Influence of the inter-chain hydrogen bonds on the thermoresponsive swelling behavior of UCST-like microgels

UCST-like microgels constituted by chains of poly(acrylic acid) (PAAc) and poly(acrylamide) (PAAm) show an intense swelling behavior when they are heated that is not comparable with the observed in neat poly(acrylamide) neither neat poly(acrylic acid) microgels. In order to figure out the microscopic changes that occur within the microgels, we have used quasielastic neutron scattering (IQNS) and FTIR-ATR techniques in order to study the polymer dynamics and the chemical groups, which are involved along this process. With IQNS we have observed that at 290 K, the co-existence of PAAc and PAAm in the microgels, produced a substantial reduction of the vibrational motion of the polymer chains as well as a reduction of the segmental diffusion coefficient. By contrast when the microgels were heated at 330 K, we observed an increment of the oscillations of the vibrational component as well as a significant increment of the segmental diffusion coefficient. The FTIR-ATR experiments indicated that during the microgel heating, there is a rupture of the intramolecular hydrogen bonds existing between the groups COOH and CONH2 of neighboring polymer chains. This process is reversible and these interactions reappear again when the system is cooled at 290 K, during the collapse of the microgels.