Volume Phase Transition Mechanism of Poly[di(ethylene glycol)ethyl ether acrylate]-Based Microgels Involving a Thermosensitive Poly(ionic liquid)

The microdynamic volume phase transition mechanism of poly[di(ethylene glycol)ethyl ether acrylate] (PDEGA)-based microgels with newly developed thermoresponsive polyionic liquid (PIL) (poly(tetrabutylphosphonium styrenesulfonate) P[P4,4,4,4][SS]) moieties was studied by applying temperature-variable Fourier transform infrared (FTIR) spectroscopy in combination with two-dimensional correlation spectroscopy (2Dcos) and the perturbation correlation moving window (PCMW) technique. It can be found that the content of hydrophilic PIL moieties plays a significant role in the thermally induced phase transition behavior of microgel systems; namely, the microgels containing fewer PIL moieties present a sharp transition behavior and a gel-like state (10%, w/v) in water whereas the microgels with more PIL moieties undergo a slightly broad phase transition process and a flowable solution state. Herein, the C═O···D2O-PIL hydrogen bonds as the interaction between PDEGA and P[P4,4,4,4][SS] moieties result in a complete dehydration process for the microgels with fewer PIL moieties and the dehydrated behavior of SO3– groups acts as the driving force during the phase transition. As for the microgels with more PIL moieties, the whole transition process is dominated by the hydrophobic interaction of C–H groups. Even though the intermolecular hydrogen bonds (C═O···D2O-PIL) appear as well, the more remarkable effect of the Coulombic repulsive force of PIL restrains the water molecules from breaking away, thus causing a gradual and incomplete dehydration process during heating.