Analysis of thermosensitive core–shell colloids by small-angle neutron scattering including contrast variation

We present an investigation of thermosensitive core–shell particles by small-angle neutron scattering (SANS). The particles consist of a solid poly(styrene) core and a shell of crosslinked poly(N-isopropylacrylamide) (PNIPA) chains. These latex particles are dispersed in water and have a diameter of ca. 150 nm. At ambient temperature the PNIPA-network in the shell is swollen but at higher temperature water is expelled and the shell undergoes a continuous volume transition. The radial extension of the shell is investigated as a function of temperature by use of SANS. The analysis by SANS is performed at different contrasts using appropriate mixtures of H2O and D2O. It demonstrates that the shell has a well-defined compact structure above the volume transition. The swelling of the shell upon cooling can be described in terms of an affine expansion of the network. This is followed by a slight decrease of the volume fraction with increasing distance to the surface of the cores. The analysis by SANS demonstrates that the phase behavior of the network in the shell may be undertaken in terms of average volume fractions. It thus supplements the previous analysis by SAXS in a decisive manner.