Mineral liquid crystalline polymers

NIR transmittance tuneability under a magnetic field of colloidal suspensions of goethite (α-FeOOH) nanorods

Goethite (α-FeOOH) nanorods were synthesized and their size and shape were controlled by synthesis parameters. Stable colloidal suspensions were prepared and their transmittance in NIR range was tuned by modifying magnetic field direction and strength.

Self-assembled hierarchically structured organic-inorganic composite systems

Designing bio-inspired, multifunctional organic-inorganic composite materials is one of the most popular current research objectives. Due to the high complexity of biocomposite structures found in nacre and bone, for example, a one-pot scalable and versatile synthesis approach addressing structural key features of biominerals and affording bio-inspired, multifunctional organic-inorganic composites with advanced physical properties is highly challenging. This article reviews recent progress in synthesizing organic-inorganic composite materials via various self-assembly techniques and in this context highlights a recently developed bio-inspired synthesis concept for the fabrication of hierarchically structured, organic-inorganic composite materials. This one-step self-organization concept based on simultaneous liquid crystal formation of anisotropic inorganic nanoparticles and a functional liquid crystalline polymer turned out to be simple, fast, scalable and versatile, leading to various (multi-)functional composite materials, which exhibit hierarchical structuring over several length scales. Consequently, this synthesis approach is relevant for further progress and scientific breakthrough in the research field of bio-inspired and biomimetic materials.

Morphology control in biphasic hybrid systems of semiconducting materials

Simple blends of inorganic nanocrystals and organic (semiconducting) polymers usually lead to macroscopic segregation. Thus, such blends typically exhibit inferior properties than expected. To overcome the problem of segregation, polymer coated nanocrystals (nanocomposites) have been developed. Such nanocomposites are highly miscible within the polymer matrix. In this Review, a summary of synthetic approaches to achieve stable nanocomposites in a semiconducting polymer matrix is presented. Furthermore, a theoretical background as well as an overview concerning morphology control of inorganic NCs in polymer matrices are provided. In addition, the morphologic behavior of highly anisotropic nanoparticles (i.e. liquid crystalline phase formation of nanorod-composites) and branched nanoparticles (spatial orientation of tetrapods) is described. Finally, the morphology requirements for the application of inorganic/organic hybrid systems in light emitting diodes and solar cells are discussed, and potential solutions to achieve the required morphologies are provided.

Hierarchically structured vanadium pentoxide-polymer hybrid materials

Biomimetic composite materials consisting of vanadium pentoxide (V2O5) and a liquid crystal (LC) "gluing" polymer were manufactured exhibiting six structural levels of hierarchy, formed through LC phases. The organic matrix was a polyoxazoline with pendant cholesteryl and carboxyl units, forming a lyotropic phase with the same structural orientation extending up to hundreds of micrometers upon shearing, and binding to V2O5 via hydrogen bridges. Composites consisting of V2O5-LC polymer hybrid fibers with a pronounced layered structuring were obtained. The V2O5-LC polymer hybrid fibers consist of aligned V2O5 ribbons, composed of self-assembled V2O5 sheets, encasing a chiral nematic polymer matrix. The structures of the V2O5-LC polymer composites strongly depend on the preparation method, i.e., the phase-transfer method from aqueous to organic medium, in which the polymer forms LC phases. Notably, highly defined micro- and nanostructures were obtained when initiating the synthesis using V2O5 tactoids with preoriented nanoparticle building units, even when using isotropic V2O5 dispersions. Shear-induced hierarchical structuring of the composites was observed, as characterized from the millimeter and micrometer down to the nanometer length scales using complementary optical and electron microscopy, SAXS, μCT, and mechanical nanoindentation.

Aqueous liquid crystals of graphene oxide

The formation of liquid crystals (LCs) is the most viable approach to produce macroscopic, periodic self-assembled materials from oriented graphene sheets. Herein, we have discovered that well-soluble and single-layered graphene oxide (GO) sheets can exhibit nematic liquid crystallinity in water and first established their isotropic-nematic solid phase diagram versus mass fraction and salt concentration. The zeta potential of GO dispersion is around -64 mV, and its absolute value decreases with increasing salt concentration, implying that the electrostatic repulsive force between negatively charged GO sheets is the dominant interaction in the system of GOLCs and also explaining the salt-dependent phase behavior. For single-layer GO sheets with average diameter of 2.1 μm and polydispersity index of 83%, the isotropic-nematic phase transition occurs at a mass concentration of ∼0.025%, and a stable nematic phase forms at ∼0.5%. Rheological measurements showed that GO aqueous dispersions performed as typical shear flows and confirmed the isotropic-nematic transition. The ordering of GO sheets in aqueous dispersions and the solid state is demonstrated by the characterizations of polarized-light optical microscopy, small-angle X-ray scattering, scanning electron microscopy, and transmission electron microscopy. The direct, real-time fluorescent inspections by confocal laser microscopy further reveal that the individually dispersed fluorescent GO sheets align with orientational directions along their long axes. These novel findings shed light on the phase behaviors of diversely topological graphenes and lay the foundation for fabrication of long-range, ordered nano-objects and macroscopically assembled graphene-based functional materials.

Lyotropic liquid crystalline self-assembly in dispersions of silver nanowires and nanoparticles

We report demixed nematic lyotropic liquid crystalline phase formation in dispersions of silver nanowires and spherical nanoparticle aggregates in ethylene glycol and water. This phase is observed in samples in spite of the high density, large aspect ratio, and long relaxation times of the nanowires which have an average length of 6.8 microm. Remarkably, in the biphasic region, the nanowire-rich liquid crystalline phase exhibits a strandlike morphology which has only previously been reported for single-walled carbon nanotube liquid crystals. Shearing predominantly liquid crystalline dispersions results in both significant nanowire alignment and nanowire-aggregate demixing. The results of this research suggest that the nanoparticle contaminants common to many synthesis schemes facilitate liquid crystalline phase formation and that these dispersions can be processed into aligned coatings.

Lamellar phase in colloidal suspensions of positively charged LDHs platelets

The lamellar phase in colloidal suspensions of positively charged platelets of layered double hydroxides (LDHs) was studied with synchrotron small angle X-ray scattering and optical observations.

Phase diagrams of Wyoming Na-montmorillonite clay. Influence of particle anisotropy

Natural Na-Wyoming montmorillonite was size fractionated by successive centrifugation. Polydisperse particles with average sizes of 400, 290, and 75 nm were then obtained. As the structural charge of the particles belonging to three fractions (determined by cationic exchange capacity measurements) is the same, such a procedure allows studying the effect of particle anisotropy on the colloidal phase behavior of swelling clay particles. Osmotic stress experiments were carried out at different ionic strengths. The osmotic pressure curves display a plateau whose beginning systematically coincides with the sol/gel transition determined by oscillatory stress measurements. The concentration corresponding to the sol/gel transition increases linearly with particle anisotropy, which shows that the sol/gel transition is not directly related to an isotropic/nematic transition of individual clay particles. Indeed, a reverse evolution should be observed for an I/N transition involving the individual clay particles. Still, when observed between crossed polarizer and analyzer, the gel samples exhibit permanent birefringent textures, whereas in the "sol" region, transient birefringence is observed when the samples are sheared. This suggests that interacting clay particles are amenable to generate, at rest and/or under shear, large anisotropic particle associations.

Physical properties of aqueous suspensions of goethite (alpha-FeOOH) nanorods. Part II: In the nematic phase

At volume fractions larger than 8.5%, aqueous suspensions of lath-like goethite (alpha-FeOOH) nanorods form a lyotropic nematic phase. In this article, we first discuss the nematic ordering within statistical-physics models of the isotropic/nematic phase transition. We then describe the influence of a magnetic field on the nematic phase. Because the nanorods bear permanent magnetic moments, the nematic suspensions have dipolar order and very low Frederiks thresholds. Moreover, the nematic phase aligns parallel to a small magnetic field but realigns perpendicular to a high field because of a competition between the permanent moments of the nanorods and their negative anisotropy of magnetic susceptibility. This magneto-optical study of the nematic phase is completely consistent with that of the isotropic phase of the same suspensions published in Part I (this issue, p. 291). Besides, we demonstrate the field-induced biaxiality of a nematic single domain aligned perpendicular to the field. We also describe here preliminary experiments where an a.c. electric field is applied to the nematic phase. Both field amplitude and frequency were found to control the alignment direction and homeotropic-to-planar alignment transitions were observed. From this data, simple models were used to estimate some physical constants of the nematic phase.

Physical properties of aqueous suspensions of goethite (alpha-FeOOH) nanorods. Part I: In the isotropic phase

Depending on volume fraction, aqueous suspensions of goethite (alpha-FeOOH) nanorods form a liquid-crystalline nematic phase (above 8.5%) or an isotropic liquid phase (below 5.5%). In this article, we investigate by small-angle X-ray scattering, magneto-optics, and magnetometry the influence of a magnetic field on the isotropic phase. After a brief description of the synthesis and characterisation of the goethite nanorod suspensions, we show that the disordered phase becomes very anisotropic under a magnetic field that aligns the particles. Moreover, we observe that the nanorods align parallel to a small field (< 350 mT), but realign perpendicular to a large enough field (> 350 mT). This phenomenon is interpreted as due to the competition between the influence of the nanorod permanent magnetic moment and a negative anisotropy of magnetic susceptibility. Our interpretation is supported by the behaviour of the suspensions in an alternating magnetic field. Finally, we propose a model that explains all experimental observations in a consistent way.