We have performed small angle neutron scattering measurements (SANS) on dilute aqueous dispersions of polymer microgel particles as a function of temperature, $T$. The microgel particles are spherical crosslinked assemblies of a loose gel network of a poly-N-isopropylacrylamide (NIPAM) polymer. When the temperature is raised beyond a critical temperature, $T_{\mathrm{lc}}\approx 32°\mathrm{C}$, the polymer becomes more strongly attracted to itself than the solvent, and the microgel particles contract. The measured form factor, $F\left(q\right)$, for dilute suspensions of uniform microgel particles exhibits many peaks that are characteristic of solid polymer nanospheres. The position and amplitude of the peaks as a function of wave number, $q$, provide insight into the density profile of poly-NIPAM within the microgels. These peaks can be described well over a wide range of temperature by a model of the polymer density within the particles that is constant up to an inner radius, $R_1$, and decreases linearly to zero at an outer radius, $R_2$.

• Received 8 November 2004

DOI:https://doi.org/10.1103/PhysRevE.71.040801