par Capponi, Simona;Napolitano, Simone ;Wudie;bbenhorst, Michael
Référence Non-equilibrium Phenomena in Confined Soft Matter: Irreversible Adsorption, Physical Aging and Glass Transition at the Nanoscale, Springer International Publishing, page (227-244)
Publication Publié, 2015-08
Partie d'ouvrage collectif
Résumé : Recent models of the glass transition rationalize the relevant features of vitrification by introducing the concept of medium range order in supercooled liquids. In particular, a crucial role is assigned to the orientational order induced by intermolecular bonding (bond orientational order, BOO). Although this idea is very appealing, severe difficulties limit the experimental determination of the BOO in molecular liquids: because of the small difference in entropy and in form factor between the isotropic liquid and the phase rich in orientational order, calorimetry and scattering techniques cannot be used to assess the BOO. In this chapter we describe an innovative model to investigate BOO via broadband dielectric spectroscopy. We verified that ultrathin films of polyols deposited by physical vapor deposition below their glass transition temperature show an extraordinary enhancement in bond orientational, indicated by a huge enhancement of the dielectric strength with respect to the bulk values. Hint of an underlying phase transition was found from a liquid phase enriched in bond orientational order, with a metastable character, towards an ordinary liquid phase. The kinetic stability of the metastable phase could be tuned by varying the deposition conditions and the molecular size. The impact of film thickness and of the presence of a solid substrate on the persistence of enhanced BOO were also investigated. We demonstrate that confinement stabilizes the non-equilibrium character of our supercooled liquids with enhanced orientational order.