Thermal and flammability properties of a silica–poly(methylmethacrylate) nanocomposite

PMMA, poly(metheylmethacrylate), nanocomposites were made by in situ radical polymerization of MMA, methylmethacrylate, with colloidal silica (ca. 12 nm) to study the effects of nanoscale silica particles on the physical properties and flammability properties of PMMA. Transparent samples resulted and the dispersity of the particles was examined by transmission electron microscopy and atomic force microscopy. The addition of nanosilica particles (13% by mass) did not significantly change the thermal stability, but it made a small improvement in modulus, and it reduced the peak heat release rate roughly 50%. Last, the flame‐retardant mechanism provided by the addition of nanosilica particles in PMMA is discussed. Published 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 2072–2078, 2003     

On the use of the atomic force microscope to monitor physical degradation of polymeric coating surfaces

The atomic force microscope (AFM) was used to monitor changes in surface features of an acrylic melamine coating that was exposed to a variety of conditions. Exposure to ultraviolet (UV) radiation and high relative humidity caused general roughening of the surface and the formation of pits. Further, the damage of the coating surface was much more substantial for exposure to high relative humidity compared to exposure to dry environments. This difference in degradation rates correlated with measurements of chemical degradation determined using infrared spectra that were acquired along with the AFM images. Building Materials Division, 100 Bureau Dr., Stop 8621, Gaithersburg, MD 20899-8621.     

Computer analysis of a polymer coating exposed to field weather conditions

A heat and moisture transfer model was used to study weathering of polymeric materials, such as paint, asphalt, sealants, plastics, textiles, and polymeric composites. Three damage indices, related to temperature changes, humidity changes, and time of wetness, respectively, were introduced to quantify adverse effects of climate. The variation of these indices was investigated for a hot and humid climate (Miami, FL) and a hot and dry climate (Phoenix, AZ). In addition, the relative effects of solar radiation, surface wetting by rain and dew condensation, and variations in outdoor temperature and relative humidity on the three damage indices were investigated. 100 Bureau Dr., Stop 8621, Gaithersburg, MD 20899-8621.     

The sub-micron scale energy dissipative deformation mechanisms of Kevlar fibrils

In this study, the energies to indent and scratch Kevlar 49 and Kevlar KM2 single fiber surfaces are investigated along with the associated deformation mechanisms of the fiber fibrillar network. For both indentation and scratching, values of energy are determined from the measured forces and displacements under a number of different contact conditions, which include variations in probe size, indentation/scratch depth, and scratch length. These values are correlated to the observed fibrillar deformation mechanisms. The total indentation energy is primarily recoverable for indentations made at low depths and/or by probes that impart low levels of effective strain. Indentations made to greater depths by probes imparting greater effective strains result in additional forms of deformation, which correlate with an increase in the percentage of the total energy that is absorbed and an increase in the total specific energy (energy per volume) of indentation. The energy of a constant depth scratch exceeds the energy of indentation for scratch lengths greater than ~1.5–3 times the indentation depth. The total specific energy of scratching decreases with the onset of microstructure failure, in the form of fibrillation, during scratching. The energies to indent and scratch Kevlar KM2 are generally lower than Kevlar 49, which is likely related to the decreased properties of the Kevlar KM2 shell and/or a difference in the local properties of the two fibers. The fibrillar deformation mechanisms critical to optimizing the specific energies of indentation and scratching, which depend on strain, are identified and discussed.     

Metrology for characterizing scratch resistance of polymer coatings

Current methods for scratch resistance assessment are often based on “relative but not quantitative” types of measurements, such as visual inspection, gloss changes, and changes in gray scale level or lightness. Most results are used for qualitative assessment purposes, which result in the lack of a repeatable and reliable standardized test method for the polymer materials community. To implement a scientifically based standardized test method for quantifying scratch resistance, it is vital to understand the relationships between material mechanical properties, morphology, and appearance (optical properties) of surface and subsurface deformation. In this article, preliminary results from a scratch testing protocol to identify the “onset” of plastic deformation in poly(methyl methacrylate) and poly(propylene) commercial samples are presented. Recent advances in optical scattering measurements to identify the onset of plastic deformation by analyzing specular and off-specular intensities are also presented. Presented at the 82nd Annual Meeting of the Federation of Societies for Coatings Technology, October 27–29, 2004, in Chicago, IL.     

Fusion of soft computing and hard computing in industrial applications: an overview

Soft computing (SC) is an emerging collection of methodologies which aims to exploit tolerance for imprecision, uncertainty, and partial truth to achieve robustness, tractability, and low total cost. It differs from conventional hard computing (HC) in the sense that, unlike hard computing, it is strongly based on intuition or subjectivity. Therefore, soft computing provides an attractive opportunity to represent the ambiguity in human thinking with real life uncertainty. Fuzzy logic (FL), neural networks (NN), and genetic algorithms (GA) are the core methodologies of soft computing. However, FL, NN, and GA should not be viewed as competing with each other, but synergistic and complementary instead. Considering the number of available journal and conference papers on various combinations of these three methods, it is easy to conclude that the fusion of individual soft computing methodologies has already been advantageous in numerous applications. On the other hand, hard computing solutions are usually more straightforward to analyze; their behavior and stability are more predictable; and, the computational burden of algorithms is typically either low or moderate. These characteristics. are particularly important in real-time applications. Thus, it is natural to see SC and HC as potentially complementary methodologies. Novel combinations of different methods are needed when developing high-performance, cost-effective, and safe products for the demanding global market. We present an overview of applications in which the fusion of soft computing and hard computing has provided innovative solutions for challenging real-world problems. A carefully selected list of references is considered with evaluative discussions and conclusions.     

Review of Instrumented Indentation

Instrumented indentation, also known as depth-sensing indentation or nanoindentation, is increasingly being used to probe the mechanical response of materials from metals and ceramics to polymeric and biological materials. The additional levels of control, sensitivity, and data acquisition offered by instrumented indentation systems have resulted in numerous advances in materials science, particularly regarding fundamental mechanisms of mechanical behavior at micrometer and even sub-micrometer length scales. Continued improvements of instrumented indentation testing towards absolute quantification of a wide range of material properties and behavior will require advances in instrument calibration, measurement protocols, and analysis tools and techniques. In this paper, an overview of instrumented indentation is given with regard to current instrument technology and analysis methods. Research efforts at the National Institute of Standards and Technology (NIST) aimed at improving the related measurement science are discussed.     

The influence of surface microstructure on the scratch characteristics of Kevlar fibers

In this work, nanoindentation and nanoscratching experiments are combined with atomic force microscopy to investigate the relationships between contact geometry, apparent friction, and deformation modes of two grades of Kevlar^® (Dupont) fiber—Kevlar KM2 and Kevlar 49. Changes in the relative angle between the scratching probe and the fiber surface, often termed as the attack angle, result in changes in deformation mode, which correlate with the changes in the apparent friction. As attack angle increases, the observed deformation modes of the fiber surface change from a smoothing of the surface, often termed as ironing, to fibrillation, in which the fibrils break and coalesce in front of the progressing probe. A mixture of these two modes occurs at intermediate attack angles. When fibrillation occurs, material pile-up forms in front of the progressing probe. This pile-up introduces an additional component to the frictional response that is largely responsible for an increase in apparent friction with an increasing attack angle and/or scratch length. The level of friction associated with fibrillation is measured to be up to approximately three times higher than previously reported for Kevlar yarn–yarn friction. Fibrillation of Kevlar KM2 occurs at larger attack angles as compared to Kevlar 49, which is believed to be related to a near-surface region of reduced modulus and hardness previously observed in KM2 fibers. A detailed discussion of the measured response is given based on the interactions between the scratching probe and the fibrillar network and the resulting deformation mechanisms.     

Seizure onset from periventricular nodular heterotopias: depth-electrode study

The association between gray matter heterotopias and seizures is well established; whether seizures originate from these lesions is not known. We evaluated three patients with intractable complex partial seizures and periventricular nodular heterotopias (PNHs) with video-EEG monitoring with multiple depth electrodes, including placement in the PNH, to determine whether seizures originate from the PNH. In two of the three patients, all seizures arose from the PNH as low-voltage beta activity. In the third patient, 80% arose from the hippocampi and 20% from the heterotopia. PNHs may serve as an epileptogenic focus in patients with intractable epilepsy.     

A buckling-based metrology for measuring the elastic moduli of polymeric thin films

As technology continues towards smaller, thinner and lighter devices, more stringent demands are placed on thin polymer films as diffusion barriers, dielectric coatings, electronic packaging and so on. Therefore, there is a growing need for testing platforms to rapidly determine the mechanical properties of thin polymer films and coatings. We introduce here an elegant, efficient measurement method that yields the elastic moduli of nanoscale polymer films in a rapid and quantitative manner without the need for expensive equipment or material-specific modelling. The technique exploits a buckling instability that occurs in bilayers consisting of a stiff, thin film coated onto a relatively soft, thick substrate. Using the spacing of these highly periodic wrinkles, we calculate the film's elastic modulus by applying well-established buckling mechanics. We successfully apply this new measurement platform to several systems displaying a wide range of thicknessess (nanometre to micrometre) and moduli (MPa to GPa).