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 colloidal core-shell particles by synchrotron small-angle X-ray scattering. The latex-particles are dispersed in water and consist of a solid poly(styrene) core (diameter: 107 nm) and a shell of a thermosensitive crosslinked polymer chains. The thermosensitive shell is built up from poly(N-isopropylacrylamide) chains (PNIPA) crosslinked by N,N'-methylenbisacrylamide (BIS). The scattering intensities measured as function of temperature may be decomposed into a part deriving from the overall structure and a part originating from the fluctuations within the network. The analysis of the overall structure leads to the volume fraction of the swollen network at different temperatures. The intensity caused by the fluctuations of the network becomes the leading contribution at high scattering angles. It may be described by a Lorentzian below and above the volume transition. The analysis demonstrates that critical fluctuations of the network around the transition temperature are fully suppressed. This finding is explained by the strong steric constraint of the network by its confinement within a shell of colloidal dimensions: i) The swelling and shrinking can only take place along the radial direction, and ii) the chains are bound to the solid surface of the cores which remains constant during the transition.