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Osmotic shrinkage in star/linear polymer mixtures
A. Wilk1,2, S. Huißmann3, E. Stiakakis4,5, J. Kohlbrecher1, D. Vlassopoulos4,6*, C. N. Likos3,7, G. Meier5, J. K. G. Dhont5, G. Petekidis4,6 and R. Vavrin1
Laboratory for Neutron Scattering, ETH Zurich & Paul Scherrer Institut, 5232, Villigen PSI, Switzerland
2 Faculty of Physics, A. Mickiewicz University, Umultowska 85, 61-614, Poznan, Poland
3 Department of Physics, Heinrich-Heine-University of Düsseldorf, 40225, Düsseldorf, Germany
4 Institute of Electronic Structure and Laser, FORTH, 71110 Heraklion, Crete, Greece
5 Institut für Festkörperforschung, Forschungszentrum Jülich, 52425, Jülich, Germany
6 Department of Materials Science & Technology, University of Crete, 71300 Heraklion, Crete, Greece
7 Faculty of Physics, University of Vienna, Sensengasse 8/12, A-1090, Vienna, Austria
* e-mail: email@example.com
Accepted: 26 May 2010
Published online: 2 July 2010
Multiarm star polymers were used as model grafted colloidal particles with long hairs, to study their size variation due to osmotic forces arising from added linear homopolymers of smaller size. This is the origin of the depletion phenomenon that has been exploited in the past as a means to melt soft colloidal glasses by adding linear chains and analyzed using dynamic light scattering experiments and an effective interactions analysis yielding the depletion potential. Shrinkage is a generic phenomenon for hairy particles, which affects macroscopic properties and state transitions at high concentrations. In this work we present a small-angle neutron scattering study of star/linear polymer mixtures with different size ratios (varying the linear polymer molar mass) and confirm the depletion picture, i.e., osmotic star shrinkage. Moreover, we find that as the linear/star polymer size ratio increases for the same effective linear volume fraction ( c/c *with c *the overlapping concentration), the star shrinkage is reduced whereas the onset of shrinkage appears to take place at higher linear polymer volume fractions. A theoretical description of the force balance on a star polymer in solution, accounting for the classic Flory contributions, i.e. elastic and excluded volume, as well as the osmotic force due to the linear chains, accurately predicts the experimental findings of reduced star size as a function of linear polymer concentration. This is done in a parameter-free fashion, in which the size of the cavity created by the star, and from which the chains are excluded, is related to the radius of the former from first principles.
© EDP Sciences, SIF, Springer-Verlag Berlin Heidelberg, 2010