National Technical University of Athens
School of Chemical Engineering
Department of Materials Science and Engineering
Computational Materials Science and Engineering Group (Co.M.S.E.)

On the 17th of September 2017, Prof. Doros N. Theodorou was awarded with the European Materials Medal, in recognition of development of new statistical mechanics-based molecular and multiscale simulation methods for the calculation of thermodynamic, structural, interfacial, and permeability properties of polymeric materials, and zeolites.

Sincerest congratulations to our Master thesis student Dora Argyropoulou on winning a postgraduate studies scholarship of the Bodossaki Foundation.

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Polymer melts at interfaces

Understanding the fascinating and complex dynamics of melts of large flexible polymer coils close to solid substrates has been an ongoing challenge for many decades. From the point of view of molecular simulations, the spectrum of length and time scales associated with polymer melts of long chains poses a formidable challenge to studying the long-time dynamics.

We have formulated a method, based on combining self-consistent field theory with dynamically corrected transition state theory, for estimating the rates of adsorption and desorption of end-constrained chains e.g., by cross-links or entanglements) from a polymer melt onto a solid substrate. This approach has been successfully applied on a polyethylene/graphite system, where the whole methodology was parameterized by atomistically detailed molecular simulations. For short-chain melts, which can still be addressed by molecular dynamics simulations with reasonable computational resources, the self-consistent field approach gives predictions of the adsorption and desorption rate constants which are gratifyingly close to molecular dynamics estimates.

Figure 1: Schematic illustration of an end-constrained subchain of 50 CH2 units (marked in red), whose residence times in the adsorbed and desorbed state, in the course of an MD simulation, were used for estimating adsorption and desorption kinetics via hazard-plot analysis. External link:

To the best of our knowledge, this the first time that the solution of the polymer SCF problem next to a solid surface has been used to provide insight into the dynamics of the system. This work suggests that SCF, coupled with transition-state theory, can give good results for the rate of adsorption/ desorption between polymers in a melt and a surface. SCF theory is less computationally demanding than atomistic simulations, and thus SCF can be used to examine regimes not practically accessible to atomistic simulations, such as the regimes of low adsorption or low desorption rates. Furthermore, the parameters in SCF have been directly connected to atomistic parameters, suggesting that SCF may provide a more faithful description of the problem than alternative, coarse-grained simulation methods. In the regime where molecular dynamics simulation of adsorption rates is practical (short chains, very close to the surface), our results for the adsorption rate are in good agreement with the results from molecular dynamics simulation. Finally, a reasonable agreement between the melt adhesion tension computed from SCF and that measured in atomistic simulations and in experiment, was found.

Figure 2: Adsorption rate constant, predicted by Self Consistent Field / Transition State Theory calculations, is presented as a function of the distance of the ends of an end-constrained subchain from the graphite surface. Both ends are kept at the same distance from the graphite surface, and we consider chain lengths of N = 50 and N = 100 methylenes. Adsorption rates for an end-constrained C50 subchain in a C200 polyethylene melt next to graphite obtained by MD simulation using a detailed united-atom model are also shown. External link:

Relevant publications

[1] Daoulas, K. C.; Theodorou, D. N.; Harmandaris, V. A.; Karayiannis, N. C.; Mavrantzas, V. G. "Self-Consistent-Field Study of Compressible Semiflexible Melts Adsorbed on a Solid Substrate and Comparison with Atomistic Simulations" Macromolecules 2005, 38, 7134-7149.
[2] Theodorou, D. N.; Vogiatzis, G. G.; Kritikos, G. "Self-Consistent-Field Study of Adsorption and Desorption Kinetics of Polyethylene Melts on Graphite and Comparison with Atomistic Simulations" Macromolecules 2014, 47, 6964-6981.

Relevant projects

[1] EU COMPNANOCOMP, Multiscale computational approach to the design of polymer-matrix nanocomposites.