Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session W4: Computational Challenges in Simulations of Macromolecular Assemblies |
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Sponsoring Units: DPOLY DCOMP Chair: Grant Smith, University of Utah Room: Colorado Convention Center Korbel 2B-3B |
Thursday, March 8, 2007 2:30PM - 3:06PM |
W4.00001: Confined Self-Assembly of Block Copolymers Invited Speaker: Spontaneous formation of ordered structures from amphiphilic molecules has attracted tremendous attentions in the last decades. Among the many different amphiphilic systems, block copolymers with their rich phase behavior and ordering transitions have become a paradigm for the study of structural self-assembly. In a physically confined environment, structural frustration, confinement-induced entropy loss and surface interactions can strongly influence the molecular organization. In particular, it is possible that confinement can lead to unusual morphologies which are not accessible in the bulk, thus providing opportunities to engineer novel structures. For confined asymmetric diblock copolymers, a rich variety of novel morphologies, ranging from helices to toroids to complex networks, is expected. The complexity of the possible structures presents computational challenges in simulations of macromolecular assemblies under confinement. We have used a combination of simulated annealing method and self-consistent field theory to exam the self-assembly of block copolymers confined in different geometries. A generic structural evolution path is obtained for the confined systems. The study demonstrates that confined self-assembly of amphiphilic molecules provides a robust method to produce nanoscopic structures which are not accessible in the bulk phases. [Preview Abstract] |
Thursday, March 8, 2007 3:06PM - 3:42PM |
W4.00002: Simulation of driven self assembly of complex polymeric systems across multiple length scales Invited Speaker: The self assembly of macromolecular systems is often driven or facilitated by the application of external fields, including flow, voltage, or confinement. The structures that arise when external fields are applied often depend on the history of the sample, and it is therefore important to develop theoretical and computational methods capable of describing the order formation process across multiple length and time scales. Over the past several years we have developed several new classes of multiscale modeling techniques for study of the structure and properties of polymeric materials under external fields, including confinement or flows. For systems at equilibrium, these systems permit precise calculation of the free energy. For systems beyond equilibrium, these methods include the effects of fluctuating hydrodynamic interactions (for dilute and semidilute systems) and the effects of constraining molecules (for concentrated melts). These models and methods can be used to investigate the equilibrium structure and relaxation of a variety of fluids, including solutions of biological macromolecules. The usefulness and limitations of our proposed approach will be discussed in the context of three applications. The first application is concerned with the elongation and presentation of long DNA molecules in nanofluidic channels. A multiscale model, that includes fluctuating hydrodynamic interactions, has been used to design a gene mapping device and to interpret experimental data pertaining to the structure and dynamics of confined chromosome-length DNA. The second application is concerned with the study of liquid-crystal based biosensors. A multiscale model has been used to design a liquid-crystal based device in which nanoscale particles suspended in a liquid crystal self assemble into highly regular structures, including chains, upon exposure to proteins or virions. The third application focuses on the formation of ordered block copolymer structures on nanopatterned substrates. Results from mesoscopic multiple length and time scale simulations will be presented to explain the effects of surfaces and different types of confining walls on the free energy of a variety of morphologies. [Preview Abstract] |
Thursday, March 8, 2007 3:42PM - 4:18PM |
W4.00003: Chemistry Unified Language Interface: a Novel Toolkit for Hybrid Macromolecular Models Invited Speaker: In our everyday life, we appreciate the enormous diversity of industrial formulation in many applications, from personal and health care, to plastics and coatings, to novel nanotechnology. In all of these cases the systems are compound, containing solvents mixtures, polymers, surfactants, actives and colloids or fillers. CULGI (Chemistry Unified Language Interface) is a library of many modeling functions and approaches, encompassing molecular, mesoscopic and macroscopic chemical modeling and machine learning. We discuss several of the important theoretical challenges that we face: (a) the lack of unification in classification and description of common chemical entities, such as molecules, colloids and surfactants, (b) the curse of loss of thermodynamic accuracy in large scale molecular dynamics, and (c), most importantly: how to derive interaction models and parameters for real mixed systems of industrial relevance on a coarse-grained level. We illustrate the challenges by some resent results relevant to industrial formulation and application: the in situ phase formation of polyolefin blends, the lyotropic phase structure of concentrated surfactant ? polyacid mixtures, and dissolution rates of sparingly soluble drugs from polymer stabilized suspensions. [Preview Abstract] |
Thursday, March 8, 2007 4:18PM - 4:54PM |
W4.00004: Modeling Microcapsule-Substrate Interactions: Repairing Damages Surfaces and Separating Damaged Cells. Invited Speaker: We model two different scenarios that involve capturing the behavior of macromolecular assemblies. In the first study, we model the rolling motion of a fluid-driven, particle-filled microcapsule along a heterogeneous, adhesive substrate to determine how the release of the encapsulated nanoparticles can be harnessed to repair damage on the underlying surface. We integrate the lattice Boltzmann model for hydrodynamics and the lattice spring model for the micromechanics of elastic solids to capture the interactions between the elastic shell of the microcapsule and the surrounding fluids. A Brownian dynamics model is used to simulate the release of nanoparticles from the capsule and their diffusion into the surrounding solution. We focus on a substrate that contains a damaged region (e.g., a crack or eroded surface coating), which prevents the otherwise mobile capsule from rolling along the surface. We isolate conditions where nanoparticles released from the arrested capsule can repair the damage and thereby enable the capsules to again move along the substrate. Through these studies, we establish guidelines for designing particle-filled microcapsules that perform a ``repair and go'' function and thus, can be utilized to repair damage in microchannels and microfluidic devices. In the second study, we extend the above model of fluid-filled, elastic spheres rolling on substrates to three dimensions and thereby demonstrate a useful method for separating cells or microcapules by their compliance. In particular, we examine the fluid-driven motion of these capsules over a hard adhesive surface that contains soft stripes or a weakly adhesive surface that contains ``sticky'' stripes. As a result of their inherently different interactions with the heterogeneous substrate, particles with dissimilar stiffness are dispersed to distinct lateral locations on the surface. Since mechanically and chemically patterned surfaces can be readily fabricated through soft lithography and can easily be incorporated into microfluidic devices, our results point to a facile method for carrying out continuous ``on the fly'' separation processes. [Preview Abstract] |
Thursday, March 8, 2007 4:54PM - 5:30PM |
W4.00005: Non-equilibrium dynamics at polymer surfaces and interfaces Invited Speaker: The ability to predict the rheological and flow properties of polymer blends and other multi-phase systems relies critically on the ability to understand how the interfacial dynamics affect the overall properties. Continuum theories tend to break down in the near interfacial region since they do not account for local molecular orientation and other related effects. We have shown, using MD simulations, that the behavior at interfaces tends to be controlled by the stress transfer between the two phases and this is dependent on the local chain structure and conformation at the interface. Studies on nanoparticles added to polymer blends also illustrates the importance of controlling the interfacial region. To account for all these effects we have formulated a dynamic self-consistent field theory that couples local chain conformation to a constitutive equation that describes the rheological properties of the system. We illustrate this approach by studying the effect of slip at polymer/polymer interfaces. [Preview Abstract] |
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