DOI: 10.1140/epje/i2002-10063-7
Structure of colloidal complexes obtained from neutral/poly- electrolyte copolymers and oppositely charged surfactants
J.-F. Berret1, G. Cristobal1, P. Hervé1, J. Oberdisse2 and I. Grillo31 Complex Fluids Laboratory, CNRS - Cranbury Research Center Rhodia Inc., 259 Prospect Plains Road CN 7500, Cranbury NJ 08512, USA
2 Laboratoire Léon-Brillouin, CEA Saclay, F-91191 Gif-sur-Yvette, France
3 Institute Laue-Langevin, BP 156, F-38042 Grenoble cedex 9, France
jeanfrancois.berret@us.rhodia.com
(Received 24 July 2002 / Published online: 21 January 2003)
Abstract
We report on the phase behavior and scattering
properties of colloidal complexes made from block copolymers and
surfactants. The copolymer is poly(sodium acrylate)-b-poly(acrylamide),
hereafter abbreviated as PANa-PAM, with molecular weight 5000 g/mol for the
first block and 30000 g/mol for the second. In aqueous solutions and
neutral
pH, poly(sodium acrylate) is a weak polyelectrolyte, whereas
poly(acrylamide) is neutral and in good-solvent conditions. The surfactant
is dodecyltrimethylammonium bromide (DTAB) and is of opposite charge with
respect to the polyelectrolyte block. Combining dynamical light scattering
and small-angle neutron scattering, we show that in aqueous solutions
PANa-PAM diblocks and DTAB associate into colloidal complexes. For
surfactant-to-polymer charge ratios
Z lower than a threshold (
), the complexes are single surfactant micelles decorated by few
copolymers. Above the threshold, the colloidal complexes reveal an original
core-shell microstructure. We have found that the core of typical radius
100-200 Å is constituted from densely packed surfactant micelles
connected by the polyelectrolyte blocks. The outer part of the colloidal
complex is a corona and is made from the neutral poly(acrylamide) chains.
Typical hydrodynamic sizes for the whole aggregate are around 1000 Å.
The aggregation numbers expressed in terms of numbers of micelles and
copolymers per complex are determined and found to be comprised between
100-400, depending on the charge ratio
Z and on the total concentration. We
have also shown that the sizes of the complexes depend on the exact
procedure of the sample preparation. We propose that the driving mechanism
for the complex formation is similar to that involved in the phase
separation of homopolyelectrolyte/surfactant systems. With copolymers, the
presence of the neutral blocks prevents the macroscopic phase separation from
occurring.
82.70.-y - Disperse systems; complex fluids.
82.35.Rs - Polyelectrolytes.
61.12.-q - Neutron diffraction and scattering.
© EDP Sciences, Società Italiana di Fisica, Springer-Verlag 2002
