1996 Annual Report

Technical Activities: THEORY AND MODELING

Materials science and engineering are on the threshold of a fundamental transformation. New theoretical capabilities combined with spectacular advances in computer processor power, memory, and computational methodologies now allow researchers to simulate highly complex materials behavior and microstructures. MSEL Theory and Modeling efforts in FY96 continued their focus on the behavior and properties of materials over the entire range of length scales, from atoms to bulk materials. Molecular dynamics simulations were performed to study such phenomena as fracture in brittle materials, relaxation and glass formation in supercooled liquids, and propagation of dislocations in metals. Mesoscopic phase-field and reaction-diffusion models of metal alloys, polymers, and polymer/liquid-crystal blends permitted the study of phase behavior, stability, separation kinetics, and morphological evolution in these materials, complementing ongoing experimental efforts at NIST and elsewhere in the area of materials processing. Finite-element computer codes were developed to model a variety of materials processes, such as injection molding of automotive parts, solder geometry in microelectronic interconnects, physical aging in composite materials, and mechanical properties of ceramic microstuctures.

To facilitate progress in the rapidly evolving field of computational materials science, MSEL established a distributed Center for Theoretical and Computational Materials Science (CTCMS) in 1994. Using the tremendous advances in electronic communication and collaboration, this infrastructure facilitates interactions between industry, academia, NIST and other government labs in the development and application of state-of-the-art theoretical and computational materials science techniques to industrially important materials and materials processing problems.

To use more effectively the nation's talents and resources, the CTCMS integrates ongoing research at various institutions by forming temporary multi-disciplinary and multi-institutional research teams as required to attack key materials issues. The CTCMS has three principal activities, all operating interactively: planning, research, and technology transfer. Workshops are held as the first step in defining technical research areas with significant technological impact, identifying team members, and in building and designing the infrastructure for collaborative research. NIST, in its unique position as a "third party" liaison, is well suited to play a national role in this planning process. The CTCMS provides an infrastructure and support for its members, including an interactive World Wide Web information server, and modern computing and workshop facilities. In FY96, the CTCMS addressed critical technical issues in processing, characterization, behavior, and properties of multiphase, heterogeneous materials. Current programs include theory and simulation in the areas of microelectronic interconnect design, Green's functions and boundary element methods applied to mechanical properties, microstructure and dynamics of glass formation, standards in micromagnetics modeling, morphological control of polymer-based liquid crystal display materials, solidification and dendritic growth in casting of alloys, and object-oriented finite-element modeling of ceramic microstructures.

Significant Accomplishments

• Developed a relationship between the entropy of a glass forming polymer system and its viscosity in which the viscosity is determined by the configurational free energy, rather than the configurational entropy as originally anticipated.

• Introduced new and general analytic description of transport virial coefficients of particles having general shape and arbitrary property mismatch.
  
• Validated analytic model with numerous examples in three and two dimensions corresponding to bulk and thin film polymer materials.
• Development of new numerical first-passage time random walk method for solving for polymer solution transport properties.
  

Theory and Modeling in Polymer Physics

E.A. DiMarzio

Objectives

The project objective is to develop a statistical mechanical basis for the non-Arhenius temperature dependence of viscosity in glass forming polymers.

Technical Description

In this project the viscosity of glass forming polymers is examined within the context of the entropy theory of glasses. A simple statistical mechanical model is developed to account for the non-Arhenius temperature dependence of the viscosity that is observed in polymer melts.

External Collaborations

Work on the effects of plasticizer on glass transition and on the fundamentals of the viscosity of glass forming liquids was performed in collaboration with researchers from Armstrong World Industries.

Accomplishments

Developed a statistical mechanics based relationship between the entropy of a glass forming polymer system and its viscosity. The model predicts that the viscosity is determined by the configurational free energy, rather than the configurational entropy as originally anticipated.

Outputs

Publications

E.A. DiMarzio and A.J.-M. Yang, The Configurational Entropy Approach to the Kinetics of Glass Formation, J. Res. NIST (In Press).

Presentations

E. A. Di Marzio, The use of Kinetic Energy to Derive the Kinetic Properties of Glasses, APS March Meeting, St. Louis, MO, March, 1996.

Theory and Modeling of the Properties of Polymer Blends, Suspensions and Solutions

J. Douglas

Objectives

• Develop general theory relating to the transport properties of polymer blends and solutions.
• Describe influence of complex-shaped additives on the transport properties of polymer materials.
• Calculate equilibrium and transport properties of polymers having membrane-like and sponge-like connectivity.

Technical Description

• Perform numerical calculations of the hydrodynamic radius, capacity, and Smoluchowski rate constant for diffusion-limited reactions of polymers using a novel random walk algorithm for solving the associated boundary value problems.
• Calculate transport properties (electrical and thermal conductivity, dielectric constant, refractive index, shear modulus, magnetic pemeability, shear viscosity) of polymer blends, polymer solutions and polymer materials with suspended additives by developing a perturbation expansion in the property mismatch, using numerical and Padé approximation methods.
• The critical exponents describing the average dimensions, friction coefficient and the intrinsic viscosity of polymeric random surfaces are calculated based on the Wiener sheet model with inclusion of excluded volume interactions. The growth of branched polymer structures under equilibrium and non-equilibrium diffusion-limited conditions is also treated within the same framework.

External Collaborations

Building Materials Division, NIST- Apply transport property calculations to cement modeling. finite element calculations of transport virial coefficients for complex shaped particles having a wide range of property mismatch to validate analytical model.

Biotechnology Division and Center for Advanced Research in Biotechnology- Collaborate on development of new random walk based algorithms for calculating transport properties of complex-shaped particles.

Accomplishments

• Introduced new and general analytic description of transport virial coefficients of particles having general shape and arbitrary property mismatch.
• Validation of analytic model with numerous examples in three and two dimensions corresponding to bulk and thin film polymer materials.
• Development of new numerical first-passage time random walk method for solving for polymer solution transport properties.

• Introduced new model describing the average size and transport properties of polymers, membranes and sponge-like structures in good and poor solvents.

Publications

J. F. Douglas and E. J. Garboczi, Intrinsic Viscosity and Polarizability of Particles Having a Wide Range of Shapes, Advances in Chemical Physics, 91, 85 (1995).

E. J. Garboczi and J. F. Douglas, Intrinsic Conductivity of Objects Having Arbitrary Shape and Conductivity, Physical Review E 53, 6169 (1996).

J. F. Douglas, A Dynamic Measure of Order in Glasses, Comp. Mat. Sci.4, 292 (1995).

J. F. Douglas, Swelling and Growth of Polymers, Membranes, and Sponges, Physical Review E, 54, 2677 (1996).

J. F. Douglas, C. Guttman, A. Mah, T. Ishinabe, Spectrum of Self-Avoiding Walk Exponents,

Physical Review E, (to appear).

J. Given, J. B. Hubbard, J. F. Douglas, A First-Passage Algorithm For Calculating the Friction and Capacity of Polymers, Journal of Chemical Physics, (submitted).

Presentations

J. F. Douglas, Transport Properties of Suspensions, Polymer Solutions, and Polymer Blends

Polymers Gordon Conference, Newport ,Rhode Island, July 1996.

J. F. Douglas, Polymers and Superfluids?, Physics Division Lecture Series, NIST, March 1996.