Research Highlights of Polymers Division

NIST Combinatorial Methods Center

Combinatorial techniques offer a highly parallel, automated, and often miniaturized route for experimentation that allows hundreds or thousand of specimens to be fabricated and analyzed in a rapid and efficient fashion. Such techniques provide opportunities to significantly boost productivity, increase the scope of parameter-space exploration and enhance the understanding of complex systems in a variety of scientific and technological disciplines. Indeed, in recent years the unmitigated success of combinatorial and high-throughput methods in the pharmaceutical industry and the genomics effort has spurred researchers in the materials science and chemical industries to reconsider their reliance on traditional "one-at-a-time" experimental approaches to product and knowledge discovery. However, several challenges impede the successful adaptation of combinatorial methods in these arenas. For example, pharmaceutical combi models, which emphasize product discovery, often do not provide insight into the structure/property/processing relationships that are central to materials research. In addition, techniques and instrumentation explicitly suited to the high-throughput fabrication and analysis of combinatorial libraries of interest to materials scientists need to be developed. Moreover, many institutions interested in acquiring combinatorial methods require information on how to incorporate such strategies into their research program.

In response to these technical and educational needs, the NIST Combinatorial Methods Center (NCMC) was formally established in January 2002. A NIST-wide effort centered in the Polymers Division, the NCMC includes contributions from throughout MSEL and several other laboratories including Building and Fire Research, Physics, and Chemical Science and Technology Laboratories. The NCMC's mission is twofold. As a research program focused upon the creation of novel specimen libraries and instrumentation, the NCMC advances the state-of-the-art in combinatorial methods geared towards materials research and discovery. A complimentary effort is an outreach program, which serves to disseminate NCMC research products, instrument design, best protocols and practices, and other information relevant to the combi method. This organization also provides an effective means by which industrial needs are gauged.

NCMC combinatorial methods research is rooted in the so-called gradient approach. The core of gradient combi is the gradient specimen. These samples continuously vary in one property (e.g., film thickness) along a spatial coordinate over a prescribed range. By arranging two such gradients (e.g., thickness and temperature) in an orthogonal manner, a gradient library is created that exhibits every possible combination of properties within the scope of the constituent gradient specimens. Gradient libraries are particularly useful to materials scientists because they offer a convenient and thorough way in which to map parameter space. In addition, gradient libraries are especially amenable to examination via microscopy and microanalysis, which facilitates the illumination of structure-property relationships. Finally, as opposed to many robotics-oriented discrete approaches, gradient techniques can offer a cost effective way of performing combinatorial studies, making them attractive to a wider range of institutions.

As they are the basic unit of gradient libraries, many NCMC research efforts concentrate upon the development of novel gradient specimens and processes, with polymer science being a focus in recent years. Methods for producing gradients in film thickness, composition and temperature formed the basis of previous NCMC studies. This year, several new techniques were added to the NCMC repertoire. New instrumentation for generating gradients in ultraviolet-radiation (UV) exposure was central to this effort. Here, a UV slit-source is accelerated across a planar specimen. When this source is held in close proximity (< 5 mm) to the sample UV-generated ozone can chemically modify self-assembled monolayer (SAM) treated sample surfaces resulting in a gradient in surface energy and/or chemical functionality. Depending upon the SAM employed, such surface energy gradients can span 70° in water contact angle. Larger source/sample distances are effective in producing gradients in light-induced crosslinking, polymerization, and degradation. Since the degree of UV exposure is controlled through stage motion, this instrument is particularly versatile. Gradient profile, depth and steepness are easily tuned for a wide range of applications. In another new technique, gradients in the height of topographic surface features are created. Here, topographic gratings are lithographically etched into wedges of silsesquioxane "spin-on" oxides created via flow-coating. The resulting patterns, with heights ranging from (30 to 150) nm, provide a combi strategy for examining topography-mediated templating of film properties, making them useful for the development of thin-film optoelectronic devices and MEMS.

NCMC research also generates new techniques and instrumentation for the high-throughput measurement of materials properties. This year, efforts concentrated on combinatorial methods for evaluating mechanical properties and adhesion. Gradient film libraries mounted on deformable copper grids enable combinatorial examination of polymer failure mechanisms. In another technique, libraries prepared on rubbery polydimethylsiloxane supports allow high-throughput modulus measurements. Recently completed instrumentation facilitates sophisticated combinatorial measurements of adhesion via the Johnson-Kendall-Roberts (JKR) approach using micro-lens arrays. In addition, methods for performing combinatorial tests of interfacial fracture were developed in conjunction with a modeling effort that guides interpretation of experimental data.

Due to these and previous research efforts (some of which are detailed in this report) the NCMC has established a widely recognized expertise in gradient combinatorial materials science. This impact is demonstrated by the success of the NCMC outreach program, described below.

NCMC Outreach

The foundation of NCMC outreach is a membership program that invites industrial, academic, and government institutions to participate in a consortium designed to facilitate the advancement of combinatorial materials research. This program serves as a conduit through which NCMC research products are disseminated and industrial needs are gauged. Since the NCMC emphasizes relationships and collaboration that do not involve proprietary information, Center/member and member/member communication is maximized. However, collaborations that address specific industrial problems are established when necessary.

The NCMC has currently attracted 11 members (see table below), in part due to an extensive dissemination and recruiting effort that culminated in the inaugural NCMC conference held in San Diego, CA in January 2002.

Members are served through several routes. Foremost among these is a series of tri-annual members' meetings that showcase NCMC technical advances and protocols, distribute information on research conducted at NIST and elsewhere of relevance to combinatorial methods, and provide a forum in which current issues and needs in combinatorial and high-throughput research are discussed.

The first NCMC members meeting, held on April 26, 2002, was attended by over 20 representatives from 8 industrial institutions. Entitled, "Library Design and Calibration: What you need to know and do before you look for hits", this meeting involved a variety of activities. Technical presentations detailed the design of combinatorial experiments using the gradient technique and methods for calibrating gradient libraries. The importance of these processes in combinatorial research was demonstrated through a case study that illustrated how calibration and design principles work in an actual research program. An NCMC staff-lead discussion focused on combinatorial strategies for multicomponent formulations research and management of the massive amounts of data combinatorial studies necessarily produce. Laboratory demonstrations designed to provide practical first-hand knowledge about NCMC instrumentation and gradient techniques focused upon flow coating, composition and surface energy gradients, automated hot-stage optical microscopy, adhesion measurement instrumentation, infrared-imaging, and combinatorial mechanical testing. The second NCMC meeting, "Adhesion and Mechanical Properties", is slated for October 2002.

This year has also seen numerous visits to NCMC facilities by member parties. For example, both Rhodia and the Air Force Research Laboratory sent staff to work alongside NCMC scientists on combinatorial projects. This type of interaction is especially useful to institutions that are new to combinatorial research and its effective implementation.

The new NCMC web site (http://www.nist.gov/combi) provides information on the center and its activities, research publications from the NCMC and selected sources in the public domain, instrument schematics and specifications, and automation and analysis software. Additional resources, including browsable combinatorial data libraries, are also in development.

"Combinatorial methods offer the possibility of accelerating industrial materials research. The NCMC is well suited to drive this new technology because of their excellent work in high-throughput application testing…and addressing industrial needs. Industry will benefit by getting started in combi earlier."

Wolfgang SchrofPolymer PhysicistBASF (NCMC Member)

For more information on this topic:

Cher Davis, Michael Fasolka, Alamgir Karim, Eric Amis (Polymers Division, NIST)

Visit the NCMC web site at http://www.nist.gov/combi