Nisin, an antimicrobial peptide used in food preservation was evaluated for thermal stability. Nisin retained antimicrobial activity after having been heated at 121��C for seven hours. Anhydrous nisin and concentrated nisin solutions (5-10 mg/ml) were still active after having been heated up to 190��C or 230��C. When exposed to high temperatures of 200 or 250��C for up to one hour, nisin lost all antimicrobial activity. After 3.5 hours of heating at 121��C an inactive nisin degradation product, designated nisin L, was isolated. In comparison to native nisin, nisin L had reduced activity against Pediococcus pentosaceus FBB-61 and no activity against Bacillus cereus T vegetative cells or spores. Structural changes to nisin L were studied by high performance liquid chromatography (HPLC), ion spray mass spectroscopy (MS), ��H nuclear magnetic resonance spectroscopy (NMR) and circular dichroism (CD) spectroscopy and compared to original nisin. There was no difference in molecular weight between nisin and nisin L. Both MS spectra contained nisin, with average molecular weight (MW) of 3354 daltons (D), and hydrated nisin with average molecular weight of 3372 D. Nisin L had higher proportion of hydrated molecules, and it had molecules with more then one water addition. Proton NMR analysis of nisin L indicated that dehydrobutyrine 2 and dehydroalanine 5 residues had been altered, and that several new hydrogen resonances appeared. Water additions in nisin L are likely to have occurred at dehydroresidues, making them inactive. Nisin L was found to be more polar, as would be expected for a more hydrated peptide. Analysis of CD spectra indicated that nisin L had smaller content of a helix and therefore lesser membrane spanning capability. Tandem mass spectroscopy of original nisin revealed that it was hydrated at lysine 34 residue. / Graduation date: 1996

In this study, PEEK/SiO2 nanocomposites were fabricated by means of simple compression molding technique. The performances and properties of the resulting PEEK nanocomposites were examined in terms of tensile loading, hardness, dynamic mechanical analysis (DMA), thermal mechanical analysis (TMA), thermogravimetry analysis (TGA), differential scanning calorimetry (DSC), X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The results indicated that the modified nanosilica was seen to disperse more uniformly than the unmodified counterparty. The XRD patterns of the modified-silica filled PEEK composites reveal a systematic shift toward higher angles, suggesting the smaller d-spacing of the PEEK crystallites. As for the thermal properties of the resulting PEEK nanocomposites, there is no significant difference for the melting and crystallization temperatures, as well as the degree of crystallization between the modified and unmodified silica filled PEEK nanocomposites. The TMA results show that the coefficient of thermal expansion (CTE) becomes lowered when the content of the nanosilica increases. Furthermore, the CTE of the modified-silica filled PEEK nanocomposites shows the higher CTE values, as compared with those of the unmodified counterparts. In addition, the inclusion of the nanosilica could improve the microhardness and the stiffness of the resulting PEEK nanocomposites with the sacrifice of the elongation, as evident from the tension and DMA testing.

A large amount of heat can be liberated during cement hydration, causing very large temperature increases in mass concrete members. The non-uniform temperature field produced by the cement during curing can cause very high internal stresses that may crack the concrete. Concrete thermal cracking in very large structures is a well-known phenomenon and was studied extensively during the height of dam construction in the United States. In recent years concrete bridge member sizes have increased for structural and aesthetic reasons. Recent problems in San Antonio and Houston, Texas with thermal cracking and very high internal temperatures in mass concrete bridge members has renewed interest in studying early-age thermal cracking and its mechanisms. In order to predict the early-age thermal cracking risk of a concrete member, the temperature history, autogenous shrinkage, modulus development, tensile strength development, coefficient of thermal expansion development, creep behavior, and external restraint conditions must be known. A testing procedure has been developed to measure concrete heat of hydration, mechanical property development, and free shrinkage response at different curing temperatures. The concrete free shrinkage includes thermal and autogenous shrinkage components and is measured using a newly developed free shrinkage testing apparatus. The early age concrete creep is calculated from rigid cracking frame tests performed at different varying temperatures. Trends in early age creep behavior for different concrete mixtures common in mass concrete have been found and are used to develop a statistical model relating concrete mixture proportions and constituent material properties for use in mass concrete thermal stress modeling. The results from the test methods described are used in a new concrete early-age cracking risk and durability software package called ConcreteWorks. / text

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In currently used shear rheometers, shear stress is inferred from a measurement of either total force or total torque. These methods are subject to experimental errors due to uncontrolled flow at the sample boundaries. Such errors can be avoided by measuring the shear stress locally, in the region of controllable flow, using a shear stress transducer. A new sliding plate rheometer for molten plastics has been developed to incorporate a recently developed shear stress transducer. The rheometer operates at temperatures up to 250$ sp circ$C. Static and dynamic calibrations showed that the shear stress transducer sensitivity is stable and that its frequency response is suitable for the study of molten plastics. This rheometer was equipped with a computer controlled servohydraulic linear actuator, which provided wide flexibility in shear history selection. Digital data acquisition and signal processing enabled the use of the discrete Fourier transform for nonlinear viscoelastic property determination. Important differences were observed between the locally measured shear stress and values inferred from total force in both large amplitude oscillatory shear and in reciprocating exponential shear tests. For these property measurements, free boundary errors can dominate the dynamics of total force measurements.

This work is focussed on the equilibrium properties of r-mer fluids and their mixtures with particular emphasis on the limiting case of polymer mixtures. / The Flory-Patterson-Prigogine corresponding states theory of Polymer Solutions is corrected for non-randomness and extended to multicomponent multigroup systems. / A novel equipment for vapor sorption measurements in concentrated polymer solutions is described. With this equipment five polymer-polymer-solvent and one block copolymer-solvent systems have been studied. The Flory-Huggins (chi) interaction parameters for the polymer pairs have been determined. / The experimental data have been treated with the non-random new Flory theory. Interactional constants for the polymer pairs have been determined. A new interaction parameter has been introduced which considerably simplifies the treatment of ternary systems. / Two new statistical-thermodynamic theories of the fluid state have been developed and tested against existing experimental data.

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