Susete N. Fernandes

A cellulose liquid crystal motor: a steam engine of the second kind

The salient feature of liquid crystal elastomers and networks is strong coupling between orientational order and mechanical strain. Orientational order can be changed by a wide variety of stimuli, including the presence of moisture. Changes in the orientation of constituents give rise to stresses and strains, which result in changes in sample shape. We have utilized this effect to build soft cellulose-based motor driven by humidity. The motor consists of a circular loop of cellulose film, which passes over two wheels. When humid air is present near one of the wheels on one side of the film, with drier air elsewhere, rotation of the wheels results. As the wheels rotate, the humid film dries. The motor runs so long as the difference in humidity is maintained. Our cellulose liquid crystal motor thus extracts mechanical work from a difference in humidity.

Cellulose-based liquid crystalline photoresponsive films with tunable surface wettability.

We report on a new type of liquid crystalline cellulosic films with light controllable reversible wettability. The films are prepared from a thermotropic cellulose derivative functionalized with azo-containing groups. These groups exhibit dynamic changes in interfacial properties in response to UV irradiation. The UV irradiation induces trans-to-cis isomerization in the azobenzene moiety, which causes a conformational change in the upper molecular layers of the thin films. These changes originate a hydrophobic to comparatively hydrophilic transformation of the surface. The reversible wettability of the surface results from the cis/trans photo and thermal isomerization. The UV-vis absorption spectra, as well as contact angle measurements with UV irradiation, clearly support the understanding of the phenomenon. This type of surface design enables the amplification of molecular level conformational transitions to macroscopic changes in interface properties using the means of isomerism. This opens new opportunities in surface engineering using eco-friendly cellulose manipulation.

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.

1H–2H Cross-Relaxation Study in a Partially Deuterated Nematic Liquid Crystal

A detailed study of the cross-relaxation effects between the ¹H and ²H spins systems is presented in the nematic phase of a 5-cyanobiphenyl (5CB) liquid crystal, partially deuterated at α position (5CB-αd₂). The proton spin-lattice relaxation time was measured at a frequency range from 5 kHz to 100 MHz at a temperature 5 K below the nematic-isotropic phase transition. In the low frequency domain, the spin-lattice relaxation rate (T₁⁻¹) dispersion clearly differs from that of the fully protonated 5CB homologue. At two distinct frequencies, T₁⁻¹ presents two distinct local maxima and for low frequencies T₁⁻¹ presents a stronger frequency dependence when compared with what is observed for 5CB. The T₁⁻¹ dispersion obtained for 5CB-αd₂ for frequencies above 60 kHz was interpreted in terms of the relaxation mechanisms usually accepted to interpret the spin-lattice relaxation in nematic phases in general and 5CB in particular. For lower frequencies it was necessary to consider cross-relaxation contributions between the proton and deuterium reservoirs. A detailed model interpretation of the deuterium quadrupolar dips with respect to the proton-spin relaxation is presented. The analysis of the quadrupolar relaxation independently confirms that the order director fluctuations is the dominant mechanism of proton relaxation in the low frequency domain.

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.

Revealing the Hierarchical Mechanical Strength of Single Cellulose Acetate Electrospun Filaments through Ultrasonic Breakage.

The tensile strength of single cellulose acetate electrospun fibers is determined through sonication-induced fragmentation in water using a model previously developed by Terentjev and co-workers. The fragmentation of the electrospun fibers results in a gradual shortening of their length until a constant modal length is achieved. A single electrospun CA fiber tensile strength of ≈ 150 MPa (55-280 MPa) is determined based on fracture statistics. It is also observed that the fragmented fibers show bunches of nanofilaments at their ends with similar diameters to those of round structures observed in the cross-section of the initial electrospun fibers (≈ 38 nm). The sonication of these nanofilaments gives rise to spherical particles with similar diameter dimensions, which allows the estimation of a value of the tensile strength of the order of 2 MPa for these nanostructures. The aggregation and the alignment of the nano filaments inside the electrospun fiber should be the source of its higher strength value.

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

Functional Stimuli-Responsive Gels: Hydrogels and Microgels

One strategy that has gained much attention in the last decades is the understanding and further mimicking of structures and behaviours found in nature, as inspiration to develop materials with additional functionalities. This review presents recent advances in stimuli-responsive gels with emphasis on functional hydrogels and microgels. The first part of the review highlights the high impact of stimuli-responsive hydrogels in materials science. From macro to micro scale, the review also collects the most recent studies on the preparation of hybrid polymeric microgels composed of a nanoparticle (able to respond to external stimuli), encapsulated or grown into a stimuli-responsive matrix (microgel). This combination gave rise to interesting multi-responsive functional microgels and paved a new path for the preparation of multi-stimuli “smart” systems. Finally, special attention is focused on a new generation of functional stimuli-responsive polymer hydrogels able to self-shape (shape-memory) and/or self-repair. This last functionality could be considered as the closing loop for smart polymeric gels.

Twisted, 10–12 May 2017, Luxembourg

The conference ‘Twisted’, held in Luxembourg from 10th to 12th of May 2017, was organised at the University of Luxembourg, Campus Limpertsberg, in BâtimentdesSciences surrounded by a beautiful and inspiring green garden with magnificent trees, with Prof. Jan Lagerwall and Prof. Tanja Schilling as chairs. Twisted aimed to bring together scientists working on different areas, but interested in the same precise subject. It was the first in what might become a series of conferences that hopefully will be repeated in the future, possibly with different but always concise names, which can include ‘Bent’ and ‘Splayed’, as warmly suggested by Prof. Gray at the beginning of his presentation. Twisted provided an international forum to enhance the exchange of scientific ideas and technological advances in the field of physics, chemistry and applications of cholesteric lyotropic liquid crystals (LCs) developing in colloidal suspensions of chiral nanorods. The importance of Twisted was translated in the international attendance of participants from most European countries, USA, Canada and Japan. The final programme included one satellite workshop, four keynote talks, eight invited talks, twelve oral presentations and twelve posters. The keynote lectures were given by Prof. Mark MacLachlan (University of British Columbia, Canada), Prof. Mark Wilson (Durham University, England), Prof. Derek Gray (Department of Chemistry, McGill University, Canada) and Dr Silvia Vignolini (University of Cambridge, UK). The speakers were selected due to the high significance of their work on LCs from chiral nanorods. The conference started with a satellite workshop (see Figure 1), organised by Dr Christina Schütz from the Experimental Soft Matter Physics group, University of Luxembourg. The workshop was attended by several participants and some stimulating questions were discussed regarding the production and characterisation of cellulose nanocrystals (CNCs), and the self-assembly of cellulose nanorods. The welcome dinner, hosted at the Brasserie du Cercle in the heart of Luxembourg city, presented a unique panoramic view over the Place d’Armes and a choice of regional dishes. The dinner started with some welcome words by Prof. Jan Lagerwall and Prof. Tanja Schilling. The fraternisation amongst the attendees evolved as dinner progressed and an animated atmosphere could be sensed throughout the evening (Figure 2). The vice president of the University of Luxembourg Prof. Tonie van Dam launched the conference. Twisted was organised into four main sessions where experimental and theoretical work were nicely coupled and balanced together; Session 1 – Advanced materials derived from chiral nanorods; Session 2 – Theory and simulations of cholesteric phases; Session 3 – Chiral nanorod suspensions: from particle tuning to self-assembly; and Session 4 – The route to applications (Figure 3). Under the theme ‘Advanced materials derived from chiral nanorods’, the recent advances on the applications of CNCs templating and the mesoporous silica photonic materials obtained were addressed. Some examples of plant filaments with intrinsic curvature and bio-inspired photonic materials from cellulose were presented. The helicoidal self-assembly of CNCs within spherical droplets was used as a toolbox to raise interesting questions related to the obtained pitch vs. CNC concentration and kinetic arrest either in micro droplets or in films. The possibility to use commercial CNC to obtain LC phase was highlighted. Emphasis was given to functionalised-CNC with inorganics and polymers and the possibility to use these systems as nanoscale templates with different chirality. Recent advances on the magnetic alignment of CNCs in suspension and 3D structures were shown. The control of final microstructure and properties of solid materials was addressed based on its phase and rheological behaviour. The topic ‘Theory and simulations of cholesteric phases’ led to interesting discussions on the spontaneous formation of chiral systems from achiral molecules and the phase transitions in mixtures of chiral and achiral LC molecules under an external field, as well as of cholesteric phases from flexible particles (twisted origamis). Questions were raised regarding the need of chirality to obtain twisted molecular organisations, and the percolation observed in the nematic LIQUID CRYSTALS TODAY, 2017 VOL. 26, NO. 3, 59–62 https://doi.org/10.1080/1358314X.2017.1359408