Molecular Simulations of Polymers: Melts, Networks, Nanocomposites

Lecture by Doros Theodorou

School of Chemical Engineering
National Technical University of Athens

Tuesday, May 7, 2013
Lecture at 4:00 PM
Room 1610 Engineering Hall

Reception at 3:30 p.m. Engineering Hall Lobby

Predicting the physical properties of polymeric materials from their chemical constitution is a challenging task. The extremely broad spectra of length and time scales governing the structure and molecular motion of polymers defy "brute force" molecular simulation approaches. Fortunately, statistical mechanics can serve as a basis for designing new methods and algorithms that overcome this challenge.

We will discuss predictions of the volumetric properties and molecular packing of high molar mass linear and short-chain branched polymer melts, based on united-atom models developed for small organic molecules. Here, the use of connectivity-altering Monte Carlo (MC) algorithms enables equilibration at all length scales. Predictions are validated by comparison to equation of state and scattering measurements. Melt configurations are reduced to entanglement networks via topological analysis to study how entanglement properties, such as the tube diameter invoked by reptation-based theories of rheological behavior, depend on the chemical constitution and degree of branching of chains. Tracking the temporal evolution of the networks in the course of atomistic molecular dynamics (MD) simulations reveals a broad distribution of entanglement strengths.

To study the mechanical response of elastomers, we generate model crosslinked networks with the chemical constitution of polyethylene. Cavitation of these systems upon imposition of hydrostatic stress is examined with isothermal MD simulations. Cavitation is found to exhibit the characteristics of a first-order phase transition between a homogeneous and a cavitated phase. Upon increasing the hydrostatic stress, model systems cavitate at a stress level close to the limit of mechanical stability of linear polyethylene; the initial emergence of a cavity is associated with overcoming cohesive interactions in the material. Upon decreasing the hydrostatic stress on a cavitated sample, a hysteresis loop is traced; the sample snaps back to a homogeneous state as it approaches a second limit of stability, where the stress is commensurate with the Young's modulus and with estimates from continuum mechanical analysis.

We are developing a multiscale simulation strategy to study the structure and dynamics of materials consisting of nanoparticles of roughly spherical shape, either bare or carrying surface-grafted polymer chains, within amorphous polymer matrices. This strategy encompasses atomistic MD, coarse-grained MC, and Field Theory-inspired Monte Carlo (FTiMC). Each level of representation invokes parameters that can be extracted from the previous (more detailed) level, such that all calculations are ultimately based on an atomistic force field. Simulation predictions for how the conformation and segmental dynamics of free and grafted polymer chains are affected by the nanoparticles in these systems are validated through neutron scattering measurements and used as a starting point for the calculation of mechanical properties.

Doros Theodorou

Doros Nicolas Theodorou was born in Athens, Greece in 1957. He graduated from Athens College in 1976 and obtained a Diploma in Chemical Engineering from the National Technical University of Athens (NTUA) in 1981. In September 1981 he started his graduate studies in the Department of Chemical Engineering of the Massachusetts Institute of Technology (MIT), where he obtained his Master of Science degree in 1983 working under the direction of James Wei, and his Ph.D. in 1985 under the supervision of Ulrich Suter.

From June 1985 to July 1986 he fulfilled his military service obligation in the Hellenic Navy (Naval Academy of Greece). Immediately thereafter he started his academic career as an Assistant Professor in the Department of Chemical Engineering of the University of California, Berkeley. He was promoted to the tenured position of Associate Professor in 1990 and to Full Professor in 1994. In parallel, starting in 1986, he served as Associated Faculty in the Center for Advanced Materials of the Lawrence Berkeley Laboratory (LBL), where he directed the project "Polymer-Substrate Interactions". His research group at Berkeley carried out the first predictive computer simulations of polymers at interfaces and advanced molecular simulations as a tool for understanding sorption and diffusion phenomena in zeolites.

In 1991 Doros Theodorou was elected Associate Professor in the Department of Chemical Engineering of the University of Patras in Greece, where he taught in parallel with his appointment at Berkeley for a few years. In 1995 he resigned his positions at Berkeley and LBL and relocated to Greece as Professor of Chemical Engineering at the University of Patras and adjunct senior faculty member at the Institute for Chemical Engineering and High Temperature Chemical Processes (ICE/HT-FORTH). In parallel, he served as collaborating researcher in the Institute of Physical Chemistry of NCSR "Demokritos" in Athens. Since February 2002 he has been Professor in the Department of Materials Science and Engineering of the School of Chemical Engineering of NTUA, where he directs the Computational Materials Science and Engineering group.

He has taught many undergraduate and graduate courses in the general areas of Thermodynamics, Physical Chemistry, Statistical Mechanics and Molecular Simulations, Structure-Property Relations in Materials, Polymer Science and Engineering, and Chemical Engineering Science. He has authored 151 research articles in international scientific journals, 26 book chapters and invited review articles, and the books Simulation Methods for Polymers, with M.J. Kotelyanskii, and Diffusion in Nanoporous Solids, with J. Kärger and D. Ruthven. His published work has received more than 7000 citations (h-index = 49). He has served on the editorial boards of 10 international scientific journals and organized 13 international scientific conferences. He represented Greece in the mobility of researchers program of the European Union in the period 1995-2004 and is currently a member of the National Council of Research and Technology of Greece.

His research work focuses on the development and application of new, hierarchical computational methods for understanding and predicting the properties of materials from their chemical constitution. Today, many members of his research groups at Berkeley, Patras, and Athens hold distinguished teaching appointments in universities, or managerial positions in industrial laboratories and plants around the world. His research work has received considerable recognition. For example, he was awarded the Presidential Young Investigator Award of the U.S. National Science Foundation (1988-1992), the Allan P. Colburn Memorial Lectureship of the University of Delaware (1993), the Earnest W. Thiele Lectureship of the University of Notre Dame (1993), the Robert W. Vaughan Lectureship of Caltech (1994), the Science Award of the Bodossakis Foundation in Chemistry (1996), the Danckwerts Lectureship of AIChE, IChemE, EFCE and CES (2006), and the D. Medema Award of the Dutch PTN (2009).