Poly-N-isopropylacrylamide (PNIPAM) is a thermo-responsive polymer widely investigated as a “smart” material with a variety of applications, ranging from drug delivery to sensors development. PNIPAM microgels are colloidal-scale particles characterized by cross-linked networks that swell in water at room temperature giving rise to transparent gels. In the present work, the dynamical and structural properties of PNIPAM microgels and the polymer-induced water properties variations have been investigated upon cooling by means of molecular dynamics simulations.
The PNIPAM network model is built taking into account the inhomogeneous polymer radial density within the microgel, on the basis of the value of the PNIPAM repeating units/bis-acrylamide mole ratio, PNIPAM/BIS, used during the synthesis and of the maximum degree of swelling of such microparticle. The computational methodology is thus based on previous atomistic modelling studies that characterized the PNIPAM behavior in aqueous solution [1,2]. Different hydration degrees are investigated.
The behavior of the mean squared fluctuations of the PNIPAM hydrogen atoms and the self intermediate scattering functions as a function of the temperature suggests the presence of a dynamic transition of the microgel around 250K. A coupling of the PNIPAM network dynamics to the hydration water is also detected from the translational dynamics temperature dependence of the hydration water molecules. Computational results are compared to elastic incoherent neutron scattering experiments. A clear variation of the elastic incoherent scattering intensity observed around 250K confirms the occurrence of a dynamic transition analogous to that observed for proteins .
1. E. Chiessi, G. Paradossi, “Influence of tacticity on hydrophobicity of poly(N-isopropylacrylamide): a single chain molecular dynamics simulation study” J. Phys. Chem. B 2016, 120, 3765-3776.
2. G. Paradossi, E. Chiessi, “Solution behaviour of poly(N-isopropylacrylamide) stereoisomers in water: a molecular dynamics simulation study”, Phys. Chem. Chem. Phys, 2017, 19, 11892-11903.
3. E. Zaccarelli, M. Bertoldo, F. Natali, J. Ollivier, A. Orecchini, A. Paciaroni, M. Zanatta, “Fast and slow dynamics in PNIPAM microgels” 2015 Institut Laue-Langevin (ILL) doi:10.5291/ILL-DATA.9-11-1736.