Réticulation

Résumé

En chimie des polymères, la réticulation correspond à la formation d'un ou de plusieurs réseaux tridimensionnels, par voie chimique ou physique. Des liaisons chimiques (appelées ponts) entre les chaînes macromoléculaires sont créées. Les structures réticulées sont généralement préparées à partir de pré-polymères linéaires ou ramifiés de faible masse molaire (issus d'une polymérisation partielle), réticulés sous l'action de la chaleur en présence d'un catalyseur/durcisseur (agent réticulant). L'UICPA définit une réticulation comme une petite région d'une macromolécule d'où sortent au moins quatre chaînes, et qui est formée par des réactions entre des sites ou des groupes chimiques aux molécules existantes, ou bien par des interactions entre des macromolécules existantes. Présentation La réticulation d'un polymère peut être réalisée par génération in situ de radicaux résultant de la décomposition thermique d'un peroxyde. La vulcanisation d'un élastomère (insaturé) possédant

Source officielle

https://fr.wikipedia.org/wiki/R%C3%A9ticulation

À propos de ce résultat

Cette page est générée automatiquement et peut contenir des informations qui ne sont pas correctes, complètes, à jour ou pertinentes par rapport à votre recherche. Il en va de même pour toutes les autres pages de ce site. Veillez à vérifier les informations auprès des sources officielles de l'EPFL.

Publications associées (100)

Publications associées

Chargement

Personnes associées (20)

Personnes associées

Chargement

Unités associées (15)

Unités associées

Chargement

Concepts associés (61)

Concepts associés

Chargement

Cours associés (51)

Cours associés

Chargement

Séances de cours associées (109)

Afficher plus

Séances de cours associées

Chargement

Design of High Permittivity Polysiloxane Elastomers for Dielectric Elastomer Transducers

Philip Horst Caspari

Dielectric elastomer transducers consist of a dielectric elastomer between two compliant electrodes. When the elastic capacitor is electrically charged, the dielectric elastomer elongates until a balance of elastic and electrostatic force is reached. Electrical energy is converted into mechanical work. Contrary, in the generator mode, the capacitor is stretched by an outside mechanical force. The mechanical work is converted into electrical energy due to the change of capacitance. The need of a high voltage power supply is the main obstacle for the elastomer transducer technology in various applications. Siloxane elastomers are the most frequently tested elastomers with excellent mechanical stability, processability and electrically insulating properties. It is well-known that an increase in permittivity of the silicone elastomer would allow a reduction of the driving voltage of the elastic transducer. Hence, manifold research activities have been initiated to enhance the permittivity of siloxane elastomers. In the present work, siloxane elastomers were developed that combined increased permittivity with high mechanical stability and cost-efficient processability of the siloxane materials. The synthesis of polar thiols as the first step of siloxane functionalization was studied. Serious challenges in the synthesis of polar groups that can be integrated by thiol-ene addition as side group of polysiloxanes were found. Therefore, commercially available alkylthiols were selected for the functionalization of siloxane due to their chemical stability. The alkylthiols were grafted onto the polysiloxane by thiol-ene addition and subsequently cross-linked in the well-established organotin-catalyzed condensation reaction. The impact of different types of cross-linker on the electro-mechanical properties of the polar siloxane elastomers was analyzed. A significant improvement in the performance of actuator test devices was demonstrated. In addition, the electromechanical stability of the polar siloxane elastomer was illustrated by 50.000 actuation cycles. However, solvent had to be used in the production process of the siloxane elastomers and the cross-linking reaction time was about two days. Apart from their application in thin film actuators, these polar siloxane elastomers were studied with respect to their performance in dielectric elastomer generators together with siloxane composites. Nanospring carbon nanotubes were blended as high permittivity fillers in siloxanes subsequent to cross-linking. Indeed, improved performances of the high permittivity silicone elastomers were found and a deeper understanding for the design of dielectric elastomers for generator applications was gained. The most significant result of this work was the development of polar siloxane elastomers that could be processed into thin film elastomers without any solvent within a few minutes. Two different synthetic approaches were developed based on low viscosity mixtures of cross-linkable oligo(ethylthioethyl)(methyl)siloxanes and multifunctional thiol cross-linkers. The cross-linking reaction was selectively initiated by UV-induced radical formation. These siloxane elastomers possessed an increased permittivity and showed a high electromechanical stability.

EPFL2019

Chargement

Tissue Engineering points to maintain, restore or to improve and create new tissues. This purpose typically involves the selection of a natural or synthetic biomaterial as a scaffold to create the biological substitutes. Furthermore, angiogenesis, the growth of new blood vessels from pre-existing ones, is a critical physiological process toward the success of tissue engineering. Along with this consideration, biomaterials that possess intrinsic angiogenic potential may be better candidates for tissue engineering. In the present report, we investigated the effects on the in vitro CAM assay (Folkman et al., A simple procedure for the long-term cultivation of chicken embryos, 1974; Auerbach et al., Angiogenesis assays: a critical overview, 2003) of hyaluronic acid (HA). HA derivatives were formed by reaction of the carboxylic acid groups of the soluble natural polymer with hydrazides, via carbodiimide-mediated coupling. This intermediate product was then further reacted with a cyclic thioimidate (Traut's reagent) compound for thiolation. We herein describe a physiologically compatible strategy to prepare HA hydrogels able to i) support drug delivery, and ii) support potential in situ encapsulation of cells. In this strategy, modified hyaluronic acid was cross-linked with itself and 6% (w:v) HA hydrogels were formed under physiological conditions by air oxidation of thiols to disulfides. According to a LIVE/DEAD assay, HA hydrogel showed to support cell growth and spreading. The biomaterial scaffold was loaded with plasmid and a DNA release assay was performed. However hydrogels swollen in proteases containing media indicated that only 10% of the integrated DNA remained inside after 48hours. The angiogenic properties of the hydrogel were investigated on the chicken chorioallantoic membrane assay. In addition, we also examined the involvement of plasmid Vascular Endothelial Growth Factor (VEGF) on the CAM by integrating the signal into the hydrogel. Angiogenesis was observed around the implanted biomaterial. In addition, fluorescence microscopy of perfused animal indicated the presence of blood vessels sprouting into the hydrogel matrices. We anticipate that this HA-based hydrogel is highly tunable (Shea et al., Non-viral vector delivery from PEG-hyaluronic acid hydrogels, 2007). In addition, hydrogels with this capability for localized delivery of gene therapy vectors and controllable stability have direct applications for tissue engineering and could be useful tool for several biomedical applications (Hubbell et al., Functional biomaterials design of novel biomaterials, 2001)

2008

Chargement

A new non-toxic chemically cross-linked ELP hydrogel for in situ protein delivery

Karine Ciapala

Proteins play an essential role in all biological processes and have been attracting more and more attention as drug candidates due to their high specificity and activity. However, their use as therapeutics has been seriously held back given that their ability to be effectively and conveniently delivered is problematic. Indeed, simple oral administration is ineffective since proteins are degraded in the digestive tract. Until now, these therapeutic agents were delivered by infusions or frequent injections. But this causes serious problems, as the compound does not get delivered in a prolonged continuous manner. Additionally, injections are also problematic as patients are reluctant to use needles unless there illness is life threatening. That is why focus has grown over the last years on developing novel delivery systems for proteins. Amongst the various types of delivery systems that exist, hydrogel based ones are very promising, as they possess remarkable biomimetic properties. The hydrogels that are chemically cross-linked are stronger than their counterparts, the physical hydrogels. An appealing approach to protein delivery that is more and more sought is to form the hydrogel in situ from an injected aqueous polymer solution and to use the formed gel as a depot for the sustained release of the protein. Oxygen sensitive gellation is an interesting stimulus to form these gels, but has not been successfully developed, as the gelling kinetics are too slow. Therefore, research groups have turned to the use of toxic chemicals to speed up the reaction. Elastin-like poylpeptides (ELP) are artificial biopolymers that are composed of a pentapeptide repeat. This repeat consists of Val-Pro-Gly-Xaa-Gly, where Xaa denotes a guest residue. An interesting characteristic of ELPs is that they are thermally responsive. Meaning, that when in solution they go through a lower critical solution temperature (LCST) transition. Below their LCST they are soluble in water and above their LCST they are insoluble. ELPs are well suited for drug delivery applications for many reasons such as their biocompatibility. The ELP that is studied is ELP9 Gel. Its guest residues are Val, Ala and Cys in a ratio of 1:14:1 respectively. As this ELP has cysteines it has the capacity to form a gel via cross-linking of the thiols groups by oxidation. The rheological tests that were carried out showed that this ELP9 Gel has all the characteristics of a gel. Furthermore, its gelling kinetics were greatly increased by heating it, but still remain too long for in situ applications. This suggests that the LCST behaviour of ELPs seems to play some role in favouring the cross-linking. But further controls remain to be done to confirm this hypothesized tandem process. A delivery strategy that involved this ELP9 Gel and Blue Fluorescent Proteins (BFP) with added cysteines was not successful as the molecular biology to obtain the modified proteins failed. However, the elaboration of a fusion peptide between Glucagon-Like-Peptide-1 (GLP-1) and ELP9 Gel was accomplished. The preliminary activity assay showed a reduced activity in comparison to the GLP-1 peptide alone. Unfortunately this ELP9 Gel has a setback due to its slow gelling time. Therefore, before continuing the research on the delivery strategies, it would be wiser to create a better ELP. If a new ELP with a higher cysteine concentration and lower transition temperature can be created, its gelling kinetics may be favourable for in situ protein delivery applications and thus, a chemically cross-linked hydrogel that does not require any toxic cross-linker would be available. This would be a big step in creating a very robust hydrogel for protein delivery applications but also for tissue regeneration

2009

Chargement

Design of High Permittivity Polysiloxane Elastomers for Dielectric Elastomer Transducers

Philip Horst Caspari

EPFL2019

2008

A new non-toxic chemically cross-linked ELP hydrogel for in situ protein delivery

Karine Ciapala

2009