No description available.

No description available.

Chiral nematic thin films containing responsive dopants have previously been considered as sensors and dosimeters. Some systems have been developed, with mixed results. The investigation so far has been piecemeal, with little attention paid towards understanding the properties that govern how these sensors operate. By carefully considering these properties, we believe that it is possible to predict the effectiveness of a sensor, and therefore a framework can be established which would allow only sensors with significant potential to be investigated. The applications of a variety of chiral nematic thin films incorporating responsive dopants is modelled and then investigated experimentally. The systems chosen for investigation were selected using the criteria of commercial potential and synthetic simplicity. In each case it was found that the behaviour of these systems could be predicted well from simple experimental data. The systems chosen for investigation were judged on the basis of two key criteria: They must be systems that deal with a target that is of commercial interest, so that performance expectations for 'real-life' applications can be given at the start of the research process. They must be manufactured only using materials and processes that are commercially available. These limitations were chosen to highlight the conditions under which such a model would be expected to operate. In this work, a successful model that predicts sensors behaviour from basic experimental information was developed and shown to match well with experimental results. In the process, three new sensing systems were developed, all with potential commercial significance. Additionally, some observations regarding the methods of operation which are most likely to result in high sensitivity and low cross-sensitivity have been made.

No description available.

This thesis comprises an account of research carried out into the flexoelectro-optic effect, as observed in chiral nematic liquid crystals, and its potential for application in fibre optic communications components. The flexoelectro-optic effect provides a mechanism of fast, analogue rotation of the optic axis in chiral nematic materials via the application of an electric field to the sample. In particular, bimesogenic liquid crystal materials exhibit very large flexoelectro-optic tilt angles, and a large tilt angle per unit field in comparison to other mesogenic materials. In this work a new geometry for the flexoelectro-optic effect is developed in which the chiral nematic liquid crystal is aligned with its helical axis along the normal to the cell walls and the electric field is applied in the plane of the cell. It is shown that polymer stabilization of this device by the addition of a small percentage of reactive mesogen to mixture increases greatly the ability of the device to withstand high amplitude a.c. electric fields. Applied fields of up to 6.8 V/μm are shown to induce a maximum birefringence of ∆n=0.037, due to both flexoelectric and dielectric coupling, and ∆n=0.012 due to flexoelectric coupling only in a sample based on symmetric difluorinated bimesogens. This induced birefringence is shown to consistently respond to field application and removal on the sub millisecond timescale. Polymer stabilization of the same mixtures in the uniform lying helix texture is shown to affect the electro-optic response of the samples in a manner which is dependent on the concentration of reactive mesogen used, and the temperature at which the reactive mesogen is cured. A concentration of approximately 3% weight/weight, however, has little detrimental impact on the device characteristics, and curing of the sample at the lower end of the chiral nematic temperature range is shown to allow optimization of both tilt angle and response time of the samples. The effect is also employed to demonstrate a new method of fast electrical tuning of the output wavelength from chiral nematic photonic band edge lasers. An 8nm shift was induced in these devices by a 3.5 V/μm applied field.

Chiral nematic liquid crystals or cholesteric liquid crystals (CLC) can be obtained by adding a chiral dopant into a nematic liquid crystal. Liquid crystal molecules spontaneously rotate along a long axis to form helical structures in CLC system. Both pitch size and orientation of the helical structure is determined by the boundary conditions and can be further tuned by external stimuli. Particularly, the uniform lying helical structure of CLC has attracted intensive attention due to its beam steering and diffraction abilities. Up to now, studies have worked on controlling the in-plane orientation of lying helix through surface rubbing and external stimuli. However, it remains challenging to achieve steady and uniform lying helical structure due to its higher energy, comparing with other helical configurations. Here, by varying the surface anchoring, uniform lying helical structure with long-range order is achieved as thermodynamically stable state without external support. Poly (6-(4-methoxy-azobenzene-4'-oxy) hexyl methacrylate) (PMMAZO), a liquid crystalline polymer, is deposited onto the silicon substrate to fine-tune the surface anchoring. By changing the grafting density of PMMAZO, both pitch size and orientation of lying helical structure are precisely controlled. As the grafting density increases, the enhanced titled deformation of helical structure suppresses the pitch size of CLC at the same cell thickness; as the cell thickness increases, the morphology transition from long-range order stripe to small fingerprint domain is facilitated.

No description available.

Les nanocristaux de cellulose (NCC), obtenus par hydrolyse acide des microfibrilles de cellulose native sous forme de suspensions colloïdales aqueuses, sont des nanoparticules biosourcées ayant des propriétés mécaniques et optiques particulièrement séduisantes pour la conception de nanomatériaux à haute performance. Pour éliminer certaines de leurs limitations comme la sensibilité au sel et l'absence de contrôle de leurs interactions, nous nous sommes attachés au cours de ce travail à modifier chimiquement ces NCC par greffage de chaînes de polymère thermosensible de la famille de polyétheramines Jeffamine® sur leur surface. La première méthode de greffage utilisée faisait appel à une oxydation TEMPO suivie d'un couplage peptidique. Après l'optimisation de la réaction de greffage, les nouveaux systèmes (NCC-g-Jeffamine) ont été caractérisés et de nouvelles propriétés ont été identifiées notamment leur insensibilité au sel, leur capacité de redispersion dans des solvants organiques, leur caractère tensioactif, leur stabilité thermique améliorée ainsi que leur thermoagrégation réversible. L'étude structurale par diffusion des neutrons aux petits angles a permis d'avoir des informations sur les épaisseurs des couches de polymère greffé ainsi que sur la conformation des chaînes. De nouvelles caractéristiques de l'auto-organisation en phase chirale-nématique de ces nouveaux systèmes ont été notées concernant leur diagramme de phase et leur pas cholestériques. La deuxième méthode de greffage explorée consistait en une oxydation aux ions métapériodate suivie d'une amination réductrice. Les résultats ont montré un détachement partiel des chaînes de cellulose suite à l'oxydation puis leur détachement complet après le greffage du polymère pour obtenir des copolymères cellulose-Jeffamine ayant éventuellement des propriétés intéressantes à étudier. / Cellulose nanocrystals (CNC), obtained by acid hydrolysis of native cellulose microfibrils as colloidal aquous suspensions, are bioresourced nanoparticles that have great mechanical and optical properties well adapted for the conception of new nanomaterials with high performance. In order to eliminate some of their limitations like sensitivity to salt and absence of interactions control, we studied in this work the chemical modification of these CNC by grafting thermoresponsive polymer chains (polyetheramines Jeffamine®) on the their surface. A first grafting strategy used was a TEMPO oxidation followed by peptidic coupling. After the optimization of the grafting reaction, the new systems (CNC-g-Jeffamine) were characterized and new properties were identified particularly their insensitivity to salts, their ability to be redispersed in organic solvents, their surfactant character, their enhanced thermal stability and their reversible thermoagregation. The structural study using small angles neutrons scattering gave us information about thicknesses of the polymer corona and the chains conformation. New characteristics of their self-assembly into chiral-nematic phases were noticed more particularly concerning phase diagrams and cholesteric pitches. The second grafting strategy consisted in an oxidation using periodate ion followed by reductive amination. Results showed a partial detachment of cellulose chains after oxidation. A complete detachment was observed after grafting the polymer chains to finally obtain cellulose-Jeffamine copolymers which could have interesting properties to study.

Cellulose is the most abundant biopolymer on Earth as it is found in every plant cell wall; therefore, it represents one of the most promising natural resources for the fabrication of sustainable materials. In plants, cellulose is mainly used for structural integrity, however, some species organise cellulose in helicoidal nano-architectures generating strong iridescent colours. Recent research has shown that cellulose nanocrystals, CNCs, isolated from natural fibres, can spontaneously self-assemble into architectures that resemble the one producing colouration in plants. Therefore, CNCs are an ideal candidate for the development of new photonic materials that can find use to substitute conventional pigments, which are often harmful to humans and to the environment. However, various obstacles still prevent a widespread use of cellulose-based photonic structures. For instance, while the CNC films can display a wide range of colours, a precise control of the optical appearance is still difficult to achieve. The intrinsic low thermal stability and brittleness of cellulose-based films strongly limit their use as photonic pigments at the industrial scale. Moreover, it is challenging to integrate them into composites to obtain further functionality while preserving their optical response. In this thesis, I present a series of research contributions that make progress towards addressing these challenges. First, I use an external magnetic field to tune the CNC films scattering response. Then, I demonstrate how it is possible to tailor the optical appearance and the mechanical properties of the films as well as to enhance their functionality, by combining CNCs with other polymers. Finally, I study the thermal properties of CNC films to improve the retention of the helicoidal arrangement at high temperatures and to explore the potential use of this material in industrial fabrication processes, such as hot-melt extrusion.