An x-ray scattering based metrology was conceived over 20 years ago as part of a collaboration between National Institute of Standards and Technology (NIST) and International Business Machines Corporation (IBM) to evaluate the performance of lithographic patterning materials for the semiconductor industry. This methodology treated a periodic array of lithographic structures as a diffraction grating and focused on extracting the physical dimensions of the structures in the grating by analyzing the diffraction patterns. In the early stages of the work the focus was on developing the transmission small-angle x-ray scattering (tSAXS) as a metrology tool to measure the critical dimensions (CD) of the lithographic features vital to the integrated circuit chip fabrication. Later, the focus shifted to include grazing incident small-angle x-ray scattering and x-ray reflectivity as parts of the CD metrology tool due to their unique capabilities. Frequently the term critical dimension small-angle x-ray scattering (CDSAXS) has been used as a synonym for the metrology of using tSAXS for CD measurements without mentioning transmission. Various milestones in the CDSAXS development are reviewed in this article together with some prospects regarding the future growth of x-ray-based metrology for complex three-dimensional nanostructures important to semiconductor industries.

The near-limit diffusion flame regimes and extinction limits of dimethyl ether at elevated pressures and temperatures are examined numerically in the counterflow geometry with and without radiation at different oxygen concentrations. It is found that there are three different flame regimes—hot flame, warm flame, and cool flame—which exist, respectively, at high, intermediate, and low temperatures. Furthermore, they are governed by three distinct chain-branching reaction pathways. The results demonstrate that the warm flame has a double reaction zone structure and plays a critical role in the transition between cool and hot flames. It is also shown that the cool flame can be formed in several different ways: by either radiative extinction or stretch extinction of a hot flame or by stretch extinction of a warm flame. A warm flame can also be formed by radiative extinction of a hot flame or ignition of a cool flame. A general €-shaped flammability diagram showing the burning limits of all three flame regimes at different oxygen mole fractions is obtained. The results show that thermal radiation, reactant concentration, temperature, and pressure all have significant impacts on the flammable regions of the three flame regimes. Increases in oxidizer temperature, oxygen concentration, and pressure shift the cool flame regime to higher stretch rates and cause the warm flame to have two extinction limits. At elevated temperatures, it is found that there is a direct transition between the hot flame and warm flame at low stretch rates. The results also show that, unlike the hot flame, the cool flame structure cannot be scaled by using pressure-weighted stretch rates due to the its significant reactant leakage and strong dependence of reactivity on pressure. The present results advance the understanding of near-limit flame dynamics and provide guidance for experimental observation of different flame regimes.

Due to the importance of low-temperature chemistry in the ignition of many hydrocarbons, cool flames continue to be studied in great detail. Cool flames are the direct result of a successful first-stage ignition event, and the strength of the cool flame stage is closely correlated to the likelihood of engine knock [1]. The presence of propagating cool flames in a mixture can significantly shorten the ignition delay time between the first and second stages of ignition compared to what would be expected in a homogeneous reactor [2]. Furthermore, at near-limit fuel concentrations, cool flames can possess broader flammability limits than those of classical hot flames [3, 4].

To enable discovery, development, manufacturing, and deployment of advanced materials at least twice as fast as possible today, at a fraction of the cost—that is the ambitious goal of the Materials Genome Initiative for Global Competitiveness, or the MGI as it is often called, launched by President Barack Obama in June of 2011. In the MGI white paper, a specific path to achieve this goal is outlined in the form of the Materials Innovation Infrastructure (MII) and an integrated, holistic research workflow that leverages the MII. The MII consists of three components, presented in Fig. 1: Theoretical and Computational Tools, Experimental Tools, and Digital Data and Data Informatics Tools. The Materials Innovation Infrastructure (MII). The MGI has already had a measurable impact on the way materials scientists and engineers think about and conduct research. The National Science Foundation (NSF) has launched Designing Materials to Revolutionize and Engineer our Future (DMREF), a multi-directorate activity intended to directly address the MGI via NSF funding opportunities; and the National Institute of Standards and Technology (NIST) has funded a multi-institution consortium to establish a MGI Center of Excellence, the Center for Hierarchical Materials Design (CHiMaD). Similar efforts, on both small and large scales, have been initiated at the Department of Energy (DOE) and the Department of Defense laboratories and research funding units. These are important steps in realizing the goals spelled out in the MGI, but at some point the researchers—the folks in the trenches—need to get stuff done, plain and simple. What tools do they need or what tools need improvement? Where does one start? What are the key problems that need to be addressed? How does one begin to develop, implement, and access the MII? In mid and late 2013, a series of two joint NIST-DOE-NSF MGI Grand Challenges Summits were held. The purpose of these summits was to identify Grand Challenges in materials science and engineering that need to be addressed in order to realize the vision of the MGI. The output of these Summits is a list of research problems ranging in complexity and scope from the single group / principle investigator level to the institutional level. Not surprisingly, many of the identified Grand Challenges are known problems areas in materials science and engineering, with active research already underway. However, what is new and important about this list of Grand Challenges is the realization that these are not only interesting academic problems, but also critical problems that must be addressed in order to realize the goal of the MGI. The purpose of this Special Issue is to highlight superlative examples of both industrial and academic research in polymer physics that either directly or indirectly addresses these Grand Challenges or is performed in the spirit of the MGI MII – that is, as a holistic attack on a problem, leveraging theory, simulations, and experimentation. Furthermore, this Special Issue includes select Perspective articles that highlight the importance of MGI-like workflows and the MII in order to enable effective materials discovery and development. Consider the paper and perspectives from Glenn Fredrickson, Gila Stein, and Valeriy Ginzburg highlighting the role of theory, simulation, and model-based measurement in the area of block copolymer lithography. In this trending application area for block copolymers, it is an integrated research workflow in the spirit of the MGI that is enabling significant progress. The excellent review from Alejandro Strachan, focusing on molecular-scale simulation of thermoset polymers, highlights the state-of-the-art in molecular simulations of this very important class of engineering polymers. The timely review from Daniel Read, covering recent seminal work on simulating the rheology of “real-world” polymer ensembles, is sure to be a go-to reference for any polymer scientist interested in understanding the role of polymer architecture on processing and fabrication. It is important to remember that while this collection of Articles, Reviews, and Perspectives constitute a snapshot of MGI-relevant research, many of the Grand Challenges are not represented here and development of the MII is an active endeavor. It is an exciting time for materials science and polymer physics. With Grand Challenges abound, MGI funding on the rise, and the potential for genuine societal impact, let's celebrate the MGI's third birthday and continue to push forward toward the goal of discovery, development, manufacturing, and deployment of advanced polymeric materials at least twice as fast as possible today, at a fraction of the cost. August W. Bosse ExxonMobil Corporate Strategic Research Annandale, NJ Eric K. Lin NIST Materials Science and Engineering Division Gaithersburg, MD

The frequency and energy dissipation change of a quartz crystal microbalance during moisture absorption was measured for films with thickness ranging from 3 to 205 nm. Evidence of the viscoelastic nature of the films was observed for films thicker than 90 nm through the frequency and energy dissipation changes. For sufficiently thin films (t < 40 nm), the frequency change could be effectively modeled as a simple increase in mass, as predicted by the Sauerbrey equation. The viscosity of the swollen films was independent of initial polymer film thickness (93−205 nm). The equilibrium swelling ratio was independent of film thickness for all films examined (3−205 nm). The transition between the observation of a rigid film and a film showing viscoelastic character was found to be at β1D = 0.26 ± 0.10, where β1 = 2π/λs, λs is the shear wavelength, and D is the film thickness. This transition agrees with the predictions of White and Schrag (J. Chem. Phys. 1999, 111, 11192).

Established in 1962, the Polymers Division in the Material Measurement Laboratory of the National Institute of Standards and Technology (NIST) will soon celebrate its 50 th year as a world leader in polymers research. The mis-sion of the NIST Polymers Division is to serve as the Nation's reference laboratory responsible for producing the measurement methods, standards, and data needed to advance the manufacture and use of “soft” materials (poly-mers and complex fl uids), with the goal of improving industrial competitive-ness and addressing national needs. The NIST mission is unique because the work of the organization focuses on a facilitation role for technology development in the United States through a combination of strict objectivity and the highest levels of technical expertise. In order to maintain the broadest impact in facilitating technical competiveness, the Polymers Division advances the measurement science of polymers and complex fluids rather than engage in the development of new advanced materials for commercial use. NIST focuses on the underlying measurements needed for all companies competing in a particular sector, i.e., the important pre-competitive technical work that is needed to advance the fi eld rather than one company in particular. We develop measurement methods for physical/chemical properties and structure; processing, flow, and transport; and functional properties and performance (electrical, optical, biolog-ical, mechanical) of polymers and complex fluids. Measurement development has often accompanied fundamental break-throughs in polymer science within the Division. Novel measurements have lead to theoretical advances; precision measurements of semicrystalline polymers, polymer blends, and piezoelectric polymers resulted in leading theories of polymer morphology and crystallization kinetics, phase separation kinetics, and the current model for piezoelectric polymers, respectively. Alternatively, theoretical developments have often resulted in breakthroughs in polymer measurements and standards, such as the BKZ theory of non-linear viscoelasticity resulting in improved measurements of mechanical properties and theories for polymer chains in solution resulting in methods and standards for molecular mass distribution. Currently, the Polymers Division consists of nearly one hundred scientists with a broad portfolio of research that includes advanced imaging measurements of the interaction of biological systems with polymer materials, smallangle neutron and X-ray scattering measurements of nanostructured materials, the separation and purifi cation of single-wall carbon nanotubes, and the development of new tests for the reliability of soft body armor. The Division works closely with industry, government, and university partners to ensure that the critical measurement needs of the Nation are being met. For example, the Division has worked with International SEMATECH and Intel on identifying the potential limits of next-generation photoresist materials; the National Institutes of Health on standards development for dental materials; and with university groups around the world including Seoul National University, Stanford University, the University of Delaware, and the University of Colorado. Recent progress in some of these major areas is highlighted in this special issue. The unrelenting drive to shorter lengthscales in microelectronics presents new challenges in control of chemical and physical processes at molecular dimensions. Three papers highlight recent advances understanding and controlling these processes. One paper focuses on characterization of the complex interplay of chemistry and transport for photoresist development. Another paper focuses on high-performance mold materials for nanoimprint lithography. The fi nal paper highlights progress in identifying the potential limits and power of self-assembled nanoscale patterns with block copolymer thin films. Much of the work the Polymers Division does revolves around development of new and powerful characterization techniques. Several papers highlight advances following this theme. Surface wrinkling has been developed in the Polymers Division as a tool for making very sensitive rheological measurements of thin and ultrathin polymer membranes; we include a review of this topic. Also represented is pioneering work in the Division on interfacial rheological characterization of small volume liquid samples, characterization and separation of carbon nanotube dispersions, use of combinatorial methods to characterize interactions of mammalian cells with polymeric materials, and new conceptual approaches to understanding initial cell morphological response to polymeric surfaces. We hope you enjoy the issue. More information about the Division and its programs can be found at http://www.nist.gov/mml/polymers/.

Neutron reflectometry is used to measure the rate of interdiffusion between bilayer samples of deuterated and hydrogenated poly(methyl methacrylate) (PMMA) films with the polymerpolymer interface near the native oxide surface of a silicon wafer. In this work, the effects of a favorable substrate interaction and the molecular weight of each polymer layer are determined. Both the film thickness and the molecular weight of the lower d-PMMA layer are kept constant with a thickness of approximately one radii of gyration (Rg) of the polymer and the molecular weight of the upper hydrogenated layer is varied. Earlier experiments show that the mobility of the polymer chains within Rg of the substrate is much lower than that of the bulk, suggesting that the mobility of surface-pinned polymers controls the interdiffusion rate. In this study, we find that the rate of interdiffusion is strongly dependent upon the molecular weight of the top layer.

Photoresist materials enable the fabrication of advanced integrated circuits with ever-decreasing feature sizes. As next-generation light sources are developed, using extreme ultraviolet light of wavelength 13.5 nm, these highly tuned formulations must meet strict image-fidelity criteria to maintain the expected performance gains from decreases in feature size. However, polymer photoresists appear to be reaching resolution limits and advancements in measurements of the in situ formed solid/solid and solid/liquid interface is necessary. This Review focuses on the chemical and physical structure of chemically amplified photoresists at the lithographic feature edge at length scales between 1 nm and 100 nm. Neutron reflectivity measurements provide insight into the nanometer-scale composition profiling of the chemical latent image at an ideal lithographic line-edge that separates optical resolution effects from materials processing effects. Four generations of advanced photoresist formulations were examined over the course of seven years to quantify photoresist/photoacid and photoresist/developer interactions on the fidelity of lithographic features. The outcome of these measurements complement traditional resist design criteria by providing the effects of the impacts of the photoresist and processing on the feature fidelity. These physical relations are also described in the context of novel resist architectures under consideration for next-generation photolithography with extreme-ultraviolet radiation.

The long-standing pursuit of a synthetic equivalent to tissue cartilage has sprouted significant new activities in strategies for new material synthesis, among them a noticeable one being the double-network hydrogels (DN-gels) scheme. DN-gels were prepared from the combination of an anionic polyelectrolyte network and a lightly crosslinked neutral polymer. These hydrogels exhibit an intriguing combination of properties intrinsic to natural cartilage: a low surface friction coefficient and a fracture toughness much higher than either of the constituent materials. The reinforcement of a hard, brittle polymer gel with a soft, viscoelastic neutral polymer is counter-intuitive. Based on our recent results from neutron scattering measurements, we proposed a deformation mechanism where the molecular association between these two polymers plays a pivotal role. In this work, we further evaluate the proposed mechanism by performing mechanical measurements on DN-gel samples with different polyelectrolyte network structure. The experimental results provide qualitative support for the proposed deformation mechanism.