INTRODUCTION

This report gives a description of the principal technical activities of the Polymers Division during the 1997 fiscal year. It is organized according to the program structure by which we plan our work and follows a concise format that was designed to make it easier for readers to understand the total range of our programs. Technologies can seldom be neatly categorized according to their technical content. Correspondingly, our industry customers rarely describe their needs in only one or two technical areas. This organization of the report also provides the reader with the management context in which we view individual projects in order to make our motivation for the work clearer. We hope this will encourage our customers to offer comments and advice on ways we can be more effective in our work.

The Polymers Division is responsible for providing standards, measurement methods, and fundamental concepts of polymer science to assist those U. S. industries that produce or use synthetic polymers in essential parts of their business. We plan our programs primarily to develop improved measurement capability for broad sectors of the industrial community. We rely heavily on advice from industrial and technical communities to set our priorities. This advice stems from both extensive informal visits to and from our customers and from the following formal workshops:

• NIST hosted a three day joint scientific workshop with the Hashimoto Phasing Project of the Exploratory Research and Technology Organization (ERATO) of Japan, which was attended by over 65 representatives of U.S. industry, ERATO, and NIST. In addition to further developing the scientific interactions between these two research groups, the workshop provided U.S. industrial researchers a direct view of the ERATO-supported project.
• In April 1997, SEMATECH, the Semiconductor Research Corporation and NIST conducted a workshop on Interfaces and Adhesion in Electronic Packaging and Assembly involving experts from industry, academia and government. The workshop explored new scientific opportunities for enhancing the understanding and robustness of material interfaces in semiconductor devices. As the size of microelectronic devices shrink, and their complexities increase, industry has identified material Ainterfaces and adhesion@ as a critical area where substantial advancements within the semiconductor industry are needed.
• In October 1996, the U.S. Air Force Rome Laboratory and NIST sponsored the 6^th International Conference on Moisture in Microelectronics at NIST=s Laboratories in Gaithersburg, MD. Drawing over ninety individuals from industry, academia and government, this workshop was held to allow researchers and engineers, in the microelectronics industry, to exchange new insights and knowledge on how moisture impacts the performance and reliability of today=s microelectronic products and packaging materials. Moisture is universally seen as the single largest environmental factor which threatens the manufacturability and reliability of electronic packages and interconnects.
• An industry/NIST workshop on interfacial micromechanics in May, 1997 brought together leading researchers in the field and important industrial users of composites. The important problems in interface testing were identified and routes to solving some of them agreed upon. For example, an international program on interface test standardization under the auspices of VAMAS was announced to solve the problem of poor intercomparison.

In all these programs, collaboration with others continues to be the most effective way to develop and transfer technology. In particular, industry collaborators who know both their business and technical needs are vital to planning and execution of the projects. The Division's technology transfer activities were recognized by the NIST Slichter Award to A. Bur, K. Migler and F. Wang and by the Federal Laboratory Consortium Award for Excellence in Technology Transfer to A. Bur. These awards recognize that our industrial collaborators have adopted NIST technology in their processing and R&D facilities.

This year the Polymers Division joined four industrial laboratories in the Participating Research Team (PRT) for the Advanced Polymer Beamline at Brookhaven National Synchrotron Light Source. Data taken during the commissioning phase showed a wavevector range and resolution much superior to lab based facilities. When completed this beamline will provide real-time simultaneous small and wide angle x-ray scattering with high incident flux and multiple area detectors for dynamic measurements. Beam time available through PRT membership will complement our in-house SAXS facilities and significantly enhance the capabilities available for our internal research and for our industrial and academic collaborators.

In a significant step toward increased development of biomaterial reference materials and standards, the three government agencies with responsibilities in this area - NIST (Polymers Division and Standard Reference Materials Program), the Food and Drug Administration, and the National Institute of Health - signed a Memorandum of Understanding (MOU) establishing a coordinating committee for the development of reference materials. The first step will be to transfer to NIST the reference materials previously developed by the National Heart, Lung, and Blood Institute for dissemination through our Standard Reference Materials Program. This will provide the community with one central source for such materials.

The Division has focused a majority of its resources on specific industrial sectors. This has allowed us to assess the needs of each industry group and plan a response that is appropriate to our capabilities and role. Each of the program overviews describes the industrial focus of the program even though the program outputs often have wider applicability. There are many technical accomplishments which are described within this report under each specific program. The following is a selected list of these accomplishments which gives an overview of the output of all the Polymers Division programs.

Significant Accomplishments

ELECTRONICS PACKAGING, INTERCONNECTS & ASSEMBLY

• For dielectric measurements, a coaxial sample holder has been designed and tested and measurement protocols instituted which eliminate discrepancies in the overlap regions of frequency in the several different instruments required to cover the range from DC to 1 GHz. The new holder reduces the measurement error in capacitance and loss at 30 MHz from 8% using a conventional four terminal holder to 0.1%. The same configuration can be used at frequencies up to 1 GHz where the uncertainty is about 1%. These techniques will allow the accurate measurement of dielectric constant and loss of thin film materials being evaluated for interlayer dielectrics in the microelectronics industry.
• Density profile and coefficient of thermal expansion (CTE) of ultra-thin (~100 nm) spin-on glass films have been determined by x-ray reflectivity and reported in collaboration with Dow-Corning Corp. Spin-on glass film is a primary candidate for interlayer dielectrics in future multi-level chips. Density profile and CTE are critical data needed for chip design and are not accessible with any other conventional measurement technique.
• A three terminal capacitance cell has been designed and constructed of nichrome-coated fused quartz electrodes that is capable of measuring the out-of-plane coefficient of thermal expansion of films as thin as 5 :m with a standard uncertainty of 0.1% over a 100 /C interval for polymeric materials whose properties are sufficiently stable. This measurement technique is expected to supplement the standard test method for coefficient of thermal expansion which is inadequate for providing reliable data on the thin polymer films encountered in the design of flexible circuits and microelectronic packaging.
• A new technique using twin neutron reflectivity has been developed which enables the determination of the density of thin films on many substrates including silicon wafers with a relative uncertainty of "1%. Density of thin films, especially in the ultra-thin range (#100 nm), provides important insights about the structure of such films that has not been previously determined with an accuracy of "1%.

POLYMER BLENDS and PROCESSING

• The capability of NIST-developed on-line instrumentation, using light scattering and optical microscopy to measure in-situ domain size and shape during extrusion of incompatible and reactively compatibilized blends was demonstrated. In addition to morphology studies, the instrumentation can measure velocity profiles and characterize multiphase mixing. Industrial partners, including 3M and Rohm & Haas, have begun work using this instrument to investigate effects of polymer processing aids.
• NIST's on-line microscopy extruder instrument has revealed new droplet morphologies in the high shear stress regime which are tentatively attributed to normal forces. For two component systems, with a high viscosity ratio, droplets with negative surface curvature or structures elongated orthogonal to the flow direction, were observed. This result demonstrates the power of on-line measurements to complement typical post processing studies.
• In collaboration with Packard Electric, the determination of specific changes in the spectrum of a fluorescent probe provided simultaneous measurement of temperature and crosslinking in a polyethylene/polyvinyl acetate copolymer. This measurement can be used to control and monitor continuous extrusion of wire insulation during which crosslinking is activated by raising the temperature in the later stages of processing. During the early stages of the extrusion, limited crosslinking is allowed so that the rheology remains uniform.
• Dendrimer molecules of convergent types have been characterized with a combination of small angle neutron scattering, small angle x-ray scattering, and transmission electron microscopy. Four critical observations arise from the solution characterization which contrast dendrimers with other classes of polymers: Dendrimers are spherical in shape and have a narrow size distribution even at relatively low molar mass; the segment density distribution profile is nearly uniform within the dendrimer; the terminal units of the dendrimer are located within a relatively narrow shell toward the periphery of the molecule; and with increasing concentration the dendrimers appear to collapse and pack in a random close packing arrangement with little interpenetration. These characteristics suggest possible applications as size and molecular mass calibration standards for SANS, SAXS, TEM, AFM, MALDI, SEC, and filtration.
• In collaboration with Exxon, large pressure induced demixing in metallocene polyolefin blends was demonstrated through optical cloud point measurements. The observed linear dependence of the demixing temperature on pressure is 0.24 /C/MPa. High pressures are typical in most polymer processing, including polyolefins, and the strong dependence on pressure will greatly shift phase boundaries.

POLYMER COMPOSITES

• A database of permeability and other reinforcement properties was released in collaboration with the NIST Standard Reference Data Division. This data will aid the composites industry in designing liquid molding processes by providing critical input to mold filling simulation software.
• A Lattice-Boltzmann microflow simulation was compared to experimental data for flow in a model of the porous tows of composite reinforcements. The formation of voids and their subsequent size reduction was successfully recreated, and the permeability of the model porous medium was accurately predicted. The Lattice-Boltzmann method is much more computationally efficient than conventional finite element methods for multiphase flows in complex geometries, enabling the rapid assessment of fiber architecture effects on composite processing and void formation.
• The ability of Optical Coherence Tomography to image internal fabric architecture and residual porosity distribution was demonstrated for refractive index-matched glass reinforced epoxy and vinylester composites. This new technology uses visible and near infrared light, is fast, and may be a low cost alternative to X-ray imaging, at least for 1 cm thicknesses, in a number of commercially important composite systems.
• The ability of an optical fiber sensor system to use existing thermocouple ports in structural reaction injection molding equipment used by the automotive industry was demonstrated. The ability to collect data without disrupting production equipment is an important step towards more widespread use of sensors in industry.

POLYMER CHARACTERIZATION PROGRAM

• Recertification of SRM 1482, a polyethylene narrow molecular mass standard, was completed and a report issued. The standard is intended for use in calibration and performance evaluation of instruments used to determine molecular mass and its distribution by size exclusion chromatography.
• Nearly 100 different procedures for preparing samples of synthetic polymers for analysis by matrix assisted laser desorption ionization mass spectrometry (MALDI-MS) were taken from the literature and placed on the World Wide Web for use by practitioners of MALDI-MS.
• The influence of the static magnetic field on spectral resolution in ¹³C NMR under magic angle spinning conditions was quantified and the findings reported to the NMR user community to facilitate design of experiments that optimize resolution.
• A statistical procedure was developed to characterize uncontrolled systematic errors and noise effects on Small Angle Scattering (SAS) data. Modeling studies and real data demonstrate that adequate characterization of the precision of SAS data is not possible unless both noise and systematic error are presented to the end user. This finding will provide important input into the development of standardized data reduction and data transport tools for the SAS community.
• An international round robin was initiated to study the thermoviscoelastic behavior of a research grade polycarbonate. The co-leader on the project is Prof. J.M. Hutchinson of the Aberdeen University in Scotland with eight confirmed participants and five tentative participants. This round robin will help to resolve the fundamental issues involved in the reports from various laboratories of different (or not) fictive temperatures for different processes in glass forming liquids.
• A transition in craze growth rate under stress relaxation conditions was observed for a styrene-acrylonitrile copolymer. The logarithmic craze growth rate decreases by approximately a factor of five in going from below to above the transition aging time. If this is a universal behavior for all polymers it implies that the long term performance may be enhanced due to the aging process stabilizing the material against craze growth.
• A hybrid viscoelastic constitutive model was developed that can predict multi-step stress relaxation and creep results for polyurethane elastomers to within approximately 10% when the history is primarily one of increasing deformation or stress. This approach to nonlinear viscoelasticity will lead to simple test methods for characterizing the non-linear material parameters needed in computer simulations of, for example, blow molding processes.

DENTAL & MEDICAL MATERIALS

• The acid catalysts that bring about the desired ring-opening polymerization of spiro orthocarbonate (SOC) monomers were shown to also cause the rapid hydrolytic degradation of the monomers. The efficient competition between these two processes will affect how the SOC resins can be used as potential dental materials with minimal polymerization shrinkage.
• N-phenyliminodiacetic acid (PIDAA) was shown to be a multi-functional dental primer with the ability to etch dentin and at the same time stabilize collagen in demineralized dentin, thereby facilitating adhesive resin infiltration. In addition, PIDAA has the ability to initiate interfacial polymerization. This unique combination of properties in one adhesive agent permits the development of simplified, highly effective dental adhesive systems.
• Facile syntheses were developed for acrylic organosilsesquioxanes from readily available silane coupling agents. These reactive oligomers are readily polymerized and have potential for use in dental composites, sealants and adhesives. Other applications, e.g. in electronic packaging, that require fast-curing resins yielding polymers with high thermal stabilities and low dielectric properties also may be feasible.
• The microbond test was successfully adapted to probe adhesion to a variety of flat substrates, e.g., quartz, dentin, enamel. This new technique facilitates the evaluation of a variety of adhesive agents with diverse substrates.