Polyelectrolyte-Surfactant Complexes as Thermoreversible Organogelators
Poly(N,N-dimethyl-n-octadecylammonium p-styrenesulfonate) (PSS-DMODA) polymers were prepared and investigated as organogelators for low-polarity aromatic solvents. Gels were prepared by heating polymer solutions (2.5–20% w/v polymer) at elevated temperature and then cooling in an ice bath. Gelation was confirmed by the formation of self-supporting samples that did not flow when inverted 180°. Measurement of the gel transition temperature by inversion testing showed a dependence on the concentration of the polymer, the molecular weight of the polymer, and the gelled solvent. Cavitation rheology measurements on a subset of the gels demonstrated that they were viscoelastic solids. Scanning electron microscopy measurements of freeze-dried xerogels and polarized optical microscopy measurements showed the formation of network structures and birefringent samples, respectively. Aging studies showed syneresis of the gels especially at low concentration and temperature. Gelation was interpreted using a model for reversibly associating polymers. The gelation was attributed to the clustering of the ionic groups to form a physically cross-linked network that restricts the motion of the chains. These polyelectrolyte–surfactant complexes should be a useful class of organogelators as a number of characteristics of the polymer (molecular weight, ionic groups, side-chain length) can be independently varied to tune the properties of the resultant organogels.