  Combining elastic incoherent neutron scattering and differential scanning
calorimetry, we investigate the occurrence of the volume phase transition (VPT)
in very concentrated PNIPAM microgel suspensions, from a polymer weight
fraction of 30~wt\% up to dry conditions. Although samples are arrested at the
macroscopic scale, atomic degrees of freedom are equilibrated and can be probed
in a reproducible way. A clear signature of the VPT is present as a sharp drop
of the mean square displacement of PNIPAM hydrogen atoms obtained by neutron
scattering. As a function of concentration, the VPT gets smoother as dry
conditions are approached whereas the VPT temperature shows a minimum at about
43~wt\%. This behavior is qualitatively confirmed by calorimetry measurements.
Molecular dynamics simulations are employed to complement experimental results
and gain further insights into the nature of the VPT, confirming that it
involves the formation of an attractive gel state between the microgels.
Overall, these results provide evidence that the VPT in PNIPAM-based systems
can be detected at different time- and length-scales as well as in overcrowded
conditions.
