Supramolecular main-chain liquid crystalline polymers and networks with competitive hydrogen bonding: networks and linear polymers created from tris- and bis-functionalised pyridyl networking agents

A series of supramolecular polymers and networks with variable liquid crystalline characteristics have been created. These species are formed through the benzoic acid/pyridine hydrogen bonding of a flexible bis-acid and a mixture of a rigid bis-pyridyl and non-mesogenic tris- and bis-pyridyl molecules. The tris networked systems displayed liquid crystalline characteristics up to and including 31.0% netpoint inclusion. Above this concentration, only crystalline and melting behaviours were observed. The bis-containing polymer system displayed liquid crystalline characteristics up to and including 22.5% inclusion. The phenomenon observed in the tris system would seem to be linked to a statistical correlation of hydrogen bond acceptors and donors. The elimination of liquid crystallinity at lower concentrations of the bis-pyridyl dopant could be attributed to the lower melting nature of that species. Smectic phases were found in both series of complexes in loadings up to 10% of the non-liquid crystalline component. There was also no observed phase segregation of the species after multiple heat/cool cycles and extended periods of time in the isotropic state. This would indicate that the thermodynamically more stable mesogenic phase cannot out-compete the non-liquid crystalline network.     

Photomechanical effects in liquid crystalline polymer networks and elastomers

Liquid crystals are widely employed as stimuli‐responsive materials. Liquid crystallinity can be retained in polymeric form. Photoinduced mechanical effects in liquid crystalline polymer networks and elastomers have been a topic of considerable recent research. This review details the historical underpinnings and recent advances in the synthesis and the corresponding photomechanical response of these materials. In nearly all cases, the conversion of light into mechanical work has employed azobenzene as either a guest additive or covalently attached to the network. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018 , 56, 695–705     

Influence of molecular weight on liquid crystalline behavior of linear and star mesogen‐jacketed liquid crystal polymers

A series of novel multi‐armed (di‐, tri‐ and tetra‐armed) mesogen‐jacketed liquid crystal polymers (MJLCPs) were synthesized by atom transfer radical polymerization (ATRP) of {2,5‐bis[(4‐methoxyphenyl)oxycarbonyl]styrene}(MPCS) using di‐, tri‐ and tetrafunctional initiator, respectively. The results show that the number average molecular weight (M_n,GPC) was increased versus monomer conversion, and the polydispersities were quite narrow (<1.19), which is the characteristic of controlled polymerization. The chemical structures of these multi‐armed mesogen‐jacketed liquid crystal polymers were confirmed by ¹H NMR. The liquid crystalline behavior of these multi‐armed MJLCPs with arms ranging from two to four was studied by differential scanning calorimetry (DSC), polarized optical microscopy (POM) and wide‐angle X‐ray diffraction (WAXD). It was found that liquid crystalline phases appeared simply when the number molecular weights (M_n,GPC) of these multi‐armed MJLCPs was higher than a certain critical values, that is, M_n,GPC > 1.87 × 10⁴ g/mol, 1.84 × 10⁴ g/mol, 2.69 × 10⁴ and 3.68 × 10⁴ g/mol, which were initiated by coil difunctional initiator, hard difunctional initiator, trifunctional initiator and tetrafunctional initiator, respectively. All the liquid crystalline phase was found to be stable up to the decomposition temperature of these multi‐armed MJLCPs. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 3232–3244, 2005     

Photopolymerization of nematic liquid crystal monomers for structural applications: Linear viscoelastic behavior and cure effects

This article is concerned with molecular orientation in liquid crystal (LC) monomers and the retention of orientation in crosslinked network polymers formed from them by photopolymerization. This is of importance because anisotropic mechanical and physical properties can be beneficial in certain structural applications. To this end, linear viscoelastic behavior of liquid crystal photo-monomers was investigated with dynamic mechanical analysis, and molecular order was studied by infrared dichroism measured with Fourier transform infrared spectroscopy. Although the order parameter of the monomer could vary from 0.45 to 0.70, depending on temperature, the order parameters of the polymer samples varied only from 0.50 to 0.62 and depended on polymerization temperature and extent of cure. The mechanical anisotropy was found to be a complicated phenomenon that depended not only on the molecular order, but also on other factors such as free volume and network structure. The difference in the elastic modulus parallel and perpendicular to the alignment direction was as high as a factor of two in the glassy state, and a factor of three above T_g. In addition, different amounts of mechanical anisotropy could be induced by varying the cure conditions. Finally, different postcuring schemes could cause variations in mechanical behavior by advancing cure or by inducing secondary reactions. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1081–1089, 1998     

Dynamic mechanical behavior of a side-chain liquid crystalline polyacrylate

The phenomenology of the α- and β-relaxation processes in an amorphous and a crystallized specimen of a side-chain LC polyacrylate, based on the azobenzene mesogenic unit, was investigated by thermal and dynamic mechanical methods. The activation energy of the β relaxation of the crystallized sample is equal to that of the amorphous sample. The α-transition process of the amorphous sample was described by the WLF equation. In contrast, the α-relaxation behavior of the amorphous part of the semicrystalline sample was described by a double Arrhenius law broken at T = T_g. This peculiar behavior has been tentatively related to the decrease of the motion characteristic length of the macromolecules confined in the multidomain structure of the semicrystalline state. © 1996 John Wiley & Sons, Inc.     

Morphology of polymer networks polymerized in highly ordered liquid crystalline phases

The morphology and optical properties of polymer stabilized liquid crystals formed in a more highly ordered low molecular weight liquid crystal solvent were studied. Tetrafunctional, mesogenic monomers (with and without flexible spacers) were polymerized in isotropic, nematic and smectic phases of the LC solvent (4′-octyl-4-cyanobiphenyl) and studied with scanning electron microscopy and cross-polarized light microscopy. The network morphology of the nematic and isotropic phase polymerizations showed strong similarities with the corresponding polymerizations in other solvents. Polymerization in the smectic phase, however, resulted in marked increases in network order and directionality. Most dramatically, even the polymer without flexible spacer formed a fibrous network of rodlike units, in contrast to the random, beaded texture formed by the same polymer in nematic or isotropic conditions. Correspondingly, a large increase in birefringence demonstrated significant polymer orientation and more effective orientational interaction with the liquid crystalline solvent.     

Fluorene‐based liquid crystalline networks with linearly polarized blue emission

A series of fluorene‐based luminophores containing methacrylic end groups have been prepared and incorporated into uniaxially oriented liquid crystalline films by in situ photopolymerization. Various structural modifications on the 2‐(4‐cyanophenyl)fluorene core, which include alkyl chains at the 9‐position and elongation of the rigid core with one additional phenyl ring, have been investigated to generate emitters with adjusted liquid crystal compatibility, improved luminescence and dichroic properties. Polarized blue‐emitting films were produced that had an acceptable photostability, and it was found that the polarization emission was better for samples with low (5%) cross‐linker contents. Polarization of the luminescence was favored by the liquid crystalline properties of the luminophore. In addition, the detrimental effect of the alkyl substituent at the fluorene core on the mesomorphism and on the emission polarization can be overcome by lengthening the π‐system. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 4804–4817, 2007     

Synthesis and thermal behavior of side-chain liquid crystalline polymethacrylates containing tolane-based mesogenic side groups

The synthesis and characterization of nine polymethacrylates containing 4-alkoxy-4′-trifluoromethyltolane, 4-alkoxy-4′-cyanotolane, and 4-alkoxy-4′-nitrotolane side groups were described in this study. The phase behavior of the prepared monomers and polymers was characterized by differential scanning calorimetry, optical polarizing microscopy, and x-ray diffraction. All of the obtained monomers exhibit no mesophase, while most of the synthesized polymers reveal enantiotropic mesomorphism. The polymethacrylate containing 4-propanyloxy-4′-nitrotolane side groups was the only one which shows no mesomorphic behavior. Both the spacer length and the nature of terminal groups have profound influence on the phase transition temperatures and thermal stability of the mesophase. The polymers with longer spacers tend to form a more ordered mesophase with a wider temperature range. Among three polymers with the same spacer length, the polymer with a trifluoromethyl terminal end group is inclined to form a more ordered mesophase than the other two polymers. No side chain crystallization occurred for all obtained polymers. © 1994 John Wiley & Sons, Inc.     

Preparation and electro‐optical properties of polymer dispersed liquid crystal films with relatively low liquid crystal content

In this paper, polymer dispersed liquid crystals (PDLC) films with LC content as low as 40 wt% were prepared, and the electro‐optical properties were carefully investigated. To accomplish this, different (meth)acrylate copolymerizaiton monomers have been used. The electro‐optical properties and morphologies of the PDLC films were strongly influenced by the chemical structure of copolymerization monomers (hydroxypropyl methacrylate (HPMA), glycidyl methacrylate, hydroxypropyl acrylate) and their feed ratio. Lower driven voltage and higher contrast ratio were achieved when the PDLC films showed a morphology with suitably LC domain size. At high HPMA content, a thin polymer film was formed on the surface of PDLC samples, which is beneficial to decrease the total LC content in PDLC devices. Copyright © 2013 John Wiley & Sons, Ltd.     

Synthesis and properties of reactive liquid crystal monomers and side-chain liquid crystalline polymers

Two nematic liquid crystal (LC) monomers containing double bonds in the side chain were designed and synthesised. Length of the side groups varied from 1 to 2 methylene units. The side-chain polymers were synthesised by hydrosilylation reaction. The molecular structures of the intermediates and the LC monomers were characterised by Fourier transform infrared, elemental analysis and nuclear magnetic resonance spectroscopy. The thermal phase behaviour of the monomers and polymers were investigated by differential scanning calorimetry and polar optical microscopy coupled with hot stage. The LC monomers showed only one nematic mesophase in the cooling process. And, the two polymers exhibit an enantiotroppic nematic mesophase either in the heating or in the cooling process.     

Copolymers of 2,5‐bis[(4‐methoxyphenyl) oxycarbonyl]styrene with styrene and methyl methacrylate: Synthesis,monomer reactivity ratios,thermal properties,and liquid crystalline behavior

Copolymers of a liquid crystalline monomer, 2,5‐bis[(4‐methoxyphenyl)oxycarbonyl]styrene (MPCS), with St and MMA were prepared by free radical polymerization at low conversion in chlorobenzene with 2,2′‐azobisisobutyronitrile (AIBN) as initiator. The copolymers of poly(MPCS‐co‐St) and poly(MPCS‐co‐MMA) were characterized by ¹H NMR and GPC. The monomer reactivity ratios were determined by using the extended Kelen–Tudos (EKT) method. Structural parameters of the copolymers were obtained from the possibility statistics and monomer reactivity ratios. The influence of MPCS content in copolymers on the glass transition temperatures of copolymers was investigated by DSC. The thermal stabilities of the two copolymer systems increased with an increase of the molar fraction of MPCS in the copolymers. The liquid crystalline behavior of the copolymers was also investigated using DSC and POM. The results revealed that the copolymers with high MPCS molar contents exhibited liquid crystalline behaviors. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 2666–2674, 2005     

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     

Control of the microstructure of polymer network and effects of the microstructures on light scattering properties of UV‐cured polymer‐dispersed liquid crystal films

Polymer‐dispersed liquid crystals (PDLC) films were prepared from ultraviolet (UV) irradiation‐induced polymerization of the photopolymerizable monomers in photopolymerizable monomers/nematic liquid crystal (LC) mixtures. The effects of the composition of the mixtures, the curing temperature, and the UV light intensity on the microstructure of the polymer network in the PDLC films were investigated. Furthermore, the effects of the microstructures on the light scattering properties of the PDLC films in the wavelength region of 300–2500 nm were studied experimentally and theoretically based on the combination of three kinds of classical light scattering theories: the Rayleigh‐Gans (RG) approach, the anomalous diffraction (AD) approach, and the geometrical optics (GO) approach. It was found that the sizes of LC domain in PDLC films increased with the increase of the LC content as well as the decrease of the UV curing intensity, while increased at first and then decreased with the increase of the curing temperature. Moreover, smaller LC domain sizes could exhibit strong scattering properties in a smaller VIS wavelength region and the transmittance in NIR region (especially in the wavelength range of 1300–2500 nm) obviously decreased with the increasing sizes of LC domain. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 2090–2099, 2008     

Highly thermal conductive benzoxazine‐epoxy interpenetrating polymer networks containing liquid crystalline structures

A diamine‐based benzoxazine monomer (Bz) and a liquid crystalline epoxy monomer (LCE) are synthesized, respectively. Subsequently, a benzoxazine‐epoxy interpenetrating polymer network (PBEI) containing liquid crystalline structures is obtained by sequential curing of the LCE and the Bz in the presence of imidazole. The results show that the preferential curing of LCE plays a key role in the formation mechanism of liquid crystalline phase. Due to the introduction of liquid crystalline structures, the thermal conductivity of PBEI increases with increasing content of LCE. When the content of LCE is 80 wt %, the thermal conductivity reaches 0.32 W m^?1 K^?1. Additionally, the heat‐resistance of PBEI is superior to liquid crystalline epoxy resin. Among them, PBEI55 containing equal weight of Bz and LCE has better comprehensive performance. Its thermal conductivity, glass transition temperature, and the 5 % weight loss temperature are 0.28 W m^?1 K^?1, 160 °C, and 339 °C, respectively. By introducing boron nitride (BN) fillers into PBEI55, a composite of PBEI/BN with the highest thermal conductivity of 3.00 W m^?1 K^?1 is obtained. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55 , 1813–1821     

Time‐dependent deformation of structurally chiral polymer networks in stabilized cholesteric liquid crystals

Polymerization of crosslinkable liquid crystal monomers in chiral liquid crystalline media stabilizes the phase and enables distinct electro‐optic properties relative to small‐molecule analogs. Particularly interesting are cases where the polymerization forms a crosslinked polymer network that maintains a “structural” chirality. Recent reports have employed this methodology to realize a diverse set of electro‐optic responses in polymer stabilized cholesteric liquid crystals (PSCLCs) including reflection bandwidth broadening, reflection wavelength tuning, and dynamic scattering modes. It has been proposed that the mechanism at the root of these electro‐optic responses is an ion‐mediated, electromechanical deformation of the stabilizing and structurally chiral polymer network. In an effort to better understand the nature of these deformations, here we have characterized the electro‐optic response of PSCLCs with different polymer concentrations and crosslink densities. The dynamic response of PSCLCs to electric fields exhibits a time‐dependent behavior reminiscent of the creep of polymeric materials to mechanical deformations. The electro‐optic response can be described as the superposition of two contributions: the fast deformation of a relatively soft component of the polymer network (1–2 s) and the slower (10–20 s) deformation of a harder component. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018 , 56, 1087–1093     

Polymerization of mono-substituted acetylenes with a liquid crystalline moiety by Ziegler–Natta and metathesis catalysts

Polymerization of five monomers, 1-[p-(trans-4′-alkylcyclohexyl)phenoxy]alkyne (i.e., PCH001A, where PCH, 0, 01 and A represent phenylcyclohexyl mesogenic moiety, number of carbon in an alkyl group attached to cyclohexyl group, ether linkage + number of methylenic units in the spacer between phenoxy and acetylenic groups, and terminal acetylene, respectively) were carried out using Ziegler-Natta and metathesis catalysts. All polymers were soluble in organic solvents such as benzene, chloroform, and THF. A liquid crystalline phase was observed in the polymers of PCH303A and PCH503A through the polarized optical microscope and DSC measurements. Polymerization of PCH003A by the Fe(acac)₃-AlEt₃ catalyst yielded a high-molecular-weight polymer with M_w = ca. 3 × 10⁶. © 1993 John Wiley & Sons, Inc.     

Polarization and viscosity measurements of ferroelectric liquid crystalline monomer polymer mixtures

Measurements of spontaneous polarization, tilt angle and response time on mixtures of a ferroelectric side chain polyacrylate with a low molecular mass liquid crystal of a very similar structure are reported. From the obtained values the rotational viscosity for the ferroelectric switching process is calculated. All mixtures exhibit an Arrhenius-like temperature dependence of the rotational viscosity, the values for the polymer being three orders of magnitude higher than for the low molecular mass compound.     

Main chain 1,1′‐ferrocene‐containing polyelectrolytes exhibiting thermotropic liquid‐crystalline and fluorescent properties

A series of 1,1′‐ferrocene‐containing polyelectrolytes ( 3, 4 ) were prepared when 1,1′‐bis(N,N‐dimethylaminomethyl)ferrocene ( 1a ) or 1, 1′‐bis{[1‐(2‐methyl)imidazol‐1‐yl]methyl}ferrocene ( 1b ) was quaternized with 1,4‐dibromobutane or α, α′‐dibromo‐p‐xylene. The counterion was bromide or bis(trifluoromethanesulfonyl)‐amide formed after metathesis with the lithium salt. Their chemical structures were determined by IR and NMR spectra. Molecular weights in the range of ～5400 ( 4a )– ～14,700 ( 4c ) for number‐average molecular weights (M_n) over narrow molecular weight distributions were determined for polymers 4 by gel permeation chromatography. Thermal properties of these materials were obtained by differential scanning calorimetry and thermogravimetric analysis that showed the polymers had thermal stabilities ranging between 172 and 330 °C. Liquid‐crystalline behavior was investigated on a hot stage polarizing optical microscope. Polymers 3a , 4b , and 4d formed either a high‐order or a low‐order smectic phase above their melting or fusion temperatures, and exhibited smectic‐to‐isotropic transitions. The ranges of the liquid‐crystalline phases for these materials were 22, 46, and >55 °C. Compounds 3b , 4a , and 4c are crystalline before melting or decomposing. All of the polymers exhibited absorption bands at ～430 nm. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 974–983, 2005     

Effect of Monomer Composition on the Performance of Polymer‐Stabilized Liquid Crystals with Two‐Step Photopolymerization

Polymer‐stabilized cholesteric liquid crystal (PSCLC) films with broadband reflection based on two‐step photopolymerization are fabricated. Owing to the helical twisting power (HTP) value of the chiral dopant (CD) decreasing with increasing temperature, PSCLC films with broadband reflection are obtained by two‐step polymerization anchoring helical pitch of different length at two different temperature points. The effect of monomer composition on the PSCLC reflection properties before and after polymerization was studied. The results show that the free‐radical monomers with appropriate concentration and cationic monomers with sufficient concentration are vital for the formation of PSCLC films with broadband reflection. In addition, the experiments show that the increase in the functionality and rigidity of the cationic monomer has a positive effect on the broadening of the reflection band. This study can provide guidance and reference for the selection of monomer species and concentration in PSCLC preparation. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019 , 57, 1126–1132     

Photopolymerization kinetics in and of self‐assembling lyotropic liquid crystal templates

Photopolymerization in and of lyotropic liquid crystal (LLC) template phases shows great promise for generating nanostructure in organic polymers. Interestingly, the order imposed on the polymerization system in LLCs significantly alters polymerization kinetics. The rate of polymerization of hydrophilic monomers increases with increasing LLC order, primarily due to monomer/polymer association with surfactant and the resulting decrease of growing polymer chain diffusion. Conversely, as LLC order increases, hydrophobic monomers become less segregated as nonpolar volume increases, which decreases polymerization rate. The efficiency of initiators is also dependent on LLC template order, further contributing to polymerization rate changes. When reactive surfactants are used, LLC mesophase, location of reactive group, and aliphatic tail length also affect polymerization kinetics. Overall, these photopolymerization kinetics directly relate to the segregation behavior and local order of reactive groups and thus can be used to probe nanostructure evolution, facilitating understanding and control of ultimate polymer nanostructure. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 471–489