  PNIPAm microgels synthesized via free radical polymerization (FRP) are often
considered as neutral colloids in aqueous media, although it is well known,
since the pioneering work of Pelton and coworkers [Langmuir 1989, 5, 816-818],
that the vanishing electrophoretic mobility characterizing swollen microgels
largely increases above the lower critical solution temperature (LCST) of
PNIPAm, at which microgels partially collapse. The presence of an electric
charge has been attributed to the ionic initiators that are employed when FRP
is performed in water and that stay anchored to microgel particles. Combining
dynamic light scattering (DLS), electrophoresis, transmission electron
microscopy (TEM) and atomic force microscopy (AFM) experiments, we show that
collapsed ionic PNIPAm microgels undergo large mobility reversal and reentrant
condensation when they are co-suspended with oppositely charged
polyelectrolytes (PE) or nanoparticles (NP), while their stability remains
unaffected by PE or NP addition at lower temperatures, where microgels are
swollen and their charge density is low. Our results highlight a somehow
double-faced electrostatic behavior of PNIPAm microgels due to their tunable
charge density: they behave as quasi-neutral colloids at temperature below
LCST, while they strongly interact with oppositely charged species when they
are in their collapsed state. The very similar phenomenology encountered when
microgels are surrounded by polylysine chains and silica nanoparticles points
to the general character of this twofold behavior of PNIPAm-based colloids in
water.

Soft Condensed Matter;Materials Science