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Soft Matter and Biological Physics

Eur. Phys. J. E 7, 141-151 (2002)
DOI: 10.1140/epje/i200101149

Microphase separation in weakly charged hydrophobic polyelectrolytes

O. Braun1, F. Boué2 and F. Candau1

1  Institut Charles Sadron (UPR 22 du CNRS), 6 rue Boussingault, F-67083 Strasbourg Cedex, France
2  Laboratoire Léon Brillouin (UMR 12 du CNRS et du CEA), CEA-Saclay, F-91191 Gif-sur-Yvette, France


(Received 1 October 2001)

Aqueous solutions of a well-defined poly( N-isopropylacrylamide-co-sodium 2-acrylamido-methyl propanesulfonate) (NIPAM/NaAMPS in a 95/5 molar ratio) have been investigated by means of small-angle neutron scattering (SANS) and rheological experiments as a function of temperature ( $25\un{^\circ C} \leqslant
T \leqslant 60\un{^\circ C}$ ) and polymer concentration ( $0.5\un{wt\%} \leqslant
C \leqslant 12\un{wt\%}$ ). The solutions remain optically transparent and isotropic over the whole temperature range, in contrast with the homopolyNIPAM which precipitates above its lower critical solution temperature (LCST = $32\un{^\circ C}$). Upon addition of salt, the systems undergo a micro-macrophase separation. At temperatures above $45\un{^\circ C}$, the SANS spectra exhibit a sharp peak at a scattering wave vector, $q_{\rm max}$, which increases slightly with temperature. At high temperature ( $T \sim 60\un{^\circ C}$), the scattered intensity follows a power law $I(q) \sim q^{-4}$ in the asymptotic regime, characteristic of two-density media with sharp interfaces, and $q_{\max}$ is found to vary with polymer concentration as $q_{\max} \sim C^{{0.22}}$. Estimates of the typical sizes give values between 40 Å and 200 Å. These results provide a strong evidence of a thermally induced microphase separation, which is corroborated by the very sharp increases of the viscosity (over 2 decades) and of the stress relaxation time of the solutions, occurring in the temperature range where the scattering peak is observed. The results are discussed and compared with the theoretical models proposed for weakly charged polyelectrolytes in a poor solvent.

82.35.Jk - Copolymers, phase transitions, structure.
82.35.Rs - Polyelectrolytes.
83.85.Cg - Rheological measurements - rheometry.
61.12.Ex - Neutron scattering techniques (including small-angle scattering).

© EDP Sciences, Società Italiana di Fisica, Springer-Verlag 2002