Molecular Weight Nematic Liquid Crystals Research Articles

Shear flows of low molecular weight nematic liquid crystals between parallel circular plates

Shear flows of low molecular weight nematic liquid crystals between parallel circular plates

Optical reversible behavior of poly(ethylene-b-ethylene oxide) block copolymer dispersed liquid crystal blends

Optical properties of novel polymer dispersed liquid crystal (PDLC) blends based on two different low molecular weight nematic liquid crystals, 4′-(hexyloxy)-4-biphenylcarbonitrile (HOBC) and N-(4-ethoxybenzylidene)-4-butylaniline (EBBA), and poly(ethylene-b-ethylene oxide) block copolymer (PE-b-PEO), were studied to develop optically reversible PDLC blends applying a temperature gradient. Differential scanning calorimetry (DSC) showed that the liquid crystals, HOBC and EBBA, maintain their nematic-isotropic transition in designed PDLC blends, which was visualized by optical microscopy (OM). Results obtained using photoluminescence and UV–vis spectroscopies indicated that HOBC and EBBA liquid crystals maintain their reversible optical behavior passing from opaque to transparent state when temperature gradient was applied. The reversible switching process was proved in four consecutive heating/cooling cycles. Designed thermo-responsive PDLC blends possessed interesting optical properties, which make them potential candidates to be use as temperature sensors or thermo-optical devices.

Optical sensor platform based on cellulose nanocrystals (CNC) – 4′-(hexyloxy)-4-biphenylcarbonitrile (HOBC) bi-phase nematic liquid crystal composite films

The preparation of composite materials has gained tremendous attention due to the potential synergy of the combined materials. Here we fabricate novel thermal/electrical responsive photonic composite films combining cellulose nanocrystals (CNC) with a low molecular weight nematic liquid crystal (NLC), 4′-(hexyloxy)-4-biphenylcarbonitrile (HOBC). The obtained composite material combines both intense structural coloration of photonic cellulose and thermal and conductive properties of NLC. Scanning electron microscopy (SEM) results confirmed that liquid crystals coated CNC films maintain chiral nematic structure characteristic of CNC film and simultaneously, transversal cross-section scanning electron microscopy images indicated penetration of liquid crystals through the CNC layers. Investigated composite film maintain NLC optical properties being switchable as a function of temperature during heating/cooling cycles. The relationship between the morphology and thermoresponsive in the micro/nanostructured materials was investigated by using transmission optical microscopy (TOM). Conductive response of the composite films was proved by Electrostatic force microscopy (EFM) measurement. Designed thermo- and electro-responsive materials open novel simple pathway of fabrication of CNC-based materials with tunable properties.

Polarization axis-selective realignment of a photoreactive liquid crystalline composite with homogeneous alignment

We investigated the photoalignment properties of a liquid crystalline composite. The composite consisted of a low molecular weight nematic liquid crystal (LC) and a photoreactive liquid crystalline polymer containing 4-(4-methoxycinnamoyloxy)biphenyl side groups. Homogeneously aligned LC composite cells were fabricated using rubbed substrates and were then exposed to a linearly polarized (LP) ultraviolet (UV) laser beam. The alignment states of the LC composites were characterized using a LP visible laser beam. The LC composite in the cell was realigned by LP UV exposure and subsequent annealing. The direction of the realignment could be controlled by the polarization direction of the LP UV beam. The relationship between the realignment direction and the rubbing strength of the substrates was also investigated. A large realignment was induced in the LC composite cell when the azimuthal anchoring strength of the rubbed substrates was relatively weak.

Switchable photoluminescence liquid crystal coated bacterial cellulose films with conductive response

Three different low molecular weight nematic liquid crystals (LCs) were used to impregnate bacterial cellulose (BC) film. This simple fabrication pathway allows to obtain highly transparent BC based films. The coating of BC film with different liquid crystals changed transmittance spectra in ultraviolet–visible region and allows to design UVC and UVB shielding materials. Atomic force microscopy results confirmed that liquid crystals coated BC films maintain highly interconnected three-dimensional network characteristic of BC film and simultaneously, transversal cross-section scanning electron microscopy images indicated penetration of liquid crystals through the three-dimensional network of BC nanofibers. Investigated BC films maintain nematic liquid crystal properties being switchable photoluminiscence as a function of temperature during repeatable heating/cooling cycles. Conductive response of the liquid crystal coated BC films was proved by tunneling atomic force microscopy measurement. Moreover, liquid crystal coated BC films maintain thermal stability and mechanical properties of the BC film. Designed thermoresponsive materials possessed interesting optical and conductive properties opening a novel simple pathway of fabrication liquid crystal coated BC films with tuneable properties.

Thermal and optical behavior of poly(ethylene-b-ethylene oxide) block copolymer dispersed liquid crystals blends

Different block copolymer dispersed liquid crystals blends based on poly(ethylene-b-ethylene oxide) (PE-b-PEO) and modified with low molecular weight nematic liquid crystals, 4′-(hexyloxy)-4-biphenylcarbonitrile (HOBC) and N-(4-ethoxybenzylidene)-4-butylaniline (EBBA) were prepared and characterized. The miscibility between each block of PE-b-PEO block copolymer and nematic liquid crystals was studied using differential scanning calorimetry (DSC). DSC results were in good agreement with the miscibility prediction supported by the solubility parameters and miscibility effect in polymer dispersed liquid crystals blends based on polyethylene oxide (PEO) homopolymer. Thermal stability of PE-b-PEO/HOBC and PE-b-PEO/EBBA blends was also studied. Block copolymer dispersed liquid crystals blends maintained the nematic/isotropic transition in function of the ratio between PE-b-PEO block copolymer and nematic liquid crystals indicating the potential application of the designed materials.

Origin of the echo phenomenon upon flow reversal in low molecular weight nematic liquid crystals

We analyse the transient shear stress response in tumbling nematic liquid crystals upon flow reversal within the framework of Leslie–Ericksen theory. In particular, we focus our attention on the echo phenomenon, i.e. the progressive re-emergence, upon flow reversal, of the transient oscillations observed at the flow start-up in low molecular weight (LMW) liquid crystals. We show that it is possible to interpret this phenomenon if the director distribution that develops after start-up contains a reversible and an irreversible component. In short, the formation of an echo results from the reversible component while the irreversible component attenuates its amplitude. Within this model, the relative proportion of reversible and irreversible contributions to the total director distribution determines the magnitude of the echo and its rate of decay with the increasing of elapsed time between start-up and flow reversal. The model proposed in this paper is fully analytical; in particular, we give an analytical expression for the intensity of the echo. This model is in good agreement with published experimental data for the MSHMA/5CB nematic mixture (Gu et al. J Rheol 37:985–1001, 1993; Gu and Jamieson, Macromolecules 27:337–347, 1994). We end this paper with a brief discussion contrasting the reappearance of the oscillations in liquid crystal polymers and surfactants immediately after the flow reversal, with the observation of the echo phenomenon around t = 2tR in LMW liquid crystals, where tR is the time interval between the start-up and the reversal of the flow.

Electro-rheology study of a series of liquid crystal cyanobiphenyls: experimental and theoretical treatment

In this work we study the electro-rheological behaviour of a series of four liquid crystal (LC) cyanobiphenyls with a number of carbon atoms in the alkyl group, ranging from five to eight (5CB–8CB). We present the flow curves for different temperatures and under the influence of an external electric field, ranging from 0 to 3 kV mm−1, and the viscosity as a function of the temperature, for the same values of electric field, obtained for different shear rates. Theoretical interpretation of the observed behaviours is proposed in the framework of the continuum theory of Leslie–Ericksen for low molecular weight nematic LCs. In our analysis, the director alignment angle is only a function of the ratio between the shear rate and the square of the electric field – boundary conditions are neglected. By fitting the theoretical model to the experimental data, we are able to determine some viscosity coefficients and the dielectric anisotropy as a function of temperature. To interpret the behaviour of the flow curves near the nematic–isotropic transitions, we apply the continuum theory of Olmsted–Goldbart, which extends the theory of Leslie–Ericksen to the case where the degree of alignment of the LC molecules can also vary.

Gel formation in a mixture of a block copolymer and a nematic liquid crystal

The viscoelastic properties of a binary mixture of a mesogenic side-chain block copolymer in a low molecular weight nematic liquid crystal are studied for mass concentrations ranging from the diluted regime up to a liquid crystalline gel state at about 3%. In the gel state, the system does not flow, exhibits a polydomain structure on a microscopic level, and strongly scatters light. Below the gelation point, the system is homogeneous and behaves like a usual nematic, so the continuum theory of liquid crystals can be applied for interpreting the experimental data. Using the dynamic Fréedericksz transition technique, the dependence of the splay elastic constant and the rotational viscosity on the polymer concentration have been obtained. Comparing the dynamic behavior of block copolymer solutions with the respective homopolymer solutions reveals that, above a mass concentration of 1%, self-assembling of the block copolymer chain segments in clusters occurred, resulting in a gel state at higher concentrations. The effective cluster size is estimated as a function of the concentration, and a scaling-law behavior near the sol-gel transition is confirmed. This technique may serve as an alternative method for determining the gelation point.

Phase Behavior of Mixtures of Low Molecular Weight Nematic Liquid Crystals and Photochemically Crosslinked Polyacrylates

The present work deals with theoretical and experimental studies to explore some physical properties of composite materials made of crosslinked poly(n-butylacrylate) networks and low molecular weight nematic liquid crystals (LCs). The chemically crosslinked polymers were obtained by exposure to UV radiation of initial solutions composed of a reactive monomer, n-butylacrylate, a small amount of a crosslinking agent, hexanedioldiacrylate, and a photoinitiator. To obtain different polymer network densities, the ratio of n-butylacrylate to hexanedioldiacrylate was varied prior to exposure to UV irradiation. Immersion in excess LC solvent of the obtained polymer networks allowed for the measurement of the solvent uptake by determining the size increase as function of temperature by polarized optical microscopy. A strong change of the swelling ratio was found around the nematic-isotropic transition temperatures of the two LCs involved, i.e., 4-cyano-4′-n-pentylbiphenyl known as 5CB, and the eutectic commercial mixture of cyanoparaphenylene derivatives (E7). The obtained swelling data were used to establish phase diagrams in terms of composition and temperature for three different network densities and two LCs. The phase diagrams exhibit upper criticial solution temperatures and isotropic network + nematic solvent (I + N), isotropic network + isotropic solvent (I + I), and isotropic swollen network (I) coexistence regions. The results were successfully analyzed within a theoretical model using a combination of the Flory-Rehner theory of isotropic mixing and the Maier-Saupe theory of nematic ordering.

Electro-mechanical effects in liquid crystals

Linear and quadratic electromechanical effects of liquid crystals are reviewed. Among the linear effects the piezoelectric and flexoelectric effects and their analogues in low molecular weight fluid, nematic, smectic and columnar liquid crystals, in polymeric and elastomer liquid crystals, are discussed and compared with the piezoelectricity of conventional solid piezoelectrics. As the most important quadratic electromechanical effect, an overview is given of the electrostriction effects in fluid, polymeric and rubber-like liquid-crystalline systems. These topics are rarely discussed in the liquid crystal literature, although they are becoming increasingly important for future energy conversion and micro-energy-generating devices.

UV tuning of the electro‐optical and morphology properties in polymer‐dispersed liquid crystals

Polymer‐dispersed liquid crystal (PDLC) films operating in reverse mode are transparent electro‐optical devices, which can be turned into an opaque state by application of a suitable electric field. The effect was investigated of different UV powers, used during the polymerization process, on the electro‐optical and morphology properties of PDLCs, working in reverse mode operation. Films were obtained by UV polymerization of mixtures of a low molecular weight nematic liquid crystal and a photopolymerizable liquid crystal monomer, homeotropically aligned by rough conductive surfaces. The electro‐optical and morphology properties of samples were related to the polymerization conditions. Samples polymerized by lower UV powers exhibited “polymer ball” morphology and an electro‐optical response due to the liquid crystal director reorientation, whereas samples obtained at higher UV powers showed a “Swiss cheese” morphology and an electro‐optical response due to dynamic scattering. In addition, we observed by conductivity and IR measurements that UV exposure induces a degradation of the nematic liquid crystal.

New cellulose derivatives composites for electro-optical sensors

In this work we used hydroxypropylcellulose (HPC) and acetoxypropylcellulose (APC) to produce free standing solid films (∼60 μm) for electro-optical devices. Cellulose derivatives films were prepared from crosslinked both HPC and APC isotropic and liquid crystalline solutions without and with a low molecular weight nematic liquid crystal mixture (LC) E7. Films prepared from anisotropic solutions presented a band texture and E7 droplets of micron and submicron size were found to coexist with the band texture in the films prepared with E7. The optical cells were composed by the cellulose derivatives films covered on both free surfaces by a layer of the LC E7 and placed between two transparent conducting substrates. All the electro-optical cells prepared showed switching times in the range of some milliseconds to hundreds of milliseconds. The on-voltage ( V on) is lowered for cells prepared with the ester derivative film obtained from isotropic solutions. The maximum transmission values show a clearly tendency to decrease for the cellulosic matrix prepared from anisotropic solutions with and without nematic liquid crystal droplets.

Transient Shear Stress Response After Flow Start-up and Flow Reversal in Low Molecular Weight Nematic Liquid Crystals

ABSTRACT Within the framework of the Leslie–Ericksen theory we develop a unified model for the interpretation the complex shear stress response of tumbling nematics after flow start-up and flow reversal. The model accounts quantitatively for: (i) the damping of the oscillations, the vanishing of the doublet structure and eventually a pronounced decay after flow start-up, and (ii) the progressive reappearance of the oscillations after flow reversal (echo phenomenon). Within this model the loss of director coherence, induced partially by noise and partially by a distribution of velocity gradients, is a significant additional cause of damping. A primary cause (but not sufficient) is the director drift towards the vorticity axis as pointed out by other authors. On the other hand, the reappearance of the oscillation is related to reversibility dictated by the mechanical Leslie–Ericksen equations combined with the velocity gradient distribution.

Liquid Crystal Influenced Self-assembly in Mesogenic Polymer and Liquid Crystal Blends

ABSTRACTIn this work, we found that by performing photo-polymerization of the mesogenic monomer RM257 in liquid crystals, well organized polymer walls were formed through out the cell by self-assembly of the polymer within the anisotropic host – the liquid crystal. The polymerization conditions, which were parameterized as UV intensity, photo-reactivity (characterized by photo initiator concentration), curing temperature, monomer concentration and mesophase of the liquid crystal host, were systematically varied. Different liquid crystal host also raised some difference in polymer network. We believe this kind of morphology comes from spinodal decomposition and the anisotropic elastic property of the liquid crystal host. It is found that for RM257 and low molecular weight nematic liquid crystals a monomer concentration of 5% is sufficient to use liquid crystal hosts to work as templates for well structured polymer walls.

PHYSICAL PROPERTIES OF ELECTRON BEAM-CURED TRIPROPYLENEGLYCOLDIACRYLATE AND LIQUID CRYSTAL SYSTEMS

Some salient features of physical properties of Tripropyleneglycol diacrylate (TPGDA) and the low molecular weight nematic liquid crystal E7 are discussed. A variety of experimental techniques giving complementary information of monomer and Electron Beam (EB) cured systems are employed to illustrate the effect of curing. A detailed thermophysical analysis is made first to assess the conditions of miscibility, determine the order-disorder transition and the glass transition temperature. DSC spectra yield the energy change at the transitions from which important parameters are readily extracted such as solubility limits and amount of liquid crystal dissolved in the polymer. Polarized Optical Microscopy enables us to follow phase transitions and morphology changes in different regions of the phase diagram. Consistent data are obtained from mechanical measurements in particular with regards to the variation of the glass transition temperature with liquid crystal concentration and the related plasticizing effect.

Swelling and Deswelling of Poly (n-butylacrylate) Networks in Isotropic and Nematic Solvents

We report swelling/deswelling data of Poly(n-butyl acrylate) networks in isotropic and anisotropic solvents. The investigated systems are prepared via polymerization induced phase separation processes of initial mixtures composed of a reactive polymer precursor, a cross-linker, a photoinitiator and a low molecular weight solvent by ultraviolet (UV) radiation. A comparison of data obtained from isotropic organic solvents and a low molecular weight nematic liquid crystal is made in terms of temperature and cross-linking density of the network. Hexanediol-diacrylate (HDDA), a di-functional monomer is used as a cross-linker and the conditions of preparation are set in order to obtain well defined model networks. Effects of temperature and composition are evaluated by measuring the size via microscopy and the mass of dry and swollen networks. Simple modelling is used to rationalize the data.

Noise Effects on the Transient Shear Flow of Tumbling Nematic Liquid Crystals

This paper describes a stochastic approach to nematodynamics in a specific case. We start from the director equation derived from the Leslie-Ericksen theory for a monodomain and we assume that the shear rate is a random function of time. This assumption is a simple way to account for the occurrence of noise in the velocity field produced by any thermal or mechanical fluctuations. We show that this approach provides a new framework allowing for the description of transient rheology upon flow inception in low molecular weight nematic liquid crystals. A bidimensional model in which the director stays in the shear plane is developed and compared to experimental data for the tumbling nematic formed by the dissolution of a side-chain polymer in 5CB.

Photo-alignment of low-molecular mass nematic liquid crystals on photoreactive polyimide and polymethacrylate film by irradiation of a linearly polarized UV light

Photocrosslinkable polyimide and polymethacrylate comprising chalcone moieties in the side chain were synthesized for fabricating the alignment layer of liquid crystals. Chalcone group was introduced into the side chain unit of the polymer backbone. We observed a photodimerization behavior either in solution or in film by irradiation of a UV light. Linearly polarized UV irradiation on the chalcone-polymer films induced optical anisotropy of the polymer film and the resultant film can be used for alignment layers for low molecular weight nematic liquid crystals.

Photo-alignment of Low-molecular Mass Nematic Liquid Crystals on Photochemically Bifunctional Chalcone-epoxy Film by Irradiation of a Linearly Polarized UV Light

Photocrosslinkable chalcone-epoxy compound comprising 1,3-bis-(4-hydroxy-phenyl)-propenone was synthesized for fabricating the photo-alignment layer of liquid crystals. Chalcone group was introduced into the main chain unit of the epoxy oligomer. We observed a photodimerization behavior and an optical anisotropy of this material by irradiation of a linearly polarized UV(LP-UV) light. With a trace amount of cationic photoinitiator (TRS-HFA), polymerization of epoxy groups was also conducted at the similar wavelength range used for photodimerization. Linearly polarized UV irradiation on the chalcone-epoxy films with cationic photoinitiator induced optical anisotropy of the film and the resultant film can be used for alignment layers for low molecular weight nematic liquid crystals.