Volume phase transition mechanism of poly[oligo(ethylene glycol)methacrylate] based thermo-responsive microgels with poly(ionic liquid) cross-linkers.

Thermo-dynamic volume phase transition mechanisms of poly[oligo(ethylene glycol)methacrylate] (POEGMA) based microgels with poly(ionic liquid) (PIL) cross-linking moieties are investigated in detail on the basis of temperature-dependent Fourier transform infrared (FTIR) spectroscopy. The original FTIR data are further analysed by two-dimensional correlation spectroscopy (2Dcos) with the perturbation correlation moving window (PCMW) technique. It is observed that the content of hydrophilic PIL cross-linking structure strongly affects the temperature induced volume phase transition mechanism of microgels in which the less cross-linked microgel exhibits a sharp volume phase transition process while the highly cross-linked microgel presents a broad transition behavior. Peculiarly, the dehydration of C-H groups acts as the driving force for the whole phase transition process within the less cross-linked microgel network and cooperative response of chemical groups is identified. It is deduced that the hydrophilic PIL moieties develop polymer-water-polymer interactions with C=O groups as C=O···D2O-PIL hydrogen bonds emerge in the less cross-linked system. As regards the highly cross-linked microgel system, the phase transition process is driven by the disruption of hydrogen bonds between C=O groups and water molecules while the response of C-H groups becomes insensitive. PIL moieties passively dehydrate following the dehydration of C-H groups on oligo(ethylene glycol) side chains and no hydrogen bond between C=O group and IL-D2O association appears during the phase transition process.