Une étude détaillée des matériaux à base de polymères et cristaux liquides, préparés par le processus de séparation de phases induite par rayonnement ultraviolet a été effectué pour des mélanges comprenant un cristal liquide nématique de la famille des cyanobiphényls, un monomère acrylique et un photoamorceur. Ces matériaux possèdent des fonctionnalités électro-optiques intéressantes notamment dans le domaine des vitrages à transparence contrôlée. Une deuxième méthode de préparation consiste à élaborer un gel chimique de polymère-cristal liquide qui est constitué par un réseau de polymère gonflé dans un solvant de type cristal liquide. La synthèse, le gonflement et la structure de plusieurs gels de polymères (de type cristal liquide) ont fait l'objet d'une étude intensive. Ces gels sont potentiellement utiles dans divers domaines tels que les écrans d'affichage ou les muscles artificiels et constituent une nouvelle catégorie de gels intelligents. L'influence de la méthode de préparation et de la composition des systèmes polymère/cristal liquide a été étudiée à l'aide de plusieurs techniques expérimentales et en particulier par spectrocopies infrarouge, diélectrique, mécanique. calorimétrie différentielle, et par microscopie optique à lumière polarisée. / A detailed study of materials based on polymers and Iiquid crystals, prepared by the process of phase separation by 1ultraviolet radiation. was made for mixtures incIuding a nematic liquid crystal of the family of cyanobiphenyls, an acrylic monomer and a photoinitiator. These materials possess interesting electro-optical features particularly in the field of smart windows. The second method of preparation of these materials consists in elaborating a chemical gel of polymer / liquid crystal which is established by a polymer network swollen in a solvent of liquid crystal type. The synthesis, the swelling and the structure of several polymer gels in liquid crystals were the object of an extensive study. These gels are potentially useful in several domains such as display screens or artificial muscles. The influence of the method of preparation and the composition of the polymer liquid crystal systems was studied by means of several experimental teclmiques and in particular by infrared, dielectric, and mechanical spectrocopies. differential scanning calorimetry, and by opticaI microscopy with polarized light.

Dans le langage courant, le terme « gel »réfère à un grand nombre de matériaux qui ont des comportements intermédiaires entre solides et liquides. Ils présentent en général une faible contrainte seuil, et peuvent aussi bien être mous et ductiles que durs et cassants. Le terme "gel" fait donc d'abord référence à un type de comportement macroscopique. A l'échelle microscopique, la matière dont ils sont formés, colloïdes ou polymères, est suspendue et diluée dans un liquide, qui assure un caractère essentiellement incompressible à l'ensemble. Les polymères, comme les colloïdes, sont soumis à l'agitation thermique qui leur confère une certaine capacité à se réorganiser. Cela permet à certains gels de s'écouler comme des liquides. Mais par définition, les gels présentent aussi une contrainte seuil qui leur permet d'être stables en l'absence de sollicitation mécanique : cela est possible parce que les colloïdes ou polymères qui les composent ont tendance à s'attacher entre eux de sorte à former une structure en réseau. Il est donc essentiel que ces éléments soient sujet à des interactions attractives, qui permettent la formation d'un réseau faible, mais mécaniquement stable, au sein du fluide porteur. L'objectif de cette thèse est de construire des outils numériques de type dynamique moléculaire pour étudier la formation et le comportement mécanique de gels. Pour cela, nous utilisons un modèle de ``particules à patchs'' tiré de la littérature. Les particules sont des objets de forme sphérique à la surface desquels sont greffés des sites ponctuels, les « patchs » qui s'attirent entre eux. Les interactions entre particules combinent une répulsion centrale aux efforts entre patchs. En considérant ces particules comme des « briques élémentaires » qui constituent des polymères, nous avons construit et caractérisé un modèle de polymères linéaires semi-flexibles de rigidité contrôlée. Puis, nous avons étudié la cinétique de réticulation de gels chimiques ainsi que sa réponse mécanique quand celui-ci est soumis à une déformation constante. Nous avons ainsi pu mettre en évidence que la topologie du réseau dépend très sensiblement des conditions de réticulation, et joue un rôle déterminant dans la réponse élastique du matériau. Ce même modèle de particules à patchs a aussi été utilisé pour construire un modèle de gels physiques colloïdaux, dont l'étude est restée cependant préliminaire / In the current language, the term “gel” refers to a large number of materials that have intermediate behavior between solid and liquid. They generally have a low yield stress, and can be soft and ductile as well as hard and brittle. So, the term "gel" refers primarily to a type of macroscopic behavior. At the microscopic scale, these materials are formed by small elements, colloids or polymers, suspended in a liquid, which ensures a global incompressibility. Colloids, as well as polymers, are subject to thermal motion which gives them a certain ability to reorganize. This allows some gels to flow like liquids. Yet, by definition, gels also exhibit a yield stress which allows them to be stable in the absence of mechanical stress: this is possible because the colloids or polymers that compose them tend to stick together and form a network structure. It is thus essential that these elements develop mutually attractive interactions that are responsible for the formation of a weak, yet mechanically stable, network within the suspending fluid. The objective of this thesis is to build numerical tools, based on molecular dynamics, to study the formation and the mechanical behavior of gels. For this, we use a model of ``patchy'' particles taken from the literature. It consists of spherical objects at the surface of which are grafted small patches that attract each other. The interaction between patchy particles thus combine a central repulsion with attractive potentials between patches. Considering these particles as ``building blocks '' that form polymers, we have built and characterized a model of semi-flexible linear polymers of controlled rigidity; we have then studied the crosslinking kinetics of chemical gels and their mechanical (elastic) response under a constant strain. We were able to show that the network topology depends significantly of crosslinking conditions, and determines quite significantly the elastic response of the material. A preliminary study of colloïdal physical gels is also presented in the last part of the document

The role of hydrophobic substituents on the gelation mechanism of highly esterified pectin and the cellulose derivatives methylcellulose and hydroxypropylmethylcellulose (HPMC) has been explored by monitoring the behaviour of the amphiphilic polysaccharides in varying combinations of an ethylene glycoVwater solvent. The gelling ability (mechanical spectroscopy, visual observation) of very highly esterified (- 100%) pectin in high concentrations of ethylene glycol (>60%) is greatly reduced, however, the polymer still undergoes conformational ordering (CD, OR). A model for gel formation involving a two stage process has been proposed, comprising adoption of the ordered structure stabilised by hydrogen bonding between OH groups of contiguous polysaccharide chains, followed by (or coincident with) aggregation of the ordered sequences by 'hydrophobic' clustering of the fundamental structural subunits to form the three dimensional gel network. It has been found that ethylene glycol promotes the fIrst stage (ordering) but is antagonistic to the second (aggregation). The reversibility (mechanical spectroscopy) of the thermo-gelling cellulose derivatives can be largely abolished in the presence of ethylene glycol (40% for methylcellulose, 10% for HPMC), attributed to solubilisation of the proposed ordered 'bundle' structure at low temperatures removing the enthalpic advantage (DSC) of gel melting. The increased sensitivity of HPMC to modification of the solvent environment is due to the presence of the polar hydroxypropyl substituent causing an inceptive destabilisation of the 'bundle' structure. It is suggested that gelation is driven by the entropic advantage of melting-out 'cages' of structured water surrounding the hydrophobic groups giving rise to intermolecular 'hydrophobic' aggregation.

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Thesis (Ph. D.)--University of Washington, 1990. / Vita. Includes bibliographical references (leaves [287]-307).

Gels are used in many applications ranging from bioengineering and pharmaceuticals to food technology and soft-robotics because of their tunable physical and mechanical properties. In many of these applications, the materials need to sustain large deformation. The microstructure of gels changes significantly at large strain values, causing a deviation in the stress responses from that at low strain. The desired mechanical responses of gels can be obtained by tuning their microstructure, therefore, the structure-property relationship for gels is required to be understood for their practical applications. This dissertation discusses two types of gels, one consists of chemical crosslinking and hydrophobic associations, and the other gel only consists of physical crosslinking. The microstructure of these two gel systems is investigated and related to their mechanical responses. The gel system with chemical and physical crosslinking mimics properties of biomaterials like resilin. Resilin is a protein-elastomer that enables biological species for power amplified activities by taking benefits of specific responses of hydrophilic and hydrophobic segments. Inspired by the microstructure and mechanical properties of resilin, a stretchable and resilient hydrogel was synthesized through a simple free radical polymerization technique. These gels retract from the stretched state to the original state with high speed over a short time, such behavior has not been frequently reported for synthetic hydrogels. This gel is also capable of performing a power-amplified activity like catapulting an object. In addition to retraction experiments, the mechanical properties of this gel were investigated in tensile and cyclic loading to determine their resilience. The hydrophobic polymer concentration affects the swelling behavior and mechanical responses such as stretchability and resilience. The second gel system considered here is a physically assembled ABA triblock copolymer dissolved in a B-selective solvent. Here, two different triblock copolymers with different concentrations were utilized. The real-time microstructural change was captured using a RheoSAXS setup with a high flux X-ray beam. The real-time microstructure of these gels subjected to temperature, varying oscillatory strain amplitude, and during relaxation after step strain was captured. This dissertation advances the understanding of the structure-property relationship of microstructurally complex gels towards their potential practical applications.

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