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Seminar 10/7/2015 - Doros Theodorou, National Technical University of Athens: Multiscale Simulations of Polymer-Matrix Nanocomposites
Abstract: In polymer-matrix nanocomposites the quantitative relationships between composition and size of polymer chains and nanoparticles, processing conditions, degree of dispersion of the nanoparticles, dynamics of the matrix chains, and macroscopic properties are still elusive. Molecular simulation holds great promise as a means for understanding and predicting these relationships, but faces serious challenges associated with the broad spectra of length and time scales governing nanocomposite properties. We are developing a multiscale simulation strategy for materials consisting of nanoparticles of roughly spherical shape within amorphous polymer matrices. This strategy encompasses atomistic molecular dynamics (MD), coarse-grained connectivity-altering Monte Carlo (MC), Field Theory-inspired Monte Carlo (FTiMC), and Brownian Dynamics/Transition State theory (BD/TST) calculations. Each level of representation invokes parameters that can be extracted from the previous (more detailed) levels, such that all predictions are ultimately based on an atomistic force field. 

Atomistic MD is useful for elucidating the details of molecular packing and in quantifying how segmental dynamics is affected by the presence of nanoparticles.  Coarse-grained MC is of strategic importance in achieving equilibration at all length scales.  By developing coarse-grained effective potentials from detailed atomistic ones via the Iterative Boltzmann Inversion method, vigorous MC sampling of the coarse-grained models with connectivity-altering algorithms, and reverse-mapping back to the atomistic level, one can generate well-equilibrated atomistic configurations to study structure and dynamics. We have applied this strategy to quantify the effects of incorporating fullerenes on the segmental motion and the glass transition of long-chain atactic polystyrene (PS) [1].

In the FTiMC approach polymer chains are represented as freely jointed sequences of statistical segments, and nanoparticles as single spherical entities. Polymer non-bonded interactions are taken into account via a functional of local density, while nanoparticle-polymer and nanoparticle-nanoparticle interactions are represented via integrated atomistic potentials. This approach can elucidate changes in the conformation and spatial extent of polymer chains resulting from the presence of the nanoparticles. We have applied FTiMC to systems consisting of silica nanoparticles dispersed in monodisperse atactic PS. The nanoparticles carry monodisperse surface-grafted PS chains of prescribed molar mass and grafting density.  Predicted scattering curves from the grafted polymer corona are validated against Small Angle Neutron Scattering measurements [2].

BD/TST calculations are targeted at tracking the temporal evolution of the nanocomposite over time scales in excess of milliseconds and predicting its stress relaxation modulus. The polymer is modeled as a set of coarse-grained beads connected along the chain contours by “entropy springs”, with “slip springs” representing entanglements between different chains.  A coarse-grained free energy function incorporating spring and nonbonded contributions is used as a starting point for deriving the dynamics. Adsorption/desorption of beads on the nanoparticle surfaces is also included.  To extract rate constants for the latter from molecular-level information, a new approach combining self-consistent field theory and Kramers theory has been developed and validated [3].

[1] Vogiatzis, G.G.; Theodorou, D.N. Macromolecules 2014, 47, 387-404.
[2] Vogiatzis, G.G.; Theodorou, D.N. Macromolecules 2013, 46, 4670-4683.
[3] Theodorou, D.N.; Vogiatzis, G.G.; Kritikos, G. Macromolecules 2014, 47, 6964-6981.

Bio: Doros Theodorou was born in Athens, Greece in 1957.  He obtained a Diploma in Chemical Engineering from the National Technical University of Athens (NTUA) in 1982.  In September 1981 he started his graduate studies in the Department of Chemical Engineering at M.I.T., where he earned a Master of Science degree in 1983 and a Ph.D. in 1985.

After fulfilling his military duty in Greece, he started his academic career as Assistant Professor of Chemical Engineering at the University of California, Berkeley (UCB) in 1986.  There he was promoted to Associate Professor in 1990 and to Full Professor in 1994.  In parallel, he was Associated Faculty at the Center for Advanced Materials of the Lawrence Berkeley Laboratory (LBL), where he directed a new research effort on polymer-surface interactions.

In 1995 he resigned his position at  UCB and LBL and relocated to Greece as Professor of Chemical Engineering at the University of Patras and adjunct faculty member at ICE/HT-FORTH.  In parallel, he worked as a Collaborating Researcher at the Institute of Physical Chemistry of NCSR “Demokritos”.   In 2002 he moved from Patras to Athens as Professor in the School of Chemical Engineering at NTUA and director of the Computational Materials Science and Engineering (CoMSE) group. 

His research focuses on the development of new computational techniques for understanding and predicting properties of materials based on their chemical constitution.  He has authored and co-authored 160 research papers, 26 chapters in books and review articles, and the books Simulation Methods for Polymers with M.J. Kotelyanskii  and Diffusion in Nanoporous Solids with  J. ?ärger ??? D. Ruthven.  His work has received more than 8500 citations (?SI, h index=52).  He represented Greece to the mobility of researchers program of the European Union (1995-2004) and was a member of the National Council of Research and Technology of Greece (2010-2013).

His work has been recognized with the Presidential Young Investigator Award of the ?SF  (1988-1992), the Science Award of the Bodossakis Foundation in Chemistry (1996), the Danckwerts Lectureship of the AIChE (2006), the D. Medema Award of the Dutch PTN (2009) and the Bird-Stewart-Lightfoot Lectureship of the University of Wisconsin (2013).  In February 2015 he was elected to the U.S. National Academy of Engineering. 

All seminars are held on Wednesdays from 12:00 noon-1:00 p.m. in the Bowen Hall Auditorium Room 222. A light lunch is provided at 11:30 a.m. in the Bowen Hall Atrium immediately prior to the seminar.
Bowen Hall Auditorium  ·  12:00 p.m. 1:00 p.m.
Thursday, October 8
Friday, October 9
Saturday, October 10