How small can poly(N-isopropylacrylamide) nanogels be prepared by controlling the size with surfactant?

Despite the increasing demand for 'as small as possible' responsive poly(N-isopropylacrylamide) (pNIPAm) nanogels with well-defined internal structure, up to date there is no systematic investigation to provide guidance what are the lower limits of nanogel size and what interactions limit its further decrease. In this work both classical batch precipitation polymerization (at 60, 70 and 80 °C) and monomer-feeding precipitation polymerization (at 80 °C) is used to determine how small pNIPAm particles could be prepared by surfactant addition. The collapsed particle size levels off with increasing surfactant concentration in each case but at a strongly temperature dependent value. The plateau values of the microgel size show non-monotonic temperature dependence. To gain deeper insight into the interactions controlling the variation of microgel size with surfactant concentration and temperature a simple model was developed and fitted to the experimental data. The model fitting clearly showed that microgel size is controlled by two key parameters: the underlying adsorption isotherm of surfactant adsorption on the collapsed microgel particles and the effective initiator concentration (the amount of initiator fragments providing surface charge for the precursor particles in unit volume of reaction mixture). The significance of the formation of carboxylic groups in the polymer network is also discussed.