Microphase separation in weakly charged hydrophobic polyelectrolytes

Aqueous solutions of a well-defined poly (N-isopropylacrylamide-co-sodium 2-acrylamido-methylpropanesulfonate) (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 degreesC less than or equal to T less than or equal to 60 degreesC) and polymer concentration (0.5 wt% less than or equal to C less than or equal to 12 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 degreesC). Upon addition of salt, the systems undergo a micro-macrophase separation. At temperatures above 45 degreesC, the SANS spectra exhibit a sharp peak at a scattering wave vector, q(max), which increases slightly with temperature. At high temperature (T similar to 60 degreesC), the scattered intensity follows a power law I(q) similar to 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) similar to C-0.22. Estimates of the typical sizes give values between 40 Angstrom and 200 Angstrom. 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.