College Park, Maryland      June 6 - 10 , 2004

M2-B2 (11:00 AM): Volume transition and internal structures of small poly(N-isopropylacrylamide) microgels

R. P. Hjelm (Los Alamos Neutron Science Center, Los Alamos National Laboratory), X. Xia (Department of Physics, University of North Texas, Denton, TX), L. Arleth (Los Alamos Neutron Science Center, Los Alamos National Laboratory), J. Wu (Department of Chemical and Environmental Engineering, University of California, Riverside, CA), Z. Hu (Department of Physics, University of North Texas, Denton, TX)

Monodispersed poly(N-isopropylacrylamide) (PNIPAM) nanoparticles, with the hydrodynamic radius Rh less than 50 nm at room temperature (about two orders of magnitude smaller than those reported previously), have been synthesized in the presence of a large amount of emulsifiers. These microgel particles undergo a swollen-collapsed volume transition in an aqueous solution when the temperature is raised to around 34C. The volume transition and structure changes of the microgel particles as a function of temperature are probed using laser light scattering and small angle neutron scattering (SANS) with the objective of determining the small particle internal structure, particle-particle and particle-solvent interactions. We find that, within the resolution of the experiments, these particles have a uniform radial cross-linker density on either side of the transition temperature, in contrast to previous reports on the heterogeneous structures of larger PNIPAM microgel particles, but in good agreement with recent reports based on computer simulations of smaller microgels. Furthermore, the particle interactions change across the transition temperature: at temperatures below the transition, the interactions appear to be described by a hard sphere contact potential, although there is evidence for a strong hard-sphere-like repulsive particle interaction; above the volume transition temperature, the potential is best described by an attractive interaction.

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