Monte Carlo simulations of properties of side‐chain liquid‐crystal polymers

This paper presents a computational conformational study of side‐chain liquid‐crystal polymers to predict the optical and liquid‐crystalline properties of a series of polyepicholorohrdrin, polyacrylate, poly(methyl acrylate), and polystyrene‐based side‐chain polymers using a Monte Carlo simulation method. Some of the simulated side‐chain polymers were synthesised by chemical modification or polymerisation. The predictive capability of the orientational order parameter has been utilised to predict the liquid‐crystalline isotropic transition temperature of the investigated polymers, which was used to infer the type of distribution in the synthesised polymers. The predictive possibilities of this criterion are explored in the estimation of the nematic–isotropic transition temperatures of the simulated polymers. Evidence is presented to suggest that for side‐chain liquid‐crystalline polymer molecules the nematic to isotropic transition occurs when the order parameter reaches a value of 0.43 according to Maier–Saupe mean‐field theory. Copyright © 2006 Society of Chemical Industry     

Self-assembly of bacterial and tunicate cellulose nanowhiskers

The self-assembly and phase transition diagrams of cellulose nanowhiskers derived from tunicates, food grade bacterial cellulose (nata-de-coco) and lab cultured bacterial cellulose are reported. The phase transition diagram for isotropic to liquid crystalline phase transition has been developed for cellulose nanowhiskers obtained from all these sources by recording phase separation behaviour and polarised optical microscopy (POM). The phase separation stops at 10, 4, and 12 wt% for tunicate cellulose, food grade and lab cultured bacterial cellulose, respectively, which also corresponds to the transition seen by POM. The phase diagrams and transition behaviours are explained on the basis of aspect ratio distributions. The liquid crystalline phase obtained after transition shows characteristics of both nematic and chiral nematic phases.     

Behaviour of the miscibility of poly(ethylene oxide)/liquid crystal blends

The microscopic behaviour of blends of poly(ethylene oxide) with two different low molecular weight liquid crystals (LC) was studied in order to evaluate miscibility. One of the liquid crystal components had a phase transition temperature lower than the melting temperature of poly(ethylene oxide) (PEO), and the other a higher value. The low molecular weight liquid crystal components were 4-cyano-4′-n-heptylbiphenyl (7CB) and p-cyanophenyl p-pentyloxybenzoate (pCP). Thermal analysis and polarized optical and scanning electron microscopy were employed. The melting temperature (T_m) depression of PEO increased with LC content in the blend, suggesting that the PEO was miscible with both liquid crystals in the isotropic phase. The spherulitic structural morphology of the semicrystalline components is affected by the presence of liquid crystals. © 1998 SCI.     

Mesomomorphism enhancement of modified side‐chain liquid‐crystalline polysiloxanes by copolymerization

The liquid‐crystalline (LC) monomer 4‐allyoxybenzoyloxy‐4′‐buthylbenzoyloxy‐p‐phenyl (M₁), whose LC phase appeared at lower temperatures, from 137 to 227°C, and the modified mesogenic monomer 4‐allyoxybenzoyloxy‐4′‐methyloxybenzoyloxy‐p‐biphenyl (M₂), whose LC phase appeared at higher temperatures, from 185 to 312°C, were prepared. A series of side‐chain LC polysiloxanes containing M₁ and M₂ were prepared by graft copolymerization. Their LC properties were characterized by differential scanning calorimetry, thermogravimetric analysis, polarizing optical microscopy, and X‐ray diffraction. The results show that the introduction of the modified mesogenic monomer M₂ into the polymeric structure caused an additional increase in the clearing point (isotropic transition temperature) of the corresponding polysiloxanes, compared with unmodified polysiloxanes, but did not significantly affect the glass‐transition temperature. Moreover, the modified polysiloxanes exhibited nematic phases as the unmodified polymer did. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 1196–1201, 2005     

Pitch precursors for advanced carbon materials — rheological aspects

The rheological behaviour of pitch precursors is important in modern processing methods for engineering carbon materials such as composites and fibres. Precursors may be isotropic or liquid crystalline, single or biphase, with the second phase solid or liquid at processing temperatures. The glassy nature of these precursors is discussed, and it is emphasized that the glass transition temperature, T_g, is an important characteristic parameter affecting the dependence of viscosity on temperature. Factors affecting T_g and its variation during thermal treatment are illustrated. A transformation diagram is described that accounts for the pitch-mesophase-carbon transformation, thermal stability regions and partially describes rheological behaviour. An experimentally determined diagram is presented, and finally some applications of this approach are outlined.     

Self-assembly of a liquid crystal ABA triblock copolymer in a nematic liquid crystal solvent

An ABA type triblock copolymer, consisting of liquid crystalline polymer (LCP, poly(4-cyanobiphenyl-4-oxyundecylacrylate)) ‘A’ end blocks and a deuterated polystyrene (dPS) ‘B’ mid block (LCP–dPS–LCP) was successfully synthesized by atom transfer radical polymerization (ATRP). The number average molecular weight (M_n) of LCP–dPS–LCP was LCP (7.1 K)–dPS (19.4 K)–LCP (7.1 K) with a polydispersity index (PDI) of 1.41. LCP–dPS–LCP was self-assembled in a nematic liquid crystal solvent of 4-pentyl-4′-cyanobiphenyl (5CB) into spherical micelles with a LCP corona and a dPS core, in which dPS was folded to produce a V-shape structure. Micellar structures of LCP–dPS–LCP in 5CB were examined by small angle neutron scattering at various block copolymer concentrations and temperatures using a curve fitting method. The critical micelle concentration was 0.25 wt% and the self-assembled micelles dissociated into unimers at 33 °C, which is lower than the nematic to isotropic transition temperature (T_ni) of 5CB (36 °C). The entropic penalty imposed on dPS by the ordered nematic state of the 5CB solvent caused phase separation of the flexible dPS block to form micelles, which vanished above the T_ni of the 5CB solvent. Magnetic field-induced global orientation of 5CB revealed the structure of the dPS core of the micelle to be prolate (an elongated sphere) oriented with its long axis along the direction of the applied magnetic field.     

Rheology of low nematic transition temperature thermotropic liquid crystalline copolyester HBA/HQ/SA

The rheology of a liquid crystalline copolyester of hydroxybenzoic acid, hydroquinone, and sebacic acid (HBA/HQ/SA copolyester) was studied on both a rotational and a capillary rheometer. DSC studies show that the copolyester has a crystalmesophasic and a broad mesophasic-isotropic transition at 170°C and 220°C. Optical texture observations show the mesophase is characterized by line defect textures, which are characteristic of a nematic structure. At 220°C, both isotropic and nematic phases coexist with the latter being the major. As temperature reaches 250°C, a clear dominance of isotropic phase is observed. At this temperature, the nematic phase of irregular shapes randomly disperses within the isotropic matrix. Subsequent rheological studies were thus conducted in crystal/nematic biphase, single nematic phase, nematic/isotropic biphase, and the near single isotropic phase. Dynamic strain sweep measurements show that a linear viscoelastic region exists at all temperatures tested. The maximum strain amplitude for the linear viscoelastic region is found to be highly structure dependent; it is > 100% in the nematic phase, ∼20% in the biphases, and only about 5% in the isotropic phase. The concurrence of curves obtained at different temperatures in a Cole-Cole plot of G′ vs. G″ indicates similar structures in the nematic phase and biphases. Measurements of steady shear viscosity using a rotational rheometer and a roundhole capillary rheometer show that in the nematic phase the copolyester behaves as a shear thinning fluid for a wide shear rate range of 1 ∼ 10,000 s⁻¹, in which the power law index is about 0.6 ∼ 0.8, and the viscosity is < 10 Pa.s at shear rates >1 s⁻¹.     

Synthesis and thermotropic liquid‐crystalline behaviour of novel main‐chain poly(ether ether ketone ketone)s containing a lateral tert‐butyl group

The synthesis of novel poly(ether ether ketone ketone)s containing a lateral group via the random copolymerization of 4,4′‐biphenol, tert‐butylhydroquinone and 1,4‐bis(p‐fluorobenzoyl)benzene is described. The copolymers were characterized by differential scanning calorimetry (DSC), wide‐angle X‐ray diffraction (WAXD) and polarized optical microscopy (POM) observation. The results showed that the thermotropic liquid‐crystalline properties were achieved in the copolymers containing 30 mol% and 50 mol% tert‐butylhydroquinone, which have relatively lower melting temperatures due to the copolymerization effect. Both the crystalline–liquid‐crystalline transition (T_m) and the liquid‐crystalline–isotropic phase transition (T_i) were observable in the DSC thermograms, while the biphenol‐based poly(aryl ether ketone) has only one melting transition. The hydroquinone‐based polymer was shown to be amorphous. Thermogravimetric analysis (TGA) results showed that these copolymers are all high‐temperature resistant with higher glass transition temperature between 147 and 149 °C, and higher decomposition temperature T_d in the range 480–520 °C. © 2000 Society of Chemical Industry     

Main‐chain liquid‐crystalline ionomers bearing potassium sulfonate groups

A series of thermotropic main‐chain liquid‐crystalline (LC) ionomers were prepared, which contained potassium sulfonate groups pendent to the chains. The polymers were prepared in an esterifying reaction with potassium ion contents ranging between 0 and 3.9 wt %. The content of potassium ion was characterized by spectrophotometric analysis with sodium tetraphenylboron as the titrant. Chemical structures were determined by various experimental techniques including Fourier transform IR spectroscopy and ¹H‐NMR. LC properties were characterized by differential scanning calorimetry, polarizing optical microscopy, and X‐rays. All of the polymers displayed nematic or smectic mesophases. With increasing potassium sulfonate ionic concentration in the polymers, the melting temperatures and isotropic transition temperatures changed little, whereas the temperature of the smectic A–nematic phase transition increased. The ionic aggregation was tangled with the rigid mesogenic groups of LC segments to form multiple block domains, leading the soft main chains to fold and form a lamellar structure due to their electrostatic interactions. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 96: 2021–2026, 2005     

Side‐chain copolymers containing smectic monomer and chiral reagent—Synthesis and characterization

A series of new chiral side‐chain liquid‐crystalline polymers were prepared containing smectic‐nematic monomer and nonmesogenic chiral monomer. All polymers were synthesized by graft polymerization using polymethylhydrosiloxane as backbone. The mesomorphic properties were investigated by differential scanning calorimetry, polarizing optical microscopy, thermogravimetric analyses, and X‐ray diffraction measurements. The chemical structures of the monomers and polymers obtained were confirmed by Fourier transform infrared, proton nuclear magnetic resonance spectra (¹H NMR). M₁ showed smectic (S_B, S_C) and nematic phase on the heating and the cooling cycle. Polymers P₀–P₂ were in chiral smectic A phase, while P₃–P₅ were in cholesteric phase, P₆ has bad LC properties, and P₇ has no LC properties. Experimental results demonstrated that nonmesogenic chiral moiety and LC mesogenic with long carbochain offered the possibility of application because of its lower glass transition temperature, and the glass transition temperatures and isotropization temperatures and the ranges of the mesophase temperature reduced with increasing the contents of chiral agent. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Photo-actuated bending of chromatic liquid crystal polymer strips and laminates

Photochromic liquid crystal polymer (LCP) films can undergo spontaneous mechanical deformations upon light illuminations. The light-induced strain is due to photoisomerization and the nematic–isotropic phase transition. A gradient strain exists due to absorption decay of the light intensity and will produce bending of LCP strips. Beam bending models are studied for single layered and bilayered LCP strips with substrate. The results show that the multilayered structures can have much larger bending curvature due to the utilization of the average light-induced contractions which were mostly not active for single-layered LCPs. Some structural optimizations for bilayered laminate are also discussed.     

A phenomenological potential and ferroelectric properties of BaTiO3–CaTiO3 solid solution

A modified phenomenological potential was constructed for BaTiO₃-CaTiO₃ solid solution single crystal based on the Landau–Devonshire theory. The Ba_1−xCa_xTiO₃ solid solution phase diagram of temperature vs concentration (T-x) is obtained and shown to perfectly agree with experimental observations. Sustained Curie temperature can be obtained by the increase of Ca concentration x, while the transition temperatures from tetragonal to orthorhombic phase, and further to rhombohedral phase decrease with increasing Ca content. In this work, spontaneous polarization and dielectric constant dependence on concentration are studied and the relationship between hydrostatic pressure and transition temperatures is established to pave the way for the generation of temperature-concentration-pressure phase diagram.     

Effect of the length of the carbochain on the phase behavior of side‐chain cholesteric liquid‐crystalline elastomers

In this study, we prepared two series of new side‐chain cholesteric liquid‐crystalline elastomers (PI and PII) derived from the same chiral bisolefinic crosslinking units and different nematic liquid‐crystalline monomers. The chemical structures of the monomers and polymers obtained were confirmed by Fourier transform infrared and ¹H‐NMR spectroscopy. The phase behavior properties were investigated by differential scanning calorimetry, thermogravimetric analysis, and polarizing optical microscopy. The effect of the length of the carbochain on the phase behavior of the elastomers was investigated. The polymers of the PI and PII series showed similar properties. Polymer P₁ showed a nematic phase, P₂–P₇ showed a cholesteric phase, and P₄–P₇, with more than 6 mol % of the chiral crosslinking agent, exhibited a selective reflection of light. The elastomers containing less than 15 mol % of the crosslinking units displayed elasticity, a reversible phase transition with wide mesophase temperature ranges, and a high thermal stability. The experimental results demonstrate that the glass‐transition temperatures first decreased and then increased; the isotropization temperatures and the mesophase temperature ranges decreased with increasing content of crosslinking agent. However, because of the different lengths of the carbochain, the glass‐transition temperatures and phase‐transition temperatures of the PI series were higher than those of the PII series, and the PI and PII elastomers had their own special optical properties. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 96: 1204–1210, 2005     

Synthesis and characterization of a series of smectic liquid crystalline elastomers

A series of new smectic and cholesteric liquid crystalline elastomers was prepared by graft polymerization of mesogenic monomer with the chiral and nonmesogenic crosslinking agent using polymethylhydrosiloxane as backbone. The chemical structures of the monomers and polymers obtained were confirmed by Fourier transform infrared (FTIR), proton nuclear magnetic resonance spectra (¹H‐NMR). The mesomorphic properties were investigated by differential scanning calorimetry (DSC), polarizing optical microscopy (POM), and X‐ray diffraction measurements (XRD). M₁ showed smectic (S_B, S_C, S_A) and nematic phases during the heating and the cooling cycles. Polymer P₀ and elastomer P₁ exhibited smectic B phase, elastomers P₂–P₅ showed smectic A phase, P₆ and P₇ showed cholesteric phase, and P₈ displayed stress‐induced birefringence. The elastomers containing less than 15 mol % M₂ displayed elasticity and reversible phase transition with wide mesophase temperature ranges. Experimental results demonstrated that the glass transition temperatures decreased first and then increased; melting temperatures and the isotropization temperatures and the mesophase temperature ranges decreased with increasing content of crosslinking unit. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 498–506, 2005     

Synthesis and characterization of a novel liquid crystalline star‐shaped polymer based on α‐CD core via ATRP

Well‐defined side‐chain liquid crystalline star‐shaped polymers were synthesized with a combination of the “core‐first” method and atom transfer radical polymerization (ATRP). Firstly, the functionalized macroinitiator based on the α‐Cyclodextrins (α‐CD) bearing functional bromide groups was synthesized, confirmed by ¹H‐NMR, MALDI‐TOF, and FTIR analysis. Secondly, the side‐chain liquid crystalline arms poly[6‐(4‐methoxy‐4‐oxy‐azobenzene) hexyl methacrylate] (PMMAzo) were prepared by ATRP. The characterization of the star polymers were performed with ¹H‐NMR, gel permeation chromatography (GPC), differential scanning calorimetry (DSC) and thermal polarized optical microscopy (POM). It was found that the liquid crystalline behavior of the star polymer α‐CD‐PMMAzo_n was similar to that of the linear homopolymer. The phase‐transition temperatures from the smectic to nematic phase and from the nematic to isotropic phase increased as the molecular weight increased for most of these samples. All star‐shaped polymers show photoresponsive isomerization under the irradiation with Ultraviolet light. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Properties of some polysilane polymers: (n-PrSiMe)n, (i-PrSiMe)n, and (sec-BuSiMe)n

Three polysilane polymers, (n-PrSiMe) _n , (i-PrSiMe) _n , and (sec-BuSiMe) _n , were synthesized and characterized by DSC. UV spectroscopy, wide-angle X-ray diffraction, and optical microscopy, all at variable temperatures. The known thermochromic transition of (n-PrSiMe) _n at 48 C is associated with a change from an orthorhombic to an isotropic phase. (i-PrSiMe) _n was examned as an insoluble and soluble (lowM _w) fraction, both existing mainly in an orthohombic lattice at room temperature. (sec-BuSiMe) _n has a mesophase structure at 25 C, undergoes a weak endothermic transition to a second (nematic) mesophase near 65 C, and becomes isotropic at 160 C.Dedicated to the memory of Professor Zygmunt Lasocki, a fine chemist and a kind and gentle person.     

Preferential solvation of the eutectic mixture of liquid crystals E7 in a polysiloxane

Blends of the nematic liquid crystal E7 and poly(methylphenylsiloxane) (PMPS) with molecular weight 120,000 g/mol are investigated by high performance liquid chromatography (HPLC) measurements. This study was prompted by observations made recently while analyzing the phase behavior of poly(siloxane)/E7 and poly(acrylate)/E7 systems. A remarkable increase of the nematic to isotropic transition temperature T_NI was found when polymer was added to the liquid crystal. Surprisingly, the increase of T_NI was enhanced with the polymer concentration up to 80 wt%, where it reached its highest value. This behavior could be interpreted by invoking a preferential solvation of the constituents of E7 towards the polymer. The present investigation provides an evidence of this phenomenon using HPLC data.     

Phase equilibria of modified ethylene glycol–maleic anhydride–phtalic anhydride polyester resins

This paper describes the influence of a termination of ended carboxyl groups of ethylene glycol–maleic anhydride–phtalic anhydride polyesters with cyclohexanole on a position of phase interface of the unsaturated polyester–styrene system. At given temperatures, optimal molecular weights were determined for the achievement of maximal miscibility of this polymer with styrene. Simultaneously, a model implicit equation was proposed, which is based on the assumption that the dependence of the interaction parameter χ on number-average molecular weights is similar to that dependence of the interaction parameter χ on temperature. Solution of these implicit equation are isotherms in the phase diagram M _n??₂?T_{cloude point}.     

Phase behavior of laundry surfactants in polar solvents

Laundry surfactants are usually mixtures of ionic and nonionic detergents that exhibit a complex phase behavior. Here the ternary phase behavior of an isotropic and a liquid crystalline (LC) surfactant mixture has been examined in water/solvent systems. The size of the LC area in the ternary phase diagram was correlated to solvent parameters including the dielectric constant and the Gordon cohesiveness parameter. The Gordon parameter was found to have a linear relationship with the amount of solvent needed to go from an LC to an isotropic state over a wide range of solvents from polar to apolar. For solvents in which no surfactant aggregation (micellar or inverted micellar) is expected, the size of the LC area is linear with the reciprocal of the dielectric constant of the solvent. On diluting practical detergent liquids with water, a large LC area can be avoided by using solvents with a relatively low dielectric constant and with a relatively low molecular weight. The aggregated state of the surfactant mixtures in the isotropic regions of the phase diagram was studied using the solvatochromic fluorescent probe Nile Red. In the water corner of the phase diagram, the surfactants are aggregated into micelles. In strongly polar solvents, such as glycerol, ethylene glycol, formamide, and ethanolamine, the surfactants are also aggregated into micelles. In somewhat less polar solvents, such as methanol, ethanol, and t-butanol, the surfactant molecules are randomly distributed. In the surfactant-rich corner of the phase diagram of the isotropic mixture, the surfactant forms inverted micelles. An inverted micelle-to-micelle transition could be observed on dilution in ethylene glycol as a discontinuity in the trend of the Nile Red fluorescence maxima.     

Alignment layer relaxation—a technique for assessing thermal transitions in polymer films

We introduce a method for assessing the glass transition temperature (T_g) of thin polymer films. The technique may be applied to any polymer film that can effect liquid crystal alignment, and is demonstrated here for a commercial polyamide-imide. The method leverages the ability of the polymer film to align nematic liquid crystals on its surface, when that surface has been prepared by mechanical brushing. Relaxation of the alignment layer, brought about by thermal cycling through T_g, is seen to affect liquid crystal alignment, and thus serve as a T_g indicator. The technique reveals a three-order-of-magnitude change in the measured property. The method allows the assessment of that portion of the film responsible for aligning liquid crystals, and provides an indication of the efficacy of alignment. Our results imply that the relaxation of the surface in Probimide 32 occurs over a range of temperatures, and relaxation is not complete until the film is heated to a temperature above the glass transition of the bulk polymer.