A hydrogel‐forming liquid crystalline elastomer exhibiting soft shape memory

Research in the field of liquid crystalline polymers has recently witnessed the introduction of liquid crystalline hydrogels. Here, we report the synthesis and characterization of a new liquid crystalline network featuring elastomeric softness, water‐swelling and shape memory characteristics. By comparing with a nonmesogenic network prepared using the same procedure, we reveal structure–property relationships of the liquid crystalline and crystalline polymer networks. Wide angle and small angle X‐ray scattering studies were used to examine the liquid crystalline ordering in both dry and hydrated states. Such ordering was found to be related to the observed shape memory and actuation (two‐way shape memory) properties and these phenomena are highlighted with demonstrations of shape change in response to heat and water stimuli. This study provides insight into the mechanisms affecting the shape evolution of water activated anisotropic liquid crystalline hydrogels and enables the future design of materials or devices for a variety of applications such as biomaterials interacting with body fluids in a hydrated state. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 38–52     

Nanosized Shape‐Changing Colloids from Liquid Crystalline Elastomers

A method to prepare shape‐changing nanospheres from liquid crystalline elastomers is reported. The nanosized colloids are prepared by a miniemulsion process. During this process, colloids are prepared from a liquid crystalline (LC) main‐chain polyester and subsequently crosslinked into a nanometer‐sized LC elastomer. The ability of these LC elastomers to change their shape at the phase transition temperature from the smectic A to the isotropic phase was detected by temperature‐dependent transmission electron microscopy. The phase transition‐induced shape change leads to strongly shape anisotropic nanosized elastomer particles.

     

Structure and internal dynamics of a side chain liquid crystalline polymer in various phases by molecular dynamics simulations: a step towards coarse graining

Side chain liquid crystalline polymer with relatively long spacer was modeled on a semiatomistic level and studied in different liquid crystalline phases with the aid of molecular dynamics simulations. Well equilibrated isotropic, polydomain smectic and monodomain smectic phases were studied for their structural and dynamic properties. Particular emphasis was given to the analysis on a coarse-grained level, where backbones, side chains, and mesogens were considered in terms of their equivalent ellipsoids. The authors found that the liquid crystalline phase had a minor influence on the metrics of these objects but affected essentially their translational and orientational order. In the monodomain smectic phase, mesogens, backbones, and side chains are confined spatially. Their diffusion and shape dynamics are frozen along the mesogen director (the one-dimensional solidification) and the reorientation times increase by one to one-and-half orders of magnitude. In this phase, besides obvious orientational order of mesogens and side chains, a stable detectable order of the backbones was also observed. The backbone director is confined in the plane perpendicular to the mesogen director and constantly changes its orientation within this plane. The backbone diffusion in these planes is of the same range as in the polydomain smectic phase at the same temperature. A detailed analysis of the process of field-induced growth of the smectic phase was performed. The study revealed properties of liquid crystalline polymers that may enable their future fully coarse-grained modeling.     

Novel structures of a smectic C* phase with high spontaneous polarization in free-standing films

New stable structures of the liquid crystalline smectic C* phase are observed in free standing films of a material with high spontaneous polarization. A stable configuration of thick films is a stripe of state with an in-plane rotation of the director. In ultra-thin films an anisotropic state was observed. The director fields of the observed structures are modelled and compared with theoretical predictions.     

Simultaneous chirality transfer and structured aggregation of a cyclopeptide in a liquid crystal

We report the finding that a chiral cyclopeptide dissolved in a nematic liquid crystal (LC) host could aggregate in a manner that is controlled by the texture (LC director configuration) of a cholesteric phase that is induced by the cyclopeptide itself. On one hand, with the fingerprint texture, where the helical axis formed by rotating LC molecules, that lies in the substrate plane, the cyclopeptide can use the LC texture as a template to aggregate and form long-range-ordered ribbons that mimic the helical configuration of the LC director. On the other hand, with the planar texture, where the helical axis is normal to the substrate plane, the cyclopeptide can migrate into the "oily-streak" defect regions of the cholesteric phase and stabilize a network of defects that dictates the electrooptical response of the LC. This is the first example of a molecular species exhibiting such a structured aggregation and defect stabilization effect in a cholesteric LC, but similar phenomena were previously reported for platinum nanoparticles and silica colloidal particles, respectively, dispersed in a cholesteric LC host. This study provides more evidence for the potential interest of exploring LCs as an anisotropic medium for mediating the aggregation and assembly of cyclopeptides.     

Microscopic characterisation of self-assembled colloidal particles in electrohydrodynamic convection of a low-birefringence nematic liquid crystal

Electrokinetics of small particles immersed in anisotropic fluids is attracting attention in recent years. Here we focus on microscopic appearance of single as well as self-assembled particles moving in the electrohydrodynamic convection (EHC) of a nematic liquid crystal with low birefringence. Characterisation of the birefringent properties is made by polarised light microscopy under different illumination conditions. Because of the small optical anisotropy, the director distortion around the particles clearly exhibits distinctive colours on both sides depending on the height in the cell. The observation can be explained as the change in the net phase retardation of the light. It is also found that a caterpillar-like motion is possible by tuning temperature, although the horizontal size of the EHC rolls is relatively narrow.     

Thermodynamics of soft anisotropic contact lines

Contact lines arising from the intersection of interfaces between liquids and nematic liquid crystals are representative models of soft anisotropic contact lines. This paper presents the thermodynamics of soft anisotropic contact lines and the derivation of the one dimensional (1D) Gibbs-Duhem adsorption equation. Consistency between the 1D Gibbs-Duhem equation and the classical equations of lineal nematostatics is shown. Using a phase space that takes into account thermodynamics, liquid crystalline order, and geometric variables, the generalized nematic line Gibbs-Duhem equation reveals the presence of couplings between curvature, torsion, adsorption, temperature, and average molecular orientation. Merging the thermodynamic analysis with nematostatics results in a model for contact line shape and orientation selection. The ability of an adsorbed solute to orient the director and to bend and twist the contact line is predicted. The thermodynamic origin of preferred orientation at a straight contact line is established.     

可多次使用的液晶型类玻璃高分子

通过高温下酯交换反应的进行,含酯键的液晶型类玻璃高分子(liquid crystalline vitrimer),能够通过简单拉伸进行取向,获得随温度变化可逆伸缩的智能材料.在目前已报道的此类主链型高分子中,酯交换剧烈发生需要的临界温度(Tv),与液晶弹性体发生可逆形变的温度(Ti,即液晶相-各向同性相转变温度)相隔较近,导致材料的使用温度范围比较窄,而且多次升降温后,取向及可逆形变会消失.为解决此问题,本文在原来体系的基础上,通过共聚合另外一种液晶基元,有效地降低了Ti,从而拓宽Ti与Tv之间的距离.这不仅使材料的使用次数明显增加,还能延长此类液晶弹性体的使用期限.     

A novel gel deformation technique for fabrication of ellipsoidal and discoidal polymeric microparticles

A simple fabrication technique for anisotropic particles of ellipsoidal/discoidal shape has been developed, based on stretching/compressing of oil-in-water emulsion templates embedded into an elastic aqueous gel; a range of solid anisotropic microparticles have been fabricated by polymerising of the deformed oil drops in the elastic gel matrix and their shape and aspect ratios have been studied as a function of the gel deformation.     

Using switched angle spinning to simplify NMR spectra of strongly oriented samples

This contribution describes a method that manipulates the alignment director of a liquid crystalline sample to obtain anisotropic magnetic interaction parameters, such as dipolar coupling, in an oriented liquid crystalline sample. By changing the axis of rotation with respect to the applied magnetic field in a spinning liquid crystalline sample, the dipolar couplings present in a normally complex strong coupling spectrum are scaled to a simple weak coupling spectrum. This simplified weak coupling spectrum is then correlated with the isotropic chemical shift in a switched angle spinning (SAS) two-dimensional (2D) experiment. This dipolar-isotropic 2D correlation was also observed for the case where the couplings are scaled to a degree where the spectrum approaches strong coupling. The SAS 2D correlation of C(6)F(5)Cl in the nematic liquid crystal I52 was obtained by first evolving at an angle close to the magic angle (54.7 degrees ) and then directly detecting at the magic angle. The SAS method provides a 2D correlation where the weak coupling pairs are revealed as cross-peaks in the indirect dimension separated by the isotropic chemical shifts in the direct dimension. Additionally, by using a more complex SAS method which involves three changes of the spinning axis, the solidlike spinning sideband patterns were correlated with the isotropic chemical shifts in a 2D experiment. These techniques are expected to enhance the interpretation and assignment of anisotropic magnetic interactions including dipolar couplings for molecules dissolved in oriented liquid crystalline phases.     

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

Summary: 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.     

Double emulsions with controlled morphology by microgel scaffolding

Double emulsions are valuable structures that consist of drops nested inside bigger drops; they can be formed with exquisite control through the use of droplet-based microfluidics, allowing their size, composition, and monodispersity to be tailored. However, only little control can be exerted on the morphology of double emulsions in their equilibrium state, because they are deformable and subject to thermal fluctuations. To introduce such control, we use droplet-based microfluidics to form oil-in-water-in-oil double emulsion drops and arrest their shape by loading them with monodisperse microgel particles. These particles push the inner oil drop to the edge of the aqueous shell drop such that the double emulsions adopt a uniform arrested, anisotropic shape. This approach circumvents the need for ultrafast polymerization or geometric confinement to lock such non-spherical and anisotropic droplet morphologies. To demonstrate the utility of this technique, we apply it to synthesize anisotropic and non-spherical polyacrylate-polyacrylamide microparticles with controlled size and shape.     

A single-site anisotropic soft-core model for the study of phase behavior of soft rodlike particles

A novel mesoscopic simulation model is proposed to study the liquid crystal phase behavior of the anisotropic rodlike particles with a soft repulsive interaction,which possesses a modified anisotropic conservative force type used in dissipative particle dynamics.The influences of the repulsion strength and the particle shape on the phase behavior of soft rodlike particles are examined.In the simulations,we observe the formation of the nematic phase and smectic-A phase from the initially isotropic phase.More...     

Localised Actuation in Composites Containing Carbon Nanotubes and Liquid Crystalline Elastomers

A liquid crystalline elastomer–carbon nanotube (LCE‐CNT) composite displays a reversible shape change property in response to light. The development of some systems such as tactile devices requires localised actuation of this material. A method is reported that combines mechanical stretching and thermal crosslinking of an LCE‐CNT for creating sufficiently well‐aligned liquid crystal units to produce localised actuation. The method demonstrates that it is feasible to optically drive a LCE‐CNT film within a localised area, since only the walls of the stretched parts of the film contain aligned LC domains.     

Functional liquid crystalline particles and beyond

ABSTRACT

Recently the preparation and handling of liquid crystalline (LC) particles, which cover the size from some 100 nano-meters to micro- and millimetres found more interest. This review describes ways for their preparation and discusses possible properties and their use and especially the relation between both. The primary shape of these particles can thereby be varied from simple sphere like to prolate or oblate spheres, to tubes, to Janus-like particles or irregular objects. Special attention is given to the manipulation of these functional LC-objects in space, either their movement (including rotation) or their shape variation. Microfluidics allow the preparation of most of these objects and, in addition, control of the director pattern.     

Broad band dielectric relaxation spectroscopy of molecular reorientation in smectic liquid crystalline phases (SmA, SmB, and CrE)

Broad band dielectric measurements reveal that the reorientation of non-chiral rod-shaped low molecular mass liquid crystals is active around their molecular long axis and a short axis, in the smectic A and hexatic smectic B phase, respectively, as well as in the soft crystalline E phase of two isomeric stilbene compounds possessing an equal molecular length of their all-trans -conformations. One ('generalized') Arrhenius equation describes the temperature dependence of the reorientation around a molecular short axis for each of these phases of both compounds. A change of the activation energy related to the reorientation around a molecular short axis is accompanied by a slowing down of the reorientation around the molecular long axis in the soft crystalline E phase in one of these compounds, compelling evidence for a coupling of both reorientations. This result is discussed with respect to the biaxiality of the soft crystalline E phase.     

Orientation dynamics in isotropic phases of model oligofluorenes: glass or liquid crystal

Orientation molecular dynamics were investigated in a series of "defect-free" oligofluorenes by depolarized dynamic light scattering and dynamic NMR spectroscopy. Typical liquid crystalline pretransitional dynamics were observed upon cooling the isotropic phase to the liquid crystalline phase with strong increase of the scattered intensity and slowing down of the characteristic time of the probed collective relaxation. This is well accounted for by the Landau-de Gennes theory, however, with a strong temperature dependence of the viscosity coefficient, reflecting the proximity of the glass transition. For the trimer the two transitions almost overlap and the molecular orientation coincide with the alpha-relaxation associated with the glass transition. The NMR measurements confirm that the time scale of the dynamics is completely governed by the glass process, yet the geometry of the motion is anisotropic, yielding order parameters ranging from 0.15 to 0.25 for the long axis in the liquid crystalline phase. The glass transition is therefore geometrically restricted with poorly ordered mesophase which is consistent with the weak transverse phonons in the light scattering experiment down to Tg+20 K.     

Self-alignment of dye molecules in micelles and lamellae for three-dimensional imaging of lyotropic liquid crystals

We report alignment of anisotropic amphiphilic dye molecules within oblate and prolate anisotropic micelles and lamellae, the basic building blocks of surfactant-based lyotropic liquid crystals. Absorption and fluorescence transition dipole moments of these dye molecules orient either parallel or orthogonal to the liquid crystal director. This alignment enables three-dimensional visualization of director structures and defects in different lyotropic mesophases by means of fluorescence confocal polarizing microscopy and two-photon excitation fluorescence polarizing microscopy. The studied structures include nematic tactoids, Schlieren texture with disclinations in the calamitic nematic phase, oily streaks in the lamellar phase, developable domains in the columnar hexagonal phase, and various types of line defects in the discotic cholesteric phase. Orientational three-dimensional imaging of structures in the lyotropic cholesterics reveals large Burgers vector dislocations in cholesteric layering with singular disclinations in the dislocation cores that are not common for their thermotropic counterparts.     

On the nematic-nematic phase transition in mixtures composed of sheet-shaped palladium organyls and apolar organic solvents

The three sheet-shaped palladium organyls 1a-c exhibit lyotropic nematic properties in apolar organic solvents. The occurrence of two nematic lyomesophases was verified by the investigation of the orientational properties of these liquid crystalline systems. In cells with obliquely deposited SiO_x layers, the director of the high temperature phase N₂ is aligned in a homogeneous, planar manner. In the low temperature phase N₁, a different orientational pattern is observed. The specific pattern type depends on the deposition conditions of the SiO_x layers. In samples homeotropically aligned in the N₂ phase, the director is tilted away from the cell normal after the transition into the N₁ phase. These changes of orientation are reversible. The experimental observations presented here support our proposed model for the structural change at the nematic-nematic phase transition.