The Chiral Twist-Bend Nematic Phase (N*TB)

The twist-bend nematic, N_TB, phase has been observed for chiral materials in which chirality is introduced through a branched 2-methylbutyl terminal tail. The chiral twist-bend nematic phase, N*_TB, is completely miscible with the N_TB phase of the standard achiral material, CB6OCB. The N*_TB phase exhibits optical textures with lower birefringence than those observed for the achiral N_TB phase, suggesting an additional mechanism of averaging molecular orientations. The N*−N*_TB transition temperatures for the chiral materials are higher than the N_TB−N transition temperatures seen for the corresponding racemic materials. This suggests the double degeneracy of helical twist sense in the phase is removed by the intrinsic molecular chirality. A square lattice pattern is observed in the N* phase over a temperature range of several degrees above the N*_TB–N phase transition, which may be attributed to a non-monotonic dependence of the bend elastic constant.     

Functional polyisobutylenes via a click chemistry approach

1‐(ω‐Azidoalkyl)pyrrolyl‐terminated polyisobutylene (PIB) was successfully synthesized both by substitution of the terminal halide of 1‐(ω‐haloalkyl)pyrrolyl‐terminated PIB with sodium azide and by in situ quenching of quasiliving PIB with a 1‐(ω‐azidoalkyl)pyrrole. Azide substitution of the terminal halide was carried out in 50/50 heptane/DMF at 90 °C for 24 h using excess azide. The 1‐(ω‐haloalkyl)pyrrolyl‐PIB precursors included 1‐(2‐chloroethyl)pyrrolyl‐PIB, 1‐(2‐bromoethyl)pyrrolyl‐PIB, and 1‐(3‐bromopropyl)pyrrolyl‐PIB. In situ quenching involved direct addition of 1‐(2‐azidoethyl)pyrrole to quasiliving PIB initiated from 5‐tert‐butyl‐1,3‐di(1‐chloro‐1‐methylethyl)benzene (bDCC)/TiCl₄ at ?70 °C in hexane/CH₃Cl (60/40, v/v). ¹H NMR analysis of the quenched product revealed mixed isomeric end groups in which PIB was attached at either C₂ or C₃ of the pyrrole ring (C₂/C₃ = 0.40/0.60). SEC indicated the absence of coupled PIB under optimized conditions, confirming exclusive mono‐substitution on each pyrrole ring. 1‐(3‐Azidopropyl)pyrrolyl‐PIB was reacted in modular fashion with various functional alkynes, propargyl alcohol, propargyl acrylate, glycidyl propargyl ether, and 3‐dimethylamino‐1‐propyne, via a Huisgen 1,3‐dipolar cycloaddition (Click) reaction, using Cu(I)Br/N,N,N′,N″,N″‐pentamethyldiethylenetriamine or bromtris(triphenylphosphine)Cu(I) as catalyst. The reactions were quantitative and produced PIBs bearing terminal hydroxyl, acrylate, glycidyl, or dimethylaminomethyl groups attached via exclusively four‐substituted triazole linkages. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 2533–2545, 2010     

Fructose production

Amyloglucosidase, pullulanase, and glucose isomerase were coimmobilized onto granular chicken bone (BIOBONE^TM). Enzyme ratios showing optimum glucose and fructose production (0.7:10:22.3 U amyloglucosidase: pullulanase: glucose isomerase) resulted in 14.4±1.9% of activity bound relative to an equal amount of free enzyme. The estimated specific activity for these enzymes decreased 4.6-fold with immobilization. Reaction_pH strongly influenced the yield and ratio of glucose and fructose produced. Net hexose production from the immobilized system was optimal at_pH 6.5 and 55°C with a fructose yield of about 20%.     

Soluble tin(II) macroinitiator adducts for the controlled ring‐opening polymerization of lactones and cyclic carbonates

Polyesters and poly(ester carbonates) were synthesized via ring‐opening polymerization with new tin(II) macroinitiator adducts containing oligomeric L ‐lactide (LLA), rac‐lactide (rac‐LA), and ?‐caprolactone (CL). The novel initiating species were synthesized by the reaction of LLA, rac‐LA, or CL with Sn(OEt)₂ (monomer concentration/initiator concentration ≤20) and then were dissolved in methylene chloride or toluene and stored in a stoppered flask for the subsequent ring‐opening polymerization of cyclic esters and carbonates. The soluble tin alkoxide macroinitiators yielded predictable and quantitative initiation of polymerization for up to 1 month of storage time at room temperature. The resulting polymers displayed low polydispersity (≤1.5), and a high monomer conversion (>95%) was obtained within relatively short polymerization times (≤2 h). Adjusting the monomer/macroinitiator ratio effectively controlled the molecular weights of the polymers. NMR was used to characterize the initiating species and polymer microstructure, and size exclusion chromatography was used to determine the molecular weight properties of the polymers. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 3434–3442, 2002     

Syntheses and crystal structures of two N-substituted thio-imidazoles

The syntheses and structures of two N-substituted thio-imidazoles are reported. The geometrical parameters for both compounds, including essentially planar imidazole rings, are consistent with previous structural studies of related materials. The only possible non-van der Waals’ interactions influencing the molecular packing are weak C–H⋯π bonds. Crystal data: C₁₂H₁₄N₂S (N-methyl-N′-2-phenylethyl-imidazol-2-thione), M _r = 218.31, monoclinic, P2₁/n, (No. 14), a = 6.8441(2) ?, b = 12.9960(4) ?, c = 13.4703(4) ?, β = 97.7729(16)°, V = 1187.12(6) ?³, Z = 4, R(F) = 0.039, wR(F ²) = 0.104. C₁₉H₂₀N₂S (N,N′-bis((s)-1-phenylethyl)imidazol-2-thione), M _r = 308.43, orthorhombic, P2₁2₁2₁ (No. 19), a = 10.4060(3) ?, b = 10.6712(3) ?, c = 14.8932(3) ?, V = 1653.81(7) ?³, Z = 4, R(F) = 0.038, wR(F ²) = 0.085.     

Poly(styrene‐b‐isobutylene) multiarm star‐block copolymers

Multiarm star‐branched polymers based on poly(styrene‐b‐isobutylene) (PS‐PIB) block copolymer arms were synthesized under controlled/living cationic polymerization conditions using the 2‐chloro‐2‐propylbenzene (CCl)/TiCl₄/pyridine (Py) initiating system and divinylbenzene (DVB) as gel‐core‐forming comonomer. To optimize the timing of isobutylene (IB) addition to living PS⊕, the kinetics of styrene (St) polymerization at −80°C were measured in both 60 : 40 (v : v) methyl cyclohexane (MCHx) : MeCl and 60 : 40 hexane : MeCl cosolvents. For either cosolvent system, it was found that the polymerizations followed first‐order kinetics with respect to the monomer and the number of actively growing chains remained invariant. The rate of polymerization was slower in MCHx : MeCl (k_app = 2.5 × 10⁻³ s⁻¹) compared with hexane : MeCl (k_app = 5.6 × 10⁻³ s⁻¹) ([CCl]_o = [TiCl₄]/15 = 3.64 × 10⁻³M; [Py] = 4 × 10⁻³M; [St]_o = 0.35M). Intermolecular alkylation reactions were observed at [St]_o = 0.93M but could be suppressed by avoiding very high St conversion and by setting [St]_o ≤ 0.35M. For St polymerization, k_app = 1.1 × 10⁻³ s⁻¹ ([CCl]_o = [TiCl₄]/15 = 1.82 × 10⁻³M; [Py] = 4 × 10⁻³M; [St]_o = 0.35M); this was significantly higher than that observed for IB polymerization (k_app = 3.0 × 10⁻⁴ s⁻¹; [CCl]_o = [Py] = [TiCl₄]/15 = 1.86 × 10⁻³M; [IB]_o = 1.0M). Blocking efficiencies were higher in hexane : MeCl compared with MCHx : MeCl cosolvent system. Star formation was faster with PS‐PIB arms compared with PIB homopolymer arms under similar conditions. Using [DVB] = 5.6 × 10⁻²M = 10 times chain end concentration, 92% of PS‐PIB arms (M_n,PS = 2600 and M_n,PIB = 13,400 g/mol) were linked within 1 h at −80°C with negligible star–star coupling. It was difficult to achieve complete linking of all the arms prior to the onset of star–star coupling. Apparently, the presence of the St block allows the PS‐PIB block copolymer arms to be incorporated into growing star polymers by an additional mechanism, namely, electrophilic aromatic substitution (EAS), which leads to increased rates of star formation and greater tendency toward star–star coupling. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 1629–1641, 1999     

Synthesis and characterization of multiarm star-branched polyisobutylenes: Effect of arm molecular weight

A series of star-branched polyisobutylenes with varying arm molecular weights was synthesized using the 2-chloro-2,4,4-trimethylpentane/TiCl₄/pyridine initiating system and divinylbenzene (DVB) as a core-forming comonomer (linking agent). The resulting star-branched polymers were characterized with regard to the weight-average number of arms per star molecule (N̄_w) and dilute solution viscosity behavior. As the molecular weight of the arm (M̄_{w, arm}) was increased, dramatically longer star-forming reaction times were needed to produce fully developed star polymers. It was calculated that N̄_w varied from 50 to 5 as the M̄_{w, arm} was increased from 13,000 to 54,000 g/mol. The radius of gyration, R_g, of the star polymers was observed to increase as M̄_{w, arm} was increased. The solution properties of the star polymers were evaluated in heptane using dilute solution viscometry. It was determined that the stars had a much higher [η] compared to the respective linear PIB arms, but a much lower [η] compared to a hypothetical linear analog of an equivalent molecular weight. The dependence of [η] on temperature for the stars and linear arms was very small over the temperature range 25 to 75°C, with only a very slight decrease with increasing temperature. [η]_star was also determined to increase with increasing M̄_{w, arm}, but decrease with increasing M̄_{w, star}. The branching coefficient, g′, calculated for the stars at 25°C, increased as N̄_w decreased and agre ed well with literature values for other star polymer systems. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35: 3767–3778, 1997     

Dynamics of Bond‐Fluctuation Model Chains in Good and Theta Solvents

The dynamic Monte Carlo algorithm is employed to explore the dynamics of flexible linear chains. The chains are represented by the bond‐fluctuation model with and without attractions between non‐bonded units placed at close distances. This mimics the behavior of real chains in the good and poorer solvents. We obtain the chain sizes, diffusion coefficients, Rouse modes, and their relaxation times. We also evaluate the time correlation function of the end‐to‐end vector at different concentrations. Subsequently, we compare the dependence of the simulation results on chain length, solvent quality, concentration, and mode order with the corresponding theoretical predictions. We observe a retardation of diffusion for non‐dilute systems close to the theta state. This retardation is too high to be exclusively attributed to the increase of global friction and can be caused by temporary adherence of the chains to transient clusters.     

Proposal to build a neutrino observatory in Sudbury, Canada

Summary The proposal to build a heavy-water Čerenkov detector deep underground is discussed. The main physics objectives are to measure the⁸B solar neutrino flux, to identify neutrino oscillations and to watch for gravitational collapse. The measurements of the sources of background and the optical properties of heavy water, which are necessary to prove the feasibility of the detector, are reported. The present status of the project is indicated.

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Riassunto Si discute il progetto di costruire un rivelatore Čerenkov ad acqua pesante a grande profondità. Gli obiettivi fisici principali sono quelli di misurare il flusso di neutrini solati di⁸B, d'identificare le oscillazioni neutriniche e di osservare il collasso gravitazionale. Si riportano le misure delle sorgenti di background e le proprietà ottiche dell'acqua pesante, che sono necessarie per provare la fattibilità del rivelatore. Si indica lo stato attuale del progetto.

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Резюме Обсуждается преложение о строительстве черенковского детектора на тяжелойводе. Основные физические задачи—измерение потока⁸B солнечных нейтрино, идентификация нейтринных осцилляций и наблюдение гравитационного коллапса Сообщаются результаты измерений источников фона и оптических свойств тяжелой воды, необходимые для вьшолнения этого детектора. Описывается состояние проекта в настоящее время.

     

Calculations of magnetostatic mode frequencies in a 70 μm-thick film of YIG

Some microwave measurements performed on a film of YIG and described recently in this journal by Borghese et al. have been examined. It is shown that, for the parallel-field case, quantitative agreement can be obtained between most of the observed lines and the results of calculations based on that theory of Damon and Eshbach. It is also suggested that, for such films of YIG, 4πM₀ is about 1800 G, which is significantly higher than for the bulk material.