Towards efficient dispersion of carbon nanotubes in thermotropic liquid crystals

in Advanced Functional Materials (2010), 20(19), 3350-3357

Motivated by numerous recent reports indicating attractive properties of composite materials of carbon nanotubes (CNTs) and liquid crystals (LCs) and a lack of research aimed at optimizing such composites ... [more ▼]

Motivated by numerous recent reports indicating attractive properties of composite materials of carbon nanotubes (CNTs) and liquid crystals (LCs) and a lack of research aimed at optimizing such composites, the process of dispersing CNTs in thermotropic LCs is systematically studied. LC hosts can perform comparably or even better than the best known organic solvents for CNTs such as N-methyl pyrrolidone (NMP), provided that the dispersion process and choice of LC material are optimized. The chemical structure of the molecules in the LC is very important; variations in core as well as in terminal alkyl chain influence the result. Several observations moreover indicate that the anisotropic nematic phase, aligning the nanotubes in the matrix, per se stabilizes the dispersion compared to a host that is isotropic and thus yields random tube orientation. The chemical and physical phenomena governing the preparation of the dispersion and its stability are identified, taking into account enthalpic, entropic, as well as kinetic factors. This allows a guideline on how to best design and prepare CNT–LC composites to be sketched, following which tailored development of new LCs may take the advanced functional material that CNT–LC composites comprise to the stage of commercial application. [less ▲]

On the balance between syn- and anticlinicity in smectic phases formed by achiral hockey-stick mesogens with and without chiral dopants

in Journal of Materials Chemistry (2009), 19(19), 2950-2957

A series of achiral hockey-stick-shaped mesogens forming tilted smectic liquid crystal phases of synclinic SmC- as well as anticlinic SmCa-type was prepared and characterized. While all homologues exhibit ... [more ▼]

A series of achiral hockey-stick-shaped mesogens forming tilted smectic liquid crystal phases of synclinic SmC- as well as anticlinic SmCa-type was prepared and characterized. While all homologues exhibit both phases, the balance shifts from anticlinic to synclinic order upon elongation of the terminal chain at the meta-position, defining the hockey-stick shape. The elongation also leads to an increased kinetic hindrance of the transition between syn- and anticlinic phases and a decreased transition enthalpy. These observations indicate that a well-defined kink (short meta-substituted chain) promotes the anticlinic structure while a higher flexibility between kinked and rod-shape (long meta-substituted chain) promotes synclinic order. An intermediate chain-length homologue was selected as host material for doping with syn- and anticlinic rod-shaped chiral dopants, respectively, at varying concentrations. Opposite of what might be expected the balance between syn- and anticlinic order was not simply dictated by the choice of dopant. Instead, both types of tilting order prevailed with roughly the same strength as in the achiral host regardless of which chiral material was added, up to concentrations well beyond normal doping conditions. Thus, at least with hockey-stick-shaped achiral hosts, syn- as well as anticlinic chiral compounds can be used effectively as chiral dopants without necessarily having an important impact on the clinicity of the resulting mixture. The hockey-stick design concept should be useful in producing achiral anticlinic-forming mesogens for low-polarization, long-pitch antiferroelectric liquid crystal mixtures. Finally, we point out that a mixture study like the one carried out here yields a conclusive means of establishing the clinicity of achiral tilted smectics, an endeavour that can sometimes be far from trivial. [less ▲]

Molecular model for de Vries type smectic-A–smectic-C phase transition in liquid crystals

in Physical Review. E ,Statistical, Nonlinear, and Soft Matter Physics (2007), 75(6), 060701

We develop both phenomenological and molecular-statistical theory of smectic-A-smectic-C phase transition with anomalously weak smectic layer contraction. Using a general mean-field molecular model, we ... [more ▼]

We develop both phenomenological and molecular-statistical theory of smectic-A-smectic-C phase transition with anomalously weak smectic layer contraction. Using a general mean-field molecular model, we demonstrate that a relatively simple interaction potential suffices to describe the transition both in conventional and de Vries type smectics. The theoretical results are in excellent agreement with experimental data. The approach can be used to describe tilting transitions in other soft matter systems. [less ▲]

Carbon nanotubes in liquid crystals as versatile functional materials

in Physica Status Solidi B. Basic Research (2007), 244(11), 4212-4217

Liquid crystals can be easily aligned in desired directions by treated surfaces or by external fields. The least ordered liquid crystal phase, the nematic, exhibits orientational order that can be easily ... [more ▼]

Liquid crystals can be easily aligned in desired directions by treated surfaces or by external fields. The least ordered liquid crystal phase, the nematic, exhibits orientational order that can be easily transferred onto carbon nanotubes dispersed in it. The alignment of the carbon nanotubes can be demonstrated by po- larized Raman spectroscopy. Carbon nanotubes not only well integrate in the matrix but also, even at very low concentration, have a detectable effect on the liquid crystal properties that can be very attractive for display applications. The presence of big aggregates of carbon nanotubes, on the other hand, interfere strongly with the switching behaviour of the liquid crystal, as we can show following the local switching of liquid crystal molecules with Raman spectroscopy. [less ▲]

Current topics in smectic liquid crystal research

in Chemphyschem : A European Journal of Chemical Physics and Physical Chemistry (2006), 7(1), 20-45

Interest in the smectic liquid-crystalline state of matter received a substantial boost with the discovery by Meyer in the mid-1970s that a chiral smectic C (SmC*) phase exhibits a spontaneous elec- tric ... [more ▼]

Interest in the smectic liquid-crystalline state of matter received a substantial boost with the discovery by Meyer in the mid-1970s that a chiral smectic C (SmC*) phase exhibits a spontaneous elec- tric polarization, and with the subsequent demonstration by Clark and Lagerwall of the surface-stabilized SmC* ferroelectric liquid crystal at the beginning of the 1980s. Since then, chiral smectic phases and their plethora of polar effects have dominat- ed the research in this field, which today has reached a mature state where the first commercial microdisplay applications are now shipping in millions-per-year quantities. In this Review we discuss some of the topics of highest interest in current smectic liquid crystal research, and address application-relevant research (de Vries-type tilting transitions without defect generation and high-tilt antiferroelectric liquid crystals with perfect dark state) as well as more curiosity-driven research (the nature and origin of the chiral smectic C subphases and their intermediate frustrated states between ferro- and antiferroelectricity). [less ▲]

The peculiar optic, dielectric and x-ray diffraction properties of a fluorinated de vries asymmetric-diffuse-cone-model ferroelectric liquid crystal

in Liquid Crystals (2006), 33(1), 17-24

A new semi-fluorinated chiral smectic liquid crystal, W504, is investigated by electro-optic, dielectric and X-ray scattering experiments. It exhibits a huge dielectric soft mode response, strong ... [more ▼]

A new semi-fluorinated chiral smectic liquid crystal, W504, is investigated by electro-optic, dielectric and X-ray scattering experiments. It exhibits a huge dielectric soft mode response, strong electroclinic effect and a birefringence which increases considerably with the director tilt angle theta; typical characteristics of a SmA - SmC transition following the de Vries asymmetric diffuse cone (ADC) model in which the non-zero director tilt in SmC arises through an ordering of tilting directions rather than an actual increase in average molecule tilt <theta(mol)>. In W504 a small increase in <theta(mol)> of about 4 degrees is however detected in the SmC phase. Although the increase in molecule inclination is much less than the increase in director tilt h, saturating close to 30 degrees, it leads to a shrinkage of the smectic layers by about 1 angstrom, a result of the large initial molecule tilt in the SmA phase, <theta(mol)>(SmA) approximate to 30 degrees. The tilting transition in W504 is thus mainly an ADC model disorder - order transition, but it also has a component of a structural transition. The semi- fluorinated molecular structure of W504 leads to a very weak electron density modulation along the layer normal, giving a vanishing form factor in bulk samples which exhibit no (001) X-ray scattering peak. In thin films the (001) peak is however observed, indicating that the electron density modulation is enhanced by the breaking of the head - tail symmetry of the liquid crystal phase at the LC - air interface. [less ▲]

Chiral smectic C subphases induced by mixing a bistereogenic antiferroelectric liquid crystal with a non-chiral liquid crystal

in Ferroelectrics (2005), 315

By mixing a bistereogenic antiferroelectric liquid crystal (AFLC) compound, exhibiting only the SmQ and SmCa mesophases, with the achiral N-SmC liquid crystal HOAB we could induce all three AFLC SmC-type ... [more ▼]

By mixing a bistereogenic antiferroelectric liquid crystal (AFLC) compound, exhibiting only the SmQ and SmCa mesophases, with the achiral N-SmC liquid crystal HOAB we could induce all three AFLC SmC-type subphases, SmCalpha, SmCbeta and SmCgamma. This seems to be in contradiction with two recent postulations regarding the subphase stability, one of which suggests that the subphases appear as a result of strong chiral interactions, the other that these phases require high smectic order something one would generally not expect in mixtures. We have studied the helical pitch, optical tilt angle, spontaneous polarization and the x-ray diffraction due to the smectic layering, as a function of mixing ratio in order to better understand the relation between phase sequence and mixture composition. The smectic layer spacing shows a strongly non-linear behavior suggesting that the basic structure of the pure AFLC substance is retained up to a HOAB content of about 75\%. [less ▲]

Differences between smectic homo- and copolysiloxanes as a consequence of microphase separation

in Liquid Crystals (2005), 32(5), 533-538

This paper compares smectic phases formed from LC-homo- and LC-co-polysiloxanes. In the homopolysiloxane, each repeating unit of the polymer chain is substituted with a mesogen, whereas in the ... [more ▼]

This paper compares smectic phases formed from LC-homo- and LC-co-polysiloxanes. In the homopolysiloxane, each repeating unit of the polymer chain is substituted with a mesogen, whereas in the copolysiloxanes mesogenic repeating units are separated by dimethylsiloxane units. Despite a rather similiar phase sequence of the homo- and co-polysiloxanes—higher ordered smectic, smectic C* (SmC*), smectic A (SmA) and isotropic—the nature of their phases differs strongly. For the copolymers the phase transition SmC* to SmA is second order and of the ‘de Vries’ type with a very small thickness change of the smectic layers. Inside the SmA phase, however, the smectic thickness decreases strongly on approaching the isotropic phase. For the homopolymer the phase transition SmC* to SmA is first order with a significant thickness change, indicating that this phase is not of the ‘de Vries’ type. This difference in the nature of the smectic phases is probably a consequence of microphase separation in the copolymer, which facilitates a loss of the tilt angle correlation between different smectic layers. This has consequences for the mechanical properties of LC- elastomers formed from homo- and co-polymers. For the elastomers from homopolymers the smectic layer compression seems to be rather high, while it seems to be rather small for the copolymers. [less ▲]

Generation of frustrated liquid crystal phases by mixing an achiral n–smc mesogen with an antiferroelectric chiral smectic liquid crystal

in Journal of Chemical Physics (2005), 122(14), 144906

By mixing the achiral liquid crystal HOAB, exhibiting a nematic (N)-smectic-C (SmC) mesophase sequence, with the chiral antiferroelectric liquid crystal (AFLC) (S,S)-M7BBM7, forming the antiferroelectric ... [more ▼]

By mixing the achiral liquid crystal HOAB, exhibiting a nematic (N)-smectic-C (SmC) mesophase sequence, with the chiral antiferroelectric liquid crystal (AFLC) (S,S)-M7BBM7, forming the antiferroelectric SmCa phase, at least seven different mesophases have been induced which neither component forms on its own: a twist-grain-boundary (TGB) phase, two or three blue phases, the untilted SmA phase, as well as all three chiral smectic-C-type ``subphases,'' SmCalpha, SmCbeta, and SmCgamma. The nature of the induced phases and the transitions between them were determined by means of optical and electro-optical investigations, dielectric spectroscopy, and differential scanning calorimetry. The induced phases can to a large extent be understood as a result of frustration, TGB at the border between nematic and smectic, the subphases between syn and anticlinic tilted smectic organization. X ray scattering experiments reveal that the smectic layer spacing as well as the degree of smectic order is relatively constant in the whole mixture composition range in which AFLC behavior prevails, whereas both these parameters rapidly decrease as the amount of HOAB is increased to such an extent that no other smectic-C-type phase than SmC/SmC exists. By tailoring the composition we are able to produce liquid crystal mixtures exhibiting unusual phase sequences, e.g., with a direct isotropic-SmCa transition or a temperature range of the SmCbeta subphase of about 50 K. (C) 2005 American Institute of Physics. [less ▲]

A chameleon chiral polar liquid crystal: Rod-shaped when nematic, bent-shaped when smectic

in Chemistry of Materials (2004), 16(19), 3606-3615

The first antiferroelectric liquid crystal (AFLC) exhibiting a (chiral) nematic phase, a combination which has long been the goal of synthetic chemists working with polar liquid crystals but which at the ... [more ▼]

The first antiferroelectric liquid crystal (AFLC) exhibiting a (chiral) nematic phase, a combination which has long been the goal of synthetic chemists working with polar liquid crystals but which at the same time represents a fundamental contradiction in terms of translational order, was recently reported by Nishiyama and co-workers. We have investi- gated this chiral twin dimer by optic, electrooptic, and dielectric methods and conclude that it is not an ordinary AFLC material, but one where the peculiar properties of bent-core smectics are combined with those of ordinary rod-shaped liquid crystals. The compound exhibits a new type of nematic-smectic phase transition, connected with a change of molecule conformation from rod- to bent-shaped. This also has an important impact on the chiral interactions in the system. Toward the high-temperature end of the smectic phase, the energy balance between bent conformation smectic and straight conformation nematic can be shifted by an electric field such that the transition to the nematic phase with stretched-out molecules can be field-induced. [less ▲]