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.

Read all news...
Multiscale Simulation of Liquid-Crystals, Polymer Melts and Rubbers

Multiscale modeling is a valuable tool for the study of phenomena taking place at different time and length scales and is able to contribute complementary information to experiments and theories. Liquid crystals exhibit a number of intermediate phases between the isotropic and crystalline ones. From a general point of view, they can be divided into thermotropic and lyotropic liquid-crystals, depending on whether the liquid-crystalline phase appears due to a change of temperature or in the concentration of a solvent, respectively. The first category contains either rod-like or disk-shaped molecules, whereas the second category includes at least two-component systems, such as lipid bilayers (e.g DPPC membrane). Both thermotropic and lyotropic liquid crystals are included in the research interests of COMSE group. We simulate these systems at atomistic and mesoscopic levels, paying special attention to the coarse-graining procedure, which is based on a systematic method (e.g Iterative Boltzmann Inversion or Force Matching). After the development of reliable mesoscopic models, we study structural, dynamical and thermodynamic properties. In the area of lipid bilayers, interactions with other molecules (e.g drugs) are of tremendous importance. Our group is conducting both molecular dynamics simulations and umbrella sampling simulations by employing the wham algorithm. Another area of research is the computation of rheological properties of polymer melts and mechanical properties of rubbers. Due to the nature of polymeric systems, atomistic simulations are not able to cover the needed time and length scales and therefore we have to resort to appropriate mesoscopic models. One approach is to use Brownian dynamics, while the entanglement effect is introduced through hopping/creation/destruction of slip-springs based on a kinetic Monte Carlo scheme. In order to simulate uniaxial elongation of rubbers, a deformation scheme has been developed, based on imposing the strain and Poisson's ratio and measuring the stresses along the elongation direction. The stress-strain curve of the simulated material is obtained from this procedure and the elastic modulus (E) is computed from the slope of the linear region of this curve.

Figure 1: Snapshot from a molecular dynamics simulation of a discotic liquid crystal at mesoscopic level.

Figure 2: Reverse mapping of a discotic liquid crystal from the coarse-grained to atomistic level.

(a) (b)

Figure 3: (a) Schematic representation of a hydrated lipid bilayer containing one drug molecule. (b) Potential of mean force for a drug along a selected reaction coordinate of the lipid bilayer using the wham algorithm. (figure provided by Grigorios Megariotis in the context of the publication: Fotakis, C.; Megariotis, G.; Christodouleas, D.; Kristi, E.; Zoumpoulakis, P.; Ntoutaniotis, D.; Zervou, M.; Potamitis, C.; Hodzic, A.; Pabst, G.; Rappolt, M.; Mali, G.; Baldus, J.; Glaubitz, C.; Papadopoulos, M. G.; Afantitis, A.; Melagraki, G.; Mavromoustakos, T. Biochimica et Biophysica Acta-Biomembranes 2012, 1818, 3107.[REF])

Figure 4: (a) Visualisation of a simulation box containing polymeric chains at the level of Kuhn segments (obtained from Field Theory Inspired Monte Carlo simulation). (b) Visualization of the simulation box containing polymeric chains at the level of beads, each bead consisting of a predefined number of Kuhn segments (coarse-graining of (a)).

(a) (b)

Figure 5: (a) Stress relaxation function of a cis-1,4 polyisoprene melt computed by a Brownian dynamics simulation coupled with a kinetic Monte Carlo scheme for the hopping of slip-springs (used for the inclusion of entanglement effect). (b) Loss and storage moduli computed by a Fourier Transform of the stress relaxation function.

Figure 6: Stress-strain curve of a rubber under uniaxial elongation.

Relevant publications

[1] Megariotis, G.; Vyrkou A.; Leygue, A.; Theodorou D. N. "Systematic Coarse Graining of 4-Cyano-4'-pentylbiphenyl" Ind. Eng. Chem. Res. 2011, 50, 546.

Relevant projects

[1] EU META-ASSEMBLY, Self-Assembly and Dynamics in Metastable States. From Molecular and Supramolecular to Mesoscopic Systems.
[2] Mesoscopic Simulations of Viscoelastic Properties of Networks.
[3] EU MNIBS, Multiscale Modeling of Nanostructured Interfaces for Biological Sensors.