Bharath Rangarajan

Kinetic parameters of a two-phase model forin situ epoxidation of soybean oil

The process ofin situ epoxidation consists of a two-phase system that involves reactions in both phases, mass transfer between phases, and thermodynamic driving forces for the mass transfer. In this paper, we present a model that treats the process as a two-phase system and uses local phase concentrations to calculate reaction and mass transfer rates. The process ofin situ epoxidation has been broken down into a set of systematic steps, and rate constants for each step have been determined. A conventional stirred tank reactor, equipped with cooling coils, eliminated the heat and mass transfer limitations so that the true kinetics ofin situ epoxidation were observed. It is shown that significantly larger rates (larger by factors of 2–10) are obtained when heat and mass transfer limitations are removed. The two-phase model adequately predicts the epoxidation kinetics over a wide range of temperatures (50–90°C). In addition, the model also correctly predicts the effect of adding an inert solvent.

Biomedical applications of polyelectrolytes

Polyelectrolytes are used in a variety of biomedical systems, including dental adhesives and restorations; controlled release devices; polymeric drugs, prodrugs, and adjuvants; and biocompatible materials. This article provides a review of biomedical applications of polyelectrolytes with emphasis on recent developments. For completeness, an overview of the methods for polyelectrolyte synthesis is provided along with a description of the unique properties of polyelectrolyte solutions, gels, and complexes which make them useful in biomedical applications. The discussion of dental materials focuses on the recent developments in glass-ionomer cements and novel organic polyelectrolyte adhesives since these materials are replacing the traditional zinc carboxylates. The section on controlled release applications includes a brief overview of recent developments in the mature areas of coatings, matrices and binders; and provides more in-depth discussions of the advanced responsive, bioadhesive, and liposomal systems that have emerged in recent years. Finally, descriptions of the recent work in polyelectrolytes as biocompatible materials as well as drugs or prodrugs are provided.

Production of aerogels

Abstract Critical point drying of gels with carbon dioxide is used in the preparation of highly porous silica monoliths. In this work, silica with a reproducible density of 0.2 g/cc is obtained. Shrinkage during drying is observed using a high pressure view cell. Shrinkage is found to occur primarily during depressurization. A process has been developed to permit drying in less than 12 hours.

Characterization of hydrogels using luminescence spectroscopy

In this contribution the literature relevant to characterization of polyelectrolytes, polyelectrolyte complexes and hydrogels using luminescence spectroscopy is reviewed. A brief introduction to the fundamentals of luminescence is followed by a description of the various types of spectroscopic studies which may be used to characterize hydrogels. In addition to experiments based upon the addition of a viscosity, temperature- or polarity-sensitive fluorescent probe to characterize the local environment, experiments and techniques based upon luminescence quenching, fluorescence polarization, phosphorescence depolarization and excimer fluorescence are discussed. This review succinctly describes the utility of luminescence spectroscopy in the characterization of hydrogel systems, with a focus on recent developments in the characterization of hydrogels and polyelectrolyte complexes.

Novel methodology for postexposure bake calibration and optimization based on electrical linewidth measurement and process metamodeling

By combining electrical linewidth measurements and neural-network (NN) process metamodeling, lithography simulators can be calibrated in an efficient way. In this work we present a novel methodology for characterizing postexposure bake using a very large experimental data set, so that the calibrated model can be used as a truly predictive tool. The adoption of a special test reticle mask allowed us to collect more than 700u2009000 critical dimensions CDs from 24 silicon wafers for a matrix of postexposure bake (PEB) time, and temperature conditions. The lithographic patterns included isolated, semidense and dense lines for structures of 0.25, 0.20, 0.175, and 0.15 μm nominal size replicated across the exposure field and across the wafer. As a result of this particular metrology, each measured CD was associated with both topological (position on the wafer and position within the field) and process information (exposure dose, PEB time, and temperature). Database management techniques were implemented in order t...

Interpretation of Aerogel Shrinkage During Drying.

A variety of silica aerogels have been prepared by the hydrolysis of TEOS, and dried using supercritical CO 2 . The shrinkage which occurs during the drying process is dependent on the gel formulation and the extent of aging of the gels in their pore liquor. Such aging normally results in an increased density, modulus and pore size of wet gels. Upon drying the corresponding aerogels show the opposite behavior for modulus and density, which decrease with the extent of aging. Both drying and aging shrinkage were not observed for base-catalyzed gels, and were very small for HF-catalyzed gels. The use of formamide resulted in reduced drying shrinkage and a slightly larger amount of syneresis. Drying shrinkage is associated with the presence of micropores. Shrinkage during drying has been observed using a high pressure view cell and it was found that most of the shrinkage occurred during depressurization. An explanation consistent with the above is proposed.

The effect of aging on acid-catalyzed aerogels

Abstract Acid-catalyzed silica aerogels have been prepared using supercritical CO 2 . During aging of the wet gel, the density and modulus increases. Drying these gels results in additional shrinkage, with the youngest gels shrinking the most and providing the most dense aerogels with the highest modulus. The structural changes (dissolution and reprecipitation) which occur on aging result in reduced stresses during hypercritical drying and therefore a lower density aerogel.