Conformational changes of poly(vinylcaprolactam) macromolecules and their complexes with ionic surfactants in aqueous solution

Dynamic light scattering measurements have been performed for aqueous solutions of linear poly(vinylcaprolactam) (PVCa) at several polymer concentrations and over a wide range of ionic surfactant concentration. The intermolecular aggregation of PVCa accompanied by a sharp increase of the light scattering intensity is observed in pure water at 33 degrees C. The effect of ionic surfactants (cetylpyridinium chloride (CPC) and sodium dodecyl sulfate (SDS)) on the behavior of solutions of linear PVCa was studied as a function of temperature. For polymer concentrations below the overlap concentration (2.5 mg/mL) a decrease of the macromolecular hydrodynamic diameter is observed upon the addition of ionic surfactant (SDS, CPC) at low surfactant concentrations. This effect is in contrast to the behavior of the complexes of surfactant with another thermosensitive polymer, poly(N-isopropylacrylamide) (PNIPA). A further increase of the surfactant concentration leads to the reentrant swelling of macromolecules. In the presence of ionic surfactant the temperature increase results in the sharp drop of the hydrodynamic diameter of the particles at some critical temperature. This critical temperature becomes higher with increasing surfactant concentration. For higher polymer concentrations (5 mg/mL) the particle size always increases upon the addition of ionic surfactants, because of intermolecular aggregation.