Visible-Light Switching of Metallosupramolecular Assemblies**

A photoswitchable ligand and palladium(II) ions form a dynamic mixture of self-assembled metallosupramolecular structures. The photoswitching ligand is an ortho-fluoroazobenzene with appended pyridyl groups. Combining the E-isomer with palladium(II) salts affords a double-walled triangle with composition [Pd₃L₆]⁶⁺ and a distorted tetrahedron [Pd₄L₈]⁸⁺ (1 : 2 ratio at 298 K). Irradiation with 410 nm light generates a photostationary state with approximately 80 % of the E-isomer of the ligand and results in the selective disassembly of the tetrahedron, the more thermodynamically stable structure, and the formation of the triangle, the more kinetically inert product. The triangle is then slowly transformed back into the tetrahedron over 2 days at 333 K. The Z-isomer of the ligand does not form any well-defined structures and has a thermal half-life of 25 days at 298 K. This approach shows how a thermodynamically preferred self-assembled structure can be reversibly pumped to a kinetic trap by small perturbations of the isomer distribution using non-destructive visible light.     

Dynamic plastic response of foil gages

Little research has been devoted to the accuracy of the response of resistance strain gages to dynamic plastic strain because of the lack of an easy-to-use comparative strain-measuring technique. The dynamic interferometric strain gage provides a suitable standard of comparison. Foil-gage response is compared directly with interferometric strain-gage response in a longitudinal-impact experiment. The accuracy of the foil gage does not decrease with increasing maximum strain (up to 8 percent). The error in shape of the strain wave does not vary drastically with distance from the point of impact. In contrast to previous work, foil gages are found to be reasonably accurate strain transducers for dynamic plastic strain.     

An extension of the concept of material symmetry

Summary The nature of a material is in large part described by its symmetry group. It is proposed to extend this concept to a more general type of symmetry transformation, and the use of such an extension is illustrated by using it to define several classes of materials and presenting the corresponding representation theorems.

---

Sommario La natura di un materiale è in gran parte descritta dal suo gruppo di simmetria. Si propone di estendere questo concetto a un tipo più generale di trasformazione simmetrica, e l'uso di questa estensione è illustrato nel definire parecchie classi di materiali e nel formulare i corrispondenti teoremi di rappresentazione.

     

Perturbation solution for secondary bifurcation in the quadratically-damped Mathieu equation

This paper concerns the quadratically-damped Mathieu equation:

     

A new method of representation of fiber creep data tensile creep behavior of some acrylic fibers

Summary Fiber creep data are usually represented as plots of extension versus time at constant stress and temperature. Fiber samples, however, exhibit some degree of variability in structure from filament to filament. Consequently, even when initial applied stresses are nominally equivalent (as determined by measurement of cross-sectional area, and load), the creep of individual fibers can differ significantly, and the creep behavior of a sample can only be characterized statistically.Structural parameters such as orientation, which affect fiber properties greatly, and therefore creep, are not considered in the conventional representation of creep data. Orientation, in turn, is associated with stretch or extension. On this basis, creep, i. e. creep rate, can be expected to depend on extension.In the proposed method for representing creep data, the rate of creep is related to the extension at a constant time and temperature. In these so-called isochronal creep rate curves, the use of stress as an independent variable is omitted, and is replaced by extension. This method of representation discloses common creep characteristics in fibers, which, though taken from the same sample, exhibit widely different creep behavior when treated conventionally. In this way, the creep behavior of the whole sample can be conveniently represented. These plots reveal more detail than the ordinary creep curves, and indicate different creep mechanisms within certain extension regions as well as the stretch-relaxation history of the particular fiber sample. In this paper, the aspects and limitations of these two methods of representation of creep data are discussed, and their relative merits are demonstrated using experimental acrylic fibers.     

An intermediate wavelength, weakly nonlinear theory for the evolution of capillary gravity waves

A new nonlinear evolution equation is derived for surface solitary waves propagating on a liquid-air interface where the wave motion is induced by a harmonic forcing. Instead of the traditional approach involving a base state of the long wave limit, a base state of harmonic waves is assumed for the perturbation analysis. This approach is considered to be more appropriate for channels of length just a few multiples of the depth. The dispersion relation found approaches the classical long wave limit. The weakly nonlinear dispersive waves satisfy a KdV-like nonlinear evolution equation with steeper nonlinearity.     

On the measurement of elastoplastic stresses

In principle, one should be able to measure elastoplastic stresses in the same manner as one does elastic stresses; i.e., measure the strains and compute the stresses from the constitutive law. In practice, this is rarely done because of the more complicated material response and the anisotropy of the plastic behavior. Further, elastoplastic stresses should be computed incrementally in the general case. This paper presents procedures for computing stresses from elastoplastic strains measured incrementally in a test under microcomputer control. The approach is evaluated for four different materials—two obeying the assumptions of classical plasticity and two showing anisotropic behavior—by computing the stresses in a smooth specimen from measured principal strains. A useful application is presented by computing the stresses at a notch root from biaxial strains measured with laser-based interferometry. The general conclusion is that even in situations where the material is clearly anisotropic, this approach can give a reasonableestimate of the largest local principal stress. Paper was presented at the 1991 SEM Spring Conference on Experimental Mechanics held in Milwaukee, WI on June 9–13.     

In search of physical parameters influenced by flow patterns in a heterogeneous two-phase mixture in microchannels using concomitant measurements

Space transportation systems require high-performance thermal protection and fluid management for systems ranging from cryogenic fluid devices to primary structures, and for propulsion systems exposed to extremely high temperatures, and other space systems, e.g., integrated circuits and cooling/environment control devices for advanced space suits. Although considerable developmental effort is underway to bring promising technologies to a readiness level for practical use, new and innovative methods are still needed. One such method is the Advanced Micro Cooling Module (AMCM), essentially a compact two-phase heat exchanger constructed of microchannels and designed to rapidly remove large quantities of heat from critical systems by incorporating phase transition. This paper describes the results of experimental research in two-phase flow phenomena, encompassing both an experimental and an analytical approach to the incorporation of flow patterns for air–water mixtures flowing in microchannels. Specifically addressed are: (1) design and construction of a sensitive two-phase experimental system which measures both ac and dc components of in situ physical mixture parameters including spatial concentration, using concomitant methods; (2) data acquisition and analysis in the amplitude, time, and frequency domains; and (3) analysis of results including evaluation of in situ physical parameters, and assessment of their validity for application in flow pattern determination.     

Gas-Phase Intramolecular Protein Crosslinking via Ion/Ion Reactions: Ubiquitin and a Homobifunctional sulfo-NHS Ester

Gas-phase intra-molecular crosslinking of protein ubiquitin cations has been demonstrated via ion/ion reactions with anions of a homobifunctional N-hydroxysulfosuccinimide (sulfo-NHS) ester reagent. The ion/ion reaction between multiply-protonated ubiquitin and crosslinker monoanions produces a stable, charge-reduced complex. Covalent crosslinking is indicated by the consecutive loss of 2 molecules of sulfo-NHS under ion trap collisional activation conditions. Covalent modification is verified by the presence of covalently crosslinked sequence ions produced by ion-trap collision-induced dissociation of the ion generated from the losses of sulfo-NHS. Analysis of the crosslinked sequence fragments allows for the localization of crosslinked primary amines, enabling proximity mapping of the gas-phase 3-D structures. The presence of two unprotonated reactive sites within the distance constraint of the crosslinker is required for successful crosslinking. The ability to covalently crosslink is, therefore, sensitive to protein charge state. As the charge state increases, fewer reactive sites are available and protein structure is more likely to become extended because of intramolecular electrostatic repulsion. At high charge states, the reagent shows little evidence for covalent crosslinking but does show evidence for ‘electrostatic crosslinking’ in that the binding of the sulfonate groups to the protein is sufficiently strong that backbone cleavages are favored over reagent detachment under ion trap collisional activation conditions.      

Rational Design of Macromolecular Structures: Focusing on the Embryonic Stages of Polymerization

Acetate-terminated oligomers of fluoral, chloral and bromal, prepared with the lithium alkoxides of tert-butanol and (-)-borneol, were analyzed by gas chromatography, polarimetry, NMR and mass spectroscopies, and by single crystal x-ray diffraction methods. The configurational and conformational properties of the “embryonic” adducts were found to depend strongly on the relative size of the trihalomethyl side group in these systems. The results of our efforts, summarized here, help to shed new light on the origins of helical geometry and crystalline order in polymers of the perhaloacetaldehydes.