Our research efforts focus on the development and implementation of computational methods for: (a) the prediction of the physical properties of macromolecular materials from their chemical constitution and architecture, and (b) the elucidation of the atomistic mechanisms and interactions that shape these properties. To model structure and dynamics in such chain-like systems and to bridge the huge gap in time and length scales between motion of polymer segments and flow in processing equipment, we are following a strategy which involves research work along the following directions:

Thermodynamic and rheological properties of polymers

We develop new, very efficient Monte Carlo and Molecular Dynamics algorithms for the fast and robust equilibration of the condensed phases of polymers (melts and crystals) based on chain connectivity-altering Monte Carlo moves. These moves are designed by using linear polyethylene as an archetype model and then are extended and applied to polymers of other architectures. Emphasis is placed on polymers of industrial interest, such as melts of polyethylene (PE), cis-1,4 polybutadiene (cis-1,4 PB), polyterepthalic ethylester (PET), and branched H-shaped polyethylene molecules. Similar algorithms are used to study the solubility of small alkanes in long-chain polymer melts, the interfacial properties of polymer melt systems consisting of grafted and free chains, and the compatibility of binary polymer blends. Guided by principles of Irreversible Thermodynamics, hierarchical modeling methodologies are also designed and implemented for the direct atomistic simulation of the viscoelasticity of these polymers and their birefringence. Today, these efforts are extended to non-linear polymer systems such as the short- and long-chain branched molecules in order to simulate their unique conformational and rheological properties.

Polymers at interfaces

Interfaces between polymeric liquids and solids play a key role in many technical applications: in manufacturing surfaces with controlled friction and wear characteristics, manipulating wettability by aqueous and organic liquids surfaces, stabilizing suspensions against flocculation, improving compatibility of immiscible interfaces, improving the fracture strength of interfaces through the use of a few grafted chains, etc. Establishing quantitative relations between chemical constitution and macroscopically manifested properties in such systems is highly desirable, because this can tremendously facilitate the rational design of multiphase systems containing macromolecular chains. Of particular importance is the simulation of end-grafted polymers on solid substrates at the atomistic level, because it can provide information about the mechanisms leading to the formation of self-assembled monolayers (SAMʼs), such as those developed by alkanethiols on Au(111) and of alkyl monolayers on Si(111). We carry out such atomistic simulations in our laboratory and study the packing and orientational characteristics of these systems, with an emphasis on their thermal stability. Recently, these studies are extended to associating polymer molecules in solution, where specific hydrophobic interactions prevail (such as the aqueous solutions of n-alkyl polyoxyethylyne ethers) leading to cloud curves.

Polymer rheology in inhomogeneous fields

The interest in the study of the rheology of polymers in inhomogeneous fields and near boundaries stems from the role that adsorbed polymers play in technological applications involving flows through porous media, etc. In our laboratory, the flow behavior of polymers in the proximity of solid boundaries is modeled through a Hamiltonian model that allows one to develop thermodynamically admissible governing equations through a projection of the microscopic degrees of freedom to a coarser level of consideration. The resulting partial differential equations are solved numerically with the method of finite or spectral elements and provide information for the boundary layers that develop next to the substrate and their effect on the flow kinematics. Through this scheme, we have been able to model the rheology of polymer solution next to walls, and investigate stress-induced migration effects in the Taylor-Couette flow and in viscometric geometries. Currently, the method is extended to calculations involving flows of polymer melts at high throughput levels and the inception of cavitation and cavity growth during filament stretching of pressure-sensitive adhesives.