João P. Canejo

Self-winding of helices in plant tendrils and cellulose liquid crystal fibers

Passiflora edulis, like other climbing plants, possesses long, tender, soft, curly and flexible organs called tendrils whose circumnutation allows the plant to find support. Tendrils curl into spirals or twist into a helix, often of one handedness over half of its length and of the opposite handedness over the other half, the two halves being connected by a short straight section – a perversion, depending on whether they are supported at just one end or supported at both ends, respectively. This is a consequence of an intrinsic curvature of the tendrils. We report on liquid crystalline cellulosic fibers and jets, which mimic the shapes of helical tendril structures. Liquid crystalline and isotropic cellulosic precursor solutions of curved and straight fibers are examined using nuclear magnetic resonance imaging (MRI) and polarizing optical microscopy (POM) techniques to determine morphological and structural features contributing to fiber curvature. We study the subtle physical mechanisms responsible for self-winding behavior as a result of the intrinsic curvature due to the non-uniform deformation of filaments. In the case of liquid-crystalline cellulosic fibers, this is due to a core of disclination forming off-axis along the filament. We also highlight the critical dependence of the helical structures on temperature, which offers a potential for direct fabrication of biocompatible tunable high-surface area membranes with mechanical adaptability.

Hierarchical wrinkling on elastomeric Janus spheres

Hierarchical wrinkling on elastomeric Janus spheres is permanently imprinted by swelling, for different lengths of time, followed by drying the particles in an appropriate solvent. First-order buckling with a spatial periodicity (λ11) of the order of a few microns and hierarchical structures comprising of 2nd order buckling with a spatial periodicity (λ12) of the order of hundreds of nanometers have been obtained. The 2nd order buckling features result from a Grinfeld surface instability due to the diffusion of the solvent and the presence of sol molecules.

Cellulose‐Based Biomimetics and Their Applications

Nature has been producing cellulose since long before man walked the surface of the earth. Millions of years of natural design and testing have resulted in cellulose-based structures that are an inspiration for the production of synthetic materials based on cellulose with properties that can mimic natural designs, functions, and properties. Here, five sections describe cellulose-based materials with characteristics that are inspired by gratings that exist on the petals of the plants, structurally colored materials, helical filaments produced by plants, water-responsive materials in plants, and environmental stimuli-responsive tissues found in insects and plants. The synthetic cellulose-based materials described herein are in the form of fibers and films. Fascinating multifunctional materials are prepared from cellulose-based liquid crystals and from composite cellulosic materials that combine functionality with structural performance. Future and recent applications are outlined.

Cellulosic liquid crystals for films and fibers

ABSTRACT Cellulose, the most abundant natural polymer on earth, is used in numerous applications in our day-to-day life. However, the discovery that cellulose-based systems could lead to the formation of liquid crystalline phases only dates to the 1970s. Compared with all known applications of cellulose, the liquid crystalline behavior has been less considered. Associated with this are the low solubility of cellulose and the existence of a chiral nematic precursor solution and its processing under the action of a shear field, which is used to produce fibers and films. In this review, we first conduct a short review of the main features of cellulosic liquid crystalline phases including the main textures observed by polarizing optical microscopy and the cholesteric phase characteristics of thermotropic and lyotropic systems observed for cellulose and cellulose derivatives. Then, we focus on the rheological properties of liquid crystalline solutions and special attention is given to the formation of striations developed during shear and the formation of the band texture, which appears during the relaxation process. Among the different techniques used, special emphasis is given to the results obtained by coupling rheology with optical microscopy (Rheo-optics) and nuclear magnetic resonance (Rheo-NMR) techniques. Some examples described in the literature, related to the use of cellulose and cellulose derivatives liquid crystals to the production of structural color scaffolds, stimuli-responsive films and fibers, are addressed. In these systems, the initial cholesteric phase determines the unique properties exhibited by the films and the fibers produced from cellulosic liquid crystalline systems.

Functional Materials from Liquid Crystalline Cellulose Derivatives: Synthetic Routes, Characterization and Applications

Cellulose is a linear syndiotactic homopolymer composed of d-anhydroglucopyranose units which are linked by β-(1 → 4)-glycosidic bonds. The primary and secondary free hydroxyl groups, which decorate the polysaccharide chains, can undergo chemical substitution given rise to a high range of cellulose derivatives. It is well known that cellulose derivatives are at the origin of films and fibers, which characteristics can be diverse if prepared from liquid crystalline phases.

Liquid fibres and their networks from cellulose-based liquid crystalline solutions

ABSTRACT The production of fibres and films with enhanced mechanical properties, from liquid crystalline cellulose-based systems, has always been a challenge. Previous works indicate that the use of spinning and electrospinning allows the fabrication of non-woven membranes with optical and mechanical characteristics distinct from casting films. The subsequent interactions of the micro/nanofibres inside membranes can modify their topology and geometry so they are found crucial for tuning the membranes’ properties. In this work, we deal with the evolution of networks made of highly stretched liquid filaments. Three main mechanisms were identified: the thinning of filaments feeding growing nodes, breaking of the thinner filaments before the thicker ones and the zipping of pairs of filaments crossing at small angles. Graphical Abstract

Elastomeric patterns probed by a nematic liquid crystal

GRAPHICAL ABSTRACT ABSTRACT Soft Janus elastomers have two surfaces with diverse characteristics. In this work, by tuning the chemical composition and the surface roughness we were able to vary the wettability of thin films (thickness of 100–200 μm) and spheres (diameters in the order of 200 μm to 2 mm) and evidence the multifunction of the opposite sides. We also describe a simple and inexpensive method to reveal the wrinkled-labyrinthine patterns that appear in the Janus particles by means of a nematic liquid crystal (LC). LC contact angle measurements associated with the swelling and anchoring characteristics of the surfaces were used to image the Janus particles opening new platforms for sensor applications from flexible free-standing LCs containers.