Volume phase transition of 'smart' microgels in bulk solution and adsorbed at an interface: A combined AFM, dynamic light, and small angle neutron scattering study

In the present article the swelling behavior of copolymer microgel particles made of poly(N-isopropylacrylamide)-co-vinylacetic acid using dynamic light scattering (DLS), neutron scattering, and in situ atomic force microscopy (AFM) for various copolymerized amounts of vinylacetic acid (VA) (up to 2.5 mol%) under slightly acidic conditions is studied. The transition temperature of these microgel particles is found to be ≈32.5 ± 1 °C, independent of the VA content. Microgel particles adsorbed onto a solid substrate display a similar volume phase transition as their dissolved counterparts. However, their swelling capacity is reduced by approximately one order of magnitude compared to the bulk value. Nevertheless, the observed effect still is sufficiently large to be exploited for the use of these particles in sensors or as nanoactuators. In addition it can be concluded that the continuous character of the transition observed in solution does not arise from the polydispersity of the particles but can be attributed to the heterogeneity inside each individual microgel particle. Finally, AFM images reveal a pattern on the surface of the collapsed particles, which we attribute to globules formed by collapsed dangling polymer chains. In solution these dangling ends form a brush contributing to the hydrodynamic dimensions of the microgels.