Polyelectrolyte brushes
Introduction
If long linear polyelectrolyte chains are grafted densely to a solid surface, a polyelectrolyte brush results. In principle, such systems can be achieved either by grafting polyelectrolyte chains to planar substrate or to strongly curve surfaces of colloidal particles. Fig. 1 displays these two ways in a schematic fashion.
A third possibility consists in end-grafting of polyelectrolyte chains onto long chain macromolecules. The properties of these “bottle-brush polymers” are strongly influenced by repulsion between polyelectrolyte grafts which induce axial tension in the main chain thus reducing significantly its flexibility. Hence these “molecular polyelectrolyte brushes” represent a subject of its own. The present review will therefore be focused on polyelectrolytes grafted to solid surfaces.
Polyelectrolyte brushes constitute a new class of materials. The strong electrostatic interaction involved brings about a number of entirely new properties if such systems are compared to monolayers of end-grafted uncharged macromolecules. Polyelectrolyte brushes have therefore become the subject of intense research which is partly due to interesting fundamental questions involved when trying to understand these systems, partly due to a great number of possible application such systems may acquire in nanotechnology. Therefore a great number of theoretical and experimental papers are available so far and polyelectrolyte brushes present certainly one of the most active fields at the interface of polymer and surface science.
Theory distinguishes between two classes of polyelectrolyte (PE) brushes: If strongly dissociating polyelectrolyte chains as e.g. poly(styrene sulfonic acid) (PSS) are grafted to surfaces, a quenched PE-brush results. The degree of dissociation of ionic monomer units within these systems is independent of the pH in the system. On the other hand, an annealed PE brush is obtained if weak polyelectrolytes as e.g. poly(acrylic acid) (PAA) are affixed to surfaces. Here the state of charging of the chains depends directly on the local pH which depends both on pH and on the ionic strength in the bulk of the solution. Hence, pH becomes a decisive variable and the system acquires pH sensitive features. In both cases of quenched and annealed PE brushes interaction between charged segments can be tuned by changing the ionic strength in the system. At low concentration ca of added salt, the Debye length κ−1 is large and the properties of the PE brush are mainly governed by electrostatic interactions. On the other hand, high salt concentrations lead to a strong screening of the electrostatic interactions and the PE brush will resemble uncharged polymer brushes. In this way the pH and ca become two external variables to which PE brushes respond and which can lead to marked changes of conformations of grafted chains, e.g., the swelling behavior in water. The strength of solvent in which the brush is immersed as well as specific interactions of the PE-chains with the counterions may constitute other variables to which PE brushes may react. Hence, PE brushes may be used to construct adaptive or “smart” multi-responsive surfaces the properties of which may be tuned within wide limits by varying these external conditions.
While much of the work done on uncharged brushes may be traced back to the seminal work of Alexander [1] and de Gennes [2], [3], the field of PE brushes has been deeply influenced by work of Pincus and co-workers [4], [5] and by Borisov, Zhulina and Birshtein [6], [7]. As has been shown in this early work, essential properties of PE brushes are determined by confinement of a major fraction of counterions compensating the electrical charge of the PE-chains within the brush layer. As a consequence, the stretching of PE chains in the brush is governed by the high osmotic pressure of the confined counterions.
Up to now, several reviews on PE brushes are available [8], [9], [10], [11], [12], [13]. In particular, Ref. [10••] gives an exhaustive survey on research up to the year 2004. More recent reviews give overviews on applications in nanotechnology [12•] and for bioactive surfaces [13]. Here we wish to highlight recent research on PE brushes. Given the enormous work done so far in this field, the present survey cannot be exhaustive but is necessarily biased by the authors' own predilections. The review is organized as follows: We first give a brief survey on theory in Section 2. The synthesis of planar and curved systems is surveyed in the Section 3: subsequent section which is followed by a comparison with experimental data in Section 4. Section 5 is devoted to special applications as e.g. in nanotechnology and a brief Section 6 concludes this survey.
Section snippets
Theory
The idea of localization of counterions within infinite planar polyelectrolyte brush was first proposed in Refs. [4], [6] on the basis of analogy between PE brush and infinite uniformly charged plane in contact with semi-infinite reservoise of salt-free solution. The exact solution of the PB equation for the latter system proves that most of counterions compensating the surface charge σ are localized within the layer of thickness proportional to so-called Gouy–Chapmann length λGC = 1/2πσlB in the
Grafting techniques
In principle, there are two ways of forming a layer of end-grafted polymer chains: In a grafting-to process the charged or uncharged chains are attached to the surface by formation of a chemical bond between the end groups of the polymer and the solid [10••]. Such a process is an extension of the formation of self-assembled monolayers (SAM) of low molecular weight components at solid surfaces. For example, poly(styrene sulfonate) terminated by trichlorosilane end groups have been attached to
Brush height H
A central result of the mean-field approach discussed in Section 2 is the transition from a osmotic brush to a salted brush regime with increasing salt concentration. Helm and co-workers were the first to demonstrate this transition in an unambiguous manner [44••]. Using X-ray reflectometry they could show that the brush height H is independent of the salt concentration in the osmotic regime. In the salted brush regime, H scales with ca−1/3 as discussed in Section 2. This central finding of
“Smart” surfaces
Zhou and Huck [12•] have recently reviewed possible applications of brush systems in general and of polyelectrolyte brushes in particular. Mixed brushes have certainly a potential for the synthesis of “smart” surfaces with switchable surface properties. The strong swelling of polyelectrolyte brushes could be used for nano-actuators. Moreover, recent experiments by Huck and co-workers demonstrated that brushes can be switched from “soft matter” to “hard matter” in presence of suitable ions [71•]
Conclusion
The present review has demonstrated that polyelectrolyte brushes present a number of fundamental questions of high interest [4], [6], [10]. At the same time, these systems open the way for the fabrication of smart and adaptable surfaces [12], [13]. While the basic theory of polyelectrolyte brushes seems to be rather well understood by now, applications have started to emerge only recently. Here interesting developments in nanotechnology are to be expected which make use of the strong changes of
Acknowledgements
OVB acknowledges support of the Alexander von Humboldt Foundation. Financial support by the Deutsche Forschungsgemeinschaft, SFB 481, Bayreuth, is gratefully acknowledged.
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