Nematic Solutions Research Articles

Phase Separation and Organisation of Colloidal Spheres Suspended in Sheared Lyotropic Liquid‐Crystalline Polymers

AbstractSummary: Organisation behaviours of spherical particles suspended in sheared, lyotropic, liquid‐crystalline polymer solutions have been investigated using polarizing optical microscopy. We find that in a nematic phase the particles phase separate and adopt anisotropic chain‐like structures along the director. An earring defect is observed around a single particle whereas a cross or strings defect between neighbouring particles is found to serve as a repulsive barrier to prevent the particles from contacting each other. A theoretical analysis is presented to explain this new phenomenon.An optical micrograph of 0.01 wt.‐% glass spheres suspended in a nematic solution of 40 wt.‐% ethyl cellulose in chloroform under an external shear force.magnified imageAn optical micrograph of 0.01 wt.‐% glass spheres suspended in a nematic solution of 40 wt.‐% ethyl cellulose in chloroform under an external shear force.

Vibrational spectra of partially oriented molecules having two conformers in nematic and isotropic solutions: furfural and 2-chlorobenzaldehyde

Vibrational analysis of the two conformers of furfural and 2-chlorobenzaldehyde has been carried out on the basis of their IR and Raman spectra measured in isotropic and anisotropic (nematic liquid crystalline) solvent. The average orientation of the individual conformers in the nematic solvent has been determined by means of a recently developed approach for low symmetry planar molecules using DFT calculations of the vibrational transitions moments. The complex shape of the carbonyl band additionally split into several components is interpreted as an effect of Fermi resonance.

The structure of equilibrium solutions of the one-dimensional Doi equation

We analyse the structure of steady state solutions of the one-dimensional Doi model for rod-like molecules. We prove that if the interaction strength parameter U is less than 4, then the constant solution is the only possible steady state solution. If U is larger than 4, then there is a new solution that corresponds to the nematic phase. All other non-constant solutions are obtained from this solution by translation. We prove further that the nematic solutions have period π instead of 2π, a property that signifies the fact the rods are symmetric, i.e. they have orientations but no directions.

Some Aspects of the Rheo-Nmr Behavior of the Lyotropic Liquid Crystal Poly(γ-BENZYL-L-GLUTAMATE) in m-Cresol

We report the use of 2H NMR spectroscopy to study the director orientation in two nematic solutions of poly(γ-benzyl-L-glutamate) (PBLG) in m-cresol during simple shear flow and after its sudden cessation. In the range of applied shear rates$(\\dot{\\gamma} = 5 \\ldots 150 \\, \\hbox{s}^{\\!-\\!1})$, a steady-state NMR line shape is reached after a deformation of a few hundred units. Two different line shapes occur that can be related to distinct shear responses in mechanical rheology: the tumbling/wagging and the flow-aligning regimes. The relaxation after cessation of the shear is similar for both cases and essentially follows the “inhomogeneous reorientation” regime known from sudden-rotation NMR experiments. A fit of the relaxation curves indicates that the tumbling parameter λ is (slightly) less than one, as expected for a tumbling system.

Dynamical properties of nematic liquid crystal solutions in pulsed magnetic fields

Behavior of the rotational viscosity coefficient of a nematic liquid crystal of the N8 type and its solution in a nonmesogenic solvent (benzene) has been studied under the conditions of variable thermodynamic state parameters (p, T).

Synthesis and properties of chiral stilbene diacrylates

Two chiral and isomerizable liquid crystalline diacrylates were synthesized. The purpose of these compounds was to tune the helical twisting power of cholesteric materials containing these compounds by means of an E–Z photoisomerization of the photoactive group derived from stilbene. The photochemical behaviour of these compounds was studied with the aid of two model compounds containing the same isomerizable mesogenic group. The mesogenic group derived from 4-(4-hydroxybenzoyloxy)-4′-hydroxystilbene decomposes upon irradiation. Its isomer, derived from 4-(4-hydroxyphenoxycarbonyl)-4′-hydroxystilbene, shows a clean E–Zisomerization. The HTP of the chiral diacrylate derived from the latter mesogenic group changes from 7 to 3 μm-1 in dilute nematic solution. Colour changes in a cholesteric material containing this compound were observed. The effect was very dependent on temperature and concentration due to the strong smectic character of this diacrylate.

Linear-dicroic infrared spectral analysis on the stereo structure of 4′-cyanophenylthiolbenzoate

The stereo structure of 4′-cyanophenylthiolbenzoate (4′-CNPTB) in solid state and in liquid crystal solution is studied by means of linear dichroic infrared (IR-LD) spectral analysis. The data in solid state suggest a near to flat geometry of the investigated compound. The comparative IR-LD spectral study in nematic solution also proves the co-planar disposition of both benzene rings, but a transversal orientation of the CO bond is observed. Similar geometry requires a significant deviation of the C–S(Ph) bond to the plane of the bridging C(Ar)–C(O)–S fragment. The discussed investigation demonstrates the possibilities of the IR-LD spectroscopy for the stereo structural characterization of 1,2-diaryl-substituted organic compounds.

IR linear dichroic spectra of the tetrahedral molecules tetrapropyn-1-yl silicon and -tin in anisotropic solvents

Dissolving spherical molecules of T d symmetry in nematic liquid crystal leads to distortions thus giving rise to lowering of the molecular symmetry down to D 2 d and C 3 v . The molecules deformed in this way undergo orientation that leads to appearance of linear dichroism. This phenomenon is shown for the examples of tetrapropyn-1-yl silicon and -tin, Si(CC–CH 3) 4 and Sn(CC–CH 3) 4. Its theoretical explanation has been found on the basis of a small distortion of the polarizability and quadrupole moment of the considered molecules in the nematic solution.

Shear alignment behavior of nematic solutions induced by ultralong side-group liquid crystal polymers.

Addition of a low concentration of a very long (430 kg/mol) side group liquid crystal polymer is shown to produce dramatic changes in the flow characteristics of a calamitic nematic liquid crystal. This polymer causes a typical flow-aligning nematic liquid crystal to align near the velocity gradient direction rather than near the velocity direction, corresponding to having a tumbling parameter lambda<-1, for concentrations greater than 7.5% polymer. Such flow-aligning behavior has not been reported previously in a calamitic nematic. The large molecular weight of the present polymer relative to those examined in the prior literature is responsible for these new phenomena.

Nematic conformation of oxyethylene spacers involved in main‐chain liquid crystals

Abstract2HNMR measurements were performed on main‐chain dimer and polymer liquid crystals (LC) having oxyethylene (OE) spacers ‐(OCH2CH2)xO‐(x=2, 3). The orientational as well as conformational characteristics of these molecules have been investigated in bulk and in a nematic solution. The OE spacer was found to take spatial arrangements characteristics of the nematic phase. The nematic conformation of the spacer remains nearly invariant over a wide range of temperature and concentration. In these analyses, the ratio of the deuterium quadrupolar splittings Δν/ΔνR (Δν: spacer and ΔνR: mesogenic unit) provided an important information regarding the spatial configuration of molecules in the LC state. The results obtained in this study are consistent with our previous conclusion drawn on a series of main‐chain LC oligomers and polymers comprising n‐alkane spacers ‐O(CH2)nO‐ (n=9, 10).

Studies on the texture of nematic solutions of rodlike polymers. 3. Rheo-optical and rheological behavior in shear

The texture in shear flow is discussed for nematic solutions of rodlike poly(1,4-phenylene terephthalamide), reported here, and poly(1,4-phenylene-2,6-benzobisthiazole), reported elsewhere. These are similar, with a sequence of scattering and visual features that develop with time, terminating at a level characteristic of the shear rate in steady state. With increasing shear rate in torsion between parallel disks, steady-state flow reveals sequentially a diffuse scattering with a turbid appearance, diffuse scattering with an opalescent appearance, a broad elliptical scattering along the radial direction with poorly-defined striations, a sharp scattering along the radial direction with well-defined striations along the flow, and a clear, highly birefringent sample, with little scattering. These same features are seen, in the same sequence in time, in transient behavior leading to steady state. This behavior is attributed to the development of striations with twist distortions alternating in angle relative to the flow, and essentially in the sample plane in the fast steady-state flow, reflecting the lower free energy of that distortion in comparison with bend and splay distortions, especially near the parallel plates of the rheometer. The deterioration of this texture at slower flows is associated with increasing distortions out of the sample plane, leading to an essentially chaotic spatial distribution of the order tensor at a slow flow.

Molecular weight dependence of shear-induced structures in nematic semiflexible polymer solutions

The role of molecular weight in shear-induced structure transitions, such as striations during shear startup and banding during relaxation, was probed through small-angle light scattering and shear microscopy. The characteristic texture relaxation time for banding, τs, is molecular weight dependent. The minimum molecular weight for these shear-induced complex texture transitions to occur is estimated from τs as 27 000 for a 37 wt % poly(n-hexyl isocyanate) in 1,1,2,2-tetrachloroethane solution at room temperature. τs is independent of applied shear rate, implying that it is a material property. Thus, both of these structure transitions appear to occur only after shearing above a threshold Deborah number, γ̇τs|c, below which viscous forces would suppress these structure transitions. Both the strain dependence of the structure transitions during shear startup and the preshear rate dependence of the banding phenomenon exhibit little or no molecular weight dependence. Increasing molecular weight does appear to decrease the characteristic band correlation length and slow the decay rate of the bands.

Synthesis and Properties of Lyotropicpara-Linked Aromatic Poly(amic ethyl ester)s

The paper describes the synthesis and structure-property relations of para-linked aromatic poly(amic ethyl ester)s with the intention to realize lyotropic precursor solutions for rigid-rod polyimides. As compared to the commonly used flexible precursors, these preorganized precursors should be superior to obtain perfectly oriented polyimide films, orientation layers, and fibers. The poly(amic ethyl ester)s were synthesized by polycondensation of 2,5-diethylbis-(ethoxycarbonyl)terephthaloyl acid dichloride and several para-functionalized diamines bearing fluoro substituents, bulky side groups, or non-coplanar structures. Certain introduced structural modifications improve the solubility in NMP to concentrations up to 50 wt.-% and lyotropic nematic solutions could be obtained. Polymers without fluoro atoms in the ortho position to the polyamide bond have a strong tendency to form a birefringent gel at room temperature which turns into a shearable lyotropic phase above 50°C. This transformation was studied by temperature dependent FTIR spectroscopy and WAXS. However when the ortho position is fluorinated the precursors form a lyotropic phase at room temperature under shear. The macroscopic surface morphology of the dried gels of concentrated solutions is dramatically affected by the chain stiffness, para-Poly-(amic ester)s show a distinct wormlike morphology, indicating molecular order within the gel, whereas the corresponding flexible meta-poly(amic ester)s show a structureless, smooth surface.

Transient 1–2 plane small-angle x-ray scattering measurements of micellar orientation in aligning and tumbling nematic surfactant solutions

A new x-ray shear cell capable of probing complex fluid structure in the flow-gradient (“1–2”) plane is used to study orientational dynamics of nematic surfactants in transient shear flows. Two surfactant systems are studied, one of which (SDS/decanol) apparently exhibits shear aligning dynamics and the other of which (CPyCl/hexanol) exhibits director tumbling. Data on the SDS/decanol system exhibit what appear to be common characteristics of aligning nematics: constant orientation state in steady shear; a single undershoot of long duration in average orientation upon flow reversal; and no significant orientation change upon step increase or decrease in shear rate or upon flow cessation. These data are discussed in light of similar measurements on shear aligning thermotropic liquid-crystalline polymers, and polydomain model predictions of transient orientation for aligning nematics. Shear reversal data on CPyCl/hexanol show an oscillatory response typical of tumbling materials, but in better qualitative agreement with the Larson–Doi polydomain model than similar recent measurements on lyotropic polymer solutions.

Hierarchical self-assembly of chiral rod-like molecules as a model for peptide beta -sheet tapes, ribbons, fibrils, and fibers.

A generic statistical mechanical model is presented for the self-assembly of chiral rod-like units, such as beta-sheet-forming peptides, into helical tapes, which with increasing concentration associate into twisted ribbons (double tapes), fibrils (twisted stacks of ribbons), and fibers (entwined fibrils). The finite fibril width and helicity is shown to stem from a competition between the free energy gain from attraction between ribbons and the penalty because of elastic distortion of the intrinsically twisted ribbons on incorporation into a growing fibril. Fibers are stabilized similarly. The behavior of two rationally designed 11-aa residue peptides, P(11)-I and P(11)-II, is illustrative of the proposed scheme. P(11)-I and P(11)-II are designed to adopt the beta-strand conformation and to self-assemble in one dimension to form antiparallel beta-sheet tapes, ribbons, fibrils, and fibers in well-defined solution conditions. The energetic parameters governing self-assembly have been estimated from the experimental data using the model. The 8-nm-wide fibrils consist of eight tapes, are extremely robust (scission energy approximately 200 k(B)T), and sufficiently rigid (persistence length l(fibril) approximately 20-70 microm) to form nematic solutions at peptide concentration c approximately 0.9 mM (volume fraction approximately 0.0009 vol/vol), which convert to self-supporting nematic gels at c > 4 mM. More generally, these observations provide a new insight into the generic self-assembling properties of beta-sheet-forming peptides and shed new light on the factors governing the structures and stability of pathological amyloid fibrils in vivo. The model also provides a prescription of routes to novel macromolecules based on a variety of self-assembling chiral units, and protocols for extraction of the associated energy changes.

Magnetic field induced instabilities in nematic solutions of polyhexylisocyanates

Magnetic field induced Freedericksz transition in the twist geometry of an initially well-aligned nematic solution of a rod-like polymer, polyhexylisocyanate (PHIC), is studied. For the angle α between the magnetic field and the unperturbed director n0, one finds a homogeneous distortion if this angle is less than 45°. For angles greater than 45°, the reorientation is inhomogeneous, and is coupled to secondary flow, giving rise to spatially periodic structures. The director deformation involves an out-of-plane tilt at short times, which manifests itself as a phase grating for transmitted polarized light. It is found that the wavelength of the instability is independent of the magnetic field strength in the experimental range, contrary to prior results for nematic solutions of polybenzylglutamate. We postulate that the insensitivity of the wavelength of the periodic structure is due to the nonflow-aligning character of PHIC solutions.

Capillary technique for measurement of polarized Raman spectra of oriented nematic solutions

Raman spectra of an oriented 12% fluorene (FL) solution in the nematic phase ZLI-1695 have been recorded and studied. Orientation was achieved by introducing the solution into a capillary tube of 0.3mm inner diameter. The method represents a very simple procedure for studying Raman spectra of oriented free molecules.

Rheology and nuclear magnetic resonance measurements under shear of sodium dodecyl sulfate/decanol/water nematics

We report on the rheology and deuteron nuclear magnetic resonance (NMR) measurements performed on the nematic lyotropic system sodium dodecyl sulfate, decanol, and water. For total surfactant and cosurfactant concentrations of about 30% by weight, we prepared nematic calamitic NC and nematic discotic ND solutions. We have studied the flow behavior of the NC and ND phases in terms of the transient shear stress using different rheological histories. In flow-reversal experiments, the transient response exhibits nonperiodic oscillations in the region of small deformations (γ=20), which is followed by a long-lasting relaxation toward the stationary state. In the pseudo-Newtonian regime of the viscosity, we have found that the stress, when normalized to its long term value and plotted versus deformation, is shear-rate independent. 2H NMR measurements at rest and under shear were performed on nematic calamitic and discotic solutions prepared in D2O. At rest, the NMR spectra exhibit well-resolved doublet structures, which indicate an alignment of the director along the axis of the magnetic field. At shear rates large enough (γ̇ > 0.3 s−1), alignment is achieved with respect to the flow velocity. One finds alignment angles of 12 ° and 105 ° for the directors of the calamitic and discotic phase, respectively. Combining the results obtained by rheology and by NMR, we provide strong evidence that the NC and ND nematics of the sodium dodecyl sulfate / decanol system are textured nematics of the flow-aligning type.

Phase equilibria in an athermal solution of platelike particles

A molecular frame lattice theory of athermal solutions of platelike particles is presented. Steric repulsion between the particles is assumed to be the sole interaction present in the system (the athermal limit). The theory is developed for flat rectangular parallelepipeds, and examined in detail for two opposite shape anisotropy limits: rods and square boards. Numerical calculations show that in a pure system of either long rods or square boards, a nematic phase is formed once the shape anisotropy exceeds some critical value: for rods the critical aspect ratio x(crit)(r) is 8. 019, and for boards x(crit)(d) is 3.742. For higher values of the ratio, a narrow concentration region of coexistence for the nematic and isotropic phases, which separates the isotropic (low concentration) from the nematic (high concentration) solution, is found on dilution of each system.

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

Fibers of 21 rigid-rod aromatic polyamides with different substitution patterns at their aromatic rings, produced by polycondensation of functionalized p-phenylenediamine and functionalized terephthaloyl dichloride and spun from nematic solutions as described in the accompanying paper (B. H. Glomm, M. C. Grob, P. Neuenschwander, and U. W. Suter, Macromol. Chem. Phys.), were characterized by the mechanical properties most relevant for compressive failure. In particular, the torsional moduli G0 and the axial compressive strength σC were determined for each fiber sample before and after employing a post-spinning heat treatment optimized to improve the degrees of orientation and the crystallinity of the fibers. The dependence of the measured values on the structural parameters of the respective polymers was studied, leading to the result that the volume of the side-chains of the studied aramids seems to influence the extent of the mechanical “anisotropy” of the fibers, probably through an effect on the interchain interactions. The relationship between the torsional modulus and the axial compressive strength was scrutinized in the light of the theoretical approach of DeTeresa, Allen, and Farris, and Allen, which suggests the existence of a proportionality between G0 and σC. In general, the results provided by our experiments are consist with this theoretical approach.