Water dynamics at low temperatures by confinement in PNIPAM microgels

Water behavior in the supercooled temperature regime cannot be experimentally investigated because of the onset of crystallization. A practical strategy to work around water nucleation is that of confining water in environments such as nanopores of silica glass or proteins. In this work, we exploit the complex internal structure of a different class of systems such as PNIPAM microgels. Poly(N-isopropylacrylamide), PNIPAM, is a thermo-responsive polymer which is soluble in water at temperatures below ~305 K, the lower critical solution temperature (LCST). At the LCST a cooperative structural rearrangement takes place. PNIPAM phase behavior across the LCST has been widely investigated because, by translating the linear chains behavior into the temperature induced volume phase transition of cross-linked microgels, several applications as smart materials are obtained.Here, by using atomistic molecular dynamics simulations combined with elastic incoherent neutron scattering experiments, we probe the structural and dynamical properties of PNIPAM microgels and the polymer-induced water properties variations in an unexplored range of the phase diagram, i.e. the supercooled temperature regime. Different hydration degrees, with a PNIPAM mass fraction from 43% to 95%, are compared. The computational methodology relies on previous modelling studies that characterized PNIPAM behavior in aqueous solution [1, 2]. The temperature dependence of the mean squared fluctuations of PNIPAM hydrogen atoms and the self intermediate scattering functions detected in the simulations suggests the presence of a dynamical transition of the microgel at Td~250. Water single particle translational dynamics is found to be characterized by two dynamic regimes with a linear behavior in the Arrhenius plot. The discontinuity of water dynamics takes place at the same temperature Td indicating a coupling between PNIPAM and hydration water dynamics. Elastic incoherent neutron scattering experiments show that PNIPAM microgels efficiently prevent water crystallization, even in conditions where the majority of the suspension is made of water. A clear variation of the scattering intensity is observed at the same Td confirming the occurrence of a dynamical transition which shares many features in common with that observed for proteins [3].REFERENCES1. E. Chiessi, G. Paradossi, J. Phys. Chem. B 2016, 120, 3765-3776.2. L. Tavagnacco, E. Zaccarelli, E. Chiessi, PCCP, 2018, in press.3. M. Zanatta et al., submitted, arxiv 1802.04841.