Mechanical properties of ultra-oriented polyethylene

Semicrystalline polymers generally exhibit moduli well below their theoretical limit due to chain folding and to lack of crystal alignment. Modulus increases attainable through standard drawing procedures are limited by sample fracture before large draw ratios are reached. Using an Instron capillary rheometer which allowed a draw ratio of > 300, transparent polyethylene strands of unusually high c-axis orientation have been produced by a combination of pressure and shear. The virtually perfect crystalline orientation and evidence for extended chains confirm that a significant improvement in modulus can be realized by this technique. The dynamic tensile storage modulus was measured by Vibron over the temperature range ?160°C to +120°C. Room-temperature moduli were 7 × 10¹¹ dyne/cm², higher than any reported values for drawn polyethylene. Values also remained above 10¹¹ dyne/cm² even at 120°C. The moduli and morphological data have been related by a model consisting of an extended-chain component in paralled with a conventional drawn morphology. Experimental and calculated moduli are compared and related to available theory.     

Jet production by a gluon source in QCD

The properties of jets produced by a gluon source are examined in QCD. We give a simple derivation of the jet opening angle (up to an undetermined constant) using arguments of general applicability. At asymptotic energies, gluon jets are found to be wider than quark jets in an intuitively natural way. Unfortunately, in the energy range anticipated for PEP or PETRA, the results are quite sensitive to variations in the undetermined constant so that firm quantitative predictions cannot be made within the present approximations.     

Band crossing and the prealignment B(E2) anomaly in 126Ba

A theoretical interpretation of the reduction in E2 strengths in ¹²⁶Ba prior to backbending is presented. A shell model basis is built from normal parity orbilals organized into multiplets of a pseudo SU(3) symmetry coupled to $\text{h}{}_{\mathrm{<mtext>11</mtext><mtext>2</mtext>}}$ configurations restricted to states of seniority zero and two. Within the framework of the model the scattering of a pair of protons from normal parity to the $\text{h}{}_{\mathrm{<mtext>11</mtext><mtext>2</mtext>}}$ orbital produces band crossing and a corresponding reduction in E2 transition strengths prior to pair alignment which is the principal mechanism of the backbending.     

The effect of implanting nitrogen on the optical absorption and electron paramagnetic resonance spectra of silica

Silica samples (type III, Corning 7940) were implanted with N using multiple energies to produce a layer ∼600 nm thick in which the concentration of N was constant to within ±5%. The optical absorption spectra of the samples were measured from 1.8 to 6.5 eV. Electron paramagnetic resonance (EPR) measurements were made at ∼20.3 and 33 GHz for sample temperatures ranging from 77 K to 100 K for most measurements. The components identified in the EPR spectra, based on comparison with reported parameters, were due to E′ centers and peroxy radicals. By comparing the changes in the optical absorption at 5.85 eV with the changes in the concentrations of the various EPR components and with the reports in the literature, we conclude that there is an additional band at 5.7-5.9 eV other than the E′ center band. We conclude that the bands between 2 and 6.5 eV and the EPR spectral components produced by implantation of N are due to radiation damage processes; neither optical bands nor EPR components related to N are detected.     

Intramolecular direct arylation in the synthesis of fluorinated carbazoles

The amination of 2-chloroanilines with aryl bromides and subsequent intramolecular direct arylation can be exploited in the synthesis of a range of fluorinated carbazoles, where the fluorine substituent can be introduced via the aniline, the aryl bromide or both substrates. Depending on substitution patterns, the two steps can either be performed in tandem in one-pot under microwave heating conditions or else require a two pot approach.     

A new approach for efficient simulation of Coulomb interactions in ionic fluids

We propose a simplified version of local molecular field (LMF) theory to treat Coulomb interactions in simulations of ionic fluids. LMF theory relies on splitting the Coulomb potential into a short-ranged part that combines with other short-ranged core interactions and is simulated explicitly. The averaged effects of the remaining long-ranged part are taken into account through a self-consistently determined effective external field. The theory contains an adjustable length parameter sigma that specifies the cutoff distance for the short-ranged interaction. This can be chosen to minimize the errors resulting from the mean-field treatment of the complementary long-ranged part. Here we suggest that in many cases an accurate approximation to the effective field can be obtained directly from the equilibrium charge density given by the Debye theory of screening, thus eliminating the need for a self-consistent treatment. In the limit sigma-->0, this assumption reduces to the classical Debye approximation. We examine the numerical performance of this approximation for a simple model of a symmetric ionic mixture. Our results for thermodynamic and structural properties of uniform ionic mixtures agree well with similar results of Ewald simulations of the full ionic system. In addition, we have used the simplified theory in a grand-canonical simulation of a nonuniform ionic mixture where an ion has been fixed at the origin. Simulations using short-ranged truncations of the Coulomb interactions alone do not satisfy the exact condition of complete screening of the fixed ion, but this condition is recovered when the effective field is taken into account. We argue that this simplified approach can also be used in the simulations of more complex nonuniform systems.