Ionically Charged Block Copolymers

The results of experimental and theoretical research on water-soluble (nonstoichio-metric) IPECs based on nonlinear (branched) polyionic species (HPE) complexed with oppositely charged linear PEs (GPE) demonstrated that the main feature of such macromolecular co-assemblies is their pronounced compartmentalized structure, which results from a distinctly nonuniform distribution of the linear GPE chains within the intramolecular volume of the branched HPE. In the case of star-shaped PEs or star-like micelles of ionic amphiphilic block copolymers, this com-partmentalization leads to the formation of water-soluble IPECs with core-corona (complex coacervate core) or core-shell-corona (complex coacervate shell) structures, respectively. Water-soluble IPECs based on cylindrical PE brushes appear to exhibit longitudinally undulating structures (necklace) of complex coacervate pearls decorated by the cylindrical PE corona. 

In this contribution, we describe our recent experimental and theoretical findings on complex coacervate core micelles. We have investigated the co-assembly of several types of oppositely charged ionic-hydrophilic block copolymers into mixed micelles. In particular, we have focused on chain mixing/segregation in the micellar corona as a function of monomer type and (the ratio between the) chain length of the polymer blocks in the corona. Our aim has been to employ co-assembly in such systems as a route towards formation of reversible Janus micelles. These are micelles with a corona that exhibits two distinguishable sides (hemispheres in the case... 

Now that we have established the requirements for the formation of reversible Janus micelles, we turn our attention to the choice of ionic-hydrophilic block copolymers. The ionic blocks have to be oppositely charged to ensure co-assembly in aqueous solutions, whereas the neutral blocks have to be water-soluble. Furthermore, the unlike water-soluble polymer blocks need to segregate, not mix within the micellar corona. Since the classical works of Flory and Huggins, extended by Scott to describe binary polymer solutions [58], it is well known that two unlike polymers... 

This orientation to control the enzymatic activity was also demonstrated by the design of smart (i.e., stimuh responsive) enzyme-block copolymer systems that can be modulated by several triggers. For instance, an on-off switch of the lysozyme activity was synchronized with the reversible formation of polyion complex (PIC) micelles that are formed through electrostatic interaction between a pair of oppositely charged block copolymers with PEG segments. It was shown that an increase in the ionic strength (NaCl concentration) of the media leads to the complete disrnption of the micellar strnctnre (due to the loss of the electrostatic interactions). Lytic activity for Micrococcus luteus cells is completely inhibited when the enzyme is... 

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. 

In ionic block copolymers, the micellar assembhes can also be induced by com-plexation with oppositely charged molecules, forming a PIC at the core. The electrostatic interaction between PEO-polycation and PEO-polyanion block copolymers is the driving force for the formation of micellar assembhes in which the complex... 

To date, there has been a very limited effort devoted to developing theory for ionic block copolymers. Gonziilez-Mozuelos and Olvera da la Cruz (1994) studied diblock copolymers with oppositely charged chains in the melt state and in concentrated solutions using the random phase approximation (RPA) (de Gennes 1970). However, this work has not been extended to dilute solutions. 

In ionic block copolymers, micellization occurs in a solvent that is selective for one of the blocks, as for non-ionic block copolymers. However, the ionic character of the copolymer introduces a new parameter governing the structure and properties of micellar structures. In particular, the ionic strength plays an important role in the conformation of the copolymer, and the presence of a high charge density leads to some specific properties unique to ionic block copolymers. Many of the studies on ionic block copolymers have been undertaken with solvents selective for the ionic polyelectrolyte block, generally water or related solvents, such as water-methanol mixtures. However, it has been observed that it is often difficult to dissolve ionic hydrophilic-hydrophobic block copolymers in water. These dissolution problems are far more pronounced than for block copolymers in non-aqueous selective solvents, although they do not always reflect real insolubility. In many cases, dissolution can be achieved if a better solvent is used first and examples of the use of cosolvents are listed by Selb and Gallot (1985). 

Several synthetic strategies are used to produce block copolymers containing a cationic block. Because charged monomers are not polymerizable by ionic techniques, the synthesis of the required block copolymers can be carried out by free radical polymerization of ionic vinyl monomers using macroinitiators, by modifying one block of a block copolymer and by coupling of two readily synthesized blocks. 

Lee, A. S. Buetuen, V. Vamvakaki, M. Armes, S. R Pople, J. A. Cast, A. P. Structure of pH-dependent block copolymer micelles Charge and ionic strength dependence. Macromolecules 2002, 35, 8540-8551. 

---