Nematic Liquid Crystalline Polymers Research Articles

Complexity from simplicity: Confinement directs morphogenesis and motility in nematic polymers

Liquid crystalline polymers (LCP) that actuate under confinement adopt shapes with complexity that is upconverted beyond that encoded by their nascent patterning. Twisted nematic LCP thermally transform into simple spiral structures, when unconstrained. Confining them into rings or rigidly fixtured rectangular elasticas induces their self-assembly into supercoils, prismatic folded trusses, twisted and bent tape spring geometries. No additional material patterning, localized stimulation or training is required. Along its way, the actuation unlocks mechanical instabilities that can power impulsive responses. The self-assembled structures also sustain dynamic responses. These include motility via crawling and kinematic manipulation in a crank-rocker mechanism from an otherwise unstructured thermal stimulus. Interacting assemblages of the individually confined LCP build functionally relevant structures, including airframes and gripper-arm mimicking geometries. Thus, confinement is shown to free shape selection from a one-to-one correspondence to the blueprinted microstructure in active polymers.

Phase Structure Recording in a Nematic Side-Chain Liquid-Crystalline Polymer.

Dye-doped nematic side-chain liquid-crystalline polymers possess extraordinary large optical nonlinearity and ability to store the induced orientational deformations in a glassy state, which makes them a very promising material for photonic applications. In this study, the phase structures were generated and recorded in the bulk of a 50-μm layer of a nematic liquid-crystalline side-chain polymer, containing polyacrylate backbone, spacer having five methylene groups, and phenyl benzoate mesogenic fragment. The polymer was doped with KD-1 azodye. The director field deformations induced by the light beam close to the TEM01 mode were studied for different geometries of light–polymer interaction. The phase modulation depth of 2π was obtained for the 18-μm spacing between intensity peaks. The experimental data were analyzed based on the elastic continuum theory of nematics. The possibility to induce and record positive and negative microlenses in the polymer bulk was shown experimentally.

Photoinduced Reorientation in Thin Films of a Nematic Liquid Crystalline Polymer Anchored to Interfaces and Enhancement Using Small Liquid Crystalline Molecules.

The photoinduced reorientation of the side-chain mesogens in nematic liquid crystalline (LC) polymer thin films triggered by the axis-selective photo-Fries rearrangements of side-chain phenyl benzoate moieties is studied to understand the regulation of the anisotropic nanostructures supported by LC polymers. The influence of the substrate surface in anchoring the side-chain mesogens near the interfaces is examined by comparing the reorientation of 30- and 120-nm-thick films. Irradiation with linearly polarized ultraviolet (UV) light and subsequent annealing causes the side-chain mesogen reorientation to align perpendicular to the electric field of the incident UV light. The inplane order in the 30-nm-thick films is lower than that in the 120-nm ones. On the other hand, the annealing period required for mesogen alignment is independent of the film thickness. It is suggested that the substrate surfaces anchor the LC mesogens to fix their orientation, rather than slowing down the reorientational motion. In addition, it is demonstrated that small LC molecules miscible with the nematic LC polymer enhance photoinduced reorientation through cooperative molecular interaction with the side-chain mesogens, remarkably accelerating the orientation and improving the inplane order of the unidirectionally aligned mesogens.

Optical Fréedericksz transition and director field structure recording in dye-doped nematic liquid-crystalline polymer

Dye-doped nematic liquid-crystalline polymers exhibit much higher orientational optical response than the systems based on low-molar-mass liquid crystals. In addition, polymer materials possess the glassy state, which allows one to record their supramolecular structure. We study the effect of light field inhomogeneity on the director deformation in the geometry of Fréedericksz transition and recording of corresponding phase structures. The relationship of the light beam width with the threshold light intensity and the size of the induced and recorded deformation region was determined. An influence of the beam waist radius on the polymer director relaxation time was detected. The results of numerical calculations based on the continuum theory and the independence of the enhancement factor of the optical torque on light–polymer interaction geometry agree with the experimental data.

Highly efficient optical director reorientation of liquid-crystalline polymer induced by dye additives.

Light-induced director orientation of polymeric liquid-crystalline systems was investigated. The materials under study were composed of a nematic liquid-crystalline polymer (NLCP) and a small amount (0.05-0.5 wt.%) of conformationally active (azobenzene) or stable (anthraquinone) dye impurity. Light action on the homogeneously aligned polymer films above glass transition temperature leads to the director reorientation and, consequently, to a change in the extraordinary refractive index. The effect is associated with the dye molecule excitation and related change of intermolecular forces. In the case of NLCP with conformationally active dye dopant, an extremely high orientational optical response was detected (nonlinear coefficient is n_{2}∼0.1cm^{2}/W). In contrast, the efficiency of orientational light action on NLCP with conformationally stable dye dopant is of the same order of magnitude as that of dye-doped low-molar-mass liquid crystals. At the normal light incidence on the NLCP doped with azo-dye, the threshold director orientation is observed which is similar to the Fréedericksz transition under the action of magnetic and electric fields. The obtained high-orientational optical response of NLCP caused by azo-dye dopant in combination with the possibility of the recording of deformed structure in the glassy state, typical for polymer compounds, reveals new opportunities in photonics applications.

Unimodal Nematic Liquid Crystalline Random Copolymers Designed for Accepting Chiral Dopants

Abstract The miscibility of chiral molecules to unimodal side-chain nematic liquid crystalline (LC) polymers was studied to generate chiral nematic phases. Chiral dopants, a phenyl benzoate derivative S-811 and an isosorbide dibenzoate derivative ISO-(6OBA)2 were miscible to the nematic LC polymer PMA(4OPB) tethering phenyl benzoate moieties as side-chain mesogens by preparing mixtures in solutions rather than directly melting the powders together. Nematic LC random copolymers containing non-LC side chains were designed to promote the interaction between mesogens and chiral molecules. The introduction of hexyl chains as side chains randomly into nematic LC polymers improved the miscibility of chiral molecules of an isosorbide dibenzoate derivative with hexyl tails. The hexyl chains of the polymer provided spaces for the chiral molecules and caused alkyl–alkyl interactions. We demonstrated that the randomly introduced hexyl chains promoted the interaction of chiral molecules with LC mesogens to generate chiral nematic phases.

Light-induced orientation transition in nematic liquid crystalline polymer

Optical orientation in transparent nematic liquid crystalline polymer (NLCP) was observed for the first time. In NLCP with dye dopant, the light-induced second-order orientation transition being an analogue of the Freedericksz transition in low-frequency fields is found. The transition threshold is ~10 μW which is lower than the threshold of the light-induced Freedericksz transition in low-molecular nematics by four orders ofmagnitude and is lower than the characteristic value for low-molecular nematics with dye dopant by two orders of magnitude.

Stable selective gratings in LC polymer by photoinduced helix pitch modulation.

A cholesteric mixture based on the nematic liquid crystalline side-chain polymer doped with a chiral-photochromic compound was prepared and used as an active medium for creation of stable polarization selective gratings by phototunable modulation of the helix pitch. Such modulation was fabricated in the polymer mixture by a nonpolarized UV-irradiation with spatially modulated intensity that causes E-Z isomerization of a chiral-photochromic dopant, decreasing its helical twisting power. It was shown that the gratings recorded by UV-exposure through a mask are strongly selective to the handedness of circular polarized light. The studied polymer film forms a right-handed helical structure and, correspondingly, the diffraction of only the right-circularly polarized light was found in the transmittance mode. The maximum diffraction efficiencies were found for the wavelength values between the maxima of selective light reflection. The films obtained open very interesting possibilities for further development of materials with stable gratings operating in the entire visible spectral range. Both the position and the width of the spectral range of an efficient diffraction can be easily controlled by the UV exposure and concentration of the dopant. The materials obtained and methods developed can be used for creation of specific diffraction elements for optics and photonics.

Study on the structure, melting, and nonisothermal crystallization behaviors of isotactic polypropylene with nematic liquid crystalline polymer as a new β-nucleating agent

Wide angle X-ray diffraction (WAXD) and differential scanning calorimetry (DSC) were used to evaluate the effect of nematic liquid crystalline polymer as a new β-nucleating agent (LCP-NA) on the structure, melting, and nonisothermal crystallization behaviors of isotactic polypropylene (iPP). A high content of β-crystal could be obtained by the combined effect of the optimum LCP-NA content and crystallization temperature and time. The relative content of β-crystal increased with increasing LCP-NA content, reached a maximum value of 59% when LCP-NA content was 1.0 wt % at 130°C for 1h, and then decreased with a further increase of LCP-NA content. Avrami, Jeziorny, Ozawa, and Mo methods were applied to describe the crystallization process. Jeziorny and Mo methods showed a good agreement in describing the nonisothermal crystallization behaviors of pure iPP and iPP blends with 1.0 wt % LCP-NA, but Ozawa method could not better describe these behaviors. The addition of LCP-NA could effectively increase the crystallization temperature and the crystallization rate, and induce heterogeneous nucleation with the nonisothermal tridimensional growth of the spherulite. Kissinger’s method indicated that the iPP blends with 1.0 wt % LCP-NA needed a higher energy for crystallization per mole than pure iPP.

Effect of a Nematic Liquid Crystalline Polymer as Highly Active β-Nucleator on Crystallization Structure and Morphology of Isotactic Polypropylene

The effects of nematic liquid crystalline polymer as a new β-nucleator (LCP-N) on crystallization structure and morphology of isotactic polypropylene (iPP) were investigated using wide-angle X-ray diffraction and polarized optical microscopy. The experimental results showed that LCP-N could lead to substantial changes in the crystallization structure and morphology of iPP. The nucleating activity of LCP-N mainly depended on its content, mesogenic molecules, and thermal history of processing. A high content of β-form (K β) was obtained by the combined effect of the optimum LCP-N content and crystallization temperature. The maximum K β reached 84% when the LCP-N content and crystallization temperature and time were 0.4 wt.%, 125°C, and 1 h, respectively. In addition, the birefringence of β spherulite was stronger than that of α spherulite; this difference is related to their particular way of growing and lamellar morphology. Due to its particular sheaf-like growth, the β spherulite was brighter and more colorful.

Synthesis of a New Liquid Crystalline Polymer as a Highly Active β-Nucleator and Its Effect on Melting and Crystallization Behaviors of Isotactic Polypropylene

A new nematic liquid crystalline polymer as a highly active β-nucleator (LCP-N) of isotactic polypropylene (iPP) was synthesized and characterized. The effect of LCP-N on thermal behavior of the iPP was investigated with differential scanning calorimetry. LCP-N showed a melting transition at 85.0°C and a nematic to isotropic phase transition at 278.0°C. The incorporation of LCP-N could lead to substantial changes in the thermal behavior of the iPP. The nucleating activity of LCP-N mainly depended on its content, mesogenic molecular structure, and thermal history of processing. A high content of β-form could be obtained by the combined effect of the optimum LCP-N concentration and crystallization temperature and time. The Φβ reached 77% when the LCP-N content, crystallization temperature, and crystallization time were 0.4 wt.%, 125°C, and 1 h, respectively.

Directed self-assembly of nematic liquid crystalline polymers on a rubbed polyimide alignment layer

Amphiphilic diblock copolymers PEO-b-PMA(4OPB) comprising hydrophilic poly(ethylene oxide) (PEO) and hydrophobic nematic liquid crystalline polymethacrylate [PMA(4OPB)] subunits were synthesized by atom transfer radical polymerization. The liquid crystalline property and microphase-separated structure were investigated by differential scanning calorimetry, polarized optical microscopy, atomic force microscopy, and X-ray diffraction analysis. Unimodal PEO-b-PMA(4OPB) with 114 ethylene oxide repeating units and phenylbenzoate moieties as methacrylate side chain mesogens showed a nematic phase when the methacrylate repeating unit was around 57. Hexagonal cylinders of the PEO subunits were formed in nematic phase and supercooling states. The thin film on a rubbed polyimide alignment layer showed a microphase-separated structure independent of the rubbing direction, while horizontally aligned cylinders were observed in the presence of small nematic liquid crystalline molecules of 4-cyano-4′-pentyloxybiphenyl (5OCB). The assistance of nematic liquid crystalline molecules was necessary for directed self-assembly of the nematic liquid crystalline diblock copolymer in accordance with the rubbing direction of the polyimide alignment layer.

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.

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.

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

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.

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.

Effect of Magnetic Field on Shear Stress of Disklike Nematic Liquid Crystalline Polymer

The effect of magnetic field on shear stress of disklike nematic liquid crystalline polymers (LCPs) is analyzed with the extended Doi theory in which the molecular shape parameter is defined at negative one. The evolution equation for the probability function of the LCP molecules is solved without any closure approximations. It can be seen that the shear rate regions of the sign changes of the time-averaged first normal stress differences are completely contrary with those conclusions achieved for the rodlike LCPs.

Effect of Magnetic Field on Molecular Orientation of Disklike Nematic Liquid Crystalline Polymer

The effect of magnetic field on the flow-phase diagram of discotic nematic liquid crystalline polymers (LCPs) is analyzed with the extended Doi theory in which the molecular shape parameter is defined at negative one. The evolution equation for the probability function of the 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, are strongly affected by the magnetic fields parallel to the flow direction.

Effects of magnetic field on shear stress of nematic liquid crystalline polymer under simple shear flow

The effect of magnetic fields on the shear stress of liquid crystalline polymers (LCPs) under shear flows is numerically analyzed using the Doi theory. The evolution equation for the probability density of LCP molecules is directly solved without any closure approximations. When the magnetic field is imposed on the LCPs along the shear flow, the low-shear rate plateau of the time-averaged generalized viscosity disappears, which is due to the manner of the new aligning mode observed by Fu et al. The negative first normal stress difference becomes opposite in all the shear rate regimes with the disappearance of periodic oscillations. For the magnetic field parallel to the velocity gradient direction, the more complicated sign changes of is investigated because of the moment caused by the magnetic field vertical to the one caused by the shear flow. The ratio increases with the increasing magnetic field strength. Finally, when the molecular shape parameter is considered as 0.9, the similar discussions are achieved according to the phase diagram among tumbling, wagging, and aligning modes versus the magnetic field strength. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1919–1926, 2010