Laser abrading of carbon fibre reinforced composite for improving paint adhesion

Surface contaminations (originating from manufacturing processes), smooth surface, and poor wettability of carbon fiber reinforced polymer (CFRP) composite impair its successful paint adhesion. Surface pre-treatment of composite materials is often required. Previous approaches of using manual sand-papering result in non-uniform surface conditions and occasional damages to the fibres. Furthermore, the process is labour intensive, slow and can be hazardous to the workers if protections are not appropriate. This paper reports an investigation into a new surface treatment method based on laser multi-tasking surface abrading and surface cleaning/texturing for the improvement of paint adhesion. A KrF Excimer laser with a wavelength of 248 nm is used as the laser source. Significant improvement in paint adhesion has been demonstrated compared with as-received and sand-papered samples. This improvement is achieved by eliminating surface contaminants, minimizing chain scission and increasing in surface active functional groups as well as increasing in surface roughness. The former two play dominant roles.     

Exceptionally low shear modulus in a prototypical imidazole-based metal-organic framework

Using Brillouin scattering, we measured the single-crystal elastic constants (C(ij)'s) of a prototypical metal-organic framework (MOF): zeolitic imidazolate framework (ZIF)-8 [Zn(2-methylimidazolate)(2)], which adopts a zeolitic sodalite topology and exhibits large porosity. Its C(ij)'s under ambient conditions are (in GPa) C(11)=9.522(7), C(12)=6.865(14), and C(44)=0.967(4). Tensorial analysis of the C(ij)'s reveals the complete picture of the anisotropic elasticity in cubic ZIF-8. We show that ZIF-8 has a remarkably low shear modulus G(min) < or approximately 1 GPa, which is the lowest yet reported for a single-crystalline extended solid. Using ab initio calculations, we demonstrate that ZIF-8's C(ij)'s can be reliably predicted, and its elastic deformation mechanism is linked to the pliant ZnN(4) tetrahedra. Our results shed new light on the role of elastic constants in establishing the structural stability of MOF materials and thus their suitability for practical applications.     

A new approach to variable selection in least squares problems

The title Lasso has been suggested by Tibshirani (1996) as acolourful name for a technique of variable selection which requiresthe minimization of a sum of squares subject to an l₁ bound on the solution. This forces zero components in the minimizingsolution for small values of . Thus this bound can functionas a selection parameter. This paper makes two contributionsto computational problems associated with implementing the Lasso:(1) a compact descent method for solving the constrained problemfor a particular value of is formulated, and (2) a homotopymethod, in which the constraint bound becomes the homotopyparameter, is developed to completely describe the possibleselection regimes. Both algorithms have a finite terminationproperty. It is suggested that modified Gram-Schmidt orthogonalizationapplied to an augmented design matrix provides an effectivebasis for implementing the algorithms.     

Synthesis and characterization of two polymorphic crystalline phases and an amorphous powder of nickel(II) bisimidazolate

Nickel(II) bisimidazolate is polymorphic. Depending on the synthetic strategy adopted, two crystalline phases (alpha- and beta-Ni(im)(2)) or an amorphous material of the same composition can be prepared. The thermodynamically stable alpha-Ni(im)(2) phase, which can be prepared in water at elevated temperatures, contains a two-dimensional polymer (of nearly square meshes) with square-planar NiN(4) chromophores and exo-bidentate imidazolate ligands bridging nickel atoms that are ca. 5.73 A apart. The beta-Ni(im)(2) phase can be kinetically stabilized at lower temperatures, but the structural complexity and the lack of single crystals prevented its full structural characterization, even in the presence of an indexed powder diffraction pattern. The spectroscopic features of these crystalline phases are compared with those of the amorphous material.     

Conformational Switching of Resorcin[4]arene Cavitands by Protonation,Preliminary Communication

The synthesis of the quinoxaline‐bridged resorcin[4]arene cavitand 1 was accomplished from 2‐[3,5‐di(tert‐butyl)phenyl]acetaldehyde via formation of the intermediate octol 2 . Such cavitands are known to occur in an open `kite' conformation at low temperature (<213 K) but to adopt a `vase' conformation at elevated temperatures (>318 K). We discovered that protonation of cavitand 1 at room temperature by common acids, such as CF₃COOH, also causes reversible switching from `vase' to `kite', and that this conformational change can be conveniently monitored by both ¹H‐NMR and UV/VIS spectroscopy.     

Host-guest interactions at self-assembled monolayers of cyclodextrins on gold

We have developed synthesis routes for the introduction of short and long dialkylsulfides onto the primary side of alpha-, beta-, and gamma-cyclodextrins. Monolayers of these cyclodextrin adsorbates were characterized by electrochemistry, wettability studies, X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectrometry (TOF-SIMS), and atomic force microscopy (AFM). The differences in thickness and polarity of the outerface of the monolayers were measured by electro-chemistry and wettability studies. On average about 70% of the sulfide moieties were used for binding to the gold, as measured by XPS. Tof-SIMS measurements showed that the cyclodextrin adsorbates adsorb without any bond breakage. AFM measurements revealed for beta-cyclodextrin monolayers a quasi-hexagonal lattice with a lattice constant of 20.6 A, which matches the geometrical size of the adsorbate. The alpha-cyclodextrin and gamma-cyclodextrin monolayers are less ordered. Interactions of the anionic guests 1-anilinonaphthalene-8-sulfonic acid (1,8-ANS) and 2-(p-toluidinyl)naphthalene-6-sulfonic acid (2,6-TNS) and the highly ordered monolayers of heptapodant beta-cyclodextrin adsorbates were studied by surface plasmon resonance (SPR) and electrochemical impedance spectroscopy. The SPR measurements clearly showed interactions between a beta-cyclodextrin monolayer and 1,8-ANS. Electrochemical impedance spectroscopy measurements gave high responses even at low guest concentrations (< or = 5 microM). The association constant for the binding of 1,8-ANS (K = 289,000 +/- 13,000M-1) is considerably higher than the corresponding value in solution. (Partial) methylation of the secondary side of the beta-cyclodextrin strongly decreases the binding.     

Ball‐Milling‐Induced Amorphization of Zeolitic Imidazolate Frameworks (ZIFs) for the Irreversible Trapping of Iodine

The I₂‐sorption and ‐retention properties of several existing zeolitic imidazolate frameworks (ZIF‐4, ‐8, ‐69) and a novel framework, ZIF‐mnIm ([Zn(mnIm)₂]; mnIm=4‐methyl‐5‐nitroimidazolate), have been characterised using microanalysis, thermogravimetric analysis and X‐ray diffraction. The topologically identical ZIF‐8 ([Zn(mIm)₂]; mIm=2‐methylimidazolate) and ZIF‐mnIm display similar sorption abilities, though strikingly different guest‐retention behaviour upon heating. We discover that this guest retention is greatly enhanced upon facile amorphisation by ball milling, particularly in the case of ZIF‐mnIm, for which I₂ loss is retarded by as much as 200 °C. It is anticipated that this general approach should be applicable to the wide range of available metal–organic framework‐type materials for the permanent storage of harmful guest species.