Polyelectrolytes-Block Copolymers

Block copolymer micelles with a polyelectrolyte corona are a very important class of colloidal particles in aqueous medium and are often referred to as polyelectrolyte block copolymer micelles. The micellization behavior of these charged micelles has been very recently reviewed by Riess [14] and FOrster et al. [15]. A brief overview of the topic will therefore be presented in what follows. Amphiphilic block copolymers consisting of one hydrophobic block linked to one ionic block will only be discussed in this section. Blocks copolymers containing one hydrophilic block and one ionic block will be discussed in Sect. 4.3. 

As exemplified above, most of the studies on anionic polyelectrolyte block copolymer micelles have been carried out on P(M)AA-containing copolymers. The ionization degree of these anionic blocks is strongly dependent on... 

Fig. 11 Experimentally determined grafting distance b as function of degree of polymerization of insoluble block for polyelectrolyte block copolymer micelles at different salt concentrations salt free, o 0.3mol/l, 1 mol/1. Reprinted with permission from [15]. Copyright (2004) Springer... 

Salt effects in polyelectrolyte block copolymer micelles are particularly pronounced because the polyelectrolyte chains are closely assembled in the micellar shell [217]. The situation is quite reminiscent of tethered polymer brushes, to which polyelectrolyte block copolymer micelles have been compared, as summarized in the review of Forster [15]. The analogy to polyelectrolyte brushes was investigated by Guenoun in the study of the behavior of a free-standing film drawn from a PtBS-PSSNa-solution [218] and by Hari-haran et al., who studied the absorbed layer thickness of PtBS-PSSNa block copolymers onto latex particles [219,220]. When the salt concentration exceeded a certain limit, a weak decrease in the layer thickness with increasing salt concentration was observed. Similar results have been obtained by Tauer et al. on electrosterically stabilized latex particles [221]. 

Abstract Polyelectrolyte block copolymers form micelles and vesicles in aqueous solutions. Micelle formation and micellar structure depends on various parameters like block lengths, salt concentration, pH, and solvent quality. The synthesis and properties of more complicated block and micellar architectures such as triblock- and graft copolymers, Janus micelles, and core-shell cylinder brushes are reviewed as well. Investigations reveal details of the interactions of polyelectrolyte layers and electro-steric stabilization forces. 

Keywords Block copolymers ABC triblock copolymers Janus micelles Cylinder brushes Core-shell nanoparticles Graft copolymers Micelles Vesicles Copolyampholytes Polyelectrolyte block copolymers Aggregation... 

Polyelectrolyte block copolymers combine structural features of polyelectrolytes, block copolymers, and surfactants. It is thus not surprising that they possess quite unusual and unique properties which make them a fascinating and challenging subject for researchers. Many of these properties are taken advantage of in technological applications and play an important role in physico-chemical properties of biological cell structures. This has motivated a comprehensive investigation so that today a much clearer picture of the behavior of polyelectrolyte block copolymers has developed. 

The present review deals with the association behavior of polyelectrolyte block copolymers which is the most outstanding feature of this class of polymers. It leads to the formation of micelles, strings, and networks of sometimes quite complicated topology. The association behavior depends on many external parameters, among them pH, temperature, and salinity, which are of relevance in many technological and biological processes. 

In dilute aqueous solutions, polyelectrolyte block copolymers self-assemble into micelles consisting of a hydrophobic core and a polyelectrolyte shell. The study of their structural properties is expected to provide a basic understanding of the properties of dense polyelectrolyte layers, electro-steric stabilization mechanisms, and actuator functions based on variations in the electrostatic interactions. 

Fig. 2 TEM-image of spherical polyelectrolyte block copolymer micelles (PB-P2VP.MeI). The pronounced contrast of the polyelectrolyte shell is due to the counterions (I-) [19]...

The first theories on the micellization of polyelectrolyte block copolymers were published by Marko and Rabin [26], Dan and Tirrell [27] and Shusha-rina et al. [28]. They predict a strong influence of the polyelectrolyte blocks on the micellization behavior. Thus not unexpectedly, a qualitatively differ-... 

Fig. 4 Experimentally determined grafting distance b as a function of the degree of polymerization of the insoluble block for polyelectrolyte block copolymers at different salt concentrations [49] ( ) salt free, (O) 0.3 mol/l, ( ) 1 mol/1...

The internal structure of polyelectrolyte block copolymer micelles such as their core radius Rc and micellar radius Rm can be determined by a variety of methods involving static and dynamic light scattering (SLS, DLS), small-angle X-ray (SAXS) and neutron scattering (SANS) as well as imaging techniques such as transmission electron microscopy (TEM) or atomic force mi-... 

Fig. 5 Cryo-TEM images of polyelectrolyte block copolymer micelles (PEE-PSSH) at a NaCl-concentration of 0.003 mol/1 (a) and 3 mol/1 (b). The core/shell-structure is well visible in Fig. 5b [49]...

Fig. 8 Schematic density profile of a block copolymer micelle. Uncharged micelles exhibit a simple core/shell structure whereas polyelectrolyte block copolymer micelles can show phase separation of the corona into a dense interior and a dilute outer domain...

For quite some time, there have been indications for a phase-separation in the shell of polyelectrolyte block copolymer micelles. Electrophoretic mobility measurements on PS-PMAc [50] indicated that a part of the shell exhibits a considerable higher ionic strength than the surrounding medium. This had been corroborated by fluorescence studies on PS-PMAc [51-53] and PS-P2VP-heteroarm star polymers [54]. According to the steady-state fluorescence and anisotropy decays of fluorophores attached to the ends of the PMAc-blocks, a certain fraction of the fluorophores (probably those on the blocks that were folded back to the core/shell interface) monitored a lower polarity of the environment. Their mobility was substantially restricted. It thus seemed as if the polyelectrolyte corona was phase separated into a dense interior part and a dilute outer part. Further experimental evidence for the existence of a dense interior corona domain has been found in an NMR/SANS-study on poly(methylmethacrylate-fr-acrylic acid) (PMMA-PAAc) micelles [55]. 

Fig. 10 Cryo-TEM images of polyelectrolyte block copolymer micelles (PB-P2VPMeI) with unperturbed spherical corona (a), corona filaments (b), filament networks (c), and micellar strings. The scale bar is 50 nm [56]...

Since neutral block copolymer micelles are a convenient model system for the investigation of the steric stabilization potential, polyelectrolyte block copolymer micelles may serve to study electro-steric interaction. Similar to the case of neutral block copolymer micelles, the interaction potential u(r) can be probed by measuring the shear modulus of micellar gels as a function of distance. Assuming a simple Debye-Hiickel potential yields for the shear modulus by taking the second derivative... 

---