Synthesis of polystyrene-polysiloxane side-chain liquid crystalline block copolymers

The synthesis of a new liquid crystalline block copolymer consisting of a polystyrene block and a side-chain liquid crystalline siloxane block is reported. The synthetic approach described is based on the anionic polymerization of styrene and cyclic trimethyltrivinyltrisiloxane monomers, followed by functionalization of the siloxane block with side chain mesogens. The siloxane block has a T_g well below 25°C and is designed to exhibit a chiral smectic C^* phase at room temperature. These block copolymers are the first side-chain liquid crystalline block copolymers which contain both a high T_g glassy block and a low T_g liquid crystalline block.     

Electroclinic properties of chiral smectic a liquid crystalline polymers and block copolymers

The scope for the synthesis and investigation of chiral smectic A liquid crystalline (LC) polymers and block copolymers is discussed. LC block copolymers can combine the molecular order of liquid crystals and the supramolecular order typical of block copolymers, thus allowing to attain information on mesomorphic responses in restricted geometries. We present a general overview on several aspects of the syntheses and properties of LC block copolymers, specifically those obtained by starting from azomacroinitiators. These materials can exhibit an electroclinic effect, that is an electrically induced molecular tilt, which is characterized by a linear dependence on the applied field and a very fast response time in the paraelectric smectic A phase. The current progress in their potential application in electrooptics is outlined.     

Synthesis and characterization of H-type amphiphilic liquid crystalline block copolymers by ATRP

H-type amphiphilic liquid crystalline block copolymers containing azobenzene were synthesized by atom transfer radical polymerization （ATRP）. Macroinitiators prepared by the esterification between poly（ethylene oxide） （PEG） and 2,2-dichloroacetyl chloride were utilized to initiate the polymerization of 6-[4-（4-ethoxyphenylazo）phenoxy]hexyl rnethacrylate （M6C）. The resulting macroinitiators and block copolymers were characterized by ^1H NMR, gel permeation chromatography （GPC）. Polarizing optical microscopy （POM） and differential scanning calorimetry （DSC） preliminarily revealed the liquid crystalline property of these block copolymers. This series of liquid crystalline block copolymers are promising in some areas, such as optical data storage, optical switch, and molecular devices.     

Mesophase structure and alignment under different fields of liquid crystalline main-chain/side-group block copolymers

Liquid crystalline block copolymers are new materials in which multiple molecular interactions can provide the driving force for complex phase behaviors and states of order. Block copolymers containing both liquid crystalline main-chain polyester and side-group polymethacrylate blocks were investigated. They phase separated in the liquid crystalline state and their individual mesophases coexisted. The copolymers responded very differently when either a mechanical or a magnetic field was used for alignment. In the fibers the orientations of the side-group and main-chain smectic planes with respect to the fiber axis depended critically on the block lengths and on their distinct tendencies to align, whereas under a magnetic field the mesogens aligned collectively with their long molecular axis parallel to the field, independent of the copolymer structure.     

Design and preparation of liquid crystalline block copolymers

The design and preparation of liquid crystalline (LC) block copolymers by use of azo-macroinitiators are outlined. This approach is very versatile and makes it possible to realize diverse architectures of block copolymers, including non-LC/side-chain, non-LC/main-chain and side-chain/main-chain block copolymers. The different blocks were phase separated and underwent their individual phase transitions. In side-chain/main-chain block copolymers different LC mesophases coexisted in equilibrium.     

Influence of a liquid crystalline block on the microdomain structure of block copolymers

We report on the phase behavior and microdomain structure of two types of diblock copolymers containing a liquid crystal (LC) block joined to a flexible coil block. Consideration of the symmetry groups of the liquid crystalline phases and of the block copolymer microdomain structures provides a rationale for predicting the possible types of liquid crystalline block copolymer morphologies. Both previously reported and newly discovered structural types are identified. Possible organizational schemes are developed for the mesogens and periodic disclination defects with respect to the intermaterial dividing surfaces separating the liquid crystalline and flexible coil domains. The first type of copolymer investigated has a rod-like LC block whereas the second type copolymer has a side chain LC block. Five different rod-coil diblocks based on poly(hexyl isocyanate-b-styrene) P(HIC-b-S) were synthesized by anionic polymerization. Wavy lamellae, zig-zag and arrowhead microdomain morphologies corresponding to smectic-C and smectic-O structures were observed depending on the composition. These layered phases have the director (PHIC chain axis) tilted at various orientations with respect to the layer normal. Side-chain LC diblocks based on functionalized poly(isoprene-b-styrene) P(I-b-S) were also investigated. These polymers were synthesized using polymer analogous chemistry from P(I-b-S) precursors. Three different mesogenic groups were attached to the PI blocks: one based on biphenyl benzoate and two based on azobenzene. The microdomain structures found for the functionalized poly(isoprene side-chain LC-b-styrene) P(ILC-b-S) diblocks are typical of traditional coil-coil diblocks (lamellae and cylinders). However, these morphologies possess an additional smectic layering of the mesogens within the microdomains of the LC block. In the case of the rod-coil diblocks, the transformation from an initially isotropic state to the final microphase separated solid state occurs via nematic and then smectic liquid crystalline states, whereas for the side-chain LC-coil cases, the microphase separation transition occurs prior to development of orientational order. The long-range microdomain order of LC block-coil block copolymers can extend over very large distances due to the influence of the orientational ordering of the LC block.     

Liquid crystalline side-group block copolymers with triblock structure: Investigations on the influence of the block arrangement on the morphology and the LC-phase behavior

Liquid crystalline triblock copolymers with LC inner block and amorphous outer blocks have been synthesized by “living” anionic polymerization and investigated using DSC, TEM, and small-angle x-ray diffraction. All samples of poly[styrene-block-2-(3-cholesteryloxycarbonyloxy) ethyl methacrylate-block-styrene] (PS-b-PChEMA-b-PS) show liquid crystalline behavior and phase separation between the blocks. Compared to triblock copolymers with PS inner block (PChEMA-b-PS-b-PChEMA) and diblock copolymers (PS-b-PChEMA) the LC block copolymers with PS outer blocks have the same properties. The LC behavior and the morphology do not depend on the block arrangement; they are only influenced by the volume fractions of the blocks. Those samples in which the liquid crystalline subphase is not continuous (spheres) only a nematic phase was found, whereas in all samples with a continuous liquid crystalline subphase, the smectic A phase of the homopolymer was observed. © 1996 John Wiley & Sons, Inc.     

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.     

Robust control of microdomain orientation in thin films of block copolymers by zone casting

Block copolymers with chemically immiscible segments exhibit a variety of microphase-separated nanostructures on the scale of 10-100 nm. Controlling the orientation of these microphase separated nanostructures is vital in many applications such as lithography, membranes, data storage, and so forth. Typical strategies involve the use of external fields or patterned substrates. Here, we report a robust zone casting technique to achieve highly ordered thin films of block copolymers on centimeter-scale substrates. The robustness of this technique is its powerful control on diverse morphologies and exceptional tolerance on versatility of block copolymer chemistry as well as allowance of a wide spectrum of substrates. We demonstrate that perpendicular orientations with respect to the surface are achieved for block copolymers with both lamellar and cylindrical morphologies by controlling solution casting rate, temperatures, and block copolymer chemical structures. Thin films of both noncrystalline and crystalline block copolymers exhibit excellent orientational order and lateral order. However, the lateral order in the thin films of crystalline block copolymers shows dependence on casting temperature and melting temperature of the crystalline segment. Remarkably, all the ordering is independent of the substrates on which the block copolymer films are cast.     

Poly(3-alkylthiophene) diblock copolymers with ordered microstructures and continuous semiconducting pathways

Conjugated rod-coil diblock copolymers self-assemble due to a balance of liquid crystalline (rod-rod) and enthalpic (rod-coil) interactions. Previous work has shown that while classical block copolymers self-assemble into a wide variety of nanostructures, when rod-rod interactions dominate self-assembly in rod-coil block copolymers, lamellar structures are preferred. Here, it is demonstrated that other, potentially more useful, nanostructures can be formed when these two interactions are more closely balanced. In particular, hexagonally packed polylactide (PLA) cylinders embedded in a semiconducting poly(3-alkylthiophene) (P3AT) matrix can be formed. This microstructure has been long-sought as it provides an opportunity to incorporate additional functionalities into a majority phase nanostructured conjugated polymer, for example in organic photovoltaic applications. Previous efforts to generate this phase in polythiophene-based block copolymers have failed due to the high driving force for P3AT crystallization. Here, we demonstrate that careful design of the P3AT moiety allows for a balance between crystallization and microphase separation due to chemical dissimilarity between copolymer blocks. In addition to hexagonally packed cylinders, P3AT-PLA block copolymers form nanostructures with long-range order at all block copolymer compositions. Importantly, the conjugated moiety of the P3AT-PLA block copolymers retains the crystalline packing structure and characteristic high time-of-flight charge transport of the homopolymer polythiophene (μ(h) ~10(-4) cm(2) V(-1) s(-1)) in the confined geometry of the block copolymer domains.     

Liquid crystalline block copolymers by sequential cationic or promoted cationic and free-radical polymerizations

The scope for the study of the synthesis and properties of liquid crystalline (LC) block copolymers is briefly outlined. While there are many approaches to the synthesis of LC block copolymers, the use of azo macroinitiators is very versatile and allows one to produce diverse block copolymer architectures. Azo macroinitiators are prepared by cationic or promoted cationic polymerization of tetrahydrofuran (1) or cyclohexene oxide (2), and are then used to initiate the free-radical polymerization of various methacrylates 3,4 or acrylates 5–9 containing mesogenic azobenzene or biphenyl units thereby yielding block copolymers. The AB or ABA block copolymers are microphase-separated and form smectic and/or nematic mesophases similar to the respective LC homopolymers.     

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.     

Synthesis and properties of photochromic liquid‐crystalline copolymers containing both spironaphthoxazine and cholesteryl groups

Photochromic liquid‐crystalline copolymers consisting of a photochromic monomeric unit containing both a spironaphthoxazine group and an undecamethylene spacer, and a liquid‐crystalline monomeric unit containing both a cholesteryl group and a decamethylene spacer were prepared to investigate the effect of the thermal properties of the photochromic monomeric unit on the mesomorphic order of the side chain of the related copolymers. The photochromic liquid‐crystalline copolymers containing a photochromic liquid‐crystalline monomeric unit showed only a smectic phase. On the other hand, the photochromic liquid‐crystalline copolymers containing a photochromic non‐liquid‐crystalline monomeric unit showed a chiral nematic phase (cholesteric phase). The photochromic chiral nematic liquid‐crystalline copolymer containing 14 mol % photochromic monomeric unit reflected visible light around 104 °C. To lower the temperature range of reflection of visible light, cholesteryl oleyl carbonate was used as a chiral nematic plasticizer for the photochromic chiral liquid‐crystalline polymer systems. Photo‐induced pitch change of the mixture by means of UV irradiation was investigated and it was concluded that the pitch change observed under UV irradiation was mainly induced by thermal effect in the case of our system. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 887–894, 2000     

Synthesis of new side‐group liquid crystalline block copolymers by living anionic polymerization

A new series of AB liquid crystalline (lc) block copolymers was synthesized via living anionic copolymerization of styrene as segment A and 6‐[4‐(4‐cyanophenylazo)phenoxy]hexyl methacrylate ( 1 ) as segment B. The copolymers were successfully prepared in quantitative yields and with narrow molar mass distributions. The number‐average molar mass of the segments was varied well‐defined. The polystyrene block ranged from 3 500 to 7 400 g/mol. The content of the B segment ranged from 1 to 54 wt.‐%. Thermotropic phases were detected by differential scanning calorimetry (DSC) for block copolymers with an lc segment of more than 30 wt.‐%.     

Synthesis and characterization of block copolymers having liquid crystalline behaviour

A series of four block copolymers based on aromatic acetoxy-polysulphone (Ac-PSF), terephthalic acid (TPA), m-phthalic acid (MPA), acetoxy-benzoic acid (Ac-HBA), and a di(acetoxy)bisphenol monomer were synthesized. Their structures were determined by IR, thermal mechanical analyses and solubility tests. They all exhibit liquid crystalline behaviour.     

Block copolymers with liquid crystalline segments

This paper describes the synthesis and characterization of polystyrene-block-poly(2,2'-dimethyl-4,4'-biphenylene phenylterephthalate)-block-polystyrene and of poly(ethylene glycol)-black-poly(2,2'-dimethyl-4,4'-biphenylene phenylterephthalate)-block-poly (ethylene glycol) block copolymers. The ABA-triblock copolymers were synthesized by condensation reaction of telechelic poly(2,2'-dimethyl-4,4'-biphenylene phenylterephthalate) with ω-hydroxy polystyrene and ω-hydroxy poly(ethylene glycol) methyl ether of different molecular weights prepared by anionic polymerization. Some aspects of the liquid crystalline behavior and the phase transitions with respect to the block copolymer composition will be discussed.     

Jacketed polymers: Controlled synthesis of mesogen‐jacketed polymers and block copolymers

The controlled radical polymerization of mesogen‐jacketed liquid crystalline polymers has triggered great interests in synthesis of complex structures as well as well‐defined linear homopolymers with controlled molecular weight and narrow molecular weight distributions. This review highlights the synthetic strategies of controlled radical polymerization of linear homopolymers, star polymers, superbranched polymers, graft polymers, block copolymers, star block copolymers, and so on. The employed living methods include nitroxide‐mediated radical polymerization and atom transfer radical polymerization. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 319–330, 2009     

Synthesis, characterization and coating applications of liquid crystalline acrylic copolymers

The synthesis and characterization of the coating properties of liquid crystalline acrylic copolymers containing p -hydroxybenzoic acid as mesogenic group are described. The synthetic method involves the grafting of p-hydroxybenzoic acid onto acrylic copolymers, confirmed by characterization with IR and ¹H NMR techniques. The presence of liquid crystal was assessed by observing optical textures under the polarizing microscope and by DSC and TGA. The coating properties such as flexibility, hardness, adhesion, drying time, viscosity, etc. of acrylic copolymers as well as of liquid crystalline acrylic copolymers were studied. The results showed that LC polymers have potential for coating applications.     

Modification of the lyotropic liquid crystalline microstructure of amphiphilic block copolymers in the presence of cosolvents

This article reviews the results of recent investigations on the macroscopic (phase behavior) and microscopic (microstructure) aspects of the role of cosolvents on the self-assembly of amphiphilic copolymers. A comprehensive account of the systematic studies performed in ternary isothermal systems consisting of a representative poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) (PEO–PPO–PEO) block copolymer (Pluronic P105, EO₃₇PO₅₈EO₃₇), water and a polar cosolvent (such as glycerol, propylene glycol or ethanol) is presented. The effect of cosolvents on the copolymer phase behavior is quantified in terms of the highest cosolvent/water ratio able to maintain the liquid crystalline structures. The effect of cosolvents on the microstructure of the lyotropic liquid crystals is quantified in terms of the degree of relative swelling per cosolvent content per copolymer content, a parameter that characterizes the given cosolvent and copolymer. The set of correlations on the cosolvent effects on the phase behavior or microstructure to the cosolvent physicochemical characteristics (such as octanol/water partition coefficient or solubility parameter) have led to the development of a hypothesis that accounts for the cosolvent effects on the self-assembly of PEO–PPO–PEO block copolymers and can be used to predict them. The rich structural diversity and the potential for a precise and convenient modification of the lyotropic liquid crystalline microstructure of the PEO–PPO–PEO block copolymers is discussed in comparison to the phase behavior of the low-molecular nonionic surfactants.     

On the interaction of the morphological structure and the LC behaviour of LC side group block copolymers

The interaction between morphological structure and phase behaviour of a group of LC side group block copolymers have been investigated using DSC, TEM and small angle X-ray diffraction. Generally, phase separation between the two blocks was observed. It was found that in the case of those samples, where the liquid crystalline sub-phase is not continuous (spheres), only a nematic phase is seen, whereas in all samples in which there is a continuous liquid crystalline sub-phase, the smectic A phase of the homopolymer is formed. On the other hand, the block copolymer seems to stabilise the LC-phase and the polymer properties in general, no dependency of the clearing temperatures and of the glass transition temperatures from the molecular weight of the LC blocks has been detected.