  Microgel suspensions of Interpenetrated Polymer Network (IPN) of PNIPAM and
PAAc in D$_2$O, have been investigated through dynamic light scattering as a
function of temperature, pH and concentration across the Volume Phase
Transition (VPT). The dynamics of the system is slowed down under H/D isotopic
substitution due to the different balance between polymer/polymer and
polymer/solvent interactions suggesting the crucial role played by H-bondings.
The swelling behavior, reduced with respect to PNIPAM and water, has been
described by the Flory-Rehner theory, tested for PNIPAM microgel and
successfully expanded to higher order for IPN microgels. Moreover the
concentration dependence of the relaxation time at neutral pH has highlighted
two different routes to approach the glass transition: Arrhenius and
super-Arrhenius (Vogel Fulcher Tammann) respectively below and above the VPT
and a fragility plot has been derived. Fragility can be tuned by changing
temperature: across the VPT particles undergo a transition from soft-strong to
stiff-fragile.
