Laser control of product electronic state: desorption from alkali halides

We demonstrate laser control of the electronic product state distribution of photodesorbed halogen atoms from alkali halide crystals. Our general model of surface exciton desorption dynamics is developed into a simple method for laser control of the relative halogen atom spin-orbit laser desorption yield. By tuning the excitation laser photon energy in a narrow region of the absorption threshold, the yield of excited state chorine atoms, Cl(2P(1/2)), can be made to vary from near 0 to 80% for KCl and from near 0 to 50% for NaCl relative to the total yield of Cl atoms. We describe the physical properties necessary to obtain a high degree of product state control and the limitation induced when these requirements are not met. These results demonstrate that laser control can be applied to solid state surface reactions and provide strong support for surface exciton-based desorption models.     

Recent developments in thermal analysis of polymers: Calorimetry in the limit of slow and fast heating rates

This review focuses on new insights into the crystal melting transition and the amorphous glass transition of polymers that have been gained through recent advances in thermoanalytical methods. The specific heat capacity can now be studied under two extreme limits, that is, under quasi‐isothermal conditions (limit of zero heating rate) and, at the other end of the scale, under rapid heating conditions (heating rates on the order of thousands of degrees per second), made possible through nanocalorimetry. The reversible melting, and multiple reversible melting, of semicrystalline polymers is explored using quasi‐isothermal temperature modulated differential scanning calorimetry, TMDSC. The excess reversing heat capacity, above the baseline, measured under nearly isothermal conditions is attributed to locally reversible surface melting and crystallization processes that do not require molecular nucleation. Observations of double reversible melting endotherms in isotactic polystyrene suggest existence of two distinct populations of crystals, each showing locally reversible surface melting. The second subject of the review, nanocalorimetry, is utilized to study samples of small mass under conditions of very fast heating and cooling. The glass transition properties of thin amorphous polymer films are observed under adiabatic conditions. The glass transition temperature appears to be independent of film thickness, and is observed even in ultra‐thin films. Recrystallization and reorganization during rapid heating are studied by nanocalorimetry of semicrystalline polymers. The uppermost endotherm seen under normal DSC scanning of poly(ethylene terephthalate) is caused by reorganization, and vanishes under the rapid heating conditions (3000K/s) provided by nanocalorimetry. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 629–636, 2005     

Isotopologue fractionation during N2O production by fungal denitrification

Identifying the importance of fungi to nitrous oxide (N₂O) production requires a non‐intrusive method for differentiating between fungal and bacterial N₂O production such as natural abundance stable isotopes. We compare the isotopologue composition of N₂O produced during nitrite reduction by the fungal denitrifiers Fusarium oxysporum and Cylindrocarpon tonkinense with published data for N₂O production during bacterial nitrification and denitrification. The fractionation factors for bulk nitrogen isotope values for fungal denitrification were in the range −74.7 to −6.6‰. There was an inverse relationship between the absolute value of the fractionation factors and the reaction rate constant. We interpret this in terms of variation in the relative importance of the rate constants for diffusion and enzymatic reduction in controlling the net isotope effect for N₂O production during fungal denitrification. Over the course of nitrite reduction, the δ¹⁸O values for N₂O remained constant and did not exhibit a relationship with the concentration characteristic of an isotope effect. This probably reflects isotopic exchange with water. Similar to the δ¹⁸O data, the site preference (SP; the difference in δ¹⁵N between the central and outer N atoms in N₂O) was unrelated to concentration during nitrite reduction and, therefore, has the potential to act as a conservative tracer of production from fungal denitrification. The SP values of N₂O produced by F. oxysporum and C. tonkinense were 37.1 ± 2.5‰ and 36.9 ± 2.8‰, respectively. These SP values are similar to those obtained in pure culture studies of bacterial nitrification but quite distinct from SP values for bacterial denitrification. The large magnitude of the bulk nitrogen isotope fractionation and the δ¹⁸O values associated with fungal denitrification are distinct from bacterial production pathways; thus multiple isotopologue data holds much promise for resolving bacterial and fungal production. Our work further provides insight into the role that fungal and bacterial nitric oxide reductases have in determining site preference during N₂O production. Copyright © 2008 John Wiley & Sons, Ltd.     

The interlaboratory comparison "IMEP-19 trace elements in rice"—a new approach for measurement performance evaluation

The aim of IMEP is to present objectively the quality of chemical measurements. Participants in IMEP compare their reported measurement results with independent external certified reference values with demonstrated traceability and uncertainty, as evaluated according to international guidelines. IMEP-19 focused on measurements of trace elements in rice aiming to support the Commission Regulation (EC) No. 466/2001 on maximum levels for certain contaminants in foodstuff. Measurement results for the elements Cd, Cu, Pb and Zn were reported by 267 field laboratories involved in food analysis from 43 countries. Performance criteria for the evaluation of the reported measurement results in IMEP-19 are suggested. The chosen performance indicators not only take into account the deviation of the reported measurement value from the certified reference value, but also set criteria for maximum and minimum acceptable uncertainty. The IMEP-19 participants' performance is reviewed by means of using new simple graphical tools, called "Naji plots".     

Enhancing Teaching Assistants' (TAs') Inquiry Teaching by Means of Teaching Observations and Reflective Discourse

Recent education reform efforts advocate teaching the process of science (inquiry) in undergraduate lecture and laboratory classes. To meet this challenge, professional development for the graduate student instructors (teaching assistants, or TAs) often assigned to teach these classes is needed. This study explored the implementation of an observation protocol designed to support peer observation and reflection among TAs teaching inquiry‐based undergraduate biology laboratories. The researchers of this study and TA‐peers who are experienced in teaching inquiry used the protocol to observe novice TAs at the beginning, mid‐point, and end of a semester‐long teaching assignment. Novice TAs used a modified version of this protocol to observe an experienced and a novice TA. Analysis of post‐semester interview data indicated engaging in both sets of observations and post‐observation discussions facilitated by the protocol gave novice TAs new ways to teach content, guidance on implementing pedagogical theory, and means to improve communications with students and classroom management skills. Researchers also used quantitative data collected from the observations to document frequency of teaching and student behaviors associated with teaching inquiry and how these frequencies changed during the semester. Considerations for how to use both sets of data to inform changes in TA professional development are discussed.     

3D Printing of Silk Protein Structures by Aqueous Solvent‐Directed Molecular Assembly

Hierarchical molecular assembly is a fundamental strategy for manufacturing protein structures in nature. However, to translate this natural strategy into advanced digital manufacturing like three‐dimensional (3D) printing remains a technical challenge. This work presents a 3D printing technique with silk fibroin to address this challenge, by rationally designing an aqueous salt bath capable of directing the hierarchical assembly of the protein molecules. This technique, conducted under aqueous and ambient conditions, results in 3D proteinaceous architectures characterized by intrinsic biocompatibility/biodegradability and robust mechanical features. The versatility of this method is shown in a diversity of 3D shapes and a range of functional components integrated into the 3D prints. The manufacturing capability is exemplified by the single‐step construction of perfusable microfluidic chips which eliminates the use of supporting or sacrificial materials. The 3D shaping capability of the protein material can benefit a multitude of biomedical devices, from drug delivery to surgical implants to tissue scaffolds. This work also provides insights into the recapitulation of solvent‐directed hierarchical molecular assembly for artificial manufacturing.     

Electron spin resonance and optical spectroscopic studies of the conducting polymer precursor: Poly(3,4-diisopropylidene cyclobutene)

The doping process in a recently developed conducting polymer precursor, poly(3,4-diisopropylidene cyclobutene) (PDPCB)¹ was studied by electron spin resonance and optical spectroscopy. Thin films of PDPCB were exposed to several oxidants, I₂, AsF₅, or a combination of AsF₅ and AsCl₃ or AsF₃, and monitored in situ by ESR. At the initial stages of doping, the films developed broad ESR spectra with Gaussian line shape. Except in the case of doping with I₂, resolved hyperfine structures from cation radicals were observed. As the doping progressed, the ESR spectra gradually transformed to narrower line widths with a Lorentzian component. The Lorentzian component can be attributed to charge carrier species developed in the film through doping. The results of optical spectroscopy (UV-visible) are incorporated to elucidate the effect of doping on electronic transitions of the doped PDPCB.     

Dielectric relaxation of poly(phenylene sulfide) containing a fraction of rigid amorphous phase

The dielectric relaxation behavior of poly(phenylene sulfide), PPS, has been investigated from room temperature to 180°C. This study was undertaken to examine the mobility of the amorphous phase through the glass transition region, to determine the contribution that rigid amorphous phase material makes to the relaxation process. Semicrystalline samples contain a fraction of the rigid amorphous phase, which was determined from the heat capacity increment at the glass transition, using degree of crystallinity determined from x-ray scattering. In the dielectric experiment, we measured the temperature and frequency dependence of the real and imaginary parts of the dielectric function. ε″ vs. ε′ was used to determine the dielectric relaxation intensity, δε = ε_s–ε∞, at temperatures above the glass transition. For amorphous PPS, δε decreases as temperature increases, while for all semicrystalline PPS, δε increases with temperature. The ratio of semicrystalline intensity to amorphous intensity determines the total fraction of dipoles which are already relaxed at a given temperature. Results indicate that more and more rigid amorphous phase material relaxes as the temperature is increased. This provides the first evidence that rigid amorphous phase material in PPS contains chains that possess different levels of molecular mobility. Finally, to the temperature of the loss peak maximum, at a given frequency, we assign the value of the dielectric T_g. For both melt and cold crystallization, the dielectric T_g systematically decreases as the crystallization temperature increases, and as the fraction of rigid amorphous phase decreases.     

Thermal properties and phase transitions in blends of Nylon-6 with silk fibroin

The thermal and structural properties of binary blends of Nylon-6 (N6) and a chemically related biopolymer, Bombyx mori silk fibroin (SF), are reported in this work. Homopolymers and blends, in composition ratios of N6/SF ranging from 95/05 to 70/30, were investigated by thermogravimetric (TG) analysis, differential scanning calorimetry (DSC), Fourier transform infrared (FTIR) spectroscopy and wide angle X-ray scattering (WAXS). Silk fibroin typically degrades at temperatures just above 210°C, which occurs within the melting endotherm of N6. In TG studies, the measured mass remaining was slightly greater than expected, indicating the blends had improved thermal stability. No beta sheet crystals of SF were detected by FTIR analysis of the Amide I region. Strong interaction between N6 and SF chains was observed, possibly as a result of formation of hydrogen bonds between N6 and SF chains. DSC analysis showed that the addition of SF to N6 caused a decrease in the crystallization temperature, the melting temperature of the lowest melting crystals and the crystallinity of N6. Furthermore, the α-crystallographic phase dominates and the γ-crystallographic phase was not observed in N6/SF blends, in contrast to the homopolymer N6, which contains both phases. We suggest that the addition of SF might result in changes of the chain extension of N6, which lead to the appearance of α-rather than γ-phase crystals.