Reactivity of oxetane monomers in photoinitiated cationic polymerization

Studies of the onium salt photoinitiated cationic ring‐opening polymerizations of various 3,3‐disubstituted oxetane monomers have been conducted with real‐time infrared spectroscopy and optical pyrometry. The polymerizations of these monomers are typified by an extended induction period that has been attributed to the presence of a long‐lived tertiary oxonium ion intermediate formed by the reaction of the initially formed secondary oxonium ion with the cyclic ether monomer. Because the extended induction period in the photopolymerization of these monomers renders oxetane monomers of limited value for many applications, methods have been sought for its minimization or elimination. Three general methods have been found effective in markedly shortening the induction period: (1) carrying out the photopolymerizations at higher temperatures, (2) copolymerizing with more reactive epoxide monomers, and (3) using free‐radical photoinitiators as synergists. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 3205–3220, 2005     

On blade/casing rub problems in turbomachinery: An efficient delayed differential equation approach

This paper presents an alternative pathway in studying the ubiquitous blade/casing rub problem in turbomachinery. Bladed disks interfere with the stationary shroud (casing) for a variety of reasons, such as axial offsets, thermal expansions. Both components being compliant, time-varying interface characteristics, nonlinearities and uncertainties in the rub forces make this dynamics very complex to model and analyze. The main idea in this paper originates from the conjecture that this dynamics is inherently akin to internal machining operation which also deals with compliant cutters (blades) but relatively more rigid workpiece (casing). This analogy directs our attention to the fact that the blade/casing impingement dynamics manifests a ‘regenerative mechanism’ which is impregnated with time delays. The ensuing time-delayed system (TDS) can be stable, which is ideal. If it is unstable, however, the interference amplitudes between the blade and the casing grow, and the nonlinear effects become dominant. If the components survived the exercise, this evolution would reach a limit-cycle behavior. Existing literature indicates that this limit cycle mode is the common state of operation in most modern-day turbomachinery. Consequently, the state-of-the-art research effort is focused on minimizing its amplitude to alleviate the destructive levels of fatigue effect. In this article we consider a different perspective in looking at these problems, by proposing the conditions to achieve stable rub interference. For this, a recent mathematical tool of the authors’ group called the Cluster Treatment of Characteristic Roots (CTCR) is deployed. CTCR declares the complete stability outlook of such time-delayed systems in the space of the operational and design parameters. We show how this new capability can assist the design process of the blade-casing interface. Simulations, relevant stability observations and comparisons with a peer technique are provided for some case studies to demonstrate the capabilities of the approach.     

Protecting‐Group‐Controlled Surface Chemistry—Organization and Heat‐Induced Coupling of 4,4′‐Di(tert‐butoxycarbonylamino)biphenyl on Metal Surfaces

Like pearls on a string , molecular building blocks have been preorganized and then interlinked on a surface (see STM images). In this way both the supramolecular self‐assembly of the reactants as well as the subsequent thermal activation to release the protecting group are controlled.

     

Photodegradation of C‐PCPDTBT and Si‐PCPDTBT: Influence of the Bridging Atom on the Stability of a Low‐Band‐Gap Polymer for Solar Cell Application

The kinetics of photodegradation and the reactivity of different sites of the low‐band‐gap polymers poly[2,6‐(4,4‐bis‐(2‐ethylhexyl)‐4H‐cyclopenta[2,1‐b:3,4‐b′]dithiophene)‐alt‐4,7‐(2,1,3‐benzothiadiazole)] (C‐PCPDTBT) and poly[2,6‐(4,4‐bis‐(2‐ethylhexyl)dithieno[3,2‐b:2′,3′‐d]silole)‐alt‐4,7‐(2,1,3‐benzothiadiazole)] (Si‐PCPDTBT) are investigated as thin films and are compared to those of poly(3‐hexylthiophene) (P3HT). The decay kinetics are monitored with UV/Vis spectroscopy and the reactivity and product evolution are investigated with X‐ray photoelectron spectroscopy (XPS). Both polymers exhibit higher stability than P3HT. The bridging atom in the cyclopentadithiophene (CPDT) subunit has a significant influence on the stability. Varying oxidation rates for the different elements were observed. In the case of Si‐PCPDTBT, the silicon atom is oxidized primarily, whereas the photooxidation rates of the other elements are reduced relative to C‐PCPDTBT. Additionally, XPS experiments with varying excitation energies reveal a significant reaction gradient within a few nanometers of the surface of degraded thin films of C‐PCPDTBT.     

Synchronization of nuclear spin generator system using active backstepping design

In this letter, an active backstepping design control method is presented for synchronizing two identical nuclear spin generator system with each other. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Ca3[BN2]I3: The First Halide‐Rich Alkaline Earth Nitrido­borate with Isolated [BN2]3– Units

The title compound Ca₃[BN₂]I₃ was obtained from reactions of mixtures of the starting materials Ca₃[BN₂]₂ and CaI₂ in a 1:4 ratio in sealed Nb tubes at 1223 K. The crystal structure was solved from powder synchrotron diffraction data. Ca₃[BN₂]I₃ is the first example of a halide‐rich nitridoborate crystallizing in the rhombohedral space group R32 [no. 155, Pearson code: hR96; Z = 12; a = 16.70491(2) Å, c = 12.41024(2) Å]. The crystal structure is built up by two interpenetrating networks of condensed edge‐sharing [BN₂]@Ca₆ and [□]@I₆ trigonal antiprisms (□ = void). In Ca₃[BN₂]I₃ two crystallograhically distinct [BN₂]^3– anions are present with d(B1–N) = 1.393(2) Å and d(B2–N) = 1.369(9) Å. Their bond angles are practically linear, varying only slightly: N–B1–N = 179(1)° and N–B2–N = 180°. Vibrational spectra were interpreted based on the D_∞h symmetry of the discrete linear [N–B–N]^3– moieties, considering the site symmetry reduction and the presence of two distinct [BN₂]^3– groups.     

Dual photo‐ and thermally initiated cationic polymerization of epoxy monomers

Selective inhibition of the photoinitiated cationic ring‐opening polymerization of epoxides by dialkyl sulfides has provided dual systems that can be “activated” by UV irradiation and then subsequently be polymerized by the application of heat. It is proposed that dialkyl sulfides terminate the initial or growing polyether chains at an early stage to form stable trialkylsulfonium salts. These systems are dormant at room temperature but on thermolysis, the sulfonium salts are capable of reinitiating ring‐opening polymerization. These dual photo‐ and thermal cure systems have potential applications in adhesives, potting resins, and composites. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 6750–6764, 2006     

An Acoustically Driven Microliter Flow Chamber on a Chip (μFCC) for Cell–Cell and Cell–Surface Interaction Studies

A novel method for pumping very small volumes of liquid by using surface acoustic waves is employed to create a microfluidic flow chamber on a chip. It holds a volume of only a few μl and its planar design provides complete architectural freedom. This allows for the reconstruction of even complex flow scenarios (e.g. curvatures, bifurcations and stenosis). Addition of polymer walls to the planar fluidic track enables cell culturing on the chip surface and the investigation of cell–cell adhesion dynamics under flow. We demonstrate the flexibility of the system for application in many areas of microfluidic investigations including blood clotting phenomena under various flow conditions and the investigation of different stages of cell adhesion.