Using chiral tactoids as optical probes to study the aggregation behavior of chromonics.

Tactoids are nuclei of an orientationally ordered nematic phase that emerge upon cooling the isotropic phase. In addition to providing a natural setting for exploring chromonics under confinement, we show that tactoids can also serve as optical probes to delineate the role of temperature and concentration in the aggregation behavior of chromonics. For high concentrations, we observe the commonly reported elongated bipolar tactoids. As the concentration is lowered, breaking of achiral symmetry in the director configuration is observed with a predominance of twisted bipolar tactoids. On further reduction of concentration, a remarkable transformation of the director configuration occurs, wherein it conforms to a unique splay-minimizing configuration. Based on a simple model, we arrive at an interesting result that lower concentrations have longer aggregates at the same reduced temperature. Hence, the splay deformation that scales linearly with the aggregate length becomes prohibitive for lower concentrations and is relieved via twist and bend deformations in this unique configuration. Raman scattering measurements of the order parameters independently verify the trend in aggregate lengths and provide a physical picture of the nematic-biphasic transition.

Thermally Switchable Liquid Crystals Based on Cellulose Nanocrystals with Patchy Polymer Grafts.

A thermally "switchable" liquid-crystalline (LC) phase is observed in aqueous suspensions of cellulose nanocrystals (CNCs) featuring patchy grafts of the thermoresponsive polymer poly(N-isopropylacrylamide) (PNIPAM). "Patchy" polymer decoration of the CNCs is achieved by preferential attachment of an atom transfer radical polymerization (ATRP) initiator to the ends of the rods and subsequent surface-initiated ATRP. The patchy PNIPAM-grafted CNCs display a higher colloidal stability above the lower critical solution temperature (LCST) of PNIPAM than CNCs decorated with PNIPAM in a brush-like manner. A 10 wt% suspension of the "patchy" PNIPAM-modified CNCs displays birefringence at room temperature, indicating the presence of an LC phase. When heated above the LCST of PNIPAM, the birefringence disappears, indicating the transition to an isotropic phase. This switching is reversible and appears to be driven by the collapse of the PNIPAM chains above the LCST, causing a reduction of the rods' packing density and an increase in translational and rotational freedom. Suspensions of the "brush" PNIPAM-modified CNCs display a different behavior. Heating above the LCST causes phase separation, likely because the chain collapse renders the particles more hydrophobic. The thermal switching observed for the "patchy" PNIPAM-modified CNCs is unprecedented and possibly useful for sensing and smart packaging applications.

Curvature-Induced Twist in Homeotropic Nematic Tori.

We confine a nematic liquid crystal with homeotropic anchoring to stable toroidal droplets and study how geometry affects the equilibrium director configuration. In contrast to the case of cylindrical confinement, we find that the equilibrium state is chiral-a twisted and escaped radial director configuration. Furthermore, we find that the magnitude of the twist distortion increases as the ratio of the ring radius to the tube radius decreases; we confirm this with computer simulations of optically polarized microscopy textures. In addition, numerical calculations also indicate that the local geometry indeed affects the magnitude of the twist distortion. We further confirm this curvature-induced twisting using bent cylindrical capillaries.

Enhanced Alignment of Water-Soluble Polythiophene Using Cellulose Nanocrystals as a Liquid Crystal Template.

Cellulose nanocrystals (CNCs) are bioderived, rodlike particles that form a chiral nematic liquid crystal (LC) in water. In this work, CNCs were used to induce long-range order in a semiconducting polymer, poly[3-(potassium-4-butanoate) thiophene-2,5-diyl] (PPBT). When mixed with CNCs, it was found that PPBT was incorporated into the liquid crystal "template" to form ordered structures with highly birefringent domains, as observed under polarized light. We show that the π-π interactions between polymer chains, which contribute considerably to the energetics of the semiconducting system, are directly influenced by the presence and packing of the liquid crystal phase. Upon increasing the concentration of CNCs from the isotropic to chiral nematic regime, we observe a bathochromic shift in the UV-vis spectra and the emergence of the 0-0 vibrational peak, suggesting enhanced π-π stacking leading to chain coplanarization. Furthermore, the chiral nature of the PPBT/CNC mixture was evidenced by a negative peak in circular dichroism (CD) spectroscopy, promoting the notion that the polymer chains followed the helicoidal twist of the chiral nematic liquid crystal host. At high temperatures, the peak height ratios and overall intensities of the UV-vis and CD spectra associated with PPBT decreased as the chiral nematic pitch grew larger in size.

Macroscopic alignment of chromonic liquid crystals using patterned substrates.

We demonstrate an efficient technique to align lyotropic chromonic liquid crystals (LCLCs) using secondary sputtering lithography (SSL). Monodomains of LCLCs prepared using SSL maintained their stable alignment for days. A generalization of Berreman's theory was employed to determine the anchoring strength of LCLCs on tessellated surface patterns. The anchoring energy initially increases with the amplitude (A) of the grooves and excellent alignment of LCLCs was observed when the amplitude of the grooves is equal to half its wavelength (λ). We also note that the anchoring energy levels off above qA∼ 3 (where q = 2π/λ), which suggests that increasing qA beyond a certain value does not provide any further advantage for the alignment of LCLCs. This finding provides a useful optimization criterion for the fabrication of the patterned cells to achieve stable monodomain alignment of LCLCs. Our analysis also explains why good alignment of LCLCs has been a difficult task.

Disclination elastica model of loop collision and growth in confined nematic liquid crystals.

Theory and modeling are used to characterize disclination loop-loop interactions in nematic liquid crystals under capillary confinement with strong homeotropic anchoring. This defect process arises when a mesogen in the isotropic phase is quenched into the stable nematic state. The texture evolution starts with +1/2 disclination loops that merge into a single loop through a process that involves collision, pinching and relaxation. The process is characterized with a combined Rouse-Frank model that incorporates the tension and bending elasticity of disclinations and the rotational viscosity of nematics. The Frank model of disclinations follows the Euler elastica model, whose non-periodic solution, known as Poleni's curve, is shown to locally describe the loop-loop collision and to shed light on why loop-loop merging results in a disclination intersection angle of approximately 60°. Additional Poleni invariants demonstrate how tension and bending pinch the two loops into a single +1/2 disclination ring. The Rouse model of disclination relaxation yields a Cahn-Hilliard equation whose time constant combines the confinement, tension/bending stiffness ratio and disclination diffusivity. Based on predictions made using this three stage process, a practical procedure is proposed to find viscoelastic parameters from defect geometry and defect dynamics. These findings contribute to the evolving understanding of textural transformations in nematic liquid crystals under confinement using the disclination elastica methodology.

Mechanisms and shape predictions of nematic disclination branching under conical confinement.

Liquid crystals (LCs) are self-organizing anisotropic viscoelastic soft materials that flow like viscous liquids and display anisotropies like crystals. When a nematic liquid crystal is confined to a capillary tube with strong anchoring conditions, disclination defects of higher (+1) and lower (+1/2) topological charges can coexist, connected through a defect branch point. The shape of the +1/2 disclination lines emanating from the branch point are functions of confinement and bulk elasticity. Previous work shows that nematic liquid crystals under cylindrical confinement display a radial (one +1 line)-to-planar polar (two +1/2 lines) defect texture transition through the nucleation and uniform motion of a disclination branch point. Here we present analysis, scaling and modeling based on a non-linear non-local nematic elastic equation that shows that a branch point also can be generated from disclinations in a liquid crystal confined to different conical geometries with homeotropic anchoring conditions. The cone aperture increases the bending stiffness but decreases the curvature of the disclination. These competing effects lead to a decrease in the total disclination curvature, increase in elastic energy and volume of the branching region. The results are summarized into power laws and integrated into a shape/energy diagram that reveals the effects of confinement and its gradient (cone angle) on disclination shape selection. These new findings are useful to assess the Frank elasticity of new nematic liquid crystals and to predict novel defect structures in complex confinement, including biological microfluidics and mesophase fiber spinning.

Nanotubes of a Different Kind: Stoichiometry and Geometry of the Orange II/γ-Cyclodextrin Complex in Water.

Orange II (O-II), a water-soluble ionic azo dye, aggregates and eventually forms needle-like crystals at concentrations greater than 0.15 M. However, when equimolar amounts of γ-cyclodextrin (γ-CD) are added to solutions containing O-II at 0.025 M or higher, the solution's appearance rapidly changes presenting a viscous, birefringent liquid, a lyotropic liquid crystalline solution. Birefringence is absent when viewing aqueous solutions of only O-II or γ-CD at concentrations greater than 0.03 M. Using ultraviolet-visible (UV-vis) and fluorescence spectroscopy, coupled with conductivity measurements, we postulate a structure for the basic "building block" of the self-assembly that eventually gives rise to a rodlike superstructure, leading to the formation of a lyotropic liquid crystalline phase.

Cylindrical posts of Ag/SiO2/Au multi-segment layer patterns for highly efficient surface enhanced Raman scattering.

We fabricated a regular array of Ag/SiO2/Au multi-segment cylindrical nanopatterns to create a highly efficient surface enhanced Raman scattering (SERS) active substrate using an advanced soft-nanoimprint lithographic technique. The SERS spectra results for Rhodamine 6G (R6G) molecules on the Ag/SiO2/Au multi-segment nanopatterns show that the highly ordered patterns and interlayer thickness are responsible for enhancing the sensitivity and reproducibility, respectively, The multi-segment nanopattern with a silica interlayer generates significant SERS enhancement (~EF = 1.2 x 106) as compared to that of the bimetallic (Ag/Au) nanopatterns without a dielectric gap (~EF = 1.0 x 104). Further precise control of the interlayer distances between the two metals plays an essential role in enhancing SERS performance for detecting low concentrations of analytes such as fluorescent (Rhodamine 6G) and DNA molecules. Therefore, the highly ordered multi-segment patterns provide great sensitivity and reproducibility of SERS based detection, resulting in a high performance of the SERS substrate.

Shape separation of gold nanorods using centrifugation.

We demonstrate the use of centrifugation for efficient separation of colloidal gold nanorods from a mixture of nanorods and nanospheres. We elucidate the hydrodynamic behavior of nanoparticles of various shapes and illustrate that the shape-dependent drag causes particles to have shape-dependent sedimentation behavior. During centrifugation, nanoparticles undergo Brownian motion under an external field and move with different sedimentation velocities dictated by their Svedberg coefficients. This effects a separation of particles of different shape and size. Our theoretical analysis and experiments demonstrate the viability of using centrifugation to shape-separate a mixture of colloidal particles.

Anisotropic Responsive Microgels Based on the Cholesteric Phase of Chitin Nanocrystals.

Anisotropic stimuli-responsive microgels based upon the cholesteric phase of chitin nanocrystals and N-isopropylacrylamide were designed and synthesized. The cholesteric structure was interrogated, and the texture was shown to directly influence the microgel shape and anisotropy. Changes in the microgel volume led to changes in the texture, where microgels comprising up to six bands exhibited a twisted bipolar texture, while those with greater volumes displayed a concentric-packing structure. As designed, the imprinted cholesteric phase induced an asymmetric response to temperature, leading to a change in shape and optical properties. Furthermore, the cholesteric structure is able to deform, facilitating transport into a small channel. Access to synthetic structures having a self-assembled twisted texture derived from cholesterics embedded within a polymer matrix will provide guidelines for designing biopolymer composites with programmable motion.

Solvent evaporation induced liquid crystalline phase in poly(3-hexylthiophene).

We report on the evolution of the chain orientation of a representative π-conjugated polymer, poly(3-hexylthiophene) (P3HT), during the solution-casting process, as monitored using polarized Raman spectroscopy. These measurements point to the formation of a liquid-crystalline phase of P3HT solutions within a specific time period during solvent evaporation, which leads to a conducting channel. These conclusions are based on the angular dependence of polarized Raman scattering peaks, the anisotropy in the fluorescence background signal, analysis of the scattering-peak shape, and direct observations of the three-phase contact line in an optical microscope under crossed polarizers. These results shed new light on the evolution of chain alignment and thus materials nanostructure, specifically in solution-processed P3HT and more generally in π-conjugated systems. They may further enable the design of improved materials and processes for this important class of polymers.

Raman scattering study of phase biaxiality in a thermotropic bent-core nematic liquid crystal.

Polarized Raman spectroscopy was used to investigate the development of orientational order and the degree of phase biaxiality in a bent-core mesogenic system. The values of the uniaxial order parameters and , and biaxial order parameters , , and , and their evolution with temperature were determined. The temperature dependence of almost all order parameters reveals a second order transition from the uniaxial to biaxial nematic phase with increasing to ∼0.22 before a first order transition to the smectic-C phase, upon cooling.

Traversing the "top-down/bottom-up" divide: molecular-scale lithography of self-assembled ribbons.

Use of graphitic (hexabenzocoronene-derived) molecules produces cholesteric ribbons which serve as molecular resists in a fluorine plasma. This procedure allows the shape of the molecular assemblies to be etched into the underlying silicon, validating the concept of "molecular resists".

Imaging optical scattering of butterfly wing scales with a microscope.

A new optical method is proposed to investigate the reflectance of structurally coloured objects, such as Morpho butterfly wing scales and cholesteric liquid crystals. Using a reflected-light microscope and a digital single-lens reflex (DSLR) camera, we have successfully measured the two-dimensional reflection pattern of individual wing scales of Morpho butterflies. We demonstrate that this method enables us to measure the bidirectional reflectance distribution function (BRDF). The scattering image observed in the back focal plane of the objective is projected onto the camera sensor by inserting a Bertrand lens in the optical path of the microscope. With monochromatic light illumination, we quantify the angle-dependent reflectance spectra from the wing scales of Morpho rhetenor by retrieving the raw signal from the digital camera sensor. We also demonstrate that the polarization-dependent reflection of individual wing scales is readily observed using this method, using the individual wing scales of Morpho cypris. In an effort to show the generality of the method, we used a chiral nematic fluid to illustrate the angle-dependent reflectance as seen by this method.

Polypeptide Composite Particle-Assisted Organization of π-Conjugated Polymers into Highly Crystalline "Coffee Stains".

We demonstrate that homopolypeptides covalently tethered to anisotropically shaped silica particles induce crystalline ordering of representative semiconducting polymers. Films drop-cast from chloroform dispersions of poly(γ-stearyl-l-glutamate) (PSLG) composite particles and poly(3-hexythiophene) (P3HT) led to highly ordered crystalline structures of P3HT. Hydrophobic-hydrophobic interactions between the alkyl side chains of P3HT and PSLG were the main driving force for P3HT chain ordering into the crystalline assemblies. It was found that the orientation of rigid P3HT fibrils on the substrate adopted the directionality of the evaporating front. Regardless of the PSLG-coated particle dimensions used, the drop-cast films displayed patterns that were shaped by the coffee ring and Marangoni effects. PSLG-coated particles of high axial ratio (4.2) were more efficient in enhancing the electronic performance of P3HT than low axial ratio (2.6) homologues. Devices fabricated from the ordered assemblies displayed improved charge-carrier transport performance when compared to devices fabricated from P3HT alone. These results suggest that PSLG can favorably mediate the organization of semiconducting polymers.

Orientational Order of a Lyotropic Chromonic Liquid Crystal Measured by Polarized Raman Spectroscopy.

Lyotropic chromonic liquid crystals are distinct from thermotropic nematics from a fundamental standpoint as the structure of the aggregating columns is a function of both the temperature and concentration. We report on the thermal evolution of orientational order parameters, both the second (=scalar) (⟨P200⟩ (=S)) and fourth (⟨P400⟩) order, of sunset yellow FCF aqueous solutions, measured using polarized Raman spectroscopy for different concentrations. The order parameter increases with the concentration, and their values are high in comparison with those of thermotropic liquid crystals. On the basis of Raman spectroscopy, we provide the strongest evidence yet that the hydrozone tautomer of SSY is the predominant form in aqueous solutions in the isotropic, nematic, and columnar phases, as well as what we believe to be the first measurements of (⟨P400⟩) for this system.

X-ray and Raman scattering study of orientational order in nematic and heliconical nematic liquid crystals.

The temperature dependence of the orientational order parameters 〈P_{2}(cosß)〉 and 〈P_{4}(cosß)〉 in the nematic (N) and twist-bend nematic (N_{tb}) phases of the liquid crystal dimer CB7CB have been measured using x-ray and polarized Raman scattering. The 〈P_{2}(cosß)〉 obtained from both techniques are the same, while 〈P_{4}(cosß)〉, determined by Raman scattering is, as expected, systematically larger than its x-ray value. Both order parameters increase in the N phase with decreasing temperature, drop across the N-N_{tb} transition, and continue to decrease. In the N_{tb} phase, the x-ray value of 〈P_{4}(cosß)〉 eventually becomes negative, providing a direct and independent confirmation of a conical molecular orientational distribution. The heliconical tilt angle α, determined from orientational distribution functions in the N_{tb} phase, increases to ∼24^{∘} at ∼15 K below the transition. In the N_{tb} phase, α(T)∝(T^{*}-T)^{λ}, with λ=0.19±0.03. The transition supercools by 1.7 K, consistent with its weakly first-order nature. The value of λ is close to 0.25 indicating close proximity to a tricritical point.

Viscoelastic and mechanical behavior of recombinant protein elastomers.

Recombinant DNA synthesis was employed to produce elastin-mimetic protein triblock copolymers containing chemically distinct midblocks. These materials displayed a broad range of mechanical and viscoelastic responses ranging from plastic to elastic when examined as hydrated gels and films. These properties could be related in a predictable fashion to polymer block size and structure. While these materials could be easily processed into films and gels, electrospinning proved a feasible strategy for creating protein fibers. All told, the range of properties exhibited by this new class of protein triblock copolymer in combination with their easy processability suggests potential utility in a variety of soft prosthetic and tissue engineering applications.

Control of anchoring of nematic fluids at polymer surfaces created by in situ photopolymerization.

In situ photopolymerization of alkyl acrylate monomers in the presence of a nematic fluid provides a cellular matrix of liquid crystalline droplets in which the chemical structure of the encapsulating polymer exerts control over the alignment (anchoring) of the liquid crystalline molecules. Control is obtained by variation of the alkyl side chains and through copolymerization of two dissimilar monofunctional acrylates. For example, among a series of poly(methylheptyl acrylate)s, the 1-methylheptyl analogue prefers planar anchoring of a nematic (TL205) over the temperature range studied. However, the polymers of other methylheptyl side chains display a homeotropic-to-planar anchoring thermal transition temperature similar to that of the n-heptyl analogue. Copolymerization of two monofunctional acrylates with opposing tendencies of aligning liquid crystal leads to tunability of anchoring behavior over a wide temperature range. The broad anchoring transitions we observed provide a way of achieving highly tilted anchoring.