  The peculiar linear temperature-dependent swelling of core-shell microgels
has been conjectured to be linked to the core-shell architecture combining
materials of different transition temperatures. Here the structure of
pNIPMAM-core and pNNPAM-shell microgels in water is studied as a function of
temperature using small-angle neutron scattering with selective deuteration.
Photon correlation spectroscopy is used to scrutinize the swelling behaviour of
the colloidal particles and reveals linear swelling. Moreover, these
experiments are also employed to check the influence of deuteration on
swelling. Using a form free multi-shell reverse Monte Carlo approach, the
small-angle scattering data are converted into radial monomer density profiles.
The comparison of 'core-only' particles consisting of identical cores to fully
hydrogenated core-shell microgels, and finally to H core/D shell architectures
unambiguously shows that core and shell monomers display gradient profiles with
strong interpenetration, leading to cores embedded in shells which are bigger
than their isolated 'core only' precursor particles. This surprising result is
further generalized to different core cross linker contents, for temperature
ranges encompassing both transitions. Our analysis demonstrates that the
internal structure of pNIPMAM-core and pNNPAM-shell microgels is heterogeneous
and strongly interpenetrated, presumably allowing only progressive core
swelling at temperatures intermediate to both transition temperatures, thus
promoting linear swelling behaviour.
