Effects of hyperbranched prepolymers prepared from butyl acrylate and butyl methacrylate on the electro‐optical properties of polymer dispersed liquid crystal

Two hyperbranched prepolymers were synthesized via the reversible addition‐fragmentation chain transfer copolymerization of butyl acrylate or butyl methacrylate with divinyl benzene, respectively. These prepolymers were used in the photopolymerization‐induced phase separation process of preparing polymer dispersed liquid crystal (PDLC) films with no risk of gelation. The morphologies of PDLC samples were investigated. Of particular interest was that the driving voltage (V₉₀) of PDLC with high glass transition temperature (T_g) matrix was as low as 5 V, whereas the V₉₀ of PDLC with low T_g matrix was still high. An explanation for this phenomenon is the hypothesis that different interaction modes function at the interface of liquid crystal and polymer. Meanwhile, the hysteresis of the two systems was minor, which correlated with the hyperbranched structure of polymer. The on‐state transmittance (T_ON) of PDLC films enhanced because of the well matching between the refractive index of polymer and that of liquid crystal. Copyright © 2011 John Wiley & Sons, Ltd.     

Intermolecular Interactions in Binary Liquid Mixtures of Styrene with m-, o-, or p-xylene

The densities (ρ), ultrasonic speeds (ν), and refractive indices (n) of binary mixtures of styrene (STY) with m-, o-, or p-xylene, including those of their pure liquids, were measured over the entire composition range at the temperatures 298.15, 303.15, 308.15, and 313.15 K. The excess volumes (V^E), deviations in isentropic compressibilities (Δks), acoustic impedances (ΔZ), and refractive indices (Δn) were calculated from the experimental data. Partial molar volumes (V⁰_?,2) and partial molar isentropic compressibilities (K⁰_?,2) of xylenes in styrene have also been calculated. The derived functions, namely, V^E, Δk_s, ΔZ, Δn, V⁰_?,2, and K⁰_?,2 were used to have a better understanding of the intermolecular interactions occurring between the component molecules of the present liquid mixtures. The variations of these parameters suggest that the interactions between styrene and o-, m-, or p-xylene molecules follow the sequences: p-xylene>o-xylene>m-xylene. Apart from using density data for the calculation of VE, excess molar volumes were also estimated using refractive index data. Furthermore, several refractive index mixing rules have been used to estimate the refractive indices of the studied liquid mixtures theoretically. Overall, the computed and measured data were interpreted in terms of interactions between the mixing components.     

Effect of monoacrylate type in UV curable PU acrylate based bicontinuous polymer/liquid crystal networks

Polymer network liquid crystals (PNLC) have been prepared from ultraviolet-curable polyurethane acrylate (PUA) and a nematic liquid crystal mixture (BL002). Effects of monoacrylate type on film morphology, temperature-dependent off-state transmittance, and electro-optic performance of the film have been studied. Among three types of monoacrylates incorporated (EHA(2-ethyl hexyl acrylate), MMA (methylmethacryalte), NVP (N-vinylpyrrolidone)), EHA-based PUAs gave the greatest polymer–LC phase separation, lowest threshold (V₁₀), and operating (V₉₀) voltages, and the effect was more pronounced in monoacrylate/triacrylate systems than in monoacrylate/diacrylate systems. Contact angle measurement offers a clue to the observed morphology and electro-optic behavior. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1393–1399, 1998     

Optical and morphological characterization of polyacrylamide hydrogel and liquid crystal systems

In this work, the thermotropic liquid crystal MBBA (N-(4-methoxybenzilidene)-4-butylaniline), entrapped on hydrogels, based on cross-linked polyacrylamide (PAAm), was studied. The liquid crystalline phases of system were characterized by polarized optical microscopy (POM), refractive index, optical transmittance, scanning electron microscopy (SEM) and water loss. It was verified the presence of birefringence on hydrogel + liquid crystal. The dynamic of formation of such birefringence finished 40 days after the hydrogel synthesis. The effective birefringence Δn, i.e., the difference on refractive index of polyacrylamide hydrogel to refractive index of hydrogel + liquid crystal (Δn₁) and the difference on refractive index of liquid crystal (MBBA) to refractive index of hydrogel + liquid crystal (Δn₂) are dependent of content of acrylamide (AAm) and MBBA on hydrogel. The increase on Δn₁ and Δn₂ with the polyacrylamide content on hydrogel was attributed to decreasing of the mobility liquid crystal inside the hydrogel. Also, an increase on MBBA concentration in the polymeric matrix provides a reduction in the values of optical transmittance in the system. The morphology observed by SEM shows that hydrogel + liquid crystal is more compact that PAAm hydrogels. The presence of MBBA causes an increase in hydrophobicity. The water loss speed is favored by the increase in the amount of MBBA present in the hydrogels.     

Effects of prepolymer structure in holographic polymer dispersed liquid crystal

Three different types of polyols—poly(ethylene glycol) (PEG), poly(propylene glycol) (PPG), and poly(tetramethylene glycol)—with different molecular weights (M_n) were incorporated into the prepolymer structure, and the effects were examined with regard to the morphology and performance of holographic polymer dispersed liquid crystal. Among them, PEG showing the lowest glass transition temperature (T_g) exhibited the lowest threshold voltage (V_th), lowest operating voltage (V_op), and lowest diffraction efficiency with slow grating formation owing to its high viscosity of prepolymer mixture. The T_g decreased with increasing M_n because of the decreased crosslink density and hydrogen bonding, which reduced the anchoring strength, V_th and V_op. PPG400 gave V_th and V_op of approximately 7 and 24 V, respectively, with a contrast ratio of approximately 12. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

Order parameter studies from effective order geometry in 4O.Om and 7O.Om liquid crystalline compounds

The effective geometry parameter, α_g = n _o /n _e, is used to evaluate the orientational order parameter, S, in the case of N-(p-n-butyloxybenzylidene)-p-n-alkoxy anilines, 4O.Om and N-(p-n-heptyloxybenzylidene)-p-n-alkoxy anilines, 7O.Om compounds with m?=?3–7 and 9 in the former case and m?=?3, 5–7 and 9 in the later materials. The results obtained are compared with those calculated using the standard techniques of molecular polarisability and birefringence. The effective geometry parameter's influence on the deflection of light by the liquid crystal compounds is also studied. The variation of temperature gradient of the ordinary refractive index, dn _o /dT, and extraordinary refractive index, dn _e /dT, of the liquid crystals is also studied.     

Effects of crosslinking agent,cure temperature,and UV flux on the electro‐optical properties of polymer‐dispersed liquid crystal cells

We present results for the effects of a crosslinking agent, cure temperature, and UV flux on the electro‐optical properties of polymer‐dispersed liquid crystal (PDLC) cells. These cells were fabricated using a mixture of a liquid crystal (E8) and an acrylic monomer (CN135). The maximum in the first derivative of the transmission vs. applied, sinusoidal voltage (inflection voltage, V_inf), varies systematically with PDLC formulation and cure‐process conditions. For PDLC cells fabricated with a crosslinking agent (SR295), V_inf increases with increasing the concentration of SR295. However, for cells fabricated without the use of a crosslinking agent, V_inf decreases with increasing the UV flux and decreasing temperature. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 404–410, 2004     

Preparation of ureido group bearing polymers and their upper critical solution temperature in water

Poly(2‐ureidoethylmethacrylate) (PUEM_n) was synthesized via reversible addition‐fragmentation chain transfer (RAFT) radical polymerization and following polymer reaction. We prepared two PUEM_n samples with different degrees of polymerization (n = 100 and 49). The polymers exhibited upper critical solution temperature (UCST) in phosphate‐buffered saline (PBS) solution. The phase separation temperature (T_p) in PBS can be controlled ranging from 17 to 55 °C by changing molecular weight of the polymer, polymer concentration, and adding NaCl concentration. The polymers in PBS formed coacervate drops by liquid–liquid phase separations below T_p. Results of the dielectric relaxation measurement, the hydration number per monomeric unit was 5 above T_p. Based on a fluorescence study, the polymer formed slightly hydrophobic environments below T_p. The liquid–liquid phase separation was occurred presumably because of weak hydrophobic interactions and intermolecularly hydrogen bonding interactions between the pendant ureido groups. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 2845–2854     

Density and refractive index measurements in hexaheptyloxytriphenylene,a discotic liquid crystal

Density and refractive index measurements in the discotic liquid crystal hexaheptyloxytriphenylene were performed in the hexagonal columnar Col_ho and isotropic I phases. The temperature dependence of the density ρ(T) for this compound was obtained by combining small angle X-ray data and capillary methods. The ordinary n _o extraordinary n _e and isotropic liquid n _i refractive indices were measured using a modified Abbe refractometer to an accuracy of about 10^?3. To check the consistency of the density and the refractometry experiments we used the Lorentz–Lorenz relation. An anomaly in the empirical relationship at the Col_ho–I phase transition, which holds for many uniaxial liquid crystals, was detected. A discrepancy between low birefringence Δn～0.109 and a relatively high local electric field anisotropy for hexaheptyloxytriphenylene is discussed.     

Low-birefringence liquid crystal mixtures for photonic liquid crystal fibres application

ABSTRACT

Series of low-birefringence liquid crystal mixtures composed of alkyl alkylbicyclohexyl carbonates and modified mixtures with other compounds have been formulated, their refractive indices and electric permittivity measured upon temperature. They exhibit the ordinary refraction index n_o lower than the refractive index of silica glass in a different range of temperature. This enables to observe in photonic liquid crystal fibres (PLCFs) a change in the light propagation mechanism from photonic band gap guiding to modified total internal reflection at different temperatures. Selected applications of PLCFs are also discussed.     

Synthesis and nanoimprinting of high refractive index and highly transparent polythioethers based on thiol-ene click chemistry

This work demonstrates the UV nanoimprinting lithography (UV-NIL) of high refractive index and highly transparent polythioethers based on thiol-ene click chemistry. Herein, 9,9-bis(3-mercaptopropylphenylether)fluorene (BMPF) is designed as a new thiol monomer with a high refractive index, high transparency, and good processability for UV-NIL. Colorless polythioethers are synthesized from BMPF and ene monomers under mild thiol-ene click reaction conditions. Excellent transmittance (96%) of 400 nm light is observed in all the polymer films and high refractive index values of 1.5972–1.6382 are attained. UV-NIL using thiol-ene photopolymerization affords polymer nanoimprinting patterns with various features on the order of 100–500 nm without any fractures. To the best of our knowledge, this is the first report on UV-NIL of high refractive index and highly transparent polymers. Through proper monomer and polymer design, novel polythioethers with suitable glass transition temperature (T _g) values are developed with high refractive index, high transparency, and good UV-NIL processability. Furthermore, UV-NIL based on thiol-ene click chemistry is accomplished at the nanoscale. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 2175–2182     

Coefficients of molecular packing and intrinsic birefringence of aromatic polyimides estimated using refractive indices and molecular polarizabilities

Molecular structure dependences of the coefficients of molecular packing (K_p) and the intrinsic birefringence (Δn⁰) were investigated for aromatic polyimides (PIs) using a method based on the modified Lorentz–Lorenz equation and the Vuks equation. This method needs only average refractive indices and polarizability tensors ( ) for PI repeating units. αˆ was calculated using the density functional theory at the B3LYP/6-31+G(d) theory level that was confirmed as optimal for PI repeating units. Because bent or kinked structures in main chain hinder the ordering of polymer chains, K_p decreases in cases that ether linkages are introduced in the main chain and that kinked-structured dianhydrides are used. Introduction of side groups, trifluoromethyl ( CF₃) groups in particular, also causes decreases in K_p. On the other hand, the estimated values of Δn⁰ are proportional to the calculated anisotropy in αˆ (Δα/V_vdw), thus Δn⁰ can be well predicted by computation. The values of Δα/V_vdw are decreased not only by bent structures in the main chain (e.g., ether linkage), presence of side groups, but also by large fractions of imide rings in the polymer structures. The estimated values of Δn⁰ range from 0.338 to 0.470, which are much larger than the reported ones for optically transparent conventional polymers. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 2354–2366, 2004     

Comments on the determination of absolute rate constants in anionic polymerization

Claims have recently been made that absolute rate constants for chain propagation of the unassociated active centers can be made in systems where a high degree of association is present. Anionic polymerization of styrene in nonpolar solvents with lithium as counterion is a typical case. The conditions required to obtain these constants (and the associated aggregate dissociation constants) are described using data from styrene polymerization with lithium and potassium as counterions and data from o-methoxystyrene polymerization. The conclusion reached must be that the k_p and K_ds values obtained for styrene with counterion lithium cannot be obtained from existing literature data and are simply artifacts of the computer analysis. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 1065–1068, 1998     

High on-state clarity polymer dispersed liquid crystal films

Abstract

Results are presented which show that the on-state clarity of a UV cured polymer dispersed liquid crystal (PDLC) film depends on the refractive index of the final polymer in the PDLC film, the ordinary refractive index of the liquid crystal, the solubility of the liquid crystal in the prepolymer and the rate at which the film is cured. Liquid crystal mixtures for use in PDLC films are chosen such that the ordinary refractive index of the liquid crystal is equal to the refractive index of the polymer matrix. It has been shown previously that a large quantity of liquid crystal remains dissolved in the polymer matrix, thus increasing the mismatch between the refractive index of the polymer and the ordinary refractive index of the liquid crystal and therefore reducing the on-state clarity. For liquid crystal mixtures which have high solubility in the prepolymer (>60 per cent) the mismatch in the refractive indices can be very large and the on-state clarity of the resulting film can be very poor (T _on<70 per cent). Results are presented which show that it is possible to increase the on-state clarity of such films by increasing the rate at which these films cure. If the liquid crystal is less soluble in the prepolymer (<45 per cent), a PDLC film formed from such a liquid crystal/ prepolymer system often has very good on-state clarity (T _on>75 per cent) be it cured slowly or quickly. Results are also presented which show that in order to achieve a true measure of on-state clarity it is necessary to use a small collection angle (<3°) in the detecting optics. If larger collection angles are used, the photodetector collects light which is scattered out of the specular beam, thus leading to a false measure of on-state clarity.     

Morphology-dependent luminescence properties of poly(benzyl ether) dendrimers

Poly(benzyl ether) dendrimers with o-, m-, and p-isomers of dialkoxybenzene at their focal points [o-, m-, and p-(Gn)₂Ar], having generation numbers (n) of 0–3, were synthesized. ¹H NMR pulse relaxation times (T₁) of the exterior MeO groups of o- and m-(Gn)₂Ar (n = 0–3) all remained in the range of 0.92–1.43 s. In sharp contrast, an exceptionally short T₁ value (0.23 s) was observed for p-(G3)₂Ar. Although their absorption spectral profiles were slightly different from one another, an essential difference was observed for their fluorescence properties. When the generation number was increased, the fluorescence efficiency of o-(Gn)₂Ar increased, but that of p-(Gn)₂Ar decreased, whereas m-(Gn)₂Ar exhibited a relatively small change in the fluorescence efficiency. Fluorescence depolarization studies showed a highly efficient intramolecular energy migration in p-(G3)₂Ar as compared with o-(G3)₂Ar and m-(G3)₂Ar. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 3524–3530, 2003     

SYNTHESIS OF ABA TYPE TRIBLOCK COPOLYMERS BY RADICAL POLYMERIZATION WITH 1,4-BIS(p-TERTBUTYLPHENYLSELENOMETHYL) BENZENE AS A PHOTOINIFERTER

1,4-Bis(p-tert-butylphenylselenomethyl)benzene was used as a bifunctional photoiniferter for the polymerization of methyl methacrylate (MMA). Both the polymer yields and the number average of molecular weights ([Mbar]_n) of polymers increased with the polymerization time and the [Mbar]_n linearly increased with polymer yield. The addition of MMA to the poly(MMA) with irradiation increased the [Mbar]_n of the polymer. Photoirradiation of telechelic polystyrene having phenylseleno groups at both ends as polymeric photoiniferter in the presence of MMA or p-chloromethylstyrene afforded effectively corresponding to the ABA type triblock copolymers. On the other hand, photopolymerization of p-methylstyrene with ABA type triblock copolymer of styrene and p-chloromethylstyrene as polymeric photoiniferter afforded to multiblock copolymer of styrene and p-substituted styrenes.     

Kinetic and microstructure studies of poly(ethylene-co-p-methylstyrene) copolymers prepared by metallocene catalysts with constrained ligand geometry

This paper discusses the poly(ethylene-co-p-methylstyrene) copolymers prepared by metallocene catalysts, such as Et(Ind)₂ZrCl₂ and [C₅Me₄(SiMe₂N^tBu)]-TiCl₂, with constrained ligand geometry. The copolymerization reaction was examined by comonomer reactivity (reactivity ratio and comonomer conversion versus time), copolymer microstructure (DSC and ¹³C-NMR analyses) and the comparisons between p-methylstyrene and other styrene-derivatives (styrene, o-methylstyrene and m-methylstyrene). The combined experimental results clearly show that p-methylstyrene performs distinctively better than styrene and its derivatives, due to the cationic coordination mechanism and spatially opened catalytic site in metallocene catalysts with constrained ligand geometry. A broad composition range of random poly(ethylene-co-p-methylstyrene)copolymers were prepared with narrow molecular weight and composition distributions. With the increase of p-methylstyrene concentration, poly(ethylene-co-p-ethylstyrene)copolymer shows systematical decrease of melting point and crystallinity and increase of glass transition temperature. At above 10 mol % of p-methylstyrene, the crystallinity of copolymer almost completely disappears. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 1017–1029, 1998     

Effect of molecular weight of macro‐iniferter on electro‐optical properties of polymer dispersed liquid crystal films prepared by iniferter polymerization

Polymer dispersed liquid crystal (PDLC) films were prepared by a devised method, in which photo‐polymerization induced phase separation in a mixtures of a macro‐iniferter, methyl acrylater, and liquid crystal. The morphology of the obtained PDLC films was examined on a polarized optical microscopy, and the effect of molecular weight of MIs on the electro‐optical properties was deliberately investigated. Decreasing the molecular weight of MIs in the films led to formation of larger liquid crystal droplets and a lower V_th values. V_sat increased and the memory effect decreased because of the increased interface anchoring strength induced by the higher molecular weight of polymer matrices. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1530–1534, 2009     

Radical polymerization of ortho-(1,3-dioxolan-2-yl)phenyl ethyl fumarate involving intramolecular hydrogen abstraction and ring opening of cyclic acetal

The polymerization of o-(1,3-dioxolan-2-yl)phenyl ethyl fumarate (DOPEF) initiated with dimethyl 2,2′-azobisiso-butyrate (MAIB) was studied kinetically in benzene. The polymerization rate (R_p) at 60°C was given by R_p = k [MAIB]^0.76 [DOPEF]^0.71. The overall activation energy of polymerization was calculated to be 98.3 kJ/mol. The number-average molecular weight of resulting poly(DOPEF) was in the range of 1000–3100. ¹H- and ¹³C-NMR spectra of resulting polymers revealed that the radical polymerization of DOPEF proceeds in a complicated manner involving vinyl addition, intramolecular hydrogen abstraction, and further ring opening of the cyclic acetal at higher temperatures. From the copolymerization of DOPEF (M₁) and styrene (St) (M₂) at 60°C, the monomer reactivity ratios were obtained to be r₁ = 0.02 and r₂ = 0.20, the values of which are similar to those of the copolymerization of ethyl o-formylphenyl fumarate and St. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 563–572, 1998     

In situ observation of smectic layer reorientation during the switching of a ferroelectric liquid crystal

Abstract

A thin plastic cell containing the ferroelectric liquid crystal ZLI3654 (4 μm) was placed edge-on to a pin-hole collimated horizontal X-ray beam. In this way, the smectic layers were brought into register. Subsequently, triangular voltage waves with various peak (V _p) values were applied across the cell and diffraction photos were obtained during the application of the alternating voltage. Up to V _p = ± 30 V, no significant movements in the initial tilted orientation of the smectic layers with respect to the surfaces (chevrons) could be observed. During the application of V _p = ± 32 V an increasing fraction of the smectic layers changed their initial tilt angle with respect to the cell surfaces to make larger tilt angles. At a slightly higher voltage, the layers became upright (bookshelf structure). Upon removing the voltage and short circuiting the cell, the quasi-bookshelf structure was sustained. The new method described here can be used in combination with a fast detector for time resolved experiments.