Publication

Polyelectrolyte complex formation at defined interfaces and structure formation in solution studied by analytical ultracentrifugation

Laurent Bourdillon
2006
Thèse EPFL
Résumé

The interactions and structural formation of polyelectrolytes in solution and environmental stimuli-responsive water-soluble polymers are subjects of intense academic research, development, and application. An increasing number of novel materials are being created by synthesizing new molecules but also by assembly of existing molecules via intermolecular interactions. Such materials have found practical applications in cell and drug immobilization, environment protection, or sophisticated analytics and sensors. The reproducible synthesis, well-defined assembly, and full understanding of the molecular impact on the application behaviour require knowledge of structure-property relationships. These needs have motivated the selection of the research performed in this thesis. To establish such relationships and to gain more insight on the molecular level, powerful characterization methods must be employed. In this context, analytical ultracentrifugation was used. On the one hand, conventional analytical ultracentrifugation techniques were applied to complicated molecular characterization. On the other hand, analytical ultracentrifugation principles were extended to problems not studied before with this technique. Specifically, the following problems were addressed with polyelectrolytes as primary polymeric substances: Online study of hydrogel formation by interaction of oppositely charged polyelectrolytes using a modified synthetic boundary experiment Characterization of polydisperse branched high molar mass charged copolymers Molecular assembly of di-block copolymers containing one polyelectrolyte block Temperature-induced concentration dependent phase transition in aqueous solution. Overall, results were obtained from analytical ultracentrifugation experiments which were not accessible by other characterization techniques. Systematic online studies of hydrogel formation at the interface of aqueous solutions of oppositely charged polyelectrolytes revealed the impact of chemical structure, concentration, molar mass, and pH on the network quality of three combinations of polyanions and polycations: sodium alginate/oligochitosan, sodium alginate/poly(L-lysine), and sodium cellulose sulfate/poly(diallyldimethyl ammonium chloride). These combinations are of practical interest. The results are expected to contribute to the optimization of network properties for specific applications in medicine, biotechnology, and pharmacy. High molar mass polyelectrolytes, which were polydispersed with regard to molar mass, charge density, and chain architecture were characterized by hydrodynamic methods, particularly analytical ultracentrifugation. By combining synthetic boundary and sedimentation velocity experiments at various speeds, the degree of homogeneity of the various distributions of the samples were identified. The correlation of molecular characteristics and application properties corresponded to predictions from molecular simulations. Sedimentation velocity studies revealed concentration- and temperature-dependent aggregation of stimuli-responsive pre-associated polymers. The monotonic increase of the sedimentation coefficient with concentration and temperature confirmed pre-association prior to the phase separation. The aggregation process was proven to be completely reversible. Moreover, absorbance concentration profiles identified a temperature-dependent portion of polymer not participating in the aggregation process. Sedimentation equilibrium experiments allowed the estimation of the influence of concentration on the molar mass.

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Concepts associés (35)
Centrifugation
La centrifugation est un procédé de séparation des composés d'un mélange en fonction de leur différence de densité en les soumettant à une force centrifuge. Le mélange à séparer peut être constitué soit de deux phases liquides, soit de particules solides en suspension dans un fluide. L'appareil utilisé est une machine tournante à grande vitesse appelée centrifugeuse. Cette technique ne fait pas partie des opérations unitaires en génie des procédés. Un des principaux appareils qui sert à la centrifugation est la centrifugeuse.
Tamis moléculaire
Un tamis moléculaire est un minéral à base de silice ayant une structure cristalline tridimensionnelle présentant des cavités et des canaux dont les surfaces peuvent adsorber les petites molécules. C'est un solide poreux qui a la propriété d'agir comme un tamis à l'échelle moléculaire. Il s'agit d'une classe d'adsorbant qui a la capacité de retenir certaines molécules à l'intérieur de ses pores. Dans l'idéal, il possède des pores de petite taille distribués de manière homogène. Il a de ce fait une grande surface spécifique.
Solution (chimie)
Une solution, en chimie, est un mélange homogène (constitué d'une seule phase) résultant de la dissolution d'un ou plusieurs soluté(s) (espèce chimique dissoute) dans un solvant. Les molécules (ou les ions) de soluté sont alors solvatées et dispersées dans le solvant. La solution liquide est l'exemple le plus connu. Une solution ayant l'eau comme solvant est appelée solution aqueuse. Il est possible de mettre en solution : un liquide dans un autre : limité par la miscibilité des deux liquides ; un solide dans un liquide : limité par la solubilité du solide dans le solvant, au-delà de laquelle le solide n'est plus dissous.
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