Nematic ordering and defects on the surface of a sphere: a Monte Carlo simulation study

We examine the ordering of hard rods on the surface of a sphere using computer simulations. As predicted by previous theories of thin nematic shells we observe four s = + 1/2 defects. However, the predicted tetrahedral symmetry for the defects and the "baseball" director configuration is not observed. Instead the four defects are located, on average, on a great circle which splits the sphere into two hemispheres, each of which has a splay dominated director configuration. We argue that this result occurs as the bend elastic constant for hard rods is much larger than the splay elastic constant.     

Chemical functionality of poly(methylenephosphine): phosphine-borane adducts and methylphosphonium ionomers

The chemical functionality of poly(methylenephosphine) n-Bu[MesP-CPh(2)](n)H () is examined in reactions with two isoelectronic species, namely BH(3) and CH(3)(+). The potential reactivity of polymer is modelled by examining the reactivity of molecular phosphines bearing similar substituents as the polymer. In particular, the phosphine-borane adducts Mes(Me)P(BH(3))-CPh(2)H () and Mes(Me)P(BH(3))-CPh(2)SiMe(2)H () are prepared from the reaction of BH(3).SMe(2) with Mes(Me)P-CPh(2)H () or Mes(Me)P-CPh(2)SiMe(2)H (), respectively. Treating with MeOTf affords the methylated model compound, [Mes(Me)(2)P-CPh(2)H]OTf (). X-Ray crystal structures are reported for each model compound. The reaction of n-Bu[MesP-CPh(2)](n)H (M(n) = 3.89 x 10(4), PDI = 1.34) with BH(3).SMe(2) affords the phosphine-borane polymer n-Bu[MesP(BH(3))-CPh(2)](n)H () (M(n) = 4.13 x 10(4), PDI = 1.26). In contrast, methylation of phosphine polymer gives n-Bu[MesP-CPh(2)](x)-/-[MesP(Me)-CPh(2)](y)H.(OTf)(y) () where approximately 50% of the phosphine moieties are methylated (from (31)P NMR).     

Diphosphiranium (P2C) or diphosphetanium (P2C2) cyclic cations: different fates for the electrophile-initiated cyclodimerization of a phosphaalkene

The synthesis and crystallographic characterization of two new classes of cationic three- and four-membered organophosphorus heterocycles are reported. Treating phosphaalkene tBuP=CHtBu (1) with triflic acid (2:1) afforded an unprecedented asymmetric diphosphiranium triflate [tBuP-CH(tBu)-P(tBu)(CH2tBu)]OTf (2). Surprisingly, the analogous reaction of tBuP=CHtBu with methyl triflate (2:1) afforded a 1,3-diphosphetanium salt [tBu(Me)P-CH(tBu)-P(tBu)-CH(tBu)]OTf (3a). Structural characterization of these novel P2C and P2C2 rings confirmed their identity, and the metrical parameters were consistent with the strain expected for these ring systems. In preliminary mechanistic investigations by NMR spectroscopy, phosphenium species [tBuPCH2tBu]OTf (4) was detected in the stoichiometric reaction of 1 with HOTf while methylenephosphonium [Me(tBu)P=CHtBu]OTf (5) was observed when 1 was treated with MeOTf. The opposite reactivity observed when the P=C bond is treated with MeOTf compared that with HOTf is surprising and, to our knowledge, has not been observed previously in phosphaalkene chemistry. In addition to their fundamental interest, we are interested in these species as ring-closed forms of the propagating species in the cationic polymerization of P=C bonds.     

Implementation of a beam deflection system for studies of liquid interfaces on beamline I07 at Diamond

X‐ray optics, based on a double‐crystal deflection scheme, that enable reflectivity measurements from liquid surfaces/interfaces have been designed, built and commissioned on beamline I07 at Diamond Light Source. This system is able to deflect the beam onto a fixed sample position located at the centre of a five‐circle diffractometer. Thus the incident angle can be easily varied without moving the sample, and the reflected beam is tracked either by a moving Pilatus 100K detector mounted on the diffractometer arm or by a stationary Pilatus 2M detector positioned appropriately for small‐angle scattering. Thus the system can easily combine measurements of the reflectivity from liquid interfaces (Q_z > 1 Å^?1) with off‐specular data collection, both in the form of grazing‐incidence small‐angle X‐ray scattering (GISAXS) or wider‐angle grazing‐incidence X‐ray diffraction (GIXD). The device allows operation over the energy range 10–28 keV.     

Silicone-based polymer blends: Enhancing properties through compatibilization

Polymer blending is a cost-effective way to control the properties of soft materials, but the propensity for blends to macrophase separate motivates the development of efficient compatibilization strategies. Across this broad area, compatibilization is particularly important for polysiloxanes, which exhibit strong repulsive interactions with most organic polymers. This review analyzes state-of-the-art polysiloxane compatibilization strategies for silicone–organic polymer blends. Emphasis is placed on chemical innovation in the design of compatibilization agents that may expedite the commercialization of new silicone–organic materials. We anticipate that hybrid silicone blends will continue to play an important role in fundamental and applied materials science across industry and academia.     

The NIST natural-matrix radionuclide standard reference material program for ocean studies

In 1977, the Low-level Working Group of the International Committee on Radionuclide Metrology met in Boston, MA (USA) to define the characteristics of a new set of environmental radioactivity reference materials. These reference materials were to provide the radiochemist with the same analytical challenges faced when assaying environmental samples. It was decided that radionuclide bearing natural materials should be collected from sites where there had been sufficient time for natural processes to redistribute the various chemically different species of the radionuclides. Over the succeeding years, the National Institute of Standards and Technology (NIST), in cooperation with other highly experienced laboratories, certified and issued a number of these as low-level radioactivity Standard Reference Materials (SRMs) for fission and activation product and actinide concentrations. The experience of certifying these SRMs has given NIST the opportunity to compare radioanalytical methods and learn of their limitations. NIST convened an international workshop in 1994 to define the natural-matrix radionuclide SRM needs for ocean studies. The highest priorities proposed at the workshop were for sediment, shellfish, seaweed, fish flesh and water matrix SRMs certified for mBq per sample concentrations of ⁹⁰ Sr, ¹³⁷ Cs and ²³⁹ Pu + ²⁴⁰ Pu. The most recent low-level environmental radionuclide SRM issued by NIST, Ocean Sediment (SRM 4357) has certified and uncertified values for the following 22 radionuclides: ⁴⁰ K, ⁹⁰ Sr, ¹²⁹ I, ¹³⁷ Cs, ¹⁵⁵ Eu, ²¹⁰ Pb, ²¹⁰ Po, ²¹² Pb, ²¹⁴ Bi, ²²⁶ Ra, ²²⁸ Ra, ²²⁸ Th, ²³⁰ Th, ²³² Th, ²³⁴ U, ²³⁵ U, ²³⁷ Np, ²³⁸ U, ²³⁸ Pu, ²³⁹ Pu + ²⁴⁰ Pu, and ²⁴¹ Am. The uncertainties for a number of the certified radionuclides are non-symmetrical and relatively large because of the non-normal distribution of reported values. NIST is continuing its efforts to provide the ocean studies community with additional natural matrix radionuclide SRMs. The freeze-dried shellfish flesh matrix has been prepared and recently sent to participating laboratories for analysis and we anticipate receiving radioanalytical results in 2000. The research and development work at NIST produce well characterized SRMs that provide the world's environment-studies community with an important foundation component for radionuclide metrology.     

An alternative approach to determine the spot-size of a multi-mode laser beam and its application to diode laser beams

An alternative approach is suggested to determine the spot-size of a multi-mode laser beam. It has been shown by simulations that the suggested approach can give the beam quality factor and characteristic radius with less than 5% error. Unlike the power content method, the proposed method is applicable to the beams even with diameter one tenth of the CCD size. The new approach has been applied to a multi-mode diode laser output and it is shown that the ABCD matrix analysis can be used for beam propagation, with the measured parameters of the laser.     

Activity coefficient of hydrochloric acid in HCl/MgCl2 mixtures and HCl/NaCl/MgCl2 mixtures from 5 to 45°C

Magnesium, the dominant bivalent cation in natural seawater, exerts a substantial influence on the patterns of ion interactions in this saline medium. Mean activity coefficients of hydrochloric acid in mixtures of this acid with magnesium chloride at four ionic strengths, namelyI=0.1, 0.3809, 0.6729, and 0.8720 mole-kg^–1, were obtained from emf measurements of cells without liquid junction at nine temperatures from 5 to 45°C. The three highest ionic strengths correspond to seawater of salinities 20, 35, and 45, respectively. In addition, mixtures of HCl, NaCl, and MgCl₂ were studied atI=0.6729, the molal ratio of NaCl to MgCl₂ being maintained at 7.202 as in natural seawater. The Harned coefficients _{1 2} were found to decrease slowly with increase in temperature. The trace activity coefficient of HCl in solutions of MgCl₂ as well as in NaCl MgCl₂ mixtures was found to be nearly identical with that measured earlier in synthetic seawater of the same ionic strength but containing NaCl, MgCl₂, KCl, and CaCl₂.     

Transition layer solutions of a higher order equation in an infinite tube

A general stability and convergence theorem is established for generalized solutions of a family of nonlinear evolution equations with non-local diffusion in one space dimension. As the first application we justify the motion by crystalline energy as a limit of regularized problems. As the sec-ond application we show the convergence of crystalline algorithm for general curvature flow equations. Our general results are also important to explain that geometric evolution of crystals depends continuously on temperature even if facets appear.