Computer-aided simulation of the influence of collective effects on polymer-melt dynamics in a straight cylindrical tube: Observation of the onset stage of the corset effect

Abstract

The dynamics of polymer melts confined in porous nano and mesoscale media has been intensively stud ied in recent years . These studies were to a large extent stimulated by the unusual phenomena detected at frequencies of 0.1-100 MHz that were first observed in the experiments devoted to the frequency dependence of the nuclear spin-lattice relaxation rate of polymer melts in porous systems [5,7,16,24]. These phenomena are known as the corset effect. In general, the basis of the corset effect is as follows. The dynamic properties of polymer melts in porous sys tems may considerably differ from the corresponding properties of polymer melts without any spatial con straints at high frequencies (small times ), when the rms displacements of polymer segments 1 This work was supported by the Russian Foundation for BasicResearch (project no. 10 03 00739 a) and the DAAD program Mikhail Lomonosov II ω 1 t ∝ ω are much smaller than the squared characteris tic radius of a tube, . Note that the specified differ ences were observed even for tubes whose radii were several times larger than the characteristic lengths of polymer chains. In other words, a mutual noncross ability of polymer chains and a low compressibility of the polymer melt lead to the specific collective effect: Polymer segments span the medium via rapidly prop agating acoustic waves and respond to the presence of impermeable tube walls by changing the character of thermal fluctuations relative to those of the common melt much earlier than the direct physical contact with the tube wall ensues.At present, controversial opinions about the possi bility of the corset effect can be found in the literature. In studies denying the existence of the corset effect, it is assumed that the influence of confinements due to tube walls on the dynamics of polymer chains can be 2 ( ) r t tube 2 r Abstract-The results of the computer aided simulation of the dynamics of a polymer melt consisting of Fraenkel chains in straight cylindrical tubes and in bulk are discussed. Two different models are studied. In the first model, the dynamics of the polymer melt is simulated via the molecular dynamics simulation. The interaction of unbound polymer segments is described by the Lennard Jones potential, which excludes any chain crossing of macromolecules and generates collective acoustic waves. In the second model, which serves as a reference, the system is studied via the Brownian dynamics method, in which intermolecular interactions are allowed for phenomenologically via friction and stochastic Langevin forces. In this case, cooperative effects are absent and the effect of spatial confinements makes itself evident only in a narrow near wall layer. For the two models under consideration, there is a significant difference in the decay of dynamic correlation functions = , where averaging is performed over all macromolecular segments and is the component of the end to end segment vector ( , and the cylindrical axis of the tube is directed along the z axis). For the first model allowing for col...

Molecular Dynamics in Polymers

“…[108]. The conclusion is that surface interactions, as expected, fade away with increasing pore diameter, whereas the collective slowing down of reorientational dynamics upon mesoscopic confinement is much more persistent in this respect.…”

“…A rather tight tube diameter must therefore be concluded in the frame of this concept if the strong anisotropy of rotational fluctuations under mesoscopic confinements is to be explained. Actually, the phenomenon was recently confirmed with the aid of computer simulations [108]. In this way, the confinement effect could clearly be reproduced on a time scale beyond 10 Â t s .…”

Soft matter in hard confinement: phase transition thermodynamics, structure, texture, diffusion and flow in nanoporous media

“…Over the years it has turned out, however, that this comparison is often hampered, if one has to solely rely on diffusion data reported in the literature and extracted with different measurement techniques. Even for identical bulk or pore systems alternative experimental methods can result in deviating dynamical properties and upon confinement these differences in the measured quantities can even be more pronounced [328,329,330,331,332]. Different methods probe different length scales and thus also different time scales of diffusion processes.…”