The volume transition in thermosensitive micronetworks as observed by small-angle X-ray scattering

We present a study of the volume transition in colloidal particles composed of a temperature independent core and a shell of thermosensitive crosslink ed polymer chains by small-angle X-ny scattering (SAXS). The poly(styrene) (PS) cores of the particles have a diameter of 80 nm whereas the shell comp osed of cross-linked poly(N-isopropylacrylamide) has a thickness of 32 nm i n the swollen state at 25 degrees C which shrinks to 18 nm at 50 degrees C during a continuous volume transition. The SAXS-intensities measured at hig h scattering angles could be described by a Lorenz-type function at both st ates. This indicates the presence of marked liquid-like local concentration fluctuations of the gel with a correlation length xi of the order of a few nanometers even in the collapsed state (25 degrees C: xi = 3.2 nm; 50 degr ees C: xi = 2.1 nm). The analysis thus shows that the core-shell microgels behave in a distinctly different manner than ordinary thermosensitive gels. This is discussed in terms of the spatial constraint exerted by the core-s hell morphology onto the network.