NMR investigations of hexamethyldisilane confined in controlled pore glasses: Pore size distribution and molecular dynamics studies

In this work, the melting-point depression and molecular dynamics of hexamethyldisilane confined within five controlled pore glasses, with mean diameters ranging from 7.9 to 23.9 nm, are studied by high-field (9.4 T) nuclear magnetic resonance (NMR), and the results are discussed with reference to the bulk substance. The melting-point depression in pores with radiusR follows the simplified Gibbs-Thompson equation ΔT=k _p/(R−s) with ak _p value of 74 K · nm and ans value of 1 nm. To our knowledge, this is the first time thek _p value of hexamethyldisilane is reported. Proton spin-lattice relaxation times (T ₁), spin-spin relaxation times (T ₂), and diffusivities (D) are reported as a function of temperature. The confinement in the pores gives rise to substantial changes in the molecular dynamics and the phase behavior. The line-shape measurements reveal a two-phase system assigned to a relatively mobile component at the pore walls and a crystalline solid at the center of the pores. However, theT ₂ measurements show that the mobile phase also embraces a minor component attributed to nonfrozen liquid in pockets or micropores. The diffusivity of the major narrow-line component is approximately three orders of magnitude larger than that in the plastic bulk phase, reflecting fast diffusion of mobile molecules. Below the melting region,T ₁ of the narrow line is significantly shorter thanT ₁ of the broad line, suggesting that the molecular reorientation is more hindered close to the surface than at the center of the pore.     

Yellow‐light‐emitting cyano‐substituted poly[(1,3‐phenylene vinylene)‐alt‐(1,4‐phenylene vinylene)] derivative: Its synthesis and optical properties

A soluble cyano‐substituted poly[(1,3‐phenylene vinylene)‐alt‐(1,4‐phenylene vinylene)] derivative ( 9 ) was synthesized and characterized. Comparison between 9 and its model compound ( 10 ) showed that the chromophore in 9 remained to be well defined as a result of a π‐conjugation interruption at adjacent m‐phenylene units. The attachment of a cyano substituent only at the β position of the vinylene allowed the maximum electronic impact of the cyano group on the optical properties of the poly(p‐phenylene vinylene) material. At a low temperature (?108 or ?198 °C), the vibronic structures of 9 and 10 were partially resolved. The absorption and emission spectra of a film of 9 were less temperature‐dependent than those of a film of 10 , indicating that the former had a lower tendency to aggregate. A light‐emitting diode (LED) based on 9 emitted yellow light (λ_max ≈ 578 nm) with an external quantum efficiency of 0.03%. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 3149–3158, 2003     

Equisingular Deformations of Sandwiched Singularities

We relate the equisingular deformation theory of plane curve singularities and sandwiched surface singularities. We show the existence of a smooth map between the two corresponding deformation functors and study the kernel of this map. In particular we show that the map is an isomorphism when a certain invariant is large enough.     

The Translation Invariant Massive Nelson Model: I. The Bottom of the Spectrum

In this paper we analyze the bottom of the energy-momentum spectrum of the translation invariant Nelson model, describing one electron linearly coupled to a second quantized massive scalar field. Our results are valid for all values of the coupling constant and include an HVZ theorem, non-degeneracy of ground states, existence of isolated groundstates in dimensions 1 and 2, non-existence of ground states embedded in the bottom of the essential spectrum in dimensions 3 and 4, (i.e., at total momenta where no isolated groundstate eigenvalue exists), and we study regularity and monotonicity properties of the bottom of the essential spectrum, as a function of total momentum. Communicated by Joel Feldman submitted 04/11/04, accepted 17/02/05     

Luminescence and lifetime characteristics of microphases in KCl:KI:Pb mixed crystals

Absorption and luminiscence measurements were performed on lead-doped KCl:KI mixed single crystals. Strong differences were found between as-grown and quenched samples. New bands were observed in the absorption spectra of unquenched samples in the spectral region between the limit positions of the A bands, corresponding to chloride or iodide lattices. The strong band at 449 nm dominates the luminescence of these samples at low temperatures. Its kinetics have a complicated form and the decay time is in the interval from ns to ms. Quenched samples have a well-known luminescence spectrum caused by octahedral lead impurity centres in KCl single crystals. The kinetics, however, also indicate mixed chlorine and iodine coordination of lead in the investigated crystals.     

Magnetic poly(glycidyl methacrylate) microspheres prepared by dispersion polymerization in the presence of electrostatically stabilized ferrofluids

Fine magnetite nanoparticles, both electrostatically stabilized and nonstabilized, were synthesized in situ by precipitation of Fe(II) and Fe(III) salts in alkaline medium. Magnetic poly(glycidyl methacrylate) (PGMA) microspheres with core‐shell structure, where Fe₃O₄ is the magnetic core and PGMA is the shell, were obtained by dispersion polymerization initiated with 2,2′‐azobisisobutyronitrile (AIBN), 4,4′‐azobis(4‐cyanovaleric acid) (ACVA), or ammonium persulfate (APS) in ethanol containing poly(vinylpyrrolidone) or ethylcellulose stabilizer in the presence of iron oxide ferrofluid. The average microsphere size ranged from 100 nm to 2 μm. The effects of the nature of ferrofluid, polymerization temperature, monomer, initiator, and stabilizer concentration on the PGMA particle size and polydispersity were studied. The particles contained 2–24 wt % of iron. AIBN produced larger microspheres than APS or ACVA. Polymers encapsulating electrostatically stabilized iron oxide particles contained lower amounts of oxirane groups compared with those obtained with untreated ferrofluid. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 5827–5837, 2004     

Self-bonding in an amorphous polymer below the glass transition: A T-peel test investigation

Films of amorphous polystyrene (PS) with a weight-average molecular weight (M_w) of 225 × 10³ g/mol were bonded in a T-peel test geometry, and the fracture energy (G) of a PS/PS interface was measured at the ambient temperature as a function of the healing time (t_h) and healing temperature (T_h). G was found to develop with (t_h)^1/2 at T_h = T_g-bulk − 33 °C (where T_g-bulk is the glass-transition temperature of the bulk sample), and log G was found to develop with 1/T_h at T_g-bulk − 43 °C ≤ T_h ≤ T_g-bulk − 23 °C. The smallest measured value of G = 1.4 J/m² was at least one order of magnitude larger than the work of adhesion required to reversibly separate the PS surfaces. These three observations indicated that the development of G at the PS/PS interface in the temperature range investigated (<T_g-bulk) was controlled by the diffusion of chain segments feasible above the glass-transition temperature of the interfacial layer, in agreement with our previous findings for fracture stress development at several polymer/polymer interfaces well below T_g-bulk. Close values of G = 8–9 J/m² were measured for the symmetric interfaces of polydisperse PS [M_w = 225 × 10³, weight-average molecular weight/number-average molecular weight (M_w/M_n) = 3] and monodisperse PS (M_w = 200 × 10³, M_w/M_n = 1.04) after healing at T_h = T_g-bulk − 33 °C for 24 h. This implies that the self-bonding of high-molecular-weight PS at such relatively low temperatures is not governed by polydispersity. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1861–1867, 2004     

Calculating scale elasticity in DEA models

In the data envelopment analysis (DEA) efficiency literature, qualitative characterizations of returns to scale (increasing, constant, or decreasing) are most common. In economics it is standard to use the scale elasticity as a quantification of scale properties for a production function representing efficient operations. Our contributions are to review DEA practices, apply the concept of scale elasticity from economic multi-output production theory to DEA piecewise linear frontier production functions, and develop formulas for scale elasticity for radial projections of inefficient observations in the relative interior of fully dimensional facets. The formulas are applied to both constructed and real data and show the differences between scale elasticities for the two valid projections (input and output orientations). Instead of getting qualitative measures of returns to scale only as was done earlier in the DEA literature, we now get a quantitative range of scale elasticity values providing more information to policy-makers.     

Searching in trees

In (Discrete Math. 17 (1977)181) Rivest introduced the search complexity of binary trees and proved that among all binary trees with a fixed search complexity the smallest ones are the so-called Fibonacci trees. This result is extended for q-trees. The structure of the smallest q-trees is again Fibonacci-like but more complicated than in the binary case. In addition an upper bound for the asymptotic growth of these trees is given.