Nematic Solutions Research Articles

An investigation of novel approaches in order to provide crosslinked fully aromatic polyamide chains

Two different, novel approaches to crosslink fully aromatic, rigid-rod aramid chains were studied. First, the new rigid-rod aramid poly[1,4-phenylen-2,5-bis(prop-2-ynyloxy)-terephthalamide] with an inherent viscosity of η inh = 2.74 dL/g was synthesized by low temperature polycondensation of p-phenylendiamine and 2,5-bis(prop-2-ynyloxy)-terephthaloylchloride. The pendant alkinyl moieties allowed thermally induced crosslinking at temperatures higher than 200°C. No weight loss was found due to this treatment, but curing gave rise to the formation of stable radicals. However, no fiber spinning experiments were carried out using this material due to the insufficient stability of the polymer chains against degradation when being dissolved in sulfuric acid. Furthermore, fibers of a rigid rod polyamide containing pyrimidine moieties, produced by polycondensation of bis-silylated 2,5-diaminoprimidine and terephthaloyl dichloride, were spun from nematic solutions. Fibers were crosslinked by complexation with nickel(II)-ions in the swollen state. Both crosslinked and non-crosslinked, otherwise identically processed samples, were characterized by wide-angle X-ray diffraction (WAXD) measurements and mechanical tests. A post-spin heat treatment was employed to improve the low degrees of orientation and crystallinity that the untreated fibers in general showed. The dominating crystal structures of both fiber samples are similar to Modification II of the well characterized fibers from poly(p-phenylene-terephthalamide) (PPTA). The number and size of the morphological defects in the crosslinked fibers was significantly higher than in the non-crosslinked samples. The influence of the annealing on the mechanical fiber properties and the molecular order in the fibers was investigated. The values of all mechanical parameters were considerably lower in the case of the crosslinked fibers, probably due to the collapse of the entire supramolecular order and fiber morphology.

Arrangement of substituted, rigid-rod aramids in the highly-ordered solid state

Fibers of various rigid-rod, fully aromatic polyamides, produced by polycondensation of differently substituted p-phenylenediamines and terephthaloyldichlorides, were spun from nematic solution and analyzed with wide-angle X-ray diffraction (WAXD). A post-spin heat treatment was employed to improve the degree of orientation and crystallinity of the fibers. The dominating crystal structures were similar to those found in poly(p-phenylene-terephthalamide) (PPTA) and described in the literature, i. e., “Modification I” and “Modification II”. With the exception of only one case, the investigated fibers do not suffer a structural transformation upon heat treatment comparable to that found in PPTA. Steric and electronic effects of the ring substitution lead in most cases to a small disturbance of the solid-state structure in the fibers. Major structural changes caused by ring substitution are rare. Effects of the ring substitution pattern in the chains on the fiber crystal structure, their structural order, and their orientational properties could be observed. While the substitution of the aromatic rings seems to affect the competition between the basic crystal structures “Modification I” and “Modification II” (in analogy to PPTA) and the observed crystal structures agree with the results of detailed atomistic modeling predictions, no simple pattern seems to exist that would help predict, which candidate crystal structure is most stable.

Phase equilibrium studies on rod/solvent and rod/coil/solvent systems containing poly(?,L-glutamate) having oligo(ethylene glycol) side chains

Phase equilibrium studies for semiconcentrated solutions of rodlike poly(γ-benzyl L-glutamate) having oligo(ethylene glycol) as side chains (PBLG-g-OEG) have been investigated. The phase-boundary concentrations in isotropic and anisotropic phases for N,N-dimethylformamide (DMF) solution of PBLG-g-OEG with short side chains (PBLG2-g-380) are higher than those for solution of PBLG-g-OEG with long side chains (PBLG2-g-770). The lattice theory and the scaled particle theory for nematic solution, which don't distinguish the molecular architecture of the rodlike polymer, cannot explain this experimental result. Repulsive interaction between rodlike polymers by means of the attached side chains is proposed for the molecular orientation of PBLG-g-OEG in anisotropic state in order to describe the experimental result. Ternary phase diagrams of PBLG-g-OEG/poly(ethylene glycol) (PEG)/DMF show that the miscibility of rodlike PBLG-g-OEG and coiled PEG is most enhanced in the system of PBLG2-g-770, which has longest and largest amount of side chains. This experimental observation is explained by using the calculation based on the lattice theory and the repulsive interaction of side chains proposed above.

An evaluation of the Larson-Doi model for liquid crystalline polymers using recoil

The mesoscopic models for the rheological properties of liquid crystalline polymers proposed by Larson and Doi in 1991 and Kawaguchi and Denn in 1999 are based on phenomenological expressions that describe the evolution of the defect density and the contribution of the “texture” to the stress. In the present work, we attempt to assess some of these assumptions by monitoring how the energy stored in the texture of liquid crystalline materials evolves during shear flows. For that purpose, strain recovery is measured as a function of the applied strain for flow reversal and intermittent flow. Solutions of poly-benzylglutamate in m-cresol, hydroxypropylcellulose in water and a nematic surfactant solution are used as model systems. Although the behaviour is described qualitatively by the model, discrepancies between the predictions and the experiments are observed, especially when the shear history includes rest periods.

Studies on the Texture of Nematic Solutions of Rodlike Polymers. 2. Development and Relaxation of Loop Disclinations

Features of the development and relaxation of loop defects in the texture of nematic solutions of two rodlike polymers are discussed. Examples given for solutions of poly(1,4-phenylene terephthalamide) and poly(1,4-phenylene-2,6-benzobisthiazole) show the overall similarity of the behavior for these two materials. The development of the mottled texture observed shortly after the solution is extruded into a rectangular capillary, to form a tangle of defect lines which then evolves to form individual loops, is discussed. The differing relaxation dynamics of individual loops observed in the final stages of coarsening near the cell center and close to a surface are presented, along with novel effects observed in the interaction of such loops with an external magnetic field.

Vibrational spectra of 3-benzylidenephthalide and its 18O-labelled derivative

Infrared and Raman spectra of 3-benzylidenephthalide and 3-benzylidenephthalide-18O have been measured in solid state and in solution in the 4000–50 cm−1 frequency region. Their linear dichroic infrared spectra in oriented nematic solutions in the liquid crystal ZLI-1538 (4000–400 cm−1) have also been recorded and studied. The complete empirical assignment of the vibrational bands is performed using the group vibrational concept, isotopic shifts and polarization features of the normal modes.

Twist Viscosity of Side-Chain Liquid-Crystalline Polysiloxanes in a Nematic Solvent

Electric-field-dependent dynamic light scattering (EFDLS) was used to determine the twist elastic constant and twist viscosity of miscible nematic solutions of side-chain liquid crystal polysiloxan...

Rigid Formation of Aligned Polymer Fiber Network in Ferroelectric Liquid Crystal

A rigid polymer fiber network for fastening two substrates and anchoring a ferroelectric liquid crystal alignment was formed, for the first time, by rapidly polymerizing aligned high-concentration acrylate monomers. The µm-size polymer fibers segregated from a nematic solution of the monomers and the ferroelectric liquid crystal under high temperature and strong ultraviolet light irradiation are oriented in the rubbing direction of the alignment layers. The free-standing composite film having a concentration of the polymer of 20 wt% exhibited a fast response time of 300 µs, a high contrast ratio of more than 100:1, and grayscale capability. It is applicable to anti-shock display devices.

NMR Study of Flow and Viscoelastic Properties of PBLG/m-Cresol Lyotropic Liquid Crystal

Director distortion patterns and backflow effects in transient flows of nematic solutions of PBLG/m-cresol are studied by deuteron NMR. The simulation of the observed NMR spectra as a function of time allows to characterise the director field pattern and to evaluate most of the viscoelastic properties of the system. A good agreement with classical rheometry is observed.

Elastic constants in dilute poly(styrene)/nematic liquid crystal solutions – effects of concentration and molecular weight

We utilized the light scattering from the anisotropy of turbidity (LSAT) method and evaluated, for the first time, the three elastic constants of dilute solutions of polystyrene (PS) in 4-pentyl-4′-cyanobiphenyl (5CB) nematic solvent. Within the solubility range of this system (less than 5%), the elastic constants of splay ( K 1), twist ( K 2), and bend ( K 3) of PS/5CB solution were determined as a function of concentration and molecular weight of PS solute. The experimental results showed a systematic diversion of the elastic constants and, in particular, a large increase in K 1 with increasing concentration and molecular weight of PS. The results are in agreement with the predicted theoretical models and other experimental data. Determination of K i in polymer/nematic solutions with LSAT is a promising experimental approach to study the macromolecular chain structure of flexible, semi-flexible and rigid-rod polymers in anisotropic regimes.

Reduced IR-LD spectra of substances with two conformations oriented in nematic solutions

Infrared linear dichroic (IR-LD) spectra of three substance (2-naphthaldehyde, terephthalaldehyde and terephthalophenone) whose molecules give rise to two conformers were studied in the present article. It was aimed to understand how both conformers of each molecule orient in a nematic solution and could they be distinguished by means of IR-LD spectra. For this purpose the substances to be studied were dissolved and oriented in the nematic phase ZLI-1695 (Merck). The results thus obtained show that IR-LD spectra can help in distinguishing between the spectral bands of the conformers.

Tumbling dynamics in a nematic surfactant solution in transient shear flows

We report rheological and structural studies of a nematic surfactant solution (CPCl/hex) in transient shear flows. Upon step changes in steady shear flow conditions, the shear stress exhibits damped oscillations that scale with shear strain, attributed to director tumbling under shear. In situ small-angle x-ray scattering spectroscopy is used to measure changes in average micellar orientation during similar transient shear protocols; the measured orientation parameter also exhibits damped oscillations reflecting the underlying tumbling dynamics. Stress and structure data are compared with predictions of the Larson–Doi tumbling polydomain model. The model predictions are qualitatively similar to the experimental observations. In particular, orientation is observed to initially decrease upon shear flow reversal in CPCl/hex, in agreement with the Larson–Doi predictions but in contrast to previous results for model lyotropic solutions of poly(benzyl glutamate). Shear stress and average orientation are closely...

Suspension of spherical particles in nematic solutions of disks and rods

We report on the behavior of colloidal spheres suspended in nematic solutions of rodlike (calamitic phase) and disklike (discotic phase) micelles. The particles form anisotropic structures that depend on the shape of the surrounding micelles. An analysis of these structures indicates that dipolar and quadrupolar elastic interactions are, respectively, observed in discotic and calamitic nematics. This shape dependency can be understood by considering the entropy driven alignment of the micelles at the surface of the particles.

Chiral Nematic Cellulose Acetate Films; Reflectivity and Effect of Deacetylation

Cellulose acetate samples with a range of degrees of substitution (DS) were prepared by deacetylation of cellulose acetate (DS = 2.5). Chiral nematic solutions of the samples were prepared in formic acid. A handedness reversal from left to right was observed as the DS used to prepare the mesophase increased from 1.8 to 2.4. By selection of a suitably long-pitch mesophase, chiral nematic films cast from a CA (DS = 2.2)/formic acid solution showed a feint reflection of circularly polarized blue light. Deacetylation of this CA film gave a chiral nematic cellulose film with a more intense reflection band at the same wavelength. The improvement in intensity is attributed to the higher intrinsic birefringence of cellulose compared to cellulose acetate.

Molecular conformation of a polyaramid in nematic solution from small angle neutron scattering and comparison with theory

Zero-average contrast small angle neutron scattering measurements have been performed on an 11%(w/w) nematic aramid polymer (PPTA) solution in concentrated sulphuric acid. The radii of gyration parallel and perpendicular to the director were determined to be 250 and 70 Å, respectively, at room temperature. In addition, the temperature dependence of the radii of gyration was investigated. The values for the radii of gyration are compared to a worm-like path model for the polymer chain statistics where the segment orientation is described using a Maier–Saupe-type orientational distribution function. From this analysis, it is found that the persistence length of the aramid molecules is the same in the nematic phase as in a dilute isotropic phase, about 290 Å. We conclude that there is no effect of the nematic environment on the polymer chain statistics in this main chain polymer solution. As a consequence, the nematic phase of aramid solutions is expected to contain polymer chains with a substantial number of hairpin defects.

Nematic order in polymer-stabilized liquid crystals

We have studied both experimentally and theoretically how the presence of polymer fibrils can influence the ordering in liquid crystal systems. When such fibrils are formed by polymerization in a nematic solution, the polymer structure can reflect the director orientation at the time of polymerization. This anisotropic structure can be retained even when the liquid crystal is heated into its isotropic phase. We report measurements of this effect and also calculations based on a simple model of the mechanism by which the polymer fibrils can be expected to influence the nematic order of a liquid crystal matrix. A numerical solution of the theoretical model suggests that the discontinuity in order parameter that occurs at the transition temperature should gradually diminish as polymer concentration is increased and should vanish at a critical concentration. This is in agreement with our experimental observations.

In search of a predictive molecular-based model of nematic solutions

The athermal model of a binary rectangular-parallelepiped fluid, continuous in translations and discrete in orientational distribution, is used for calculations of infinite dilution activity coefficients, γ ∞, of nonmesogenic solutes dissolved in nematic and isotropic phases of model nematogens. The particles having D 2 h symmetry were chosen so as to model molecules of meta- and para-xylenes (nonmesogens) and the first five homologues of 4,4′-alkyloxyazoxybenzenes (nematogens). The calculations of γ i ∞ ( i= m, p denotes isomers of xylene) for various values of the relative temperature, t*, have explicated different characters of the functions γ i ∞( t*) for the isotropic and nematic phases. The ratio γ m ∞/ γ p ∞ is less than 1 and practically constant in the isotropic phase, whereas in the nematic phase it holds that γ m ∞/ γ p ∞>1, the ratio increasing with a decrease in t*. These features agree with experimentally established regularities and confirm the significance of the effect of the particle biaxiality in interpreting the separation of structural organic isomers with the aid of nematic sorbents.

Transient Shear Flow Behavior of Dilute Solutions of Side-Chain Liquid-Crystalline Polysiloxanes in 4,4‘-(n-Pentyloxy)cyanobiphenyl

The transient stress response of homeotropic monodomains of dilute nematic solutions of side-chain liquid-crystalline polysiloxanes (LCPs) in 4,4‘-(n-pentyloxy)cyanobiphenyl (5OCB) is investigated. In contrast to the flow-aligning behavior of 5OCB, strong stress oscillations are observed for the dilute LCP solutions, characteristic of flow-tumbling behavior. The dependence of the oscillation periodicity on LCP concentration and LCP structure (molecular weight and spacer length) is presented. From analysis of the stress transients, the Leslie viscosity coefficients, α2 and α3, are calculated. For all LCP structures, the increment δα2 on dissolution of LCP is negative, whereas the increment δα3 is positive. The magnitude and signs of these viscosity increments are strictly inconsistent with earlier electrorheological measurements of the Miesowicz viscosity increments, δηb and δηc, of these solutions, using as a guide a hydrodynamic model that computes the contribution of the chains to viscous dissipation in a simple shear flow. We propose that an additional dissipation mechanism should be considered, which derives from the presence of an elastic torque between director rotation and LCP orientation. Corresponding modification of the theory leads to improved agreement with experiment.

Phase behaviour of rod with flexible side chains/coil/solvent systems: poly(α,l-glutamate) with tri(ethylene glycol) side chains, poly(ethylene glycol), and dimethylformamide

Phase behaviour is investigated for ternary solutions composed of rodlike α-helical poly(α,l-glutamate) having triethylene glycol monomethyl ether at the end of the side chains (P3EGLG), randomly-coiled poly(ethylene glycol), and N,N-dimethylformamide (DMF) as solvent. The solution containing lower molecular weight poly(ethylene glycol) (PEG10) separates into an isotropic phase, which is rich in PEG10, and an anisotropic (cholesteric) phase, which is rich in P3EGLG, when the total polymer volume fraction is higher than ca 0.13. The homogeneous anisotropic region, in which a detectable amount of PEG10 exists, is recognized along the P3EGLG-DMF axis in the triangle phase diagram. When the molecular weight of poly(ethylene glycol) is larger, additional phase-separation regions are observed: That is, isotropic-isotropic biphasic region and anisotropic-isotropic-isotropic triphasic region. Theoretical treatments based on the lattice model for nematic solutions are conducted for these rod/coil/solvent ternary systems. Calculated phase diagrams with considering flexible side chains of rodlike solute and isotropic interaction parameters are found to qualitatively reproduce the variation of the experimental phase behaviour with the change of the molecular weight of the coiled component.

New aromatic benzazole polymers II: Synthesis and conductivity of benzobisthiazole-co-polymers incorporated with 4-N,N-dimethylaminotriphenylamine groups

New aromatic benzobisthiazole copolymers containing 10-70 mol % of 4-N,N-dimethylamino-triphenylamine functionality were prepared from the respective dinitrile or dicarboxylic acid monomers, terephthalic acid, and 2,5-diamino-1,4-benzene-dithiol dihydrochloride in polyphosphoric acid. At the first approximation, the copolymers containing 10 mol % or less of the triarylamino moieties in the polymer chains still preserve the capability to form anisotropic (nematic) solutions at 10 wt % polymer concentration. This is an important requirement for processing the copolymers into fibers and films with good to excellent mechanical properties. Films with good mechanical integrity were cast from the dilute methanesulfonic acid solutions of the copolymers under reduced pressure. They showed electrical conductivity values of the order of 10 -11 -10 -10 S/cm in pristine state, with four to seven orders of magnitude increase upon exposure to mild oxidizing agents such as iodine vapor. On the contrary, the parent polymer, poly(p-phenylene benzobisthiazole) is an insulator with conductivity of less than 10 -12 S/cm, and its conductivity does not improve at all with exposure to iodine vapor.