Liquid Crystalline Copolymers Research Articles

The Photoinduced Birefringence in Thick Films of Chiral Azodye Copolymers

AbstractThe photoinduced birefringence, Δnind, in 19–35 μm thick films has been studied for two series of chiral photochromic liquid crystalline side chain copolymers. The birefringence measured is induced by excitation of the n‐π* band only. The Δnind value induced in 10 min is linear upon concentration of dye moieties but achieves saturation with increase of illumination intensity. Evolution of the birefringence can be described by double exponent growth; the faster exponent is the same for both copolymer series and should relate to the photoselection process, while the slower one should relate mainly to photo‐orientation and differs strongly dependent on chemical structure of photochromic groups. The different behaviour of two copolymer series during the growth governed by the second exponent is interpreted in terms of the “frictional model”. Due to elevated thickness of the films, rather moderate Δnind values result in huge phase retardation up to 270° and more. The materials are promising therefore for holography applications.Chemical structure of the synthesised chiral photochromic copolymers.magnified imageChemical structure of the synthesised chiral photochromic copolymers.

Frictional wear in high temperature thermotropic liquid crystalline polymers: correlation with glass transitions

Wholly aromatic thermotropic main chain liquid crystalline copolymers (LCPs) with varying glass transitions ( T g) were tested for wear resistance, particularly under high friction conditions, where surface temperatures can rise. Dynamic mechanical spectroscopy and DSC were used to characterize molecular relaxations. Three copolyester LCPs which all contain a substantial fraction of main chain 1,4-phenyl groups were chosen for this study. These included semi-crystalline Vectra ® A900, a semi-crystalline LCP containing phenyl hydroquinone (phHQ–LCP), and a low crystallinity LCP containing t-butyl substituted hydroquinone ( t-butylLCP). These have glass transitions of 100, 160 and 175 °C, respectively, and heat deflection temperatures (HDTs) of 170, 260 and 174 °C, respectively. HDT is dependent in part on crystallinity. The wear performance was found to depend mainly on T g and not HDT, suggesting a microscopic failure mechanism related to the amorphous phase. This is supported by the relatively poor elevated temperature wear performance of Vectra ® compared to the higher T g LCPs. Shear strength measurements on the neat LCP resins did not correlate with wear properties of the blends, most likely because the measurements were made at room temperature and not elevated temperatures.

Segregation of Liquid Crystalline Mesophases During the Transesterification of PET with Sebacic Acid, Hydroquinone and 4-Hydroxybenzoic Acid

The phase behavior and the morphology of PET-based copolyesters have been studied by DSC, optical and electron microscopy and WAXS. The materials were prepared by transesterification of PET with equimolar amounts of sebacic acid (S) and hydroquinone (Q), and with varying amounts of 4-hydroxybenzoic acid (H). In no case a compositionally homogeneous random copolyester was obtained: even the synthesis carried out in the absence of H gave a product that, though appearing as a homogeneous liquid-crystalline copolymer when characterized by conventional calorimetric and microscopic techniques, was shown by solvent fractionation and X-ray analysis to contain a segregated phase, probably formed by aromatic-rich copolyester chains. An increase of the amount of added H and, thereby, of the overall degree of aromaticity of the mixture, was shown to favor the segregation phenomena. The results confirm that an enthalpy-driven compositional differentiation of the two phases of the investigated copolyesters takes place during synthesis.

New types of multifunctional liquid crystalline photochromic copolymers for optical data recording and storage

A new family of multifunctional chiral-photochromic liquid crystalline (LC) copolymers containing mesogenic, chiral and photoactive groups were synthesized. The new principles of photo-regulation of the helical supramolecular structure and optical properties of the binary and ternary chiral-photochromic LC polymers based on the change of helical twisting power of the chiral-photochromic monomer units, the dual photochromism and photochemical spectral gap burning were developed. It was shown, that the introduction of small amount of low-molar-mass chiral-photochromic dopants in chiral LC copolymers having different helix signs followed by light irradiation permits one to twist or untwist the helical supramolecular structure. The synthesized polymers are shown to be promising candidates for colour data recording and storage.

The Synthesis of Liquid Crystalline Copolymers with Block Copolymer Grafts

The synthesis of novel copolymers consisting of a side-group liquid crystalline backbone and poly(tetrahydrofuran)-poly(methyl methacrylate) block copolymer grafts was realized by using cationic-to-free-radical transformation reactions. Firstly, photoactive poly(tetrahydrofuran) macroinimers were prepared by cationic polymerization of tetrahydrofuran and subsequent termination with 2-picoline N-oxide. Secondly, the macroinimers and acrylate monomers containing different spaced cyanobiphenyl mesogenic groups were copolymerized to yiel the respective graft copolymers. Eventually, these were used for indirect photochmical polymerization of methyl methacrylate by UV irradiation in the presence of anthracene as a photosensitizer leading ti the final copolymers with block copolymer grafts. The liquid crystalline, semi-crystalline, and amorphous blocks were micro-phase separated in the graft copolymers.

Liquid crystalline copolymers with cholesteryl as mesogenic group

In this paper, the synthesis and characterization of copolymers obtained from free-radical copolymerization of maleic anhydride with mesogenic 1-alkene and with mesogenic acrylates are presented. The mesomorphic properties were investigated by differential scanning calorimetry (DSC) and polarizing optical microscopy.

Dielectric and IR Spectroscopy of the Macromolecular Reaction of Anhydridization in a Functionalized Side-Chain Liquid Crystalline Copolymer Containing Acrylic Acid Groups

The anhydrization reaction has been investigated in functionalized side-chain random liquid crystalline (LC) copolymers containing cyanobiphenyl mesogenic and acrylic acid (38 mol %) groups. Heat treatment of the LC copolymer at 130 °C for 5−200 h is accompanied by formation of intra- and intermolecular anhydrides and network structures causing an increase of the glass transition and clearing temperatures. The influence of annealing on the reorientational dynamics of the copolymers was investigated by broad-band dielectric relaxation spectroscopy. The local dynamics of the side group involving spacer motion (γ relaxation) and mesogen rotation about the long axis (β relaxation) do not depend on the annealing time. At the same time a considerable (more than 2 orders of magnitude) decrease in the relaxation rates of the cooperative δ and α processes was observed due to a gradual increase in the main-chain rigidity during the annealing. The molecular mechanism of all relaxation processes detected is discussed.

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

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.

Photoactive Cholesteric Polymeric Material With Dual Photochromism

ABSTRACTNew cholesteric polymeric material containing a liquid crystalline (LC) copolymer with side phenylbenzoate nematogenic and photochromic spiropyran groups and 5 mol % chiral photochromic dopant based on cinnamic acid and isosorbide was prepared. It was shown that UV irradiation and subsequent annealing of planarly-oriented mixture films lead to untwisting of the cholesteric helix and cause an irreversible shift of selective light reflection to the long-wavelength region. This process is explained by the E-Z isomerization of chiral photochromic groups of the dopant relative to a C=C bond accompanied by a decrease in their twisting ability. Upon UV irradiation of films at room temperature, a thermally reversible transformation of spiropyran groups into a merocyanine form takes place, which is followed by the appearance of an intense absorption maximum in the visible spectral region. It was shown, that the use of the material prepared in this work provides an opportunity to record two images at a time: one image is due to a change in the helical pitch and another image (above) is due to the photochromism of spiropyran groups. It is pertinent to note that using the approach developed in this work allows one to widely vary the range of materials exhibiting dual photochromism. It was demonstrated that the mixture under examination shows promise as new photosensitive material for optics, optoelectronics, and data recording.

Thermoassisted Photorecording in Chiral Smectic Azodye Copolymers

ABSTRACTIllumination of photochromic liquid-crystalline (PCLC) copolymer films with polarized light during the mesophase formation has been studied. Both thick pressed and thin spin-coated, the films show prominent increase in photoinduced orientation, as compared with conventional isothermal photorecording. Two methods of the thermoassisted recording have been suggested, one of those with simultaneous cooling of the film and another one after a shock heating and quenching. The photoinduced birefringence, Δnind, and the order parameter of photochromic groups, Sazo, have been measured. The thermally assisted recording produces holographic gratings with high diffraction efficiency, η, up to 20-30%.

Reversible UV Image Recording on a Photochromic Side Chain Liquid Crystalline Polymer

Photochromism and photoinduced optical anisotropy have been studied for a chiral comb-like liquid crystalline copolymer containing azobenzene chromophores in its side chains. In a smectic glass phase of the polymer, upon irradiation by UV light, long living cis-isomers are observed. Spin coated films, irradiated by linearly polarized light manifest a photo-induced anisotropy, related to a strong stable dichroism, which is due to an enrichment and depletion of the chosen angular direction, correspondingly, with trans-and cis-isomers of azobenzene chromophores. These films are shown to be a novel medium for image recording, sensitive to UV light. In particular, a holographic grating may be recorded on top of a hidden UV image by interfering beams of visible Ar-laser light. Therefore, a stable holographic pattern of an image recorded by incoherent UV light is produced. It can be erased and recorded again.

New Principle of Optical Data Recording Based on Reversible Transition “Selective Reflection—Absorbance” in Photochromic Cholesteric Copolymers

A new principle of information storage based on a realization of the reversible transition <<selective reflection-absorbance>> in photochromic liquid crystalline (LC) copolymers was suggested. For this purpose a new family of triple copolymers were synthesized by copolymerization of acrylic monomers, containing nematogenic (phenylbenzoate), chiral (cholesterol), and photochromic (spiropyran) fragments. Phase behaviour and optical properties of copolymers were studied. Isomerization of spiropyran groups and formation of merocyanine form under UV-irradiation (337 nm) of the polymer films was observed. It should be noted that the maximum of the absorbance of polymer films coincides with maximum of selective light reflection. As a result the disappearance of the selective light reflection in the irradiated regions of the films occurs. Scheme of the optical data recording and reading of information recorded on planarly oriented films of such copolymers was proposed.

Side-chain functionalized liquid crystalline polymers and blends, 4. Phase behavior of functionalized liquid crystalline copolymers containing maleic acid fragments

A number of new functionalized LC copolymers containing mesogenic oxycyanobiphenyl groups and 3–41 mol-% of maleic acid has been obtained by free-radical copolymerization in solution. A significant effect of intramolecular hydrogen bonds on the phase behavior of these copolymers is observed: for a maleic acid content of up to 8 mol-% LC copolymers form only the nematic and between 10–41 mol-% the SmA phase is formed. The process of formation of anhydrides in functionalized LC polymers at 130°C is investigated. It is shown that the formation of five-membered intramolecular anhydride rings does not influence the phase behavior of the LC polymers.

Side-chain functionalized liquid crystalline polymers and blends, 4. Phase behavior of functionalized liquid crystalline copolymers containing maleic acid fragments

A number of new functionalized LC copolymers contaning mesogenic oxycyanobiphenyl groups and 3-41 mol-% of maleic acid has been obtained by free radical copolymerization in solution. A significant effect of intramolecular hydrogen bonds on the phase behavior of these copolymers is observed, for a maleic acid content of up to 8 mol-% LC copolymers form only the nematic and between 10-41 mol-% the SmA phase is formed. The process of formation of anhydrides in functionalized LC polymers at 130°C is instigated. It is show a that the formation of five-membered intramolecular anhydride rings does not influence the phase behavior of the LC polymers.

Side-chain liquid-crystalline poly(ketone)s: effect of spacer length, mesogen type and mesogen density on mesomorphic behavior

Novel side-chain liquid-crystalline copolymers (SCLCPs) were synthesized via the Pd(II) catalyzed alternating copolymerization of mesogenic 1-alkenes and carbon monoxide. For methoxybiphenyl mesogens, these copolymers exhibited highly ordered smectic E mesophases and high glass transition temperatures. The transition temperatures of these polymers were tuned by altering the spacer length or by dilution of mesogen-containing alkenes with 1-hexene. Polymers with cyanobiphenyl or methoxyazobenzene mesogens exhibited nematic mesophases. These polymers were the first ever nitrogen containing copolymers of this type. Increase of the spacer length or the 1-hexene content of methoxybiphenyl-containing copolymers resulted in a decrease of the surface polarity of spincoated films. The decomposition temperature of these SCLCPs started far above the isotropization temperature of the polymers.

Phase behavior of liquid-crystalline copolymer/liquid crystal blends

Thermodynamic phase behavior was studied for blends between side-chain liquid-crystalline (SCLC) copolymers and a low molecular weight liquid crystal (LC). SCLC copolymers, Copolyx having x mol fraction of a benzoic acid unit and 1−x mol fraction of a cyanobiphenyl group in their side chain, were prepared by radical polymerization and mixed with a low molecular weight LC, 4-cyano-4′-n-hexyloxybiphenyl (6OCB). The phase diagram of the Copoly0.24/6OCB blend exhibited an upper critical solution temperature which was not observed for homopolymer systems. Employing SCLC copolymers having two different kinds of mesogens in polymer/low molecular weight LC blends enabled us to obtain the new type of phase diagram.

Understanding and controlling the morphology of styrene–isoprene side-group liquid crystalline diblock copolymers

In side-chain liquid crystalline diblock copolymers, driving forces for ordering of the material may be provided by the chemical incompatibility between the blocks and by the liquid crystalline nature of one of the blocks. We study the microstructure and its development from initially isotropic solutions in side-group liquid crystalline copolymers based on styrene–isoprene diblocks. Hierarchical structure from the 5Å to the 500Å length scale is observed, the product of coherent block copolymer microphase separation and liquid crystalline mesophase formation. The presence of cylindrical microdomains of either the poly(isoprene-LC) or poly(styrene) block markedly increased the thermal stability of the LC. Confinement of the LC to cylindrical microdomains strongly inhibited defect formation within the mesophase after suitable orientation and thermal treatment, readily manifested by the significantly improved optical clarity of these samples versus LC matrix or LC lamellar samples. We consider the relative stabilities of parallel–transverse, perpendicular–parallel, parallel–parallel and transverse–perpendicular arrangements of the microdomain and mesophase structures in interpreting the development of structure in these materials under oscillatory shear.

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.

Bistable Memory Switching of (Liquid Crystalline Copolymer/ Low Molecular Weight Liquid Crystal) Composite Systems

Bistable electro-optical switching characteristics have been investigated for composite systems in a smectic phase induced by mixing a side chain type nematic liquid crystalline polymer (nematic LCP) or its copolymer with weak polar methoxy terminal groups in the side chains and low molecular weight nematic liquid crystals (nematic LCs) with strong polar cyano end groups. The liquid crystalline copolymer (LCcoP) with weak polar methoxy terminal groups in the side chains attaching to a polysiloxane main chain was used in order to improve the response speed of the bistable light switching for the binary composite system. Although the LCcoP with 52.5 mol% substituted mesogenic side chains did not exhibit any mesophase characteristics at any temperature, this LCcoP induced a smectic state by mixing nematic LCs. A reversible and bistable electro-optical switching with a short response time (∽100 ms) was realized for the induced smectic binary composite system upon the application of an appropriate electric field at room temperature.