Liquid Crystalline Triblock Copolymers Research Articles

The Origins of Toroidal Micelles from a Liquid–Crystalline Triblock Copolymer†

Summary of main observation and conclusionIn this paper, we reported a systematic study on the formation mechanism of bent toroidal micelles from a triblock copolymer poly(acrylic acid)‐block‐(2‐cinnamoyloxylethyl methacrylate)‐block‐poly(perfluorooctylethyl methacrylate) (PAA‐b‐PCEMA‐b‐ PFMA) in the binary solvent mixture of methanol and α,α,α‐trifluorotoluene (MeOH/TFT) through a heating‐cooling procedure. It was found that the formation process of toroids was predominantly kinetic‐controlled, and can be influenced by the preparation protocol, solvent composition and stirring rate of the solutions. The toroids were formed via a complicated morphological transition process. At high temperature, the polymer formed bilayered vesicles with bumpy surface, while upon cooling, the vesicles broke and unfolded into bumpy platelet micelles, which subsequently perforated into toroidal and cylindrical micelles upon further cooling. Besides the distinct and unique bumpy vesicle and platelet structures, the transformation mechanisms from vesicular to platelet, and toroidal micelles are also very intriguing, and may provide new insights into the interconversion of micellar morphologies.

Photopatterning of Azobenzene-Containing Liquid Crystalline Triblock Copolymers: Light-Induced Anisotropy and Photostabilization.

Photochromic liquid crystalline block copolymers (PLCBCs) are currently playing a significant role as light-responsive materials because of their light controllable features over multiple length scales. Herein, a study of the photoinduced optical anisotropy derived by the combination of orientation phenomena at molecular and supramolecular levels in a novel kind of side-chain PLCBCs with mesogenic phenyl benzoate groups and pyridine units that is hydrogen bonded with azobenzene-containing phenol is reported. Based on the polymeric architectures and composition, the supramolecular configuration self-organizes in different microphases that affect the material response to the external stimuli. Simple, 1D, polarization holograms are recorded to evaluate the photoinduced birefringence. The first step, light patterning, involves the orientation of the azobenzene units and precedes a thermal treatment that amplifies the induced anisotropy through the cooperative orientation of the mesogenic units. By selective extraction, the azobenzene units can be removed, making the material transparent to the visible light. Excellent photostability of the material birefringence is obtained, whose final value is strongly affected by the block copolymer's architecture. The versatility in the molecular design, the fine control of the photoinduced features by external parameters, and, finally, the possibility to achieve photostability make these materials of great potential for developing optical and photonic devices.

Azobenzene-containing liquid crystalline block copolymer supramolecular complexes as a platform for photopatternable colorless materials

Using liquid crystalline triblock copolymer complexes with hydrogen-bonded azobenzene moieties, photopatterned colorless birefringent films were prepared by irradiation with linearly polarized light and subsequent extracting azobenzene moieties.

Polarization Gratings in Azobenzene-Based Fully Liquid Crystalline Triblock Copolymer.

Photosensitive liquid crystalline (LC) block copolymers are a universal platform for designing functional materials for optics and photonics. Here, azobenzene LC ABA triblock copolymer containing mesogenic groups in each block is studied as a medium for the recording of polarization holographic gratings. The fast recording of polarization gratings is successfully performed with two orthogonally circularly polarized light beams of a 532-nm laser. It is shown that annealing of the recorded polarization grating increases diffraction efficiency considerably, by a factor of 70, and provokes simultaneous erasing of the surface relief grating if it has been recorded. Holographic behavior of the triblock copolymer is compared with that of an azobenzene homopolymer and a random copolymer, which has the same monomer ratio as the triblock copolymer. The obtained results are expected to allow designing new materials for the polarization of optical elements and holographic data storage.

Liquid crystalline block copolymers as adaptive agents for compatibility between CdSe/ZnS quantum dots and low-molecular-weight liquid crystals

Liquid crystalline (LC) triblock copolymers were used either as (i) a nematic matrix, which can be loaded with 10 wt% of CdSe/ZnS QDs, or (ii) a compatibilizer, which allows more than 1 wt% of QDs to be introduced in low-molecular-weight cholesteric LCs.

Polymerization-induced self-assembly of liquid crystalline ABC triblock copolymers with long solvophilic chains

Polymerization-induced self-assembly was exploited to investigate the self-assembly behavior of liquid crystalline triblock copolymers with long solvophilic chains.

Thermoreversible Ion Gel with Tunable Modulus Self-Assembled by a Liquid Crystalline Triblock Copolymer in Ionic Liquid

Ion gels with tunable storage moduli are prepared through the self-assembly of an ABA triblock copolymer AOA-12 comprised of a thermotropic liquid crystalline (LC) polymer as the end-block A and poly(ethylene oxide) (PEO) as the mid-block B in a room-temperature ionic liquid (IL), 1-ethyl-3-methylimidazolium bis(trifluoromethylsufonyl)imide ([EMIM][TFSI]). The PEO mid-block is well-solvated by this ionic liquid, whereas the LC polymer end-blocks aggregate and serve as the physical cross-linkers. For comparison, a triblock copolymer AOA-0 containing a non-LC side-chain polymer with the same mesogen is also synthesized, and its corresponding ion gel is prepared. The ion gels with relatively high concentrations of the LC triblock copolymer have hierarchical structures with different microphase-separated nanostructures and the LC arrangement of the LC blocks. By incorporating the azobenzene mesogen in the side chains, transparent AOA-n/[EMIM][TFSI] (where n is the number of carbon atoms in the spacer between ...

Liquid crystalline side-chain triblock copolymers consisting of a nematic central subblock edged by photochromic azobenzene-containing fragments: their synthesis, structure and photooptical behaviour

Synthesis, phase and photooptical properties of new symmetrical fully liquid crystalline triblock copolymers.

Synthesis and dielectric properties of novel liquid crystalline triblock copolymers with cyanobiphenyl moieties and poly(n‐butyl acrylate) segments

In this work, self‐assembly method was used to improve the dielectric constant of triblock copolymers. A series of ABA triblock copolymers with a defined length of poly(n‐butyl acrylate) (PBA, B block) segment and different lengths of liquid crystalline (LC) poly[11‐(4‐cyano‐4′‐biphenoxy)undecyl methacrylate] (P11CBMA, A block) segments were synthesized by using the atom transfer radical polymerization method. The well‐defined triblock copolymers P11CBMAm‐b‐PBAn‐b‐P11CBMAm possess three different B/A ratios (n = 50, m = 17, 43, 53). Due to the supramolecular cooperative motion effect, the copolymers can form worm‐like microstructure (WLC = 52.8%), cylinder‐like nanostructure with P11CBMA phase embedded in PBA matrix (WLC = 73.9%), and wide stripe structure with LC domains distributed unevenly in a continuous PBA matrix (WLC = 77.7%) after annealed at 160°C (above Ti) under N2 for 24 h, respectively. In order to study the influence of microphase separated morphology of triblock copolymer on the dielectric properties, solvent annealing was also used to develop various nanostructures. After thermal or solvent annealing, the dielectric constants of block copolymers increased dramatically while their loss factors remained the same. For different block copolymers, the dielectric constants increased with the increase of the LC block length. For diverse treatments, dielectric permittivities of samples varied widely with different nanostructures. The results show that the dielectric constants of block copolymers could be tuned by the block ratios and the self‐assembled microstructures. These findings will inspire researchers using self‐assembly method to design and develop novel flexible materials with high dielectric permittivity. Copyright © 2014 John Wiley & Sons, Ltd.

Synthesis of photochromic liquid-crystalline triblock copolymers by pseudoliving reversible addition-fragmentation chain-transfer polymerization

Symmetric photosensitive fully liquid-crystalline triblock copolymers are synthesized by pseudo-living reversible addition-fragmentation chain-transfer radical polymerization for the first time. The polymerization of 3-[methyl(phenyl)amino]propyl acrylate mediated by three different symmetric trithiocarbonates with various leaving groups is studied. It is shown that reversible addition-fragmentation chain-transfer agents make it possible to synthesize narrowly dispersed homopolymers with controlled molecular masses. Poly[(3-[methyl(phenyl)amino]propyl acrylate) trithiocarbonates] are used as polymeric reversible addition-fragmentation chain-transfer agents in the block copolymerization of the phenyl benzoate acrylic monomer. The chemical modification of block copolymers yields desirable photosensitive triblock copolymers containing azobenzene groups. The effect of the molecular structure of triblock copolymers on their phase behavior and thermal properties is examined.

Morphology study of a series of azobenzene-containing side-on liquid crystalline triblock copolymers

A series of azobenzene containing side-on liquid crystalline ABA triblock copolymers were investigated. This triblock series possesses the same central liquid crystal block B and various lengths of the amorphous block A. Transmission electron microscopy (TEM), small angle X-rays and neutron scattering (SAXS and SANS) were used to study their morphologies. After annealing the samples over weeks at a temperature within the nematic temperature range of block B, different morphologies (disordered, lamellar, perforated layer and hexagonal cylinder) were observed by TEM. The alignment behavior of these azo triblock copolymers in the magnetic field for artificial muscle application, as well as the phase period and the order-disorder transition (ODT) were studied in situ by SANS.

Liquid-crystalline symmetric triblock copolymers

Symmetric fully liquid-crystalline triblock copolymers of various structures containing optically active mesogenic groups are for the first time synthesized via pseudoliving radical reversible addition-fragmentation chain-transfer polymerization. Their phase behavior and physicochemical and optical properties are studied. It is shown that, depending on composition, at low temperatures block copolymers can form at temperatures phase-separated structures caused by microsegregation of blocks of different chemical natures and that, with an increase in temperature, these structures can mix to form a cholesteric mesophase characterized by a helical supramolecular structure. A model illustrating the molecular packing of block copolymers with a phase-separated lamellar structure is advanced. The effect of the molecular structure of the block copolymers on their optical properties is discussed.

Tailoring Crystallization Behavior of PEO-Based Liquid Crystalline Block Copolymers through Variation in Liquid Crystalline Content

A series of liquid crystalline–semicrystalline–liquid crystalline triblock copolymers, with poly(ethylene oxide) (PEO) as the semicrystalline central block and polymethacrylate bearing side-chain cholesteryl mesogens as the liquid crystalline (LC) end blocks, are prepared using reversible addition–fragmentation chain transfer (RAFT) polymerization. Starting with 20 kg/mol PEO, the weight fractions of the LC blocks in the triblock copolymers are varied from 21 to 86 wt %. The wide-angle and small-angle X-ray scattering (WAXS and SAXS) as well as transmission electron microscopy (TEM) studies show that with the increased LC content in the triblock copolymers different hierarchical structures including “LC lamellae in PEO lamellae” and “PEO cylinders in LC matrix” are observed sequentially. Differential scanning calorimetry (DSC) study shows that the triblock copolymers with “LC lamellae in PEO lamellae” crystallize at normal undercooling conditions (crystallization temperature Tc observed at 31.0–36.4 °C, which is close to that of homopolymer PEO), while those with “PEO cylinders in LC matrix” crystallize at very large undercooling (Tc drops to −23.5 to −27.8 °C). The large variation of the undercooling conditions required for PEO crystallization is attributed to the nanoconfinement effect from different hierarchical structures at varied LC contents. Avrami analysis has been performed to understand the PEO crystallization mechanism. In “LC lamellae in PEO lamellae”, the PEO crystallization is confined within 2D microdomains between LC lamellae and follows heterogeneous nucleation mechanism with subsequent long-range crystal growth. In “PEO cylinders in LC matrix”, the PEO crystallization is confined within 1D cylindrical microdomains and dominated by homogeneous nucleation, and long-range crystal growth is prohibited by the surrounding LC matrix. This study demonstrates that microsegregated LC domains can provide efficient confinement of the PEO crystallization, and by simply increasing the LC content, amorphous PEO can be obtained at room temperature. These LC–semicrystalline–LC triblock copolymers, with room temperature amorphous PEO confined in microsegregated nanodomains, may be used as scaffolds for lithium ion batteries and solid-state electrolytes.

A new approach to the synthesis of liquid crystalline triblock copolymers with a cholesteric structure

A new approach to the synthesis of liquid crystalline triblock copolymers with a cholesteric structure

ABA type liquid crystalline triblock copolymers by combination of living cationic polymerizaition and ATRP: synthesis and self-assembly.

Lamellar and hexagonal-coil-cylinder self-assembled structures of ABA type triblock copolymers containing mesogen-jacketed liquid crystalline polymer (MJLCP) as the rod block, and polyisobutylene (PIB) as the coil middle block were discovered. PIB was synthesized by living cationic polymerization of isobutylene initiated by 1,4-bis(2-chloro-2-propyl)benzene (p-DCC), and then a small amount of styrene was introduced at the end of the PIB chains to form the difunctional PIB macroinitiator with -CH2CH(C6H5)Cl end groups for further atom transfer radical polymerization (ATRP). 2,5-Bis[(4-methoxyphenyl)oxycarbonyl]styrene (MPCS) was block-copolymerized from the difunctional PIB macroinitiators at 110 °C. The molecular characterization of the triblock copolymers was performed with 1H NMR, 13C NMR, gel permeation chromatography (GPC), and thermogravimetric analysis (TGA). Their phase structures and transitions were investigated by differential scanning calorimetry (DSC), small angle X-ray scattering (SAXS), and polarized optical microscopy experiments. It was demonstrated that the triblock copolymers formed lamellar structures at moderate rod fractions and hexagonal coil cylinders in the rod matrix at high rod fractions. The d-spacing of the microphase-separated structures was influenced by the liquid crystalline phase of rod-like PMPCS blocks.

ABA‐type liquid crystalline triblock copolymers via nitroxide‐mediated radical polymerization: Design, synthesis, and morphologies

AbstractWe have designed and synthesized rod–coil–rod triblock copolymers of controlled molecular weight by two‐step nitroxide‐mediated radical polymerization, where the rod part consists of “mesogen‐jacketed liquid crystalline polymer” (MJLCP). The MJLCP segment examined in our studies is poly{2,5‐bis[(4‐methoxyphenyl)oxycarbonyl]styrene} (MPCS) while the coil part is polyisoprene (PI). Characterization of the triblock copolymers by GPC, 1H and 13C NMR spectroscopies, TGA, DSC confirmed that the triblock copolymers were comprised of microphase‐separated low Tg amorphous PI and high Tg PMPCS blocks. Analysis of POM and 1D, 2D‐WAXD demonstrated that the triblock copolymers formed nematic liquid crystal phase. Morphological studies using TEM indicated the sample formed lamellar structure. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 5949–5956, 2007

A Rheological and SAXS Study of the Lamellar Order in a Side-on Liquid Crystalline Block Copolymer

We study the structure and shear flow behavior of a side-on liquid crystalline triblock copolymer, named PBA-b-PA444-b-PBA (PBA is poly(butyl acrylate) and PA444 is a poly(acrylate) with a nematic liquid crystal side-on mesogen), in the self-assembled lamellar phase and in the disordered phase. Simultaneous oscillatory shear and small-angle X-ray scattering experiments show that shearing PBA-b-PA444-b-PBA at high frequency and strain amplitudes leads to the alignment of the lamellae with normals perpendicular to the shear direction and to the velocity gradient direction, i.e., in the perpendicular orientation. The order-to-disorder transition temperature (T-ODT) is independent of the applied strain, in contrast to results reported in the literature for coil-coil diblock copolymers, which show an increase in T-ODT with shear rate. It is possible that in our system, T-ODT does not depend on the applied strain because the fluctuations are weaker than those present in coil-coil diblock copolymer systems.

Discotic Liquid Crystalline Triblock Copolymers: Interplay of Liquid Crystal Architecture with Microphase Separation

A triblock copolymer has been prepared in which a main chain polymeric discotic liquid crystal is capped at both ends with blocks of poly(ethylene oxide). It has been characterized by GPC, DSC, polarizing microscopy, small- and wide-angle X-ray diffraction, electron microscopy, and rheometry. The copolymer undergoes microphase separation, giving a nanostructured material consisting of a hexagonal array of poly(ethylene oxide) rods in a matrix of columnar liquid crystal. It is found that the alignment of these rods is determined by that of the liquid crystal. Hence, under shear, both the columns of the discotic and the rods of poly(ethylene oxide) align perpendicular to the shear direction. At lower temperatures the poly(ethylene oxide) regions crystallize, and a lamellar structure is obtained.

Liquid crystalline triblock copolymers. Mechanical behaviour and orientation under uniaxial strain

The stress-strain and orientation behaviour of side-chain liquid crystalline(SLCP) ABA triblock copolymers with a backbone of polystyrene-block-1,2-polybutadiene-block-polystyrene and a cyanobiphenyl mesogen in the side chain was investigated in dependence of molecular weight. The polymer shows the behaviour of a thermoplastic liquid crystalline elastomer(TPLCE) in the nematic phase in a region between the glass transitions of the polystyrene block and the SLCP. The ultimate properties and E-modulus is lower than for conventional thermoplastic elastomers. Under uniaxial strain liquid crystalline order perpendicular to the direction of strain is induced.

Thermoplastic liquid crystalline triblock elastomers, 1. Synthesis and phase behaviour

Side-chain liquid crystalline ABA triblock copolymers with a backbone of polystyrene-block-1,2-polybutadiene-block-polystyrene and 5-[(4′-cyano-4-biphenylyl)oxy]pentanoic acid as side chains were synthesized. The block copolymers possess narrow molecular weight distributions. The liquid crystalline (LC) phase behaviour was characterized by means of differential scanning calorimetry (DSC) and X-ray scattering. Nematic and smectic polymers are accessible by variation of the spacer length.