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. <br/

On the stability of (highly aggregated) polyelectrolyte complexes containing a charged block-neutral diblockcopolymer

Using light scattering and cryogenic transmission electron microscopy, we show that highly aggregated polyelectrolyte complexes (HAPECs) composed of poly([4-(2-aminoethylthio)butylene] hydrochloride)49-block-poly(ethylene oxide)212 and poly(acrylic acid) (PAA) of varying lengths (140, 160, and 2000 monomeric units) are metastable or unstable if the method of preparation is direct mixing of two solutions containing the oppositely charged components. The stability of the resulting HAPECs decreases with decreasing neutral-block content and with increasing deviation from 1:1 mixing (expressed in number of chargeable groups) of the oppositely charged polyelectrolytes, most probably for electrostatic reasons. The difference between the metastable and stable states, obtained with pH titrations, increases with increasing PAA length and increasing pH mismatch between the two solutions with the oppositely charged component

Comparison of complex coacervate core micelles from two diblock copolymers or a single diblock copolymer with a polyelectrolyte.

With light scattering titrations, we show that complex coacervate core micelles (C3Ms) form from a diblock copolymer with a polyelectrolyte block and either an oppositely charged polyelectrolyte, a diblock copolymer with an oppositely charged polyelectrolyte or a mixture of the two. The effect of added salt and pH on both types of C3Ms is investigated. The hydrodynamic radius of mixed C3Ms can be controlled by varying the percentage of oppositely charged polyelectrolyte or diblock copolymer. A simple core-shell model is used to interpret the results from light scattering, giving the same trends as the experiments for both the hydrodynamic radii and the relative scattering intensities. Temperature has only a small effect on the C3Ms. Isothermal titration calorimetry shows that the complexation is mainly driven by Coulombic attraction and by the entropy gain due to counterion releas

Assembly of polyelectrolyte-containing block copolymers in aqueous media

In this review we present an overview of the developments of (self-)assembly of linear block copolymers containing one or more polyelectrolyte blocks in aqueous solution. Different micellar structures and phase behaviour are described. The role of inter- and intramolecular complex coacervation is emphasised. Recent developments in applications of assembly of polyelectrolyte-containing copolymers are presented

Reduction of protein adsorption to a solid surface by a coating composed of polymeric micelles with a glass-like core

Adsorption studies by optical reflectometry show that complex coacervate core micelles (C3Ms) composed of poly([4-(2-amino-ethylthio)-butylene] hydrochloride)49-block-poly(ethylene oxide)212 and poly([4-(2-carboxy-ethylthio)-butylene] sodium salt)47-block-poly(ethylene oxide)212 adsorb in equal amounts to both silica and cross-linked 1,2-polybutadiene (PB). The C3Ms have an almost glass-like core and atomic force microscopy of a dried layer of adsorbed C3Ms shows densely packed flattened spheres on silica, which very probably are adsorbed C3Ms. Experiments were performed with different types of surfaces, solvents, and proteins; bare silica and cross-linked 1,2-PB, NaNO3 and phosphate buffer, and lysozyme, bovine serum albumin, ß-lactoglobulin, and fibrinogen. On the hydrophilic surface the coating reduces protein adsorption >90% in 0.1 M phosphate buffer, whereas the reduction on the coated hydrophobic surface is much lower. Reduction is better in phosphate buffer than in NaNO3, except for the positively charged lysozyme, where the effect is reversed

Internal structure of a thin film of mixed polymeric micelles on a solid/liquid interface

The adsorption of mixed micelles of poly(4-(2-amino hydrochloride-ethylthio)-butylene)-block-poly(ethylene oxide), PAETB49-b-PEO212 and poly(4-(2-sodium carboxylate-ethylthio)-butylene)-block-poly(ethylene oxide), PCETB47-b-PEO212 on solid/liquid interfaces has been studied with light, X-ray, and neutron reflectometry. The structure of the adsorbed layer can be described with a two-layer model consisting of an inner layer formed by the coacervate of the polyelectrolyte blocks PAETB49 and PCETB47 (~1 nm) and an outer layer of PEO212 blocks (~6 nm). The micelles unfold upon adsorption forming a rather homogeneous flat layer that exposes its polyethylene oxide chains into the solution, thus rendering the surface antifouling after modification with the micelles

Complex coacervate core micro-emulsions

Complex coacervate core micelles form in aqueous solutions from poly(acrylic acid)-block-poly(acrylamide) (PAAxPAAmy, x and y denote degree of polymerization) and poly(N,N-dimethyl aminoethyl methacrylate) (PDMAEMA150) around the stoichiometric charge ratio of the two components. The hydrodynamic radius, Rh, can be increased by adding oppositely charged homopolyelectrolytes, PAA140 and PDMAEMA150, at the stoichiometric charge ratio. Mixing the components in NaNO3 gives particles in highly aggregated metastable states, whose Rh remain unchanged (less than 5% deviation) for at least 1 month. The Rh increases more strongly with increasing addition of oppositely charged homopolyelectrolytes than is predicted by a geometrical packing model, which relates surface and volume of the particles. Preparation in a phosphate buffer ¿ known to weaken the electrostatic interactions between PAA and PDMAEMA ¿ yields swollen particles called complex coacervate core micro-emulsions (C3-Es) whose Rh increase is close to that predicted by the model. These are believed to be in the stable state (lowest free energy). A two-regime increase in Rh is observed, which is attributed to a transition from more star-like to crew-cut-like, as shown by self-consistent field calculations. Varying the length of the neutral and polyelectrolyte block in electrophoretic mobility measurements shows that for long neutral blocks (PAA26PAAm405 and PAA39PAAm381) the -potential is nearly zero. For shorter neutral blocks the -potential is around -10 mV. This shows that the C3-Es have excess charge, which can be almost completely screened by long enough neutral blocks