Glassy Dynamics in Nanoconfinement as Revealed by ³¹P NMR

Abstract

We investigated the glass former m-tricresyl-phosphate confined in different nanoporous silica matrices with defined pore radii from 2-150 nm. While applying different 31P NMR techniques, we were able to detect the extremely stretched correlation functions extending over 7-8 decades in time and reflecting strong dynamic heterogeneities. The experimental results were explained by a topological model for which the broad distribution of correlation times G(ln tau) becomes inhomogeneous in space; that is, the "local" dynamics given by a correlation time tau(r) depend on the distance from the pore center. As tau(r) changes with temperature, we were able to reintroduce the idea of a dynamic correlation length.

Accelerated physical aging in PMMA/silica nanocomposites

“…However, such an interpretation contrasts with simulation studies by Scheidler et al, 43 who showed that the mobility near a surface is affected only in the first 1-2 nanometres from the surface. This interpretation is also in disagreement with experimental observations by solvation dynamics 44 and nuclear magnetic resonance 45 in low molecular weight glass formers confined in nanopores, which provide evidence that the molecular mobility of the bulk system is restored at distances not larger than 4 nm from the external surface. Furthermore, recent BDS experiments on ultra-thin polymer films do not provide an indication of a change in segmental dynamics in comparison to bulk polymers at thicknesses as small as 10 nm.…”

“…In particular, deviations from the bulk aging rate are observed at length scales of confinement which are far too large to induce any change in the molecular mobility as a result of either finite size 9,40,41,42 or interfacial effects. 43,44,45 As far as polymer thin films of the order of several hundred nanometers thick are concerned, the acceleration of physical aging in comparison to the bulk polymer has been explained invoking a mechanism based on diffusion of free volume holes and their annihilation at the external surface. 27,[35][36][37][38] The idea of a diffusion mechanism for physical aging has been considered since long ago.…”

Role of Glassy Bridges on the Mechanics of Filled Rubbers under Pressure

“…Assuming that the aggregate radius does not vary with the strain amplitude and from the literature, R ag = 50 nm, R p = 5 nm and E g = 2 GP a, we find a value of approximately δ = 0.5 nm, which (order of magnitude) agrees with the value reported in previous studies. [54][55][56] Consequently, these experimental data allow an indirect measurement of this parameter within real samples.…”

“…where T ∞ g is the bulk glass transition temperature of the polymer matrix and δ ∼ 1 nm fixes the range of the T g gradient measured by some authors. [53][54][55][56] Thus, the key point of our model is the value of the glass transition temperature at the half distance T g (z = h 0 /2) between neighboring particles compared to the temperature of the experiment. Because the value of the modulus of polymer chains abruptly decreases above T g , we assume that the mechanical response of a polymer bridge is similar to that of polymer chains located at the middle of the bridge.…”

Confinement Effects on Glass-Forming Aqueous Dimethyl Sulfoxide Solutions

“…Explicitly, simulation studies found that molecular dynamics can be bulk-like in pore centers but orders of magnitude slower at pore walls [ 30 , 31 , 32 ]. Accordingly, experimental works employed core-shell models to describe motional inhomogeneity across confinements [ 33 , 34 ]. However, reorientation and diffusion dynamics may be affected to different degrees, as reports on a breakdown of the Stokes–Einstein–Debye relation for confined water indicate [ 35 , 36 ].…”