  The influence of shape fluctuations on deformable thermosensitive microgels
in aqueous solution is investigated by dynamic light scattering (DLS) and
depolarized dynamic light scattering (DDLS). The systems under study consist of
a solid core of polystyrene and a thermosensitive shell of cross-linked
poly(N-isopropylacrylamide) (PNIPA) without and with embedded palladium
nanoparticles. PNIPA is soluble in water, but has a lower critical solution
temperature at 32 C (LCST). Below the LCST the PNIPA shell is swollen. Here we
find that besides translational and rotational diffusion, the particles exhibit
additional dynamics resulting from shape fluctuations. This leads to a
pronounced apparent increase of the rotational diffusion coefficient. Above the
transition temperature the shell collapses and provides a rather tight envelope
of the core. In this state the dynamics of the shell is frozen and the
core-shell particles behave like hard spheres. A simple physical model is
presented to capture and explain the essentials of the coupling of rotational
motion and shape fluctuations.
