Control of number density and swelling/shrinking behavior of P(NIPAM–AAc) particles at solid surfaces

Temperature and pH sensitive microgel particles of poly(N-isopropyl acrylamide) (PNIPAM) and acrylic acid (AAc) comonomers were adsorbed on silicon wafers pre-coated with PEI. The amount of adsorbed microgel particles was controlled by pH and concentration of microgel dispersions and the speed of rotation during the preparation. The swelling/deswelling behaviour of adsorbed P(NIPAM-co-AAc) microgel particles was studied by scanning force microscopy (SFM) and was compared to the temperature-induced volume phase transition in the respective bulk conditions investigated by dynamic light scattering (DLS) and electrophoretic mobility measurements. Also, after the adsorption, the swelling and shrinking is still reversible over several heating and cooling cycles, although the microgel particles are strongly compressed. The particles stay fixed at low ionic strength, up to 10−3M, and intermediate pH. At higher ionic strength or in the acidic or basic regime, they can be moved by the SFM tip or even leave the surface. At low ionic strength the particles swell and shrink in the lateral and vertical directions. At higher ionic strength the particles react more pronouncedly in the vertical direction, and almost not at all in the lateral direction. In contrast to volume measurements, the particles are more strongly swollen at high ionic strength than at low salt concentrations.