(298c) Using Magnetic Nanoparticles to Probe the Stability of Polymer-Coated Colloids

Cobalt ferrite nanoparticles with narrow size distribution were prepared and functionalized with covalently bonded homo-polymers of N-isopropyl-acrylamide such as pNIPAM and pNIPMAM. The temperature induced transitions of colloidal suspensions of these nanoparticles were studied through a combination of techniques such as differential scanning calorimetry (DSC), small angle neutron scattering (SANS), dynamic light scattering (DLS), and AC susceptibility measurements. In the pNIPAM coated nanoparticles excellent agreement was found for a transition temperature of ~30ºC by all four methods, although the AC susceptibility measurements indicated aggregation which was not evident from the DLS results. For the pNIPMAM coated nanoparticles DLS and AC susceptibility measurements indicated aggregation at a temperature of ~33-35ºC, much lower than the transition temperature peak at 40 ºC observed by DSC. These experiments and observations demonstrate the possibility of using AC susceptibility measurements to probe transitions in colloidal suspensions induced by external stimuli. Because magnetic measurements do not require optical transparency, these methods could be applied even in concentrated or opaque systems, in which light scattering techniques encounter technical problems.