced reorientation behavior. Significant thermally stimulated molecular reorientation attains a dichroism (D) greater than 0.7 for all copolymer films and a birefringence of 0.113-0.181. In situ thermal hydrolysis of the oriented NBA2 groups decreases the birefringence to 0.111-0.128. However, the oriented structures of the film are maintained, demonstrating a photo-durability, even though the NBA2 side groups photo-react. The hydrolyzed oriented films show higher photo-durability without changing their optical properties.

Flow of polymer blends—Developing a deformation-induced morphology followed by analytical scanning electron microscopy

We applied a new experimental method for studying the evolution of the morphology in the flow of polymer blends. This so-called analytic scanning electron microscopy is based on spectroscopy of Auger electron transitions, which is widely used for catching foreign atoms in metals, alloys, and ceramic articles but has remained unknown in polymer science. The possibility of applying this approach to polymer blends became possible when choosing two polymers that are composed of different atoms. We have used polysulfone with sulfur in the main chain and a thermotropic liquid crystalline copolymer (LCP) of polyethylene terephthalate and p-hydroxybenzoic acid, which has significantly more oxygen atoms in its composition than polysulfone. In addition, LCP has lower viscosity. The blends with different ratios of components were extruded through a capillary at different shear rates. The components are randomly distributed, and no significant changes in the spatial distribution or the radial migration over the volume were observed during the low shear deformations. At high shear rates, phase separation takes place, emulsification of dispersed droplets of a low-viscous phase occurs, and these droplets were drawn along the streamlines, forming a self-arranged bundle of the strings. The increase in the radial transfer, leading to the phase separation, is most likely determined by the deformations and not by the shear rate. If the concentration of the low-viscous phase is high enough, it squeezed out of the extrudate body, forming a coverlayer on the convergent stream at the capillary inlet. The apparent viscosity of the blend strongly depends on the concentration of the low-viscous component. This can be explained by the combination of the predominant flow inside low-viscous strings and the existence of the low viscous “lubricant” on the periphery of the stream. The structure of the blend is destroyed at sufficiently high deformation rates apparently due to the emergence of the elastic turbulence.

Polymerization-induced self-assembly of side-chain liquid crystalline copolymers by dissipative particle dynamics simulation

Side-chain liquid crystal copolymers can self-assemble into dispersed assemblies effectively with the polymerization-induced self-assembly (PISA) technique. However, the synergy of polymerization and self-assembly result in undefined self-assembly behavior and polymerization kinetics. We explored the PISA behavior of side-chain liquid crystal copolymers in selective solvents by combining dissipative particle dynamics simulation and a probability-based reaction model. It was found that the morphologies of assemblies depend on the PISA time, copolymer concentrations, and various structural parameters. The polymerization exhibits three stages with different reaction constants during the PISA, achieving low polydispersity of the produced copolymers. The comparison with experiments demonstrated that the simulation could capture the essential characteristics of PISA. This study can help fundamentally understand the PISA behavior of side-chain liquid crystal copolymers.

Tunable Thermal Switch via Order-Order Transition in Liquid Crystalline Block Copolymer.

Organic material-based thermal switch is drawing much attention as one of the key thermal management devices in organic electronic devices. This study aims at tuning the switching temperature (TS) of thermal conductivity by using liquid crystalline block copolymers (BCs) with different order-order transition temperature (Ttr) related to the types of mesogens in the side chain. The BC films with low Ttr of 363 K and high Ttr of 395 K exhibit reversible thermal conductivity switching behaviors at TS of ∼360 K and ∼390 K, respectively. The BC films also exhibit thermal conductivity variation originating from the anisotropy of the internal structures: poly(ethylene oxide) domains and liquid crystals. These results demonstrate that the switching behavior is attributed to an order-order transition between BC films with vertically arranged cylinder domains and the ones with ordered sphere domains. This highlights that BCs become a promising thermal conductivity switching material with tailored TS.

Synthesis and Phase Behavior of (Semifluorinated Alkane)-Based Side-Chain Liquid Crystalline Copolymers.

Side-chain liquid crystalline polymer (SCLCP) usually contains a simple and flexible homopolymer as main chain, while its effect on the self-assembly behavior is often ignored. In this work, in order to increase the structural complexity and investigate the interaction between the main chain and mesogens, perfluorinated segments are introduced into the main chain using a photoinduced Step Transfer-Addition & Radical-Termination polymerization method, producing a novel series of SCLCPs containing 4-methoxyphenyl benzoate mesogens, soft hydrocarbon spacers, and a strictly alternating perfluoroalkyl and alkyl backbone. By adjusting the length of spacers or perfluoroalkyl segments, several mesophases with complex chain packing structures are achieved. This design strategy that constructing highly ordered liquid crystalline (LC) structures from SCLCPs with precise chemical structure provides a facile way toward novel LC nanomaterials.

Molecular Chirality and Morphological Structural Chirality of Exogenous Chirality-Induced Liquid Crystalline Block Copolymers

Chiral expression from the molecular to macromolecular level, as well as aggregates, plays an important role in natural physiological activity. Therefore, understanding the chirality transfer mechanism will provide guidance for the design of new functional chiral materials. Herein, we have systematically investigated the chiral assembly and transfer from the molecular to morphological level in liquid crystalline block copolymers (LC-BCPs) induced by enantiopure tartaric acid (TA) as a chiral source. Both the experimental and theoretical results indicate that the self-organization of the liquid crystal side chains and H-bonding interactions formed between the chiral additive and LC-BCPs are key factors for chiral expression in the self-assembly of LC-BCPs doped with TA. Furthermore, the various morphologies, including particles, spheres, worms, helical cylinders, and spirals, were observed in the LC-BCPs using transmission electron microscopy. The strength of the liquid crystal related to the azobenzene side chain structure has a crucial effect on the morphological transition. This work provides guidance for the design of diverse chiral nanostructures in a chiral molecule-induced achiral polymer system and may establish a fabrication platform for chiral cryptography, optical metamaterials, and other potential applications.

Polarized Fluorescence of N-Salicylideneaniline Derivatives Formed by In Situ Exchange from N-Benzylideneaniline Side Groups in Photoaligned Liquid Crystalline Copolymer Films.

Polarized fluorescence of oriented N-salicylideneaniline (SA) derivatives is explored based on the thermally stimulated photoinduced molecular reorientation of liquid crystalline (LC) copolymethacrylate with N-benzylideneaniline derivative (NBA2) and benzoic acid (BA) side groups. The LC copolymer films show significant cooperative molecular reorientation of the NBA2 and BA side groups (D > 0.7). Subsequent thermal hydrolysis of the NBA2 side groups yields free phenylamine moieties. These moieties can form oriented SA derivatives via in situ condensation with 2-hydroxybenzaldehyde derivatives. The excited-state intermolecular proton transfer of the oriented SA molecules induces polarized fluorescence at 510-548 nm with a polarization ratio up to 6.2. Direct in situ exchange from the oriented NBA2 to SA derivatives achieves polarized fluorescence similar to that of the SA side groups.

Crystallization-Induced Uniform Nanodot Formation of Titanium Dioxide Films

Abstract We propose a simple technique to fabricate a uniform nanodot array of titanium dioxide via two-dimensional shrinkage in the amorphous-to-crystalline (anatase) transformation during calcination of a precursor film. Increasing the calcination temperature from 400 °C to 600 °C enhances the crystal size diameter from ca. 10 to 20 nm, while mixing the precursor film with a thermotropic liquid crystalline amphiphilic block copolymer results in dot arrays with more uniform particle size.

Formation of hierarchical platelets with morphological control by self-assembly of an azobenzene-containing liquid crystalline diblock copolymer

The diblock copolymer PEG-b-PAzoMA self-assembled into abundant assemblies through a heating-annealing approach, including leaf-type, macro-flower-type, and rectangular platelets by manipulating the concentration and the cooling rate of the solution.

Membranes for Cation Transport Based on Dendronized Poly(Epichlorohydrin-Co-Ethylene Oxide). Part 2: Membrane Characterization and Transport Properties.

In this paper, we report on the preparation and characterization of membranes out of two side-chain liquid crystalline copolymers, dendronized at two different extents (20 and 40%, CP20 and CP40, respectively). The membranes were characterized by atomic force microscopy (AFM), field-emission scanning electron microscopy (FESEM), contact angle (CA) analysis, and water uptake. Moreover, transport properties were studied by methanol and proton conductivity experiment and by linear sweep voltammetry (LSV). For the sake of comparison, the behavior of the grafted copolymers was compared with the unmodified copolyether CP0 and with Nafion 117. Results demonstrated that in CP20 and CP40, cation transport depends on the presence of defined cationic channels, not affected by water presence; the comparison between LSV experiments performed with different alkaline cations suggests that CP40 possesses channels with larger diameters and better-defined inner structures.

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.

Random Liquid Crystalline Copolymers Consisting of Prolate and Oblate Liquid Crystal Monomers

Random Liquid Crystalline Copolymers Consisting of Prolate and Oblate Liquid Crystal Monomers

Hybrid fluorescent cholesteric materials with controllable light emission containing CdSe/ZnS quantum dots stabilized by liquid crystalline block copolymer

Hybrid fluorescent cholesteric liquid crystalline (CLC) materials are representatives of “smart” soft matter, and are characterized by light emission that can be flexibly controlled by various external stimuli. This fact is due to the many possibilities for potential applications in the fields of photonics and optics stimulating design, and study of this type of hybrid materials. Here, we report on the optical and fluorescence properties of the hybrid CLC material based on a low-molecular-weight CLC matrix and CdSe/ZnS quantum dots (QDs) stabilized by LC diblock copolymers. The hybrid CLC material is characterized by the cholesteric phase in a wide temperature range, the high loading of QDs, and no QD aggregation. We demonstrate that the cholesteric stop band alters characteristics of the QD emission due to the resonance effect. This makes the polarization state and wavelength of the QD emission thermo- and angle-dependent. This work provides a way for the design of a wide range of field-controllable photonic devices for various applications.

Employing Cholesterol Copolymerization Strategy for a Thermally Processable Organic Room‐Temperature Phosphorescence Material

AbstractDevelopment of pure organic room‐temperature phosphorescent (RTP) materials with strong emission and decent processability is highly desirable. However, in the pursuit of high quantum yield (QY) of phosphorescence, the processability of RTP materials is suppressed unwarily. Here, a liquid crystalline (LC) copolymer is envisioned, NpA‐Chol, comprising bromonaphthalimide as the phosphor and cholesterol as the LC mesogen. NpA‐Chol exhibits LC flexibility at high temperatures, thus enabling the material to possess decent processability. In addition, the liquid crystallinity of NpA‐Chol could be improved remarkably after thermal annealing, making a 7.5‐fold increase in phosphorescence QY. This cholesterol copolymerization strategy paves a general avenue for addressing the major defect well: highly efficient RTP materials always suffer from suppression of thermal processability.

Composition-dependent phase transformation in side-chain liquid crystalline copolymers with mesogenic groups at different substituent positions.

Copolymerization is an effective approach to tailor the thermal and structural properties of liquid crystalline polymer materials, which is essential for various applications. In this work, two series of polynorbornene copolymers, A-r-B and A-r-C, with the biphenyl mesogenic side group at different substituent positions were synthesized via ring-opening metathesis polymerization in various compositions. The corresponding homopolymers A and C are liquid crystalline polymers, exhibiting an oblique columnar structure (Colob/p2) and lamellar structure, respectively, while homopolymer B is amorphous. The composition-dependent phase behaviors of copolymers were systematically studied with the combination of SAXS, GISAXS, AFM, DSC and POM techniques. With increasing molar content of A (xA), the self-organzied structure of copolymer A-r-B follows the sequence from amorphous to lamellar, undulated lamellar, and Colob/p2 structures, and that of A-r-C follows the sequence of lamellar, undulated lamellar, and Colob/p2 structures. Then, copolymers with undulated lamellar or Colob/p2 structures tend to enter lamellar phase first at higher temperature and then change to the isotropic state during heating. The composition-induced transition from lamellar to supramolecular columnar organization is somewhat reminiscent of block copolymers and other soft matter systems that can form ordered structures. Furthermore, the subsitituent number and position of rigid mesogenic units in the side chain can further modify the morphologies of self-organized phases.

Cooperative and Independent Effect of Modular Functionalization on Mesomorphic Performances and Microphase Separation of Well-Designed Liquid Crystalline Diblock Copolymers.

Liquid crystalline block copolymers (LCBCPs) are promising for developing functional materials owing to an assembly of better functionalities. Taking advantage of differences in reactivity between alkynyl and vinyl over temperature during hydrosilylation, a series of LCBCPs with modular functionalization of the block copolymers (BCPs) are reported by independently and site-selectively attaching azobenzene moieties containing alkynyl (LC1 ) and Si-H (LC2 ) terminals into well-designed poly(styrene)-block-polybutadienes (PS-b-PBs) and poly(4-vinylphenyldimethylsilane)-block-polybutadienes (PVPDMS-b-PBs) produced from living anionic polymerization (LAP). By the principle of modular functionalization, it is demonstrated that mono-functionalized (PVPDMS-g-LC1 )-b-PB and PS-b-(PB-g-LC2 ) not only maintain independence but also have cooperative contributions to bi-functionalized (PVPDMS-g-LC1 )-b-(PB-g-LC2 ) in terms of mesomorphic performances and microphase separation, which is evident from differential scanning calorimetry (DSC) and polarized optical morphologies (POM) and identified by powder X-ray diffractions. With the application of the new principle of modular functionalization, local-crosslinked liquid crystalline networks (LCNs) with controlled functionality are successfully synthesized, which show well-controlled phase behaviors over molecular compositions.

Liquid Crystalline Copolymers Containing Sulfonic and Light-Responsive Groups: From Molecular Design to Conductivity.

In the search for novel smart multifunctional liquid crystalline materials, we report the synthesis, thermal and structural characterisation, and the conductivity, of a set of new block and statistical copolymers, containing light-responsive mesogenic groups (MeOAzB), polar sulfonic acids (AMPS), and methyl(methacrylate) groups (MMA). By using a cascade of reversible addition-fragmentation chain polymerisations, RAFT, we have tailored different side-chain polymeric structures by controlling monomer composition (MeOAzB/AMPS/MMA) and configuration. We have yielded simultaneous liquid crystalline behaviour and appreciable conductivity in polymers with low concentrations of polar acid groups, by the formation of smectic phases in narrow aggregates. The light-responsiveness of the polymers, via reversible trans-to-cis photoisomerization of azobenzene groups, and the local activation of conductivity at relatively low temperatures, opens the possibility to prepare polymer electrolytes for energy conversion and storage, whose conductivity could be controlled and optimised by external stimuli, including light irradiation.

Composite Polymer Electrolyte based on Liquid Crystalline Copolymer with High-temperature Stability and Bendability for All-solid-state Lithium-ion Batteries

Composite Polymer Electrolyte based on Liquid Crystalline Copolymer with High-temperature Stability and Bendability for All-solid-state Lithium-ion Batteries