Technical Activities 1998

This report, covering the technical activities of the Polymers Division in fiscal year 1998, is organized according to the major programs of the Division. Each program description includes an overview that describes the motivation for the individual projects, selected accomplishments that we wish to emphasize and project summaries. Relevant publications are referenced for the reader who wants further information.

The Polymers Division provides standards, measurement methods, data and concepts of polymer behavior for the U.S. polymers industries and business enterprises that use polymers in products and services. The Division looks at trends in the production, processing and use of polymers in deciding areas of work. The following stimulates the current focus on electronic applications of polymers, polymer blends and processing, polymer composites, dental and medical materials, and characterization of polymer structure and properties.

• New catalysts systems give polymer producers the ability to control molecular architectures to levels heretofore unknown. The Division creates new tools based on recent advancements in mass spectrometry to characterize molecular structure.
• Most commercial polymeric materials are not single component systems, but mixtures of several polymers along with additives or fillers that improve processibility, enhance properties and/or improve long term performance. As new market opportunities and increased competitive pressures arise from advances in polymer synthesis and the precise control material components, there are increasing demands on polymer suppliers and processors for multifunctional materials with improved performance and ease of production. Current work to provide the measurement tools and knowledge base required in response to these demands includes on-line characterization of temperature and phase behavior, shear effects on phase behavior, morphology and dispersion, influence of compatibilizers and fillers, and control of interfacial interactions.
• The full utilization of fiber reinforced polymers for structural applications is limited by the lack of rapid, reliable, cost-effective fabrication methods and the poor understanding of and predictive capability for long-term performance. Methods are developed to monitor, model and control events that occur during composite fabrication by liquid molding technology and other new fabrication techniques. Test methods are developed for assessing important characteristics such as the resin/fiber interfacial adhesion and the subsequent degradation of this adhesion resulting from fluid attack, particularly moisture.
• Another trend is the increasing importance of polymers in the form of thin films. The microelectronics industry is one example of where polymeric films are used as photoresists, dielectric insulators and encapsulants. The Division develops x-ray and neutron reflectivity techniques to measure the properties of the submicron thick polymer films that are necessary for continued development in the field.
• Health care professionals increasingly use polymeric materials to replace diseased structures. The Division provides basic materials science, test methods and standards for the development of new or improved materials and delivery systems. The focus is on development of improved dental restorative materials with greater durability, wear resistance and clinical acceptability.

These programs are developed with input from sectors of U. S. industry that produce, process or use polymers in essential parts of their business. NIST sponsored workshops are a key way in which industry provides input into planning program direction. In fiscal year 1998, the Division sponsored workshops on Filled Polymers and Nanocomposites, on Properties and Applications of Dendritic Polymers, and on X-ray Microscopy of Synthetic and Natural Polymers.

In the past year the Division made significant contributions to the scientific and technology base that drives commercial advancements in polymeric materials and their uses. A listing of some of these accomplishments is given below; more complete lists and descriptions are contained in the individual sections of this report.

ELECTRONIC PACKAGING, INTERCONNECTION, AND ASSEMBLY PROGRAM

• A new test specimen design for use as a common dielectric test vehicle has been accepted by the National Center for Manufacturing Sciences (participants: DuPont, IBM, Delco, Raytheon, 3M) for their Embedded Capacitive Materials Project. This test specimen and its associated test procedure were developed by NIST, in collaboration with the industrial partners, for ferroelectric films in the frequency range from 0.5 GHz to 5 GHz.
• A new methodology for characterizing low-k thin film dielectrics has been developed and data have been delivered to Dow Corning Corp. and SEMATECH. A combination of x-ray reflectivity and neutron scattering was used to measure film porosity, pore size and moisture uptake. The results were forwarded to industry to support their efforts to produce new materials needed for next-generation microelectronics.
• The applicability of x-ray reflectivity measurements has been extended to films up to 1 µm thick. This new capacity is essential for many on-going thin film projects in the electronics application program because 1 µm is the nominal thickness for interlevel dielectrics used in today's VLSI circuitry.
• A new test methodology has been certified and is being submitted to the Institute for Interconnecting and Packaging Electronic Circuits as a standard test for measuring the thin film coefficient of thermal expansion normal to the film surface. Certification tests of the newly developed NIST capacitance cell were performed using an oriented Al₂O₃ single crystal as the test standard. Results were in excellent agreement with literature values.

POLYMER BLENDS AND PROCESSING PROGRAM

• An optical sensor to measure anisotropy during biaxial stretching of polypropylene film in production ovens at high temperature was designed and demonstrated in collaboration with Mobil Chemical Company.
• Spherical and cylindrical composition waves emanating from the boundaries of inclusions in a phase separating blend were predicted by simulations and subsequently confirmed by experiments investigating the microstructure of filled polymer materials.
• An analytical model predicting the viscosity for polymers filled with dispersions of platelet particles was developed and validated with data for a range of particle shapes. The threshold of rigidity where the suspension viscosity diverges is predicted to be inversely proportional to the intrinsic viscosity.
• Alignment of the morphology in a phase separating blend film to an underlying pattern of surface chemistry was shown to improve with decreasing film thickness until reaching a critical value below which spinodal dewetting causes controlled break-up into uniform nanoscale droplets.
• Interpenetrating polymer networks of hydroxyethylmethacrylate with PAMAM dendrimers were prepared, and the individual dendrimers were shown by small angle x-ray scattering and transmission electron microscopy to be uniformly dispersed.

POLYMER COMPOSITES PROGRAM

• Optical coherence tomography was used successfully to rapidly measure the porous structure of a high fiber volume fraction glass fiber reinforcement in sufficient detail to predict the permeability using a 3-D fluid flow simulation based on the lattice-Boltzmann method.
• A database of permeability and other reinforcement properties was released in collaboration with the NIST Standard Reference Data Program.
• A workshop entitled X-ray Microscopy of Synthetic and Natural Polymers: Towards Chemical Speciation at 10 nm Spatial Resolution. Assessment of Research Opportunities and the Need for X-ray Microscopy Facilities and co-sponsored by NIST, Department of Energy, Advanced Light Source, North Carolina State University, Carnegie Institution of Washington, Dow Chemical, and General Electric, was held at NIST on May 11-12, 1998. The workshop resulted in a DOE funded proposal for $500,000 to develop an X-ray imaging facility at the Advanced Light Source devoted to the study of polymers.

POLYMER CHARACTERIZATION PROGRAM

• A major obstacle to the use of mass spectrometry to determine the molecular mass distribution of synthetic polymers was overcome through improvements in sample preparation methods that reduced variation of signal intensity from 42% on samples prepared by conventional techniques to below 5% on samples prepared by the new procedures.
• Chemical shift calculations for the ethylene defect in isotactic polypropylene confirmed assignments of experimental NMR data used to establish relationships between molecular defects and crystallinity.
• Research to identify a material for a new Nonlinear Fluid Standard was completed, and an inter-laboratory comparison of the fluid involving 35 researchers from industry (instrument makers and material suppliers) and academe was organized.
• Recommendations for standards for estimation of statistical errors in SAS data sets were made to the Commission on Small-Angle Scattering of the International Union of Crystallography.
• In collaboration with General Electric Company and the University of Pittsburgh, the evolution of the viscoelastic response of a commercial epoxy thermoset during cure was successfully modeled.

DENTAL AND MEDICAL MATERIALS PROGRAM

• A series of highly fluorinated dimethacrylate monomers were prepared and evaluated in homopolymerization studies. Dental composites with enhanced durability and color stability may result from several of the new fluoropolymers that combine excellent mechanical strength with extreme hydrophobicity.
• A new accelerated wear resistance machine was developed in collaboration with the Ceramics Division and the University of Maryland under a NIST research consortium formed by CRADAs with four orthopaedic companies: Biomet Inc. and Zimmer Inc, both of Warsaw, Ind.; Johnson & Johnson Professional Inc., Raynham, Mass.; and Oesteonics Inc., Allendale, New Jersey. The new test machine allows for evaluation of new orthopaedic joint materials in five days or less, under a variety of load, load-cycle and motion conditions promises to hasten the introduction of longer lasting hip, knee, and other orthopaedic joint implants to the market. As a result of this successful development, the four companies elected to seek extension of their CRADAs to September 30, 2000.
• Fluorescent probes were successfully used for cure monitoring of dental bonding resins and bone cements. Improved products will result from the addition of one of the probes to dental bonding resins or bone cements.