Surfactant effect on pH and temperature sensitivities of poly(N-vinylcaprolactam-co-sodium acrylate) microgels

Spherical poly(N-vinylcaprolactam-co-sodium acrylate) [P(VCL-co-NaA)] microgels are pH and temperature sensitive. The addition of a small amount of anionic surfactant, sodium dodecyl sulfate (SDS), can greatly alter its pH and temperature dependence. A combination of static and dynamic laser light scattering was used to study the incorporation of surfactant in the microgels and its effect on the shrinking and swelling. When both pH and temperature were fixed, the microgel swelled as more surfactant was added due to the concentration of surfactant inside the microgel. For a fixed temperature, there existed a critical pH at which the swelling reached its maximum since the addition of NaOH had two opposite effects on electrostatic repulsion, namely, ionizing the COOH group and increasing the ionic strength. The maximum shifted to a lower pH when more surfactant was added. For a fixed pH, the microgel shrunk when the temperature rose. The shrinking temperature became high when the surfactant concentration or pH increased. In comparison with various chemical modifications of a similar system, the incorporation of a small amount of surfactant into the microgel provided a convenient way to vary its critical shrinking temperature and pH for different applications.