Temperature and pH-Responsive Polyacrylamide/Poly(Acrylic Acid) Interpenetrating Polymer Network Nanoparticles

The synthesis, by two sequential inverse microemulsion polymerizations, of interpenetrating polymer networks (IPN) formed by polyacrylamide (PAM) and poly(acrylic acid) (PAA) and their response to changes in pH and temperature are reported here. The temperature and pH responses of the IPN nanoparticles are compared with those of polyacrylamide and random copolymers of polyacrylamide and poly(acrylic acid) P(AM-co-AA) nanoparticles also made by inverse microemulsion polymerization. We found that only the IPN nanogels exhibited a sharp swelling increase with temperature associated with its Upper Consolute Solution Temperature, driven by hydrogen bonding interactions, and with pH, driven by electrostatic repulsions of the PAA carboxylic groups, especially at pHs larger than the pKa of the PAA. The ς-potentials of the PAM, P(AM-co-AA) and IPN nanogels were measured as a function of pH and temperature, to determine the effects of these two variables, which in turn, affected the swelling of the nanogels. Field emission scanning electron microscopy revealed that the IPN nanogels were spheroidal with sizes similar to those determined by dynamic light scattering.