Nematic Polymer Research Articles

AFM study of laser-induced crater formation in films of azobenzene-containing photochromic nematic polymer and cholesteric mixture

Abstract For the first time, AFM study of laser induced surface deformation and crater (holes) formation in liquid crystalline (LC) azobenzene-containing photochromic polymer systems was performed. Special set up including combination of polarizing optical, atomic force microscopies and a possibility of irradiation with a highly focused laser beam (532 nm) was used for these investigations. Films of the nematic azobenzene-containing polyacrylate polymer and the cholesteric mixture prepared by doping of this polymer with the chiral dopant were investigated. Focused laser beam irradiation results in the crater formation formed due to a mass-transfer outside center of beam. Depth of these craters lies in the range of tens of nanometers and increases by prolongation of irradiation time. It is found that this phenomenon takes place only for thick (5–10 μm) films, whereas these phenomena do not take place for the thin spin-coated films. For nematic and cholesteric films the rate of the crater formation, their depth and diameter are the same; that means the chirality of the systems has no influence on kinetics and the depth of photoinduced craters. It is shown, that for the nonaligned LC-films no preferable directed mass transport nearby photoinduced holes was found, but in the case of the uniaxially aligned nematic polymer films the mass-transport occurs only in direction along LC-director and does not depend on the laser polarization direction.

Flexible Mesoporous Photonic Resins with Tunable Chiral Nematic Structures

Colors of nature: Mimicking of the structural colors of nature was achieved by the preparation of easily accessible chiral nematic polymer composites based on phenol–formaldehyde resins templated by cellulose nanocrystals. Removal of the template led to mesoporous polymer films with unique optical and physical properties. The potential application of these materials in optical sensors was also demonstrated. As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re-organized for online delivery, but are not copy-edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Photopolymerisation kinetics and electro-optical properties in mixtures of dichroic dye-doped nematic liquid crystal and photocurable polymer

This work reports the effect of dye on the photopolymerisation and electro-optical properties of polymer dispersed liquid crystal (PDLC) composite films. Dichroic PDLC (DPDLC) films based on a photocurable polymer and nematic liquid crystal (LC) with an azo dichroic dye were prepared by photopolymerisation-induced phase separation method. Polarising optical microscopy has been used for monitoring the phase separation kinetics and two-phase morphology evolution in the DPDLC system. LC domains with radial structures during initial period of phase separation adopted a resultant morphology of bipolar configuration over the course of polymerisation. The phase separation and morphology of LC domains was found to be dependent on the amount of dye used. Moreover, the addition of small amount of dye reduced the switching voltage, and enhanced the contrast ratio with improved switching time in the PDLC films. It was shown that, under the application of an electric field, the molecular orientation and absorbance of dichroic dye can be controlled in DPDLC to induce non-linearity and colour contrast without the use of polarisers.

A nematic liquid crystalline polymer as highly active novel β-nucleating agent for isotactic polypropylene

A nematic liquid crystalline polysiloxane has been found to be a novel nucleating agent (LCP-NA2) to induce the formation of β-crystals in isotactic polypropylene (iPP). Wide angle X-ray diffraction, differential scanning calorimetry, and polarized optical microscopy were used to evaluate the nucleating activity of LCP-NA2. The effect of LCP-NA2 content, crystallization temperature, and time on the crystallization structure, morphology, and thermal behavior of the iPP was discussed. The results indicated that the nucleating activity mainly depended on LCP-NA2 content and thermal history of processing. The addition of LCP-NA2 could provide a large number of nuclei and lead to a more uniform morphology, along with a change in the nucleation and crystal growth mechanism. The relative content of β-crystal increased with increasing LCP-NA2 content or crystallization temperature, reached a maximum value of 70 % when LCP-NA2 content and crystallization temperature were 1.0 wt% and 125 °C, and then decreased with a further increase of LCP-NA2 content or crystallization temperature. In addition, a different polymorphic behavior had been observed between pure iPP and LCP-NA2/iPP blends under the same thermal treatment.

Electro-optical response of polymer-dispersed liquid crystal single layers of large nematic droplets oriented by rubbed teflon nanolayers

The surface orienting effect of rubbed teflon nanolayers on the morphology and electro-optical (EO) response of polymer-dispersed liquid crystal (PDLC) single layers of large nematic droplets was studied experimentally. In PDLC composites of the nematic liquid crystal (LC) E7 and NOA65 polymer, single droplets of LC with diameters as larger as 10 μm were confined in layers with a thickness of 10 μm, and the nematic director field was efficiently modified by nanostructuring teflon rubbing of the glass plates of the PDLC cell. For layered PDLCs arranged and oriented in this way, the modulated EO response by the dielectric oscillations of the nematic director exhibits a selective amplitude-frequency modulation controllable by both temperature and voltage applied, and is simply related to the LC droplet size. That may be of practical interest for PDLC-based modulators operating in the infrasound frequency range.

Influence of the mesogenic shape on the molecular dynamics and phase-biaxiality of liquid crystal main-chain polymers

Ever since the first theoretical phase diagram that was published by Straley in 1974, extensive theoretical efforts have been made on predicting the molecular shape and properties of mesogens that can form a biaxial-nematic phase. The theories suggest that mesogens, whose form is board-like rather than rod-like, are more likely to form biaxial phases. In this study we show that the onset of phase-biaxiality for nematic main-chain polymers can indeed, without significantly changing the glass transition temperature, be shifted to considerably higher temperatures by laterally broadening the mesogenic moieties. The investigations were performed by means of 2H-NMR spectroscopy on directly deuterated polymers and yield results that differ considerably from what could hitherto be observed in spin-probe measurements on side-chain polymers with similarly broadened mesogens.

Thermally Induced Isotropic-Nematic Phase Separation in Mixtures of Low-Molecular Weight and Polymer Liquid Crystals

Using a polarizing optical microscope, kinetics of the thermally induced phase separation in the mixtures of a nematic low-molecular weight (LMW) liquid crystal (LC) and a LC polymer was examined. As a LC polymer, either a nematic side-chain polymer (P1) or a smectic main-chainpolymer (P2) was chosen. Depending on structure of the LC polymer (P1 or P2), kinetics of the LMW LC phase growth was analytically described either by the universal law for the cluster growth or by the Avrami equation, respectively.

Confined chiral polymer nematics: Ordering and spontaneous condensation

We investigate condensation of a long confined chiral nematic polymer inside a spherical enclosure, mimicking condensation of DNA inside a viral capsid. The Landau-de Gennes nematic free-energy Ansatz appropriate for nematic polymers allows us to study the condensation process in detail with different boundary conditions at the enclosing wall that simulate repulsive and attractive polymer-surface interactions. By increasing the chirality, we observe a transformation of the toroidal condensate into a closed surface with an increasing genus, in some respects akin to the ordered domain formation observed in cryo-microscopy of bacteriophages.

Coupled effects of director orientations and boundary conditions on light induced bending of monodomain nematic liquid crystalline polymer plates

A photo-chromic liquid crystal polymers (LCPs) is a smart material for large light-activated variation or bending to transfer luminous energy into mechanical energy. We study the light induced behavior by modeling planar and homeotropic nematic network polymer plates. We effectively illustrate some reported experimental outcomes and theoretically predict some possible bending patterns. This paper constructs an understanding between the bending behaviors and interactions among the alignments, aspect ratios and boundary conditions, etc. Our work provides information on optimizing light induced bending in the process of micro-opto-mechanical system (MOMS) design.

Light-Induced Formation of Dynamic and Permanent Surface Topologies in Chiral–Nematic Polymer Networks

We report a procedure and a material to make patterned surface topologies by light, both in a dynamic way where the structures disappear after exposure or such that the deformations remain permanently. The method is based on a photosensitized cross-linked liquid crystal network with chiral–nematic molecular order. The polymer network is made photoresponsive by a small concentration of copolymerized azobenzene monomers. Upon exposure with patterned UV light, the molecular order parameter in the network is decreased by the trans-to-cis isomerization of the azobenzene, resulting in a density decrease and corresponding local volume increase. Under normal conditions, the polymer network behaves fully elastic and the surface deformation is fast and reversible. To induce permanent deformation, the polymer network needs to be adjusted by chain transfer agents to control the kinetic chain length of the network main chains and allow out of the plane reorientation of the azobenzene moieties and a corresponding loss ...

Plasmonic Variable Optical Attenuator Based on Liquid-Crystal Tunable Stripe Waveguides

A long-range surface plasmon polariton variable optical attenuator based on available nematic liquid crystals and polymers is proposed and theoretically investigated. It is demonstrated that the electro-optic control of the nematic molecular orientation is capable of tuning the level of index asymmetry of an Au stripe waveguide and the key properties of the fundamental long-range plasmonic mode, such as modal profile and propagation losses. By proper structural design and material selection, plasmonic in-line intensity modulators are designed, which exhibit very low power consumption, extinction ratios in excess of 30 dB, and insertion losses as low as 1 dB for a device length in the millimeter range. Such active plasmonic elements are envisaged to be used in interchip photonics bus interconnects.

Effect of magnetic field on discotic nematic liquid crystalline polymers under simple shear flow

AbstractThe effect of magnetic field on the discotic nematic liquid crystalline polymers (LCPs) is analyzed with the extended Doi theory, in which the molecular shape parameter (β) is defined at −1.0. The evolution equation for the probability function of the discotic nematic LCP molecules is solved without any closure approximations. The transition among flow‐orientation modes, such as tumbling, wagging, and aligning defined similar to the rodlike LCPs, is strongly affected by the magnetic fields. The new aligning flow‐orientation mode observed for the rodlike LCPs under magnetic fields also can be investigated in the lower shear rate region. On the other hand, the effect of magnetic fields parallel to the x‐ and y‐axis on the time‐averaged first and second normal stress differences ( $\bar N_1^*$, $\bar N_2^*$) are also studied. It can be seen that the shear rate regions of the sign changes of $\bar N_1^*$, $\bar N_2^*$ are completely contrary to those conclusions achieved for the rodlike LCPs. In addition, the absolute values of $\bar \eta ^*$ increase with the magnetic field strength in the lower shear rate range owing to the new aligning flow‐orientation mode. Finally, the flow‐phase diagram versus β is also discussed. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013

Periodic long-time behaviour for an approximate model of nematic polymers

We study the long-time behaviour of a nonlinear Fokker-Planck equation, which models the evolution of rigid polymers in a given flow, after a closure approximation. The aim of this work is twofold: first, we propose a microscopic derivation of the classical Doi closure, at the level of the kinetic equation ; second, under specific assumptions on the parameters and the initial condition, we prove convergence of the solution to the Fokker-Planck equation to a particular periodic solution in the long-time limit.

Stability of equilibria of nematic liquid crystalline polymers

We provide an analytical study on the stability of equilibria of rigid rodlike nematic liquid crystalline polymers (LCPs) governed by the Smoluchowski equation with the Maier-Saupe intermolecular potential. We simplify the expression of the free energy of an orientational distribution function of rodlike LCP molecules by properly selecting a coordinate system and then investigate its stability with respect to perturbations of orientational probability density. By computing the Hessian matrix explicitly, we are able to prove the hysteresis phenomenon of nematic LCPs: when the normalized polymer concentration b is below a critical value b* (6.7314863965), the only equilibrium state is isotropic and it is stable; when b* <b < 15/2, two anisotropic (prolate) equilibrium states occur together with a stable isotropic equilibrium state. Here the more aligned prolate state is stable whereas the less aligned prolate state is unstable. When b > 15/2, there are three equilibrium states: a stable prolate state, an unstable isotropic state and an unstable oblate state.

Mechanical properties of monodomain nematic side-chain liquid-crystalline elastomers with homeotropic and in-plane orientation of the director

We present the first study of the shear mechanical properties of monodomain nematic side-chain liquid-crystal elastomers (SCLCEs) prepared by cross-linking with UV irradiation a nematic side-chain liquid-crystal polymer oriented with an electric or a magnetic field. Their elastic behavior was studied in the dry, swollen and stretched states, in order to check the various theoretical descriptions of these systems. The shear measurements taken on the dry samples show that the shear anisotropy is much smaller than that of the usual twice cross-linked samples oriented by a mechanical stretching of the network formed after the first cross-linking step, demonstrating that the elasticity of the networks strongly depends on the preparation procedure used. The shear experiments performed on the swollen state of these two different types of elastomers reveal that the elasticity of the network is Gaussian for the elastomers oriented with the electric or the magnetic field, and non-Gaussian for the elastomers oriented with the usual stretching procedure. The analysis of the stress-strain curves of both types of elastomers with the neoclassical model based on Gaussian rubber elasticity confirms the Gaussian and non-Gaussian nature of their elasticity. The shear experiments performed as a function of the elongation of the homeotropically oriented elastomer when the shear is applied in a direction parallel to the elongation, do not show the decrease of the associated shear modulus, which is theoretically expected when the strain approaches the threshold value marking the beginning of the elastic plateau. However, the observation of this effect could be prevented by possible small misalignments of the director, as suggested by a calculation presented in one of the theories describing this effect.

Dependence of the dynamic moduli of heterogeneous nematic polymers on planar anchoring relative to flow direction

We analyze the dynamic moduli of nematic polymers in a parallel plate oscillatory shear experiment from a Doi–Marrucci–Greco orientation tensor formulation, paying special attention to the inherent connection between rheological properties and wall anchoring conditions. We assume standard experimental procedures in which the plates have been rubbed to achieve strong nematic anchoring parallel to the rubbing direction. We derive the heterogeneous, harmonic response of the nematic liquid in the weak oscillatory shear regime of linear viscoelasticity. The response function is parameterized by the orientational anchoring condition and, in particular, by the angle of rotation between the rubbing direction and the flow direction. From this analysis, we read off the frequency-dependent storage and loss moduli. The dominant effect is in the storage modulus where for high frequencies rubbing aligned with the vorticity axis can cause G ′ to be two to three orders of magnitude larger than rubbing in the flow direction. This anchoring dependency shows the significance of the order parameter fluctuations of tensor-based models: the Leslie–Ericksen theory predicts zero storage modulus for vorticity-aligned anchoring. For low frequencies, this effect is reversed with flow-aligned anchoring maximizing G ′ in a manner similar to the Leslie–Ericksen theory although we predict a nonzero modulus for vorticity-aligned anchoring.

Circular dichroism of cholesteric polymers and the orbital angular momentum of light

We explore experimentally if the light's orbital angular momentum (OAM) interacts with chiral nematic polymer films. Specifically, we measure the circular dichroism of such a material using light beams with different OAM. We investigate the case of strongly focussed, non-paraxial light beams, where the spatial and polarization degrees of freedom are coupled. Within the experimental accuracy, we cannot find any influence of the OAM on the circular dichroism of the cholesteric polymer.

Increasing the Isomerisation Kinetics of Azo Dyes by Chemical Bonding to Liquid‐Crystalline Polymers

It is well known that the proper substitution of the azobenzene core allows tuning the thermal cis-to-trans isomerisation kinetics of azo dyes. The thermal isomerisation process of nitro-substituted azobenzenes is accelerated up to 13 times with respect to that in the common isotropic solvents when they are doped in nematic low molar mass liquid crystals. This kinetic acceleration is even stronger when these azo dyes are covalently linked to a nematic siloxane polymer. In this environment, the isomerisation process is accelerated more than 10(3) times. This effect is presented herein for the first time. The possible application of the networks obtained as possible photo-actuators has been also considered.

Kinetics of formation of polymer dispersed liquid crystals in a liquid-crystalline polymer matrix

With the use of optical polarization microscopy, the kinetics of phase separation during cooling of molten mixtures of a nematic low-molecular-mass liquid crystal and a liquid crystalline polymer is studied to produce polymer dispersed liquid crystals. The statistical drop-size distribution of a low-molecular-mass liquid crystal is described in the terms of equilibrium thermodynamics of irreversible processes. For a nematic polymer component of a mixture, the analysis of time dependences of the average diameter of drops of a low-molecular-mass liquid crystal makes it possible to reveal two stages in the kinetics of their growth and to describe this process according to the universal law of cluster growth. For a smectic polymer component, the Avrami equation is used to quantitatively describe the kinetics of growth of low-molecular-mass liquid-crystal drops.

A Nematic Liquid-Crystalline Polymer: Synthesis, Photoresponse and Optical Patterns

A nematic liquid-crystalline polymer (PM6ABOC2) with an azobenzene unit in the side chain was successfully synthesized by polymerization of a monomer (M6ABOC2). The structure of PM6ABOC2 was characterized using Fourier transform-infrared (FT-IR) and 1H nuclear magnetic resonance (NMR) spectra. Thermal analysis and polarizing optical microscopy (POM) indicated a nematic liquid-crystalline phase. UV-vis absorption measurements under UV irradiation at 360 nm revealed that PM6ABOC2 has high photosensitivity both in solution and in the solid state. Binary patterns of the film were clearly observed by POM.