Analysis of the volume transition in thermosensitive core-shell particles by synchrotron small-angle X-ray scattering

We discuss the analysis of the volume transition in thermosensitive colloid al core-shell particles by synchrotron small-angle X-ray scattering. The la tex-particles are dispersed in water and consist of a solid poly( styrene) core (diameter: 107 nm) and a shell of a thermosensitive crosslinked polyme r chains. The thermosensitive shell is built up from poly( N-isopropylacryl amide) chains (PNIPA) crosslinked by N,N'-methylenbisacrylamide (BIS). The scattering intensities measured as function of temperature may be decompose d into a part deriving from the overall structure and a part originating fr om the fluctuations within the network. The analysis of the overall structu re leads to the volume fraction of the swollen network at different tempera tures. The intensity caused by the fluctuations of the network becomes the leading contribution at high scattering angles. It may be described by a Lo rentzian below and above the volume transition. The analysis demonstrates t hat critical fluctuations of the network around the transition temperature are fully suppressed. This finding is explained by the strong steric constr aint of the network by its confinement within a shell of colloidal dimensio ns: i) The swelling and shrinking can only take place along the radial dire ction, and ii) the chains are bound to the solid surface of the cores which remains constant during the transition.