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)] micro gels are pH and temperature sensitive. The addition of a small amount of an ionic surfactant, sodium dodecyl sulfate (SDS), can greatly alter its pH an d temperature dependence. A combination of static and dynamic laser light s cattering was used to study the incorporation of surfactant in the microgel s and its effect on the shrinking and swelling. When both pH and temperatur e were fixed, the microgel swelled as more surfactant was added due to the concentration of surfactant inside the microgel. For a fixed temperature, t here existed a critical pH at which the swelling reached its maximum since the addition of NaOH had two opposite effects on electrostatic repulsion, n amely, ionizing the COOH group and increasing the ionic strength. The maxim um shifted to a lower pH when more surfactant was added. For a fixed pH, th e microgel shrunk when the temperature rose. The shrinking temperature beca me high when the surfactant concentration or pH increased. In comparison wi th various chemical modifications of a similar system, the incorporation of a small amount of surfactant into the microgel provided a convenient way t o vary its critical shrinking temperature and pH for different applications .