Influence of molecular weight on PNIPAM brush modified colloidal silica particles

The effect of molecular weight and temperature on the phase transition and internal structure of poly(N-isopropylacrylamide) brush modified colloidal silica particles was investigated using dynamic light scattering (DLS) and small angle neutron scattering (SANS) between 15 and 45 °C. Dry particle analysis utilising transmission electron microscopy (TEM), Fourier-transform infrared spectroscopy (FTIR) and thermogravimetric analysis (TGA) all confirmed the thickness of the polymer brush shell increased as a function of polymerisation time. Hydrodynamic diameter and electrophoretic mobility results revealed that the brush modified particles transitioned from swollen shells to a collapsed conformation between 15 and 35 °C. The dispersions were electrosterically stabilised over the entire temperature range investigated, with minimal thermal hysteresis recorded. Modelling of the hydrodynamic diameter enabled the calculation of a lower critical solution temperature (LCST) which increased as a function of brush thickness. The internal structure determined via SANS showed a swollen brush at low temperatures (18 and 25 °C) which decayed radially away from the substrate, while a collapsed block-like conformation with 60% polymer volume fraction was present at 40 °C. Radial phase separation was evident at intermediate temperatures (30 and 32.5 °C) with the lower molecular weight sample having a greater volume fraction of polymer in the dense inner region at these temperatures.