Investigation of shear failure mechanisms of pressure‐sensitive adhesives

We report new experiments investigating the failure mechanisms in shear, of thin layers of acrylic pressure‐sensitive adhesives (PSA). We have developed a novel experimental device able to shear a soft adhesive layer confined between a rigid hemispherical lens and a rigid glass substrate. Using the resources of in situ contact visualization, the nonhomogeneous deformation of the layer and the shear failure processes were observed optically. Depending on the rheological properties of the adhesive, ratios of the contact radius over the layer thickness of 10–30 were achieved, mimicking well the contact conditions encountered in a thin adhesive layer within a joint. When the adhesive was weakly crosslinked, we observed a fluid‐like behavior and could measure a reasonable value for the viscosity of the PSA, implying that flow can occur in the layer and failure will occur by creep. On the other hand, for a more crosslinked adhesive, closer to what is used in applications, a stick‐slip peeling behavior was observed, which involves a coupling between peeling mechanisms at the leading edge of the contact and interfacial slippage. Such a process suggests a failure by fracture rather than by creep. Interestingly, the peeling mechanisms and the associated stress levels change significantly when the layer becomes as thin as 20 μm, implying a fracture process that is controlled by a critical energy release rate in shear G_IIc rather than by a critical shear stress causing failure of the interfacial bonds. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 3316–3330, 2005     

Internalization of a peptide into multilamellar vesicles assisted by the formation of an alpha-oxo oxime bond

As part of a drug-delivery project, we designed and synthesised a novel hydroxylamine cholesterol-based anchor to ensure the chemoselective ligation of recognition patterns onto multilamellar vesicles by oxime ligation. The entry of a glyoxylyl peptide into the vesicles was unexpectedly assisted by the formation of the alpha-oxo oxime bond. We studied extensively the kinetic and thermodynamic aspects of this phenomenon. Briefly, for a glyoxylyl peptide, the speed and ability to enter the vesicle were dependent upon 1) the presence of a hydroxylamine anchor of the type CholE3ONH2, 2) the amount of peptide engaged in the ligation and 3) the flip-flop motion permitted by the different formulations, in which the presence of cholesterol seems to play an important role.     

Fragmentation sous l'impact electronique d'imidazo[1,5-b]triazepinones-1,2,4 et d'imidazo[4,5-b]pyridinones

The mass spectra of 1,2,4-imidazo[1,5-b]triazepinones and imidazo[4,5-b]pyridinones, formed by condensation of diaminoimidazoles with β-dicarbonyl compounds, are reported and analysed. The main fragmentation pathways have been determined and the utility is shown in structure determinations of isomeric compounds.     

(2‐Thienylmethyl)ammonium ­trichlorostannate(II): a hybrid salt

The structure of the title hybrid salt, (C₅H₈NS)[SnCl₃], is built up from segregated layers of organic cations and Sn polyhedra. [SnCl₃]^? groups are linked together by weak Sn?Cl interactions to form a one‐dimensional polymeric chain of anions.     

Influence of spontaneous decay on resonantly enhanced two-photon ionisation of D (2s)

The wavelength dependence of the two-photon ionisation of metastable D(2s) by a laser beam has been measured near the 2s ? 3p resonant frequency. A sharp structure is observed in the broad resonance profile at quite moderate intensity. Its origin is shown to be the spontaneous decay of the 3p state that competes with photoionisation.     

Entropic stabilization of isolated beta-sheets

Temperature-dependent electric deflection measurements have been performed for a series of unsolvated alanine-based peptides (Ac-WA(n)-NH(2), where Ac = acetyl, W = tryptophan, A = alanine, and n = 3, 5, 10, 13, and 15). The measurements are interpreted using Monte Carlo simulations performed with a parallel tempering algorithm. Despite alanine's high helix propensity in solution, the results suggest that unsolvated Ac-WA(n)-NH(2) peptides with n > 10 adopt beta-sheet conformations at room temperature. Previous studies have shown that protonated alanine-based peptides adopt helical or globular conformations in the gas phase, depending on the location of the charge. Thus, the charge more than anything else controls the structure.     

Synthesis and characterization of [PtIn6](GeO4)2O and its solid solution [PtIn6](GaO4)(2-x)(GeO4)xOx/2 (0 < or = x < or = 2): gradual color change of the solid solution from black (x = 0) to yellow (x = 2) as a consequence of quantum dot effect

A new phase [PtIn6](GeO4)2O, a filled variant of [PtIn6](GaO4)2, and the solid solution [PtIn6](GaO4)(2-x)(GeO4)xOx/2 (0 < or = x < or = 2) were prepared and characterized. Single-crystal structure refinements show that [PtIn6](GeO4)2O is isotypic with the mineral, sulfohalite Na6FCl(SO4)2, and crystallizes in the space group Fmm (Z = 4) with a = 1006.0(1) pm. The building units of [PtIn6](GeO4)2O are isolated [PtIn6]10+ octahedra and (GeO4)4- tetrahedra, and the isolated O2- ions occupy the centers of the In6 octahedra made up of six adjacent PtIn6 octahedra. The lattice parameter of the solid solution [PtIn6](GaO4)(2-x)(GeO4)xOx/2 (0 < or = x < or = 2) varies gradually from a = 1001.3(1) pm at x = 0 to a = 1006.0(1) pm at x = 2, and the color of the solid solution changes gradually from black (x = 0) to red (x = 1) to yellow (x = 2). The cause for the gradual color change was examined by performing density functional theory electronic structure calculations for the end members [PtIn6](GaO4)2 and [PtIn6](GeO4)2O. Our analysis indicates that an oxygen atom at the center of a In6 octahedron cuts the In 5p/In 5p bonding interactions between adjacent [PtIn6]10+ octahedra thereby raising the bottom of the conduction bands, and the resulting quantum dot effect is responsible for the color change.     

Total Chemical Synthesis of an Intra‐A‐Chain Cystathionine Human Insulin Analogue with Enhanced Thermal Stability

Despite recent advances in the treatment of diabetes mellitus, storage of insulin formulations at 4 °C is still necessary to minimize chemical degradation. This is problematic in tropical regions where reliable refrigeration is not ubiquitous. Some degradation byproducts are caused by disulfide shuffling of cystine that leads to covalently bonded oligomers. Consequently we examined the utility of the non‐reducible cystine isostere, cystathionine, within the A‐chain. Reported herein is an efficient method for forming this mimic using simple monomeric building blocks. The intra‐A‐chain cystathionine insulin analogue was obtained in good overall yield, chemically characterized and demonstrated to possess native binding affinity for the insulin receptor isoform B. It was also shown to possess significantly enhanced thermal stability indicating potential application to next‐generation insulin analogues.     

Sharp error bounds for complex floating-point inversion

We study the accuracy of the classic algorithm for inverting a complex number given by its real and imaginary parts as floating-point numbers. Our analyses are done in binary floating-point arithmetic, with an unbounded exponent range and in precision p; we also assume that the basic arithmetic operations (+, ?, ×, /) are rounded to nearest, so that the roundoff unit is u = 2^?p. We bound the largest relative error in the computed inverse either in the componentwise or in the normwise sense. We prove the componentwise relative error bound 3u for the complex inversion algorithm (assuming p ≥ 4), and we show that this bound is asymptotically optimal (as p → ∞) when p is even, and sharp when using one of the basic IEEE 754 binary formats with an odd precision (p = 53, 113). This componentwise bound obviously leads to the same bound 3u for the normwise relative error. However, we prove that the smaller bound 2.707131u holds (assuming p ≥ 24) for the normwise relative error, and we illustrate the sharpness of this bound for the basic IEEE 754 binary formats (p = 24,53,113) using numerical examples.