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Combining elastic incoherent neutron scattering and differential scanningcalorimetry, we investigate the occurrence of the volume phase transition (VPT)in very concentrated PNIPAM microgel suspensions, from a polymer weightfraction of 30~wt\% up to dry conditions. Although samples are arrested at themacroscopic scale, atomic degrees of freedom are equilibrated and can be probedin a reproducible way. A clear signature of the VPT is present as a sharp dropof the mean square displacement of PNIPAM hydrogen atoms obtained by neutronscattering. As a function of concentration, the VPT gets smoother as dryconditions are approached whereas the VPT temperature shows a minimum at about43~wt\%. This behavior is qualitatively confirmed by calorimetry measurements.Molecular dynamics simulations are employed to complement experimental resultsand gain further insights into the nature of the VPT, confirming that itinvolves the formation of an attractive gel state between the microgels.Overall, these results provide evidence that the VPT in PNIPAM-based systemscan be detected at different time- and length-scales as well as in overcrowdedconditions.

Atomic scale investigation of the volume phase transition in concentrated PNIPAM microgels