  We combine small angle scattering experiments and simulations to investigate
the internal structure and interactions of composite
Poly(N-isopropylacrylamide)-Poly(ethylene glycol) (PNIPAM-PEG) microgels. At
low temperatures the experimentally determined form factors and the simulated
density profiles indicate a loose internal particle structure with an extended
corona, that can be modeled as a star-like object. Increasing temperature
across the volumetric phase transition, the form factor develops an inflection
which, using simulations, is interpreted as arising from a configuration in
which PEG chains are incorporated in the interior of the PNIPAM network. In
this configuration a peculiar density profile characterized by two dense,
separate regions is observed, at odds with configurations in which the PEG
chains reside on the surface of the PNIPAM core. The conformation of the PEG
chains have also profound effects on the interparticle interactions: While
chains on the surface reduce the solvophobic attractions typically experienced
by PNIPAM particles at high temperatures, PEG chains inside the PNIPAM network
shift the onset of attractive interaction at even lower temperatures. Our
results show that by tuning the morphology of the composite microgels we can
qualitatively change both their structure and their mutual interactions,
opening the way to explore new collective behaviors of these objects.
