Factors affecting the swelling of poly(N-isopropylacrylamide) microgel particles: Fundamental and commercial implications

A microgel particle is a cross-linked latex particle which is swollen by a good solvent. Particle swelling is intrinsically related to the nature of the interaction between the polymer and continuous phase. Microgel particles based on PNP [PNPn = poly(N-isopropylacrylamide)] are particularly interesting since the parent homopolymer undergoes a coil-to-globule transition in water when the temperature increases above 32°C. In this work, PCS (photon correlation spectroscopy) and SANS (small-angle neutron scattering) are employed in a complementary manner to study the environmentally induced de-swelling of PNP particles. Further, we show that particle de-swelling may be induced at room temperature by addition of alcohols or excluded free polymer (i.e. non-adsorbing free polymer) to the continuous phase. (The extents of particle de-swelling observed using these additives are similar to those achieved by heating the pure microgel particles in water above 32°C). Particle de-swelling in the presence of added alcohol or free polymer arises from 'co-non-solvency' and osmotic de-swelling effects, respectively. Copolymerization of N-isopropylacrylamide (NP) with acrylic acid yields microgel particles whose diameters are sensitive to both pH and temperature. These particles adsorb PbII ions from solution in a reversible manner. The latter property has potential application in water purification.