Colloids from oppositely charged polymers: reversibility and surface activity

The research described in this thesis concerns the formation, solution properties, and adsorption of polyelectrolyte complexes composed of at least one diblock copolymer with a neutral and a charged block and either an oppositely charged homopolyelectrolyte or a diblock copolymer, with a neutral block and an oppositely charged polyelectrolyte block. Upon mixing the aqueous solutions of the different polymers, the oppositely charged polyelectrolytes associate, forming a polyelectrolyte complex. Polyelectrolyte complex micelles – called complex coacervate core micelles (C3Ms) in this thesis – are the main focus of this thesis, but the formation of smaller aggregates, soluble complex particles, is also investigated. The salt concentration, pH, and the chemical structure of the polyelectrolytes are important variables in the formation of these polyelectrolyte complexes. In chapter 2 C3Ms were made from multiple polymer species; a diblock copolymer with a polyelectrolyte block and a neutral block, poly(acrylic acid)-block-poly(acryl amide), an oppositely charged polyelectrolyte, poly(N,N-dimethyl aminoethylamide), and a second diblock copolymer species with a charged block and a neutral block, poly(N,N-dimethyl aminoethylamide)-block-poly(glyceryl methacrylate). The polyelectrolyte block of the second diblock copolymer species had charged blocks that were oppositely charged to that of the first diblock copolymer species and whose neutral block was different from that of the first diblock copolymer. The effect of systematically varying the ratio of the homopolyelectrolyte and second diblock copolymer (based on the number of chargeable groups), while keeping the mixing fraction f+ (that is the number of positively chargeable groups, divided by the total number of chargeable groups) constant, was studied with light scattering. It was shown that the size of the resulting C3Ms decreased with increasing percentage of the second diblock copolymer, from 25 nm hydrodynamic radius, to 16 nm. Using a simple geometrical model and the light scattering intensities, the aggregation numbers were estimated to be in the range of 20-70 polymers. In chapter 3 the used diblock copolymer, poly([4-(2-aminoethylthio)-butylene] hydrochloride)-block-poly(ethylene oxide), has a polyelectrolyte part with a rather hydrophobic backbone which slows down the formation of the aggregates and the subsequent rearrangements. It was mixed with the oppositely charged poly(acrylic acid). Using light scattering and cryogenic transmission electron microscopy, it was shown that the complexes formed at f+ = 0.3 are initially very large (> 140 nm) and network like (as there is relatively little neutral polymer to stop the growth of the complexes), and rearrange relatively quickly, compared to the complexes formed at f+ = 0.5 and 0.7 (80 nm), towards small micellar complexes. The very large transient complexes formed at f+ = 0.3 are called highly aggregated polyelectrolyte complexes (HAPECs). The complexes formed at f+ = 0.5 are apparently most stable; that is, their size remains the same in time. It was concluded that there are at least three factors which influence the rearrangement rate of polyelectrolyte complexes; (1) high neutral blocks content, (2) excess charge, and (3) the chemistry of the polyelectrolytes. Increasing the salt concentration has previously been determined to speed up the rate of rearrangements as well. Furthermore, the radius of the complexes at f+ = 0.5 (80nm) is too large for the complexes to have the typical core-corona structure. Apparently, these large complexes are HAPECs as well. However, with different preparation procedures micelles can be obtained; if the HAPECs are forced to disassemble by changing the pH to an extreme value (either 11 or 3) and are subsequently re-assembled by changing the pH back to normal (7), the resulting C3Ms have a radius of about 15 nm. This is probably the state of minimum free energy, the stable state, whereas the highly aggregated complexes are in a metastable state (as they do not spontaneously rearrange in time). In chapter 4 complex coacervate core micro-emulsions (C3-μEs) were obtained by mixing solutions of anionic polyelectrolytes (poly(acrylic acid)) and diblock copolymers with an anionic polyelectrolyte block and a neutral block (poly(acrylic acid)-block-poly(acryl amide)) with solutions of a cationic polyelectrolyte (poly(N,N-dimethyl aminoethylamide)). By varying the fraction of the anionic polyelectrolyte and anionic diblock copolymer species, while keeping f+ constant, C3-μEs with radii varying from about 15 to 100 nm were prepared. Basically, these are C3Ms of which the core is swollen with extra polyelectrolyte complex, composed of oppositely charged homopolyelectrolytes. The solvent was shown to have a pronounced effect upon the size of the obtained complexes; in NaNO3 larger complexes were obtained which are in a metastable state. In phosphate buffer (a salt known to weaken the attractive forces between the used polyelectrolytes), smaller complexes were obtained, which are probably in the stable state. The geometrical model introduced in chapter 2 was extended and predicted a linear growth of the C3-μEs. The experimentally observed growth was however, non-linear, probably due to a transition of the neutral polymers in the corona from more star-like to more crew-cut behaviour (shown by self consistent field calculations). In chapter 5 the ability of a layer of adsorbed C3Ms with a more glass-like core (composed of poly([4-(2-aminoethylthio)-butylene] hydrochloride)-block-poly(ethylene oxide) and poly([4-(2-carboxy-ethylthio)-butylene] sodium salt)-block-poly(ethylene oxide)), to prevent protein adsorption to either silica or cross-linked 1,2 polybutadiene was investigated. With atomic force microscopy it was shown that the layer consists of closely packed adsorbed complex coacervate core micelles. Protein adsorption to the coated surfaces was generally reduced by > 80 %. The different forces and many variable parameters of the investigated system cause the time scales on which SCPs and C3Ms rearrange to span a very wide range; they can be both reversible and irreversible systems.