Nematic Copolymer Research Articles

Full‐Polymer Cholesteric Composites for Transmission and Reflection Holographic Gratings

A new type of self‐organized materials based on cholesteric networks filled with photoactive side‐chain copolymer is being developed. Supramolecular helical structure of cholesteric polymer network resulting in the selective reflection is used as a photonic scaffold. Photochromic azobenzene‐containing nematic copolymer is embedded in cholesteric scaffold and utilized as a photoactive media for optical pattering. 1D and 2D transmission diffraction gratings are successfully recorded in composite films by holographic technique. For the first time the possibility to create selective reflection gratings in cholesteric material mimicking the natural optical properties of cholesteric mesophase is demonstrated. That enables the coexistence of two selective gratings, where one has an intrinsic cholesteric periodic helical structure and the other is a holographic grating generated in photochromic polymer. The full‐polymer composites provide high light‐induced optical anisotropy due to effective photo‐orientation of side‐chain fragments of the azobenzene‐containing liquid crystalline polymer, and prevent the degradation of the helical superstructure maintaining all optical properties of cholesteric mesophase. The proposed class of optical materials could be easily applied to a broad range of polymeric materials with specific functionality. The versatility of the adjustment and material preprogramming combined with high optical performance makes these materials a highly promising candidate for modern optical and photonic applications.

Nematic copolymer network LCs for swift continuous phase modulation and opaque scattering states

ABSTRACTThe electro-optic response properties of a copolymer network liquid crystal in test cells with cell gaps of 2.5 µm and 5 µm were compared. In both test cells, responses with sums (ton + toff) < 3.5 ms were found. The responses were ≈ 1 ms faster in the thinner test cells. Moreover, the experiments with the thinner test cells showed that the response time toff increased with increasing voltage. The scattering properties of samples with lower polymer content were investigated and it was found that samples with 4% overall copolymer content could scatter more than 40% of the incident light already in a 8 µm thick LC layer at driving voltages of 8–12 V, whereas the no voltage state had a high see-through transparency.

Linear Viscoelastic Behavior of an Azobenzene Nematic Block Copolymer

We report on the rheological investigation of a nematic block copolymer of an azobenzene methacrylate (MA4) and methyl methacrylate (MMA). Relaxation processes are discussed in terms of chain architecture and viscoelastic response of the polymer. In contrast to analogous random copolymers of MMA and MA4, the present block copolymer did not show validity of the time-temperature superposition (TTS) principle for all material functions. TTS was found to work only for the storage modulus. Rheological steady-state and oscillatory measurements were thus compared in order to obtain further insight into such a peculiar phenomenon.

Effects of chemical structure and composition of the polymer matrix on the morphology and electro-optical performance of polymer-dispersed liquid crystal films

Polymer-dispersed liquid crystal (PDLC) systems on the basis of nematic liquid crystal E7 and amphiphilic binary copolymers of acrylic acid (AA) with such acrylates as 2-ethylhexyl acrylate (EHA), n-butyl acrylate (BA), and methyl acrylate (MA) are investigated. It is shown that the liquid crystal (LC) drops in the copolymer EHA–AA have submicrometre sizes, and their dependence on the composition of the photo-curable monomer mixture is described by a parabolic curve. The highly oriented domain structure in the same system is first revealed when electric field is applied. The threshold voltage for all systems begins to increase with some critical composition of a monomer mixture in which the longer the hydrocarbonic radical in an acrylate molecule, the higher the content of AA. The PDLC system based on the BA–AA copolymer with 30 wt% LC exhibits the least value of the driving voltage, 1 V μm–1, and the lowest memory effect.

Fluorescent and photo-optical properties of hydrogen-bonded polymer liquid-crystalline composites based on derivatives of stilbazole and crown ethers

New hydrogen-bonded photochromic stilbazole- and crown-ether-containing polymer LC composites are prepared and characterized. A smectic homopolymer with p-oxybenzoic groups, a nematic homopolymer carrying phenylmethoxy benzoate groups, and a nematic copolymer containing p-oxybenzoic and phenylmethoxy benzoate groups are synthesized and used as polymer matrices for these composites. The phase behavior and photo-optical properties of LC composites of various compositions are studied. A marked difference is observed in the fluorescence spectra for hydrogen-stabilized LC mixtures and model mixtures not forming hydrogen bonds. This effect is explained by the reversible transfer of proton of carboxyl groups involved in hydrogen bonding. It is shown that the complexation of crown-ether-containing groups of LC composites with potassium ions leads to a dramatic reduction in the intensity of fluorescence. This phenomenon may be used for creation of a new generation of sensors for metal ions.

Fluorescent and photooptical properties of H-bonded LC composites based on stilbazole derivative

A novel series of H-bonded LC (smectic and nematic) composites and “model” mixtures without hydrogen bonding based on the stilbazole derivative were prepared. As the matrices of the composites following polymers were synthesized: smectic homopolymer containing p-oxybenzoic groups, nematic copolymer containing p-oxybenzoic and methoxyphenylbenzoic side groups, and nematic homopolymer with methoxyphenylbenzoate groups. The spectral properties of composite films were studied in detail. Significant differences in the fluorescence spectra of H-bonded LC composites and “model” mixtures were revealed. Mechanism of the processes occurring in the films of the LC composites under the action of the UV light was proposed. The mechanism includes a reversible proton transfer involved in the H-bonds formation. It was found that in the oriented films of the nematic composite degree of fluorescence polarization is higher than in the films of a similar mixture without H-bonding. The effect of linearly polarized light on the optical properties of the composite films was studied. It was shown that the highest values of the photoinduced dichroism are achieved in nematic liquid crystals composites. It was revealed that for the irradiated LC composites annealing leads to decease of dichroism values (degree of polarized emission drops to zero), whereas for “model” LC mixture without hydrogen bonding a significant increase in these parameters was found.

Photoinduced reorientation processes in thin films of photochromic LC polymers on substrates with a photocontrollable command surface

Orientation and reorientation processes that occur in nematic and cholesteric LC polymer systems under irradiation with plane-polarized light are studied. A copolyacrylate containing phenyl benzoate and azobenzene side groups is synthesized as a nematic polymer; the cholesteric mixture is prepared via doping of the nematic copolymer with the chiral dopant, the derivative of D-isosorbide. Thin layers of the azobenzene-containing photoorientant SD-1 are first used as orienting substrates for polymer liquid crystals. Thin layers of the copolymer and of the mixture are spin-coated on the substrate after irradiation of the photoorientant layer with polarized light. It is shown that after annealing phenyl benzoate and azobenzene side groups of the nematic copolymer orient strictly along the direction of orientation of surface molecules, whereas in the case of the cholesteric mixture, a partial formation of the helical structure is observed. It is demonstrated that all the systems under examination can experience the repeated cyclic reorientation of the cooperative type under irradiation and subsequent annealing of the films.

Novel mechanical properties in lamellar phases of liquid-crystalline diblock copolymers

Structural properties of flexible nematic diblock copolymers in the lamellar phase are investigated using a mean-field model. We address two complementary questions on the mechanics of the system: 1) How does the nematic order affect the elasticity of the one-dimensional solid? 2) What effect does the block copolymer microstructure has on the orientation of the nematic director? In the limit when the microstructure does not influence the nematic director orientation we predict a soft lamellar compression mode. When the microstructure does influence the nematic director orientation, small compressions lead to conventional elasticity, until a critical strain is reached, where there is then a transition to a softer response. On the other hand, we show that an identifiable lamellar symmetry provides a direction along which the nematic director prefers to align. Our model provides avenues to explore nonlinear properties of flexible diblock copolymers in which the monomers on both sides have mesogenic side groups.

Ordering phenomena and photoorientation processes in photochromic thin films of LC chiral azobenzene-containing polymer systems

The processes of mesogen ordering in nematic and cholesteric side-chain polymer systems were studied by polarized optical spectroscopy. For this purpose, nematic azobenzene-containing copolyacrylate was synthesized and its cholesteric mixture with chiral dopant was prepared. Thin films of copolymer and mixture were prepared by spin-coating using polyimide-coated and rubbed glass and quartz substrates. Film thickness was comparable or less than helix pitch value. It was found that azobenzene and phenylbenzoate side groups of nematic copolymer are oriented after annealing along a rubbing direction. Cholesteric mixture demonstrates a more complicated behaviour, the direction of orientation of the side-chain groups is rotated at certain angle depending on the film thickness. Dichroism in azobenzene groups’ absorbance is lower for cholesteric mixture and decreases with film thickness increasing. A possible mechanism of ordering of the side-chain groups in these thin films was proposed based on the helical order formation. Photooptical switching of the orientation and an influence of the nonpolarized and polarized light on the cholesteric thin films were studied demonstrating a possibility of using such thin films of photochromic cholesterics as promising photoswitchable materials.

Photocontrol of supramolecular architecture in azopolymers: Achiral and chiral aggregation

The absorption spectra and circular dichroism responses upon irradiation with 488 nm circularly polarized light (CPL) have been studied on polymeric films processed from two nematic copolymers containing azobenzene chromophores. Influence of the aggregation and orientation of azobenzenes on the photoinduced chiroptical properties has been analyzed. Moreover, polymeric films and 4 μm planar cells filled with the copolymers were studied by polarized optical microscopy (POM) with the aim of investigating the change of macroscopic optical properties and textures of azopolymers upon irradiation with CPL.

Effect of hydrogen bonds on orientational order in liquid-crystalline polymers

The orientational behavior of nematic LC copolymers (4-{[6-(acryloyloxy)hexanoyl]oxy}phenyl-4-propoxybenzoate)-co-(4-(6-acryloyloxypropyl-1-oxy)benzoic acid) in a magnetic field has been studied by 2H NMR spectroscopy. An increase in the content of carboxyl groups in the copolymer leads to an appreciable growth of the orientational order parameter Szz.

Rheological Behavior of Azobenzene Nematic Homopolymer and Copolymer

ABSTRACT A rheological study of a methacrylate homopolymer containing azobenzene side groups and its 70/30 copolymer with methyl methacrylate was performed in the linear viscoelastic regime. The creep and oscillatory measurements enabled us to obtain the temperature dependence of the zero shear viscosity η. The time-temperature superposition principle (TTS) was found to hold over large temperature intervals; in particular no discontinuities were detected at the isotropic-nematic transition in either sample. However, the homopolymer evidenced a peculiar signature in the shear elastic complex modulus G* (ω,T) at a temperature T c in the nematic phase. This was attributed to a conformational transition driven by the nematic order.

2H-NMR of the Induced Chiral Phases of Acrylate-Metacrylate Liquid Crystalline Copolymers

ABSTRACT It is shown that the addition of 2 to 6 wt.% of a chiral dopant (CD) to a series of nematic copolymers (Pn) induces blue and cholesteric mesophases. The mesomorphic behaviour of various CD+Pn mixtures is investigated by polarized optical microscopy and the phase transition temperatures are determined by DSC measurements. Deuterium NMR spectra are recorded as a function of temperature within the mesophases and the spectral line shapes characteristic of different local orientational order parameters and orientational ordering processes of molecules in a magnetic field are observed depending on temperature, heating or cooling process, chiral dopant concentration, copolymer composition, and magnetic field strength.

Photorefractivity in polymer dissolved liquid crystal composites composed of low-molecular-weight nematic liquid crystals and copolymer comprising mesogenic side groups

This paper describes the photorefractive effect of the liquid crystalline composite materials comprising low-molecular-weight nematic liquid crystals (L-LC), copolymer with mesogenic side groups and a small amount of photosensitizer. Copolymers with four kinds of mesogenic side groups were investigated with respect to the compatibility between the L-LC and the copolymer and the photorefractivity of the composites. Two of them could be miscible with L-LC and the composite exhibited LC phase even though the copolymer itself did not show a LC phase. High gain coefficients (> cmK1) under the low applied dc electric field (<1V/mm) was observed when the composites showed a mesophase without any macroscopic phase separation.

Photo-optical behaviour of a photosensitive chiral nematic copolymer and mixtures containing non-chiral photoactive azobenzene groups

For the first time, the phenomenon of the light-induced increase in helix pitch length is described for cholesteric polymer systems containing photosensitive non-chiral fragments. This phenomenon is observed for a copolymer and mixtures containing azobenzene fragments which are able to undergo E-Z photoisomerization. In all cases, upon UV irradiation, the selective light reflection maximum is shifted to the long wavelength spectral region. This shift appears to be quite appreciable and, in some cases for mixtures, it might be as high as 300 nm. This process is known to be thermally reversible: upon annealing, the selective light reflection maximum comes back to its initial position. The specific features of the kinetics of this photoprocess were studied, and the fatigue resistance of such systems investigated. The advantages of their application for reversible recording of optical information with a possible non-destructive data reading is outlined.

Dynamics Heterogeneity and Optical Writing in a Liquid Crystalline Azobenzene Copolymer

The dynamics of the cholestane spin probe dissolved in a nematic azobenzene copolymer was studied by ESR following a suitable thermal procedure. Heterogeneities in the dynamics of the guest molecule resulted to occur only in the high temperature region, slightly above the nematic-isotropic transition temperature. A temperature region above the glass transition was signalled, in which stable optical writing could be obtained. Optical birefringence and topological writing experiments were carried out on polymer films at the macroscopic and nanometer length-scales, respectively. SNOM analysis showed a very homogenous substrate that could be suitable for optical nanowriting.

Photo-orientation phenomena in photosensitive chiral nematic copolymers

Photo-orientational phenomena have been studied for two comb-shaped cholesteric copolyacrylates containing azobenzene side groups. Copolymer I contains nematogenic phenyl benzoate groups and photosensitive chiral menthyl-containing azobenzene side groups. Copolymer II is composed of nematogenic phenyl benzoate groups, photosensitive cyanoazobenzene groups and chiral photochromic benzylidene- p -methan-3-one fragments. Under the action of polarized Ar+ laser light (488 nm), orientation of the side groups of the copolymers takes place, and this orientation is perpendicular to the vector of the electric field of the incident light. This process shows a co-operative character; that is, it involves both photosensitive azobenzene and phenyl benzoate groups. The kinetics of growth of the photoinduced orientational order parameter were studied as a function of film thickness, incident light intensity, and preliminary UV irradiation. For the planar oriented films of the copolymers, irradiation with polarized light leads to the development of photoinduced birefringence δ n ; maximum values of δ n reach 0.01. The photo-optical properties of copolymers I and II are compared. Such materials may be used for ‘dual’ data recording by varying the helix pitch, selective light reflection maximum, and photoinduced birefringence or linear dichroism.

Side-chain functionalized liquid-crystalline polymers and blends. IX. Phase behavior and structure of comb-shaped liquid-crystalline ionomers containing calcium ions

Liquid-crystalline (LC) ionomers containing 2–15 mol % calcium ions were synthesized by the exchange reaction between the nematic LC copolymer, bearing oxycyanobiphenyl mesogenic groups, and the carboxyl groups of acrylic acid, with calcium acetate. The incorporation of 2–3 mol % Ca ions in the LC copolymer leads to some rise in the clearing point and glass-transition temperature. A further increase in the concentration of metal ions (>5 mol %) is accompanied by induction of the smectic A phase where clearing point and glass-transition temperatures keep constant values. Phase behavior of the LC ionomers may be understood on the basis of a structural model that considers the dual role of calcium ions in a polymer matrix. Metal ions act as points of noncovalent electrostatic binding of the polymer chains and are capable of forming larger ionic associates (multiplets). The comparison of the phase behavior of sodium and calcium containing LC ionomers shows that the formation of ionic links may lead to the growth of structure defects suppressing a positive influence of charged groups on the mesophase clearing temperature. The orientation behavior of the LC ionomers in the magnetic field was studied. It was shown that the incorporation of calcium ions (3 mol %) in the LC copolymer matrix leads to the growth of orientation order parameter S of the nematic phase. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 3953–3959, 2001

Thermotropic Chiral−Nematic Poly(p-phenylene)s as a Paradigm of Helically Stacked π-Conjugated Systems

As an emerging class of multifunctional optical materials, chiral−nematic poly(p-phenylene)s with cyanobiphenyl and (S)-1-phenylethyl pendants were synthesized and characterized. Nematic homopolymer was also synthesized for preparation into a uniaxially aligned film. Linearly polarized photoluminescence (LPPL) and FTIR dichroism revealed that both the conjugated backbone and the nematic pendant are oriented along the director with an orientational order parameter, S = 0.62 and 0.73, respectively. Films prepared with chiral−nematic copolymers were found to selectively reflect left-handed circularly polarized light in the ultraviolet, visible, and infrared region at a decreasing chiral content. The helically stacked poly(p-phenylene)s were further characterized in terms of circularly polarized photoluminescence (CPPL). On the basis of a recent theory, S = 0.67 was found for the orientational order of conjugated backbones within quasinematic layers comprising the chiral−nematic film. Within experimental uncertainty, the S values determined with the LPPL and CPPL techniques are in good agreement, thereby validating the proposed supramolecular structure on the basis of which theories governing light absorption, emission, and propagation in structured media were constructed.

Synthesis and characterization of copolymaleimides containing 4‐cyanobiphenyl‐based side groups for nonlinear optical applications

We report the synthesis of a nematic copolymer, P(CBMS-co-M3), prepared by free radical polymerization of an equimolecular mixture of p-(4-cyanobiphenyl-4′-yloxy)methylstyrene (CBMS) and N-[3-(4-cyanobiphenyl-4′-yloxy)propyl]maleimide (M3) and two isotropic alternating copolymers, P(S-alt-Mn) (n = 3,6) prepared by chemical modification of poly(styrene-alt-maleimide), P(S-alt-M), by n-(4-cyanobiphenyl-4′-yloxy)alkan-1-ol. These copolymaleimides were characterized by NMR, DSC, and optical microscopy. Some corona poling experiments were performed and the second harmonic coefficients d31 and d33 were measured. It was shown that one can gain in net polar ordering by starting with a liquid crystalline system. The ratio d33/d31 was much larger than 3, in agreement with the molecular statistical models for electric field poling of liquid crystals. At ambient conditions, changes of d33 and d31 are 15% over 325 days. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 513–524, 1999