Nematic Polymer Research Articles

Synthesis of novel liquid crystalline organometallic polymers

Abstract

On the Frank Elastic Constants of Lyotropic Polymer Liquid Crystals

The Frank elastic constants of lyotropic polymer liquid crystals are formulated using an equivalent freely-jointed chain model, which can be uniquely defined by the parameters of the corresponding wormlike chain and the polymer concentration. The contribution of the higher virial terms to the elastic constants was taken into account on the basis of the scaled particle theory used previously for undistorted nematic polymer solutions. The fomulated elastic constants are applicable to systems with any polymer conformation from the rod limit to the coil limit and at arbitrary concentration and degree of orientation. The theoretical elastic constants for twist and bend distortions agree semiquantitatively with most experimental results for the elastic constants of lyotropic liquid crystals of poly(γ-benzyl glutamate) with no adjustable parameters.

Coupling between Thermodynamics and Conformations in Wormlike Polymer Nematics

We adapt the Gupta-Edwards theory of wormlike main-chain nematics to the case of finite-length polymers. The thermodynamic parameters of lyotropic and thermotropic phase transitions are obtained for arbitrary polymer lengths and we find that the crossover between long- and short-chain regimes takes place for polymers of relatively low chain length (shorter than the persistence length). We show that this is related to the fact that in the nematic phase there are two qualitatively different length scales, which are analogous to the persistence length in the isotropic phase. One scale, ξ⊥, defines the contour length associated with independent fluctuations of chain segments about the nematic director and decreases with the nematic field. Another scale, ξ∥, determines the correlation length of the longitudinal component of the tangent vector. This size is larger than ξ⊥ and increases exponentially with the strength of the nematic interaction. This allows one to identify an intermediate range of chain length ξ⊥ << L << ξ∥, in which the isotropic-nematic phase transition is determined by the long-chain limit but the single-molecule conformation is practically a straight line. The implications of our results for the crystallization of short polymers, such as normal alkanes, are discussed.

Theory and Simulation of Optical Banded Textures of Nematic Polymers during Shear Flow

Simulations of banded texture optical phenomena during shearing of liquid crystalline polymers are presented. A computational analysis of the polarized light transmission pattern for the nonhomogeneous orientation structure of a representative nematic polymer during shear start-up flow is presented. Simulations show that the shear-induced structure that gives rise to the banded texture is an array of tubular orientation inversion walls whose axes are normal to the flow direction. The corresponding optical computations are in good agreement with the experimentally observed banded textures of light transmission under crossed polars of sheared poly(γ-benzyl-L-glutamate) (PBLG) (Yan, N. X. ; Labes, M. M. Macromolecules 1994, 27, 7843). The computer simulations show the intimate relation between the light transmission pattern and the tubular orientation inversion wall array and, hence, shed a new light on the origin of this ubiquitous phenomenon.

RHEOLOGY AND RHEO-OPTICS OF POLYMER LIQUID CRYSTALS

The rheological and rheo-optical properties of nematic solutions of rodlike polymers are discussed. Magnetic field induced instability in the twist geometry is discussed in terms of the flow properties of these solutions. The rheological properties of these nematic solutions are compared to isotropic solutions of the same polymer. The behavior in shear flow of nematic solutions is discussed in terms of the Ericksen-Leslie equations. Deviations from those solutions are also discussed in context to polymeric nematics.

Bifurcation analysis of a molecular model for nematic polymers in shear flows

It is well known that the rigid rod model for the dynamics of nematic polymers can be formulated in terms of a non-linear partial differential equation for the orientational distribution function, the so-called Smoluchowski Equation, and the rheological predictions are then obtained by integrating this equation numerically. The integration requires cumber-some numerical schemes even for very simple flow fields, and thus techniques which reduce the complexity of the problem by transforming the partial differential to a set of a few ordinary differential equations are attractive. In these approximations, the bifurcation structure of the original model should be maintained, however. In this work, we have determined for the first time the bifurcation structure of the Smoluchowski Equation; the rigid rod model is exhaustively studied for the case of a shear flow.

Rheology of nematic wormlike micelles

In this paper we report on the nonlinear rheology of wormlike micelles in their nematic phase. Also called equilibrium or ‘‘living’’ polymers, the micelles investigated here are long, flexible, and locally cylindrical aggregates made of surfactant molecules. It is demonstrated that when subjected to a shearing flow at a constant rate, the stress response of a nematic solution of living polymers exhibits a transient regime which consists of damped oscillations, followed by a stationary regime. Peculiar attention has been paid to the transient behavior at low shear rates (γ̇<5 s−1). In this regime, it is shown that the mesoscopic domain theory developed by Larson and Doi [J. Rheol. 35, 539–563 (1991)] for textured materials applies to the present micellar system. The scaling laws predicted for the transient shear stress in the tumbling regime are, for instance, well verified. When expressed in terms of strain γ=γ̇t applied to the solution at time t, the shear stress normalized to its long time value essentially depends on the ratio γ̇/γ̇presh where γ̇presh denotes the shear rate at which the solution has been presheared. This result suggests that the effect of the rheological history experienced by a nematic solution before any measurements is crucial. On the other hand it means that a constant deformation (230 strain units in the present case) is necessary for the sample to forget its shear history. As far as the rheological properties are concerned, the present study emphasizes the strong analogy between the nematic equilibrium polymers and the liquid‐crystalline polymers (LCPs). The overall rheological data are also discussed in terms of tumbling instability and flow‐alignment regime and compared to the theoretical predictions and experimental findings on LCPs. Finally, the present data suggest that the rheology of the nematic living polymers is entirely dominated by the dynamics of the textures (or defects). In contradistinction with what was originally thought, the microscopic processes such as the reversible breaking of the micellar chains seem to play a minor role here.

Anisotropic viscoelasticity of side chain nematic polymers

A macroscopic continuum mechanical model for incompressible side chain nematic polymers, under isothermal conditions is given. The model is a synthesis of a transient network model and the standard nematorheological model. Simplifications in the model yield constitutive equations that are identical to well-known rheological models for polymer melts and for low-molar-mass nematics. The model is used to derive the anisotropic relaxation modulus, and the orientation-dependent storage and loss moduli for small-amplitude oscillatory shear. The model is partially validated with existing small-amplitude oscillatory shear data. The present model is able to predict and explain the observed differences in the storage and loss moduli between the isotropic and shear flow-aligned nematic phase of a side chain nematic polymer.

Electro‐optical properties and nonlinear response of side chain liquid crystalline polymers

AbstractAmong the NLO processes that have been studied, one of the most visually dramatic is the frequency doubling. In the field of optical information storage this process can provide the conversion of near‐infrared laser light from diode lasers into deep blue light. Compared to the more traditional inorganic NLO materials, polymers with polarizable aromatic pendant side groups are increasingly being recognized as the materials of the future. Recently it has been pointed out that the axial ordering spontaneously present in nematic and smectic A polymers can be used to enhance field‐induced polar ordering by elongating the orientational distribution function along the electric field direction. Depending on the value of the microscopic order parameters &lt;P&gt;2 and &lt;P&gt;4, the performance may be improved by a factor of 1 to 5 by using LCPs instead of ordinary amorphous polymers for SHG.1‐4)Interesting results have been obtained for copolyethers prepared by chemical modification of polyepichlorohydrin with classical 4‐cyano‐4'‐hydroxybiphenyl mesogenic group which possesses NLO properties itself.5‐8) These copolyethers afford the opportunity to fine‐tune the polymer properties by varying the concentration of the mesogenic side groups. As the concentration is increased, we move from a purely isotropic polymer to a nematic polymer. Spin‐coated films have been activated using the corona poling technique and the order parameters &lt;P&gt;2 have been determined from optical absorption spectra. The second harmonic coefficients d33 and d31 have been measured and compared with different statistical models. In view of the great practical importance, characterization of the wavelength dispersion has been carried out. It agrees well with the two‐level approximation model. The dynamics of optical SHG has been investigated. It has been shown that both the presence of LC character in the material and the temperature at which the films are stored below Tg are important in determining the stability of the SH coefficients.A polyacrylate and a polymethacrylate bearing the same 4‐cyanobiphenyl‐based side groups have also been studied.7‐8) Of particular interest is the fact that the former is nematic while the latter is purely isotropic at rest, the addition of a methyl group to each structural unit of the polyacrylate backbone creating a higher conformational barrier to mesogen packing. Studies of the temporal and thermal characteristics of the poling process have been performed to:•understand and control the poling process with the intention of maximizing poling induced nonlinearity and stability.•elucidate the influence of the polymer backbone, our data including the use of the same mesogenic unit attached to increasing flexible backbones (e.g. polymethacrylate and polyether).•establish if, in the isotropic cases, noticeable axial order can be induced by the poling field, especially when the system is pulled through nematic/isotropic transition by the electric field.

Recent advances in the observation of disclinations of main‐chain nematic polymers

AbstractExperimental observation of disclinations of main‐chain nematic polymers is reviewed. The geometry of disclination structure is discussed both from observations of schlieren texture and the theory of curvature elasticity for a two‐dimensional case with equal splay and bend moduli. The effects of non‐equal elastic moduli, the nature of the disclination core and the observation of inversion wails are briefly mentioned. Examples of experimental observation of disclinations of high topological strengths up to S=4 are given. The main experimental methods to map the director field around disclinations are 1) lamellar decoration, 2) band‐texture decoration and 3) surface microcrack decoration. The methods are discussed and compared, and examples illustrated by electron micrographs or photomicrographs are presented.

The Slightly-Bending-Rod Model of Nematic Polymers

Abstract A simple model for stiff polymers is here proposed to describe the flow behavior in the nematic phase. The model relaxes the usual assumption of a complete rigidity of the rodlike molecule. The dynamical equation is tractable if the molecular bending motion is much faster than rotation of the molecular axis.

Multistage Crosslinking of Nematic Networks

Nematic polymer melts, cross-linked in their isotropic phase, typically form polydomain nematic rubbers and gels. Such elastomers, if strained and then further cross-linked, form nematic monodomain elastomers with interesting mechanical and thermal properties. We calculate the elastic free energy of a network which has undergone multiple cross-linking and obtain the surprising result that it is identical in form to that for a sample once cross-linked from an oriented nematic monodomain. Gaussian elasticity thus predicts that it is impossible to permanently imprint complex structure (including, for instance, biaxiality) into a nematic elastomer by imposing a complex sequence of strains interspersed by cross-linking stages. A multistage cross-linked network should therefore exhibit the same behavior as a single-stage system, including recently predicted and observed nematic transitions in response to imposed strains.

Effect of Mesophase Order and Molecular Weight on the Dynamics of Nematic and Smectic Side-Group Liquid-Crystalline Polymers

The dynamic moduli of side-group liquid-crystalline polymers (SG-LCPs) having methacry- late backbone, hexamethylene spacer, and phenyl benzoate mesogens are characterized as a function of molecular weight in the isotropic, nematic, and smectic phases. Molar mass was varied from 3 x lo6 to 3 x lo6 g/mol, corresponding to the range where the onset of entanglement is observed in the isotropic phase (approximately l-lOMe,,so). Nematic order produces a profound change in the dynamics of the entangled SG-LPCs relative to the isotropic phase; however, this effect is absent in the unentangled SG-LCPs. Oscillatory shear with large amplitude (yo z 40%) is effective in inducing macroscopic alignment in the nematic phase for all the SG-LCPs studied. Smectic order increases the elastic character of the fluid, but its incremental effect in a system that is entangled is relatively small. Large-amplitude shearing can be used to alter the microstructure in the smectic liquid: shearing the smectic phase produces a decrease in modulus, whereas shearing in the nematic phase followed by cooling into the smectic phase produces an increase in modulus.

Induced circular dichroism as a probe of handedness in chiral nematic polymer solutions

Abstract Achiral dyes in isotropic (acetyl)(ethyl)cellulose (AEC) solutions display no induced CD signals irrespective of acetyl content, polymer concentration or dye content. When dissolved in anisotropic AEC solutions, the dye molecules show strong induced CD bands that disappear when the phases are heated above the anisotropic/isotropic phase transition temperature. The liquid crystal induced circular dichroism (LCICD) spectra for the dyes in a series of well-characterized AEC mesophases, whose handedness depends on solvent and acetyl content, reveal that the sign of the LCICD signal is determined by the supramolecular structural features of the host matrices. Thus, acridine orange (AO) dissolved in a left-handed liquid crystalline AEC solution displays a negative LCICD whereas a positive LCICD was observed when the AO is dissolved in a right-handed AEC mesophase. The sign and intensity of the LCICD signals change with acetyl degree of substitution, solvent and temperature. In all cases, changes in sig...

Induced Smectic Phase Formed by New Liquid Crystalline Binary Systems Consisting of Main Chain Polymer and Twin Compound

Abstract The thermal and mesomorphic properties of the new liquid crystalline binary systems, consisting of a nematic main chain liquid crystalline polymer and a nematic twin liquid crystal, have been studied. The liquid crystalline binary systems formed an induced smectic A phase, which was characterized by a uniaxial conoscopic figure, a fan texture, and a broad X-ray scattering peak at the wide angle.

Measurement of the Domain Growth Kinetics in Multidomain Nematic Liquid Crystal Polymers by Means of the Worm Like Path Model for Multiple Scattering

Abstract The rate of domain growth in nematic side chain polymer samples is measured using the worm like path model for multiple scattering of light. The worm like path model which describes multiple scattaering in terms of the optical persistence length is found to apply both to multiple scattering in liquid crystal polymer films as well as to model polystyrene despersions. The required value of the optical persistence length can be calculated from the single scattering process so that there are no adjustable parameters in this model. The rate of domain growth in liquid crystal polymer samples is found to obey a scaling law where the growth exponent is about 0.85, contrary to the expected value of ½. In addition, an unexpected effect of the layer thickness on the rate of growth is observed, even for layer thickness much larger than the domain size.

Shear-Induced Orientations and Textures of Nematic Living Polymers

We have used theology, rheo-optical observations, and small-angle neutron scattering under shear to investigate the flow, orientation, and texture properties of nematic solutions of equilibrium or ''living'' polymers. The elementary objects of these liquid crystalline phases are wormlike micelles, that is, flexible cylindrical aggregates made of amphiphile molecules. The aqueous surfactant solutions investigated here (cetylpyridinium chloride and hexanol in 0.2 M NaCl-brine) exhibit the classical structural sequence: isotropic/nematic/hexagonal with increasing surfactant concentration phi. The nematic phase observed for phi = 34%-38% consists of very long cylindrical micelles closely packed parallel to each other, thus producing long-range orientational order. We have shown that when subjected to a steady shear, the nematic phase of living polymers behaves very similarly to that of the conventional liquid crystalline polymers. From the rheo-optical observations, we demonstrated that so-called band textures show up perpendicular to the flow direction after cessation of the shearing, indicating a modulation of the director orientation. As for Liquid crystalline polymers, these textures are interpreted in terms of relaxation of the Frank distorsional energy accumulated during the now. The time evolution of the average band spacing of the texture could be determined and compared to recent theoretical predictions;e.g. in the low-shear regime the squared band spacing scales linearly with time. We also study the evolution of the order parameter of the nematic state (actually spatially averaged over the mesoscopic textures observed during the flow or at rest) determined from small-angle neutron scattering spectra obtained under shear. As remarkable features, we found out that above a characteristic shear rate, gamma(c), the nematic living polymers form an orientational monodomain which in addition is remarkably stable over a long period of time. The present data are compared to those reported for liquid crystalline polymers.

Nature of the Flow-Induced Worm Texture of Thermotropic Polymers

Nematic polymers show at low shear rates the so-called worm texture which is characterized by a large density of defects. To study these defects, a specially designed cone-and-plate rheometer has been used for quenching the flowing worm texture of a thermotropic polymer. The preparation of samples involves ultramicrotoming and etching with sulfuric acid. The observations show that the defects are half-integer twist disclination loops lying in the shearing plane. The director is oriented along the flow direction inside the loop and along the vorticity axis outside.

Numerical Simulation of Flows of Liquid Crystalline Polymers through Parallel Plates Containing a Cylinder.

Flows through parallel plates containing a cylinder are numerically calculated for nematic liquid crystalline polymers using a simplified model of the Leslie-Ericksen theory. A tensor expression for the orientation of the director is used in the basic equations in order to avoid the difficulty in calculation due to the tumbling of the director. The orientation of the director is affected by the contraction and expansion flows, and tumbling occurs. An area where the molecule aligns in the direction of the flow exists behind the cylinder. It extends downstream as the parameter λ approaches the value one. The flow field does not change significantly from that of Newtonian fluids.

Rheological features of thermotropic and isotropic poly(ester imide)s

Rheological properties of two poly(ester imide)s with a similar chemical structure are studied in this paper. Both polymers contain the same mesogenic unit and flexible spacer: the only difference is the orientation of the connecting ester groups. The liquid crystalline nature of one of the poly(ester imide)s in the range 170–235°C gives rise to typical peculiar rheological features of nematic polymer such as two shear-thinning regions in the viscosity curves, ∂η/ ∂T>0 when isotropization takes place and orientation during the injection process.