Dynamic Aggregation of Poly-N-Isopropylacrylamide Characterized Using Second-Order Scattering

A second-order scattering (SOS) method is presented for the characterization of aqueous particle suspensions undergoing aggregation. Scattering intensities are measured at 90° by a standard fluorimeter and referenced against dynamic light scattering (DLS) measurements to determine particle size increase in a metal-promoted aggregation process for 0.05 mg/mL aqueous poly-N-isopropylacrylamide (PNIPAm), MW ∼10 k g/mol. Particle size increases monotonically from 30 nm to 210 nm at temperature 308 K. A further validation of the SOS method was performed using monodisperse polystyrene reference particles sized at 52 nm, 101 nm, 151 nm, and 206 nm, which demonstrated the technique's accuracy to within 6% and its versatility with respect to sample composition. The technique is ideal for monitoring colloidal stability and macromolecular assembly and it can be performed at lower concentrations than are typically used in DLS.