Adsorption of polyelectrolytes and charged block copolymers on oxides consequences for colloidal stability

The aim of the study described in this thesis was to examine the adsorption properties of polyelectrolytes and charged block copolymers on oxides, and the effect of these polymers on the colloidal stability of oxidic dispersions. For this purpose the interaction of some well-characterised polyelectrolytes and block copolymers with oxidic substrates has been systematically studied. A set of block copolymers with one charged block and one neutral water-soluble block had to be synthesised because this type of block copolymers was not commercially available. These block copolymers were prepared by anionic polymerisation by Dr. Arnold's group in Halle (Germany).

In order to measure the amount of polymer adsorbed as a function of several experimental parameters (pH, ionic strength, type of polymer, type of substrate) we used a reflectometer equipped with a stagnation-point flow-cell. With this optical technique the adsorbed amount on an (optically flat) solid substrate is measured. This technique is also suited to follow the kinetics of the adsorption process. Information about the amount of charge in the adsorbed layer was obtained from streaming potential measurements (on flat surfaces, Ch. 5) and electrophoresis (on particles, Ch. 6). The effect of polymer on the colloidal stability of oxidic dispersions was probed by measuring the changes in the optical transmission with time (Ch. 5).

In Chapter 1 it is explained that in many applications there is a need to control the colloidal stability of oxidic dispersions, and it is described how in general the stability can be affected by polymer addition. Also, the aim, scope and outline of this study are given.

In the following three chapters we present the adsorption properties of homopolyelectrolytes (Chs. 2 and 3) and of charged block copolymers (Ch. 4) on oxides. The homopolyelectrolytes used were two polymers with a constant charge (quaternised polyvinyl pyridine, PVP ⁺, and quaternised polydimethylaminoethyl methacrylate, AMA ⁺) and one with a pH-dependent charge (polydimethylaminoethyl methacrylate, AMA). The block copolymers consisted of a charged block (AMA) and a water-soluble neutral one (dihydroxypropyl methacrylate, HMA). As the substrates we used both silicon oxide (SiO ₂ .), which is acidic in nature and the amphoteric titanium dioxide (TiO ₂ ) .

In Chapter 2 the rate of adsorption and the final adsorbed amount of the homopolyelectrolytes are studied as a function of pH and ionic strength. The initial adsorption rate is found to be equal to the rate with which the polymer molecules arrive to the surface; hence, the transport of molecules from the bulk of the solution to the surface is rate-limiting. Above a certain coverage the adsorption rate goes down, indicating that the already adsorbed molecules form a kinetic barrier for further adsorption.

The adsorbed amount at saturation depends on pH and ionic strength. For both PVP ⁺and AMA ⁺a monotonic increase in the adsorbed amount is observed with increasing pH, since the surface gets more negatively charged. For AMA, which has a pH-dependent charge, a maximum in the adsorbed amount is found when the pH, and, thus the polymer charge, is varied.

When the adsorbed charge of polyelectrolyte is compared to the bare surface charge, as determined from titration experiments, it is found that the net adsorbed charge exceeds by far the net bare surface charge (overcompensation). Part of the surplus of charge is neutralised by adjustment of the pH-dependent charges on the surface and on the polymer.

In Chapter 3 the reversibility of the adsorption of PVP and AMA is studied by taking the system out of equilibrium, e.g., by a change of the pH or the free polymer concentration. Then, the subsequent relaxation is followed. In case of complete reversibility, the adsorbed amount of the relaxed system should be equal to that for direct adsorption on a bare surface under these conditions. Also, the reversibility was investigated by examining the exchange of adsorbed polyelectrolyte molecules by molecules from the solution.

The experiments indicate that the adsorbed layer was never fully relaxed. Therefore, we conclude that the experimental systems are only partly reversible on the timescale of the experiments (30 min.). Presumably, the reorganisation of molecules in the adsorbed layer is rather slow, because of the strong (electrostatic) bond between polyelectrolyte and surface. A model for the structure of the adsorbed layer of strongly charged polyelectrolytes, allowing little reconformation, is proposed. In this model the molecules adsorb as isolated chains on the surface. These islands repel mutually, thereby forming a heterogeneous layer. An indication for the existence of empty spaces between the molecules comes from the observation that after adsorption of the polyelectrolyte neutral polymer molecules can attach to the surface.

In Chapter 4 the synthesis and characterisation of the AMA-HMA block copolymers are described. Next, the adsorption properties of the block copolymers are studied as a function of pH, ionic strength and block length ratio. A maximum in the adsorbed amount is observed when the composition of the block copolymer is varied, similar to the maximum found for homopolyelectrolytes upon variation of the segment charge (see Ch. 2). Also, the adsorbed amount follows the same trends with pH and ionic strength as does the homopolyelectrolyte AMA; almost no effect of the presence of the neutral block on the adsorption data can be detected. From these facts we Infer that the charged block in the block copolymer anchors on the surface. Probably, the neutral block can also adsorb, since charged molecules usually do not cover the surface completely, because they repel each other (see Ch. 3). Only at high adsorbed amounts the surface becomes so densely populated that the neutral block is forced into the solution, thereby forming an extended layer which may provide steric stabilisation.

In Chapter 5 we discuss the effect of addition of charged (block co)polymers on the stability of oxidic dispersions. We note that the same type of polymer can both stabilise and destabilise a dispersion. For example, a low dosage of strongly charged (say, cationic) polymer molecules to a dispersion of oppositely charged (anionic) particles may cause mosaic flocculation, whereby bare (negatively charged) and covered (positively charged) parts on neighbouring particles attract each other. When the polymer is very long bridging flocculation could also occur whereby one polymer molecule adsorbs on two particles at the same time. When, however, the dosage Is high enough to saturate the particles, the dispersion may be stabilised sterically or electrostatically, depending on the thickness of the steric layer and on the amount of charge on the particles.

As illustrated above, the effect of the polymers depends critically on the polymer charge, on the dosage and on the molar mass of the polymer. In Ch. 5 a set of requirements could be formulated to optimise the performance of the polymers for stabilisation or flocculation.

Finally, in Chapter 6 we describe the formation and stability of multilayers of polyelectrolytes. Since charge reversal occurs upon adsorption of a strongly charged (say, cationic) polyelectrolyte (see Ch. 2), an oppositely charged (anionic) polymer molecule is attracted to such a covered surface. Therefore, when cationic and anionic polymers are supplied in alternating order to a solid substrate, multilayers can be formed.

The multilayer build-up is characterised by a step-wise Increase of the adsorbed amount and the layer thickness, and by alternatingly highly positive and highly negative values for the ζ-potential. The stability of the multilayer is shown to depend strongly on the polymer charges and the ionic strength and, hence, on the electrostatic interaction between the polymers involved. When this interaction Is weak no stable multilayers form, but polycations and polyanions form complexes at the surface which then may desorb. For pairs of strongly interacting polymers, which formed very stable multilayers, the charge stoichiometry could be studied. This charge stoichiometry, which was not always 1 : 1, was found to be independent of the substrate, the pH or the ionic strength, but rather sensitive to the monomer structure.