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Forschungsarbeit

Micellar Interpolyelectrolyte Complexes

by Christopher Synatschke (17.08.2009) 

Water-soluble micellar systems are of considerable interest, as they show potential in various applications such as emulsifiers, detergents, paints and drug delivery systems.1, 2 A special sub-type among such aggregates are so-called multicompartment micelles. These are complex nanoscopic structures that can combine several different properties or functionalities within a single particle at very close proximity.

In our approach to form such particles, we utilize the self-assembly of amphiphilic triblock terpolymers of the general structure A-B-C in aqueous solutions. A, B, and C stand for three different polymeric building blocks, which are combined to a linear chain. As can be seen in Scheme 1 the polymer consists of a non-water-soluble (hydrophobic) block A, a positively charged block B and finally a negatively charged block C. Both B and C are water soluble (hydrophilic), but carry opposite charges.

Schematic picture of the self-assembly of an amphiphilic triblock terpolymer, A-B-C, in aqueous solution.[Bildunterschrift / Subline]: Scheme 1. Schematic picture of the self-assembly of an amphiphilic triblock terpolymer, A-B-C, in aqueous solution. A core-shell-corona micelle is formed, when the positively and negatively charged blocks form an intramicellar interpolyelectrolyte complex (im-IPEC) and collapse onto the hydrophobic core.

When the polymer is transferred into an aqueous solution, many chains aggregate. The hydrophobic block A tries to minimize the unfavourable interaction with the solvent molecules. Consequently, it collapses, forming the core of the micelle as shown in Scheme 1. The attraction between the opposite charges of blocks B and C induces their collapse onto the hydrophobic core, neutralizing the opposite charges. The complex formed from two oppositely charged polymer segments is called an interpolyelectrolyte complex (IPEC). In our case a non-continuous IPEC shell (hemispheres) is formed on the hydrophobic core. A continuous IPEC shell would be only a few nanometers thick and would have a large interfacial area between the hydrophobic core and the IPEC shell. This is energetically unfavourable and the interfacial energy is reduced by the formation of the hemispheres. True core-shell-corona particles are formed, because the negatively charged block is longer than the positive charged one. After the formation of the im-IPEC between the blocks B and C, part of the negatively charged chains C remains uncomplexed. These negatively charged chains are water soluble and form a corona around the multicompartment micelles. Because of the resulting coulomb repulsion the particles are stabilized in solution and cannot aggregate. Particles with a uniform size and spherical shape could be confirmed with electron microscopy and light scattering techniques.3 Figure 1 shows an exemplary image of the multicompartment micelles taken with cryogenic transmission electron microscopy (cryo-TEM), as well as the distribution of the hydrodynamic radii as measured with dynamic light scattering (DLS). The multicompartment character of the micelles can be seen in the microscopy image with the light grey hydrophobic core and the dark dots of the im-IPEC. The corona is also visible in the image. The chains are highly stretched, because of the negative charges they carry.

[Bildunterschrift / Subline]: Figure 1. Cryogenic Transmission Electron Microscopy image (left) and the distribution of the hydrodynamic radii (right) from dynamic light scattering measurements of the core-shell-corona micelles in aqueous solution.

Further complexation reactions (i.e. IPEC formation) can be performed with the residual negative charges of the corona through the addition of positively charged polymers. Sun-like structures of the second IPEC could be observed as a transition state. The relaxation of the second IPEC to a shell was completed after several days.4

The next steps aim at a further functionalization of the micelles. The introduction of a response to external stimuli, as for example temperature, pH and light can be introduced through a complexation of the compartmentalized precursor micelles (cf. Scheme 1) with appropriate block copolymers. The incorporation of metallic or inorganic nanoparticles could also add interesting properties to these highly defined micellar systems.

References:

1. Alexandridis, P.; Lindman, B., Amphiphilic Block Copolymers: Self-Assembly and Applications. Elsevier: Amsterdam, 2000.
2. Kataoka, K.; Harada, A.; Nagasaki, Y. Advanced Drug Delivery Reviews 2001, 47, (1), 113-131.
3.  Schacher F., Walther A., Mueller A. H. E., Langmuir, 2009, DOI: 10.1021/la901182c
4.  Schacher F., Betthausen E., Walther A., Schmalz H., Pergushov D. V., Mueller A. H. E., ACS Nano, 2009, accepted


Christopher Volker Synatschke
* 1984

Stationen
  • seit 2005
  • Studium der Chemie auf Diplom an der Universität Bayreuth
  • Aug. 2006
  • Diplomvorprüfung in Chemie an der Universität Bayreuth
  • Sept. 2007 - März 2008
  • Auslandsstudium an der University of New South Wales, Sydney, Australien, am „Center for Advanced Macromolecular Design”
  • seit März 2008
  • Elitestudiengang Macromolecular Sciences an der Universität Bayreuth

Veröffentlichungen
  • Sept. 2008
  • Posterpräsentation auf der „Macro- and Supramolecular Architectures and Materials Conference” in Mainz
  • Okt. 2008
  • Publikation in der Zeitschrift „Macromolecules”, 2008, 41 (21), pp 7904–7912.