Project B4: Colloidal stabilization by unattached homopolymer and copolymers

Principal Investigators: E. Bartsch (Freiburg) / A. N. Semenov (Strasbourg)
Collaborators: M. Maaloum (Strasbourg)
PhD Student: Alexey Shvets / Jochen Schneider

Current state of the research

Colloidal dispersions are important for many technologically applications. The main problem here is that the colloidal particles tend to aggregate due to van-der-Waals attractions. The polymer-induced (PI) depletion attraction in colloidal dispersions is well understood [1]. It is also known [1] that a certain amount of free polymer added to a colloidal system may enhance its stability (depletion stabilization). So far, the latter effect was largely ignored in the colloid physics community, but could open exciting new ways to control colloidal stability. Previous theoretical studies of free-PI stabilization were based on oversimplified models involving uncontrolled approximations and ad hoc assumptions [2,3]. Even the most basic features of the depletion stabilization phenomenon are yet unknown. It is unclear how the PI repulsion depends on the solution parameters, polymer structure and monomer/surface interactions. It is the aim of the project to establish how the effective colloid interaction potential depends on polymer solution parameters and to identify the regimes where the PI stabilization mechanism can be harnessed to efficiently control the colloidal properties – by using novel theoretical concepts, direct force measurements and the potential dependent properties accessible by light scattering (LS).
 

Contributions of the participating groups

Recently, Semenov et al. considered the PI interactions using a new rigorous theoretical approach [4,5]. They elucidated two main contributions to the PI repulsive interactions in the semidilute regime: (i) the anti-Casimir effect [4];  (ii) the chain-end effect [5]. The anti-Casimir repulsion is due to fluctuations of monomer concentration coupled with the chain connectivity constraints. The chain-end effect shows a maximum above the coil overlap concentration. The strongest stabilization effect is anticipated as the static correlation length ξ becomes comparable to the coil size R. Semenov et al. are therefore well prepared to study this important regime both analytically and by computer simulations, lifting the previously adopted restriction ξ << R [5]. Bartsch et al. have studied the effect of PI depletion attraction on the colloidal glass transition and were among the first to show via LS the predicted phenomenon of reentrant melting [6]. The group has a large expertise in the synthesis and characterization of model colloids with tailor-made properties [7] and in studying the connection between single-particle properties and particle interactions with scattering techniques [8].

 

Research project and collaborations

To explore the potential of PI stabilization we propose to study the stabilization phenomenon by a collaboration of two experimental and one theoretical groups involving two PhD students – one from theory and one from experiment. The aim is to set up a number of suitable model systems, optimized by the interplay of theory and experiment, which provide experimental data that can be used to quantify the depletion stabilization in various colloid/ polymer systems and to find how the stabilization strength depends on the most important polymer, colloid, and solution parameters.

For this purpose, the Bartsch group will synthesize and characterize colloidal particles which, on addition of a suitable polymer, should expose the depletion stabilization effect. The phase behavior of colloid-polymer mixtures at low colloid volume fractions will be studied to learn when depletion stabilization is effective. Guided by the theoretical work, the experi- mental PhD will determine from LS experiments the osmotic pressure as function of colloid and polymer concentration, polymer molecular weight, etc., which can then be compared with theoretical predictions from the theoretical PhD thesis. In addition, the experimental PhD student will work in the Maaloum group in Strasbourg to measure the forces between colloidal particles in semidilute solutions of the free polymer, acting as depletion agent, via colloidal probe AFM and to obtain the particles' interaction potential [9]. The force data will then be compared with the theory. In addition, the derived interaction potential will be used to calculate structure factors and/or pair correlation functions via integral equation theories. The results will be compared with experimental data from static LS and/or optical microscopy. The free polymers to start with will be homopolymers. As the theory predicts depletion stabilization to be more effective when diblock polymers or triblock copolymers are used where the (small) heteropolymer block tends to adsorb on the colloids, the experimental work will be extended in this direction. The theoretical PhD student will calculate how the depletion effect depends on the adsorption energy and structural parameters of the copolymers. In addition, the theoretical work will include a detailed analysis of an interplay between steric stabilization and free-PI stabilization effects. For the systematic study and understanding of depletion stabilization the expertise of all groups (polymer theory, polymer physics and colloidal physics&chemistry) and the combination of theory and experiment will be crucial.


References
[1]D.  H.  Napper, "Polymer Stabilization of Colloidal Dispersions", Academic, London, 1983.
[2]R. I. Feigin and D. H. Napper, J. Colloid Interf. Sci. 75, 525 (1980).
[3]J. Y. Walz and A. Sharma , J. Colloid Interf. Sci. 168, 485 (1994).
[4]A. N. Semenov and S. P. Obukhov, J. Phys: Condens. Matter 17, S1747 (2005).
[5]A. N. Semenov, Macromolecules 41, 2243 (2008).
[6]T. Eckert, E. Bartsch, Phys. Rev. Lett. 89, 125701 (2002)
[7]S. Kirsch et al., Macromolecules 32, 4508 (1999).
[8]T. Eckert, E. Bartsch, Faraday Discuss. 123, 51 (2003).
[9]A. Courvoisier, F. Isel, J. François and M. Maaloum, Langmuir 14, 3727 (1998).