Dynamics in confinement

P. A. Thompson, G. S. Grest, M. O. Robbins. Phase transitions and universal dynamics in confined fihns. Phys Rev Lett (55 3448-3451, 1992. 

A slight increase ( 10K) of Tg as determined by dilatometry is observed with decreasing film thickness. This is illustrated in the inset in Fig. 28a, giving the second numerical derivative of the sample capacitance as a function of temperature for two thicknesses, 310 nm and 17 nm. An increase of dilatometric Tg is often assigned in the literature to slower molecular dynamics in confinement... 

There could be two possible reasons for this apparently contradictory situation,. First, in the above discussion, we adopted a diffusion constant that was evaluated from the diffusion constant of polymer chains at the interface between two bulk polymeric layers [72]. Recent measurements have revealed the existence of heterogeneous dynamics in confined geometries such as thin films and nanopores [73,74]. For stacked thin films of polymers, the dynamics vary with an essential dependence on the distance of the layer of interest from the free surface or from the substrate [75]. If such dynamical heterogeneity is taken into account, the diffusion of polymer chains could be restricted by the existence of an immobile region. 

FIGURE 5.5 Arrow chart used by R. Richert in presentation at the Third International Conference on Dynamics in Confinement, Grenoble, France, April 2006. (Originally reproduced... 

M. Wittebrood, Phase Transitions and Dynamics in Confined Nematic Liquid Crystals, PhD thesis. University of Nijmegen (1997). 

Massa MV, Dalnoki-Veress K, Forrest JA (2003) Crystallization kinetics and crystal morphology in thin poly(ethylene oxide) films. Eur Phys J E 11 191-198 McKeima GB (2003) Status of our understanding of dynamics in confinement perspectives from confit 2003. Eur Phys J E 12 191... 

Concerning confined polymers exhibiting slowing-down of both the equilibrium and the out-of-equilibrium dynamics, the obvious conclusion is that there exists full interdependence between the two aspects of glassy dynamics in confinement. This agrees with findings on bulk glass former, which, as shown by several studies, display the same interdependence [20, 41, 74, 87, 153, 163, 180]. 

Free Interface and Out-of-Equilibrium Dynamics in Confined Polymers... 

In recognizing that arguments based on the rate of spontaneous fluctuations fail to describe the out-of-equilibrium dynamics and in view of recent activity on the influence of adsorption on the glassy dynamics in confinement, we identify the free interfacial surface as the main parameter determining the magnitude of negative deviations from bulk behaviour. In doing so, we emphasize how a suitable model for the description of the out-of-equilibrium dynamics in confinement must account for the presence of dominant bulk linear dynamics and the amount of free interface. We show how there exists solid indications that the FVHD model nicely fits in the idea that the free interface is the key parameter to describe the out-of-equilibrium dynamics in confinement. 

Recently, dynamic light scattering has also been widely used to study the dynamics in confined liquid crystalhne systems and a brief overview of different confinement geometries will be given in the following section. 

Mertelj, A. and Copic, M. (2000). Dynamic light scattering as a probe of orientational dynamics in confined liquid crystals, Phys. Rev. 61 1622-1628. 

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