Polyelectrolyte complexes and multilayers assembled from biopolymers : electrostatically driven formation of mesoscale structures
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- Institutt for fysikk 
Oppositely charged polyelectrolytes interact spontaneously upon mixing. This interaction can be exploited to form polyelectrolyte complexes and polyelectrolyte multilayers. In the latter case the interaction takes place at a surface thus restricting the degrees of freedom as compared to the formation of polyelectrolyte complexes in dilute solution. In this study the morphologies of polyelectrolyte complexes and multilayers assembled from biopolymers were studied by atomic force microscopy (AFM) imaging. Different biopolymers of varying chain stiffness were chosen to investigate the influence of chain stiffness on the structures formed. Experiments showed that the chain stiffness of the polyanion is a key parameter in determining the morphology of polyelectrolyte complexes, generalising the compaction behaviour of DNA to be determined by its polyelectrolyte properties. Polyelectrolyte complexes in solution form toroidal structures, depending on the chain stiffness. In particular, the toroidal dimension was found to be determined by the chain flexibility. Using AFM to take snapshots of the collapse pathway, it was furthermore found that temperature treatment of complexes can be used to drive them toward the energetically most stable state. The morphology representing the most stable state was found to depend on the specific system, identified to be toroids for xanthan-chitosan and short rods for plasmid DNA-chitosan. Employing an ethidium bromide fluorescence assay, the stability of compacted DNA towards disruption by competing polyanions was found to increase following annealing, illustrating that temperature might be an additional parameter to investigate when designing drug delivery systems. Although driven by electrostatic interactions, the exact structure formed was influenced by polymer-specific details. Furthermore, release of DNA from its compacted form could not be seen as the reverse process of compaction. The morphology of both polyelectrolyte complexes and the surface of the multilayers varied with the combination of polyanion and polycation employed in the assembly process. When used to prepare polyelectrolyte multilayers, these relatively stiff polyelectrolytes formed network-like surfaces, observed already after deposition of one polycation-polyanion bilayer. The surface morphology of multilayers obtained when alginate was employed in combination with poly- L-lysine depended on the alginate composition. By the choice of different polyanion/polycation combinations as well as deposition conditions, the surface roughness can be tuned, which has implications for the biocompatibility of the surfaces. The polyelectrolyte multilayers were additionally characterised using dual wavelength reflection interference microscopy (DW-RICM). This microinterferometric method has previously been used to study monolayers. DW-RICM was here verified as an accurate method for measuring thicknesses of multilayers. However, while thickness measurements were trustworthy, more work is needed to solve problems related to determination of the interfacial potential. The thickness of the chitosan-xanthan and chitosan-algiante multilayers depended on the ionic strength during preparation, and growth of chitosan-xanthan multilayers was not sustained when the multilayers were assembled from low ionic strength. The response of the multilayers to changes in salt concentration after preparation could be explained from the polyelectrolyte properties of the polymers. Furthermore, difference in modes of reaction between alginate and chitosan as compared to alginate and poly-L-lysine was important during the formation of multilayers. The observed mesoscale structures formed when the biopolymers were assembled into complexes in dilute solution or multilayers share the importance of electrostatics in the assembly process. Furthermore, molecular parameters such as the chain stiffness influence the morphology of the resulting structures.