We explore the dynamics of viscous propylene glycol (PG) near its glass transition for the case of soft spatial confinement. The supercooled liquid is geometrically restricted by the reverse micelles of a glass-forming PG/AOT/decalin microemulsion, with the intramicellar dynamics being probed by triplet state solvation dynamics. While hard confinement by porous solids is known to result in slower dynamics and an increased glass transition temperature T(g) of PG, the nanodroplets suspended in a more fluid environment display faster structural relaxation, equivalent to a reduction of T(g) as observed in freestanding polymer films.

Using solvation techniques we explore the dynamics of nanoconfined and interfacial supercooled liquids near their glass transition temperatures. Confinement is accomplished by the use of porous glasses with pore diameters between 4 and 7.5 nm, and by microemulsions with droplet sizes between 2.6 and 5 nm. Via the attachment of the probe molecules to the inner surface of porous glasses filled with 3-methylpentane, the interfacial layer is measured selectively for different surface curvatures and shows an increase of the relaxation time by more than three orders of magnitude over that of the bulk liquid. This frustration is most pronounced at the pore boundary and decreases gradually across the first few nanometers distance away from the interface. Nanoconfinement realized by glass-forming microemulsions with the confining material being more fluid than the intramicellar droplets reverse the situation from frustrated dynamics in hard confinement to accelerated relaxation behavior in the case of soft boundaries. Without changing the size of the geometrical restriction of propylene glycol, hard versus soft boundary conditions changes the glass transition shift ⌬T g from +6 K to − 7 K. The findings can be rationalized on the basis of certain interfacial dynamics which differ from bulk behavior, with the penetration of this effect into the liquid being governed by the length scale of cooperativity. This picture explains both the more disperse relaxation times in confinement and the dependence of the average relaxation time on pore size.

No abstract

No abstract

The slow dynamics associated with the structural relaxation of glass forming materials near the glass transition is very sensitive to the effects of small confining geometries. Based upon the experimental results of triplet state solvation dynamics, we explore the extent to which confinement effects can be rationalized solely in terms of interfacial dynamics which are modified relative to the bulk situation. The importance of the interfacial conditions is emphasized by observing the changes due to the surface chemistry, by comparing relaxation times at and further away from the surface, and by studying the effects of ‘soft’ versus ‘hard’ confining materials. While ‘hard’ confinement by porous solids is observed to result in slower dynamics and an increased glass transition temperature Tg for propylene glycol, our 4.6 nm nanodroplets suspended in a more fluid environment display faster structural relaxation, equivalent to a reduction of Tg as observed in free standing polymer films.