Abstract :
Nanometric confinement of molecular fluids is a classical route to stabilize metastable states by achieving frustrations of the bulk natural fluctuations and/or phase transitions. In the first chapter of the manuscript, we address the physics of water under confinement and in a second chapter, the specific case of a polymer melt.
Confinement of molecular liquids is a route to tune very significant temperature depressions of the melting point. This property has recently been intensively used in the quest for experimental evidences of the existence of a Low Temperature Critical Point (LTCP) in bulk liquid water, at Ts ~ 228 K and Ps ~ 100 MPa. Here, we highlight the surprisingly rich low temperature (from 100 to 300 K) dynamical behavior of interfacial water. Then, we propose a percolation model to account for the dynamical/thermodynamical transitions we observe at 150, 220 and 240 K and reach a global and coherent view of this two dimensional (2D) water. Due to dominant surface interactions, we question the relevance of confined water to prove the reality of the LTCP. Nevertheless, using interfacial water, we show that a liquid-liquid transition (a condition for the existence of the LTCP) involving water is possible.
Recently, a corset effect has been proposed: under confinement the reptation tube diameter of a polymer chain, would be only a few Angstroms i.e. one order of magnitude smaller than in bulk. In the second chapter, we describe an inelastic neutron scattering-based multiscale approach to polymer dynamics (bulk and confined) from the atomic scale at short time (ps), up to few tens of nanometers and long times (600 ns). Over this detailed study of the time and spatial dependence of the polymer relaxations we detect no corset effect.
When using nanometric confinement to obtain pure volume effects, next to the detrimental so-called surface effects evidenced in the first chapter, the significant physical insight lost by powder average of the spectroscopic observables is another limitation. In the second chapter, we illustrate how to take advantage of a macroscopically oriented confining matrix to lift this severe drawback. The ambition of the third and last chapter is to define a physical system, where macroscopic orientation meets nanometric confinement with no surface effects, to induce strong 1D pure volume effects over macroscopic distances. We discuss how such nano-pipes could enhance macroscopic flow, offering systems of prime interest to both fundamental and applied research.
Keywords: confinement, dimensionality, transport, water, polymer, NMR, neutron.
IRAMIS/LLB