Novel Technique for All-Optical Clock Extraction Using Fiber Bragg Gratings

We demonstrate a simple passive technique, using phase shifted fibre Bragg gratings, for extraction of optical clock signals from RZ optical data signals . The generation of an optical clock signal at 12.5 GHz from a 12.5 Gbs RZ optical data stream is experimentally demonstrated with simulation of optical clock extraction at 10 GHz fro m 40 Gbs OTDM data streams .     

Intercrystalline links in polyethylene crystallized from the melt

Intercrystalline links in polyethylene have been revealed by crystallizing from the melt mixtures of fractionated polymer and the linear hydrocarbon n-C₃₂H₆₆, the latter constituent being removed later by washing at room temperature in an organic solvent. These fibrous links measure up to 15,000 A. in length and are 30–300 A. in thickness. Molecular chains are oriented parallel to the lengths of the links, and apparently nucleate on tie molecules formed by the simultaneous crystallization of different parts of the same molecule on the surfaces of different, and often widely separated, crystals. The maximum length of the links found in a given polymer varies as the square root of molecular weight, and there are indications that molecules in the melt are much more extended than is predicted by conventional configurational statistics. The links are under tension and presumably exert a significant influence on physical properties.     

Electron microscopic study of deformation in polyethylene

Thin films of polyethylene prepared from blends of fractionated polymer and a linear hydrocarbon (n-C₃₂H₆₆) have been used to study the role of intercrystalline links in the deformation of semicrystalline polymer under uniaxial stress. These links have been found to be strong and virtually inextensible elements of the structure. It is shown that they are firmly attached to the chain-folded lamellar crystals they bridge (both within the same spherulite and across boundaries between adjacent spherulites) and that, by concentrating applied stress, they commonly induce these lamellae to begin yielding in regions close to their points of attachment. Where there are many closely spaced links the stress is distributed fairly evenly, and drawing is relatively smooth and uniform. With more sparsely distributed links, however, stresses tend to be concentrated at widely separated points; deformation then tends to be severe and highly localized, often resulting in failure of the material upon drawing. There are indications that stress is also transmitted between chain-folded lamellae in ways other than by intercrystalline links. One such way is by means of chain ends and molecular loops that emerge from the surfaces of these crystals and are embedded in interlamellar material. Experiments in which the deformed films were subsequently heated confirm earlier conclusions that extended chains in drawn polymer may undergo refolding during annealing.     

An electronic perspective on the reduction of an n=n double bond at a conserved dimolybdenum core

Density functional theory has been used to assess the role of the bimetallic core in supporting reductive cleavage of the N=N double bond in [Cp2Mo2(mu-SMe)3(mu-eta1:eta1-HN=NPh)]+. The HOMO of the complex, the Mo-Mo delta orbital, plays a key role as a source of high-energy electrons, available for transfer into the vacant orbitals of the N=N unit. As a result, the metal centres cycle between the Mo(III) and Mo(IV) oxidation states. The symmetry of the Mo-Mo delta "buffer" orbital has a profound influence on the reaction pathway, because significant overlap with the redox-active orbital on the N=N unit (pi* or sigma*) is required for efficient electron transfer. The orthogonality of the Mo-Mo delta and N-N sigma* orbitals in the eta1:eta1 coordination mode ensures that electron transfer into the N-N sigma bond is effectively blocked, and a rate-limiting eta1:eta1-->eta1 rearrangement is a necessary precursor to cleavage of the bond.     

Characterization of partially transesterified poly(beta-hydroxyalkanoate)s using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS) was used for the characterization of a partially transesterified poly(beta-hydroxyalkanoate), PHA, polymer produced by the bacterial strain Alcaligenes eutrophus using saponified vegetable oils as the sole carbon sources. The transesterification was carried out separately under acidic and basic conditions to obtain PHA oligomers weighing less than 10 kDa. The intact oligomers were detected in their cationized [M + Na](+) and [M + K](+) forms by MALDI-TOFMS. A composition analysis, using the MALDI-TOF spectra, indicate that the oligomers obtained via acid catalysis were terminated with a methyl 3-hydroxybutyrate end group, and those obtained by base catalysis had a methyl crotonate (olefinic) termination. In addition to HB (hydroxy butyrate), the oligomers were found to contain a small percentage of HV (hydroxy valerate). This was independently confirmed using gas chromatography/mass spectrometry (GC/MS). In comparison, the analysis of a commercial PHA polymer, transesterified under identical conditions, only showed the presence of HB, i.e. a pure PHB homopolymer. Copyright 1999 John Wiley & Sons, Ltd.     

Modification of immobilized metal complexes toward the design and synthesis of functional materials for nitric oxide delivery

Functional polymeric materials containing immobilized metal complexes with the potential to bind nitric oxide (NO) are described. The materials were synthesized using template copolymerization techniques and contain immobilized metalloporphyrin and metallosalen (N,N′‐bis(salicylidene)‐1,2‐ethylenediaminato(2‐)) sites where NO binding occurs. A potential drawback of current materials designed for NO‐delivery is the rebinding of NO to the material after release. To minimize this undesirable rebinding event, materials were prepared in which immobilized sites contain additional covalently‐linked functional groups that can bind to the metal centers after NO dissociation. A model system with a covalently attached Co^IIsalen complex and pyridine groups confirmed that the immobilized sites bind NO in the presence of the additional ligands. Attempts to immobilize an iron porphyrin, containing two axial imidazole ligands, were unsuccessful because the lability of the iron center prevented the formation of a stable template. Instead, the desired material was achieved by selecting Co^III porphyrin with axially coordinated imidazole ligands as the template complex. Once immobilized, a procedure was developed to remove the cobalt ions and incorporate iron into the sites. The site structure of these new immobilized sites was examined spectroscopically. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 2282–2292, 2006     

Influence of reptation on localized diffusion in crystallizing polymers

Crystalline texture in polymer spherulites appears to be determined in part by interplay during solidification between interface morphology and the diffusion of species segregated at crystal growth fronts; these species are molecules of lower molecular weight (fractionation) or molecules of stereoirregular structure. Early discussions of this behavior were based upon assumption of a single diffusion coefficient in each case. However, it is now known that, because of reptation, each molecule in a polymer melt diffuses with a diffusion coefficient dependent on its individual molecular weight. In this paper, the influence of reptation upon concentration profiles and diffusion ranges is examined. It is shown that such influence is slight when segregated species have relatively narrow distributions of molecular weight, such as are typical when segregation involves fractionation or is mostly confined to fractionated stereoirregular species blended with crystallizable host polymer. With broad distributions, however, concentration profiles are significantly altered and long segregated molecules dominate morphologically important behavior. Meaningful average diffusion ranges can often be derived and related to appropriately averaged molecular weights of participating molecules. Morphological implications of the various results are outlined.     

Morphology and properties of polycaprolactone-poly(dimethyl siloxane)-polycaprolactone triblock copolymers

The molecular structure, crystallization, solid-state morphology, thermal properties, and phase behavior of two copolymers consisting of a poly(dimethylsiloxane) (PDMS) mid-block coupled to polycaprolactone (PCL) end-blocks were investigated. Both copolymers (which differ only in the molecular lengths of the PCL end-blocks) were found to be lamellar systems, whose core consists of PCL chains having the same crystal structure as PCL homopolymer, and whose amorphous interlayers contain the PDMS blocks and the PCL noncrystalline segments. From x-ray and electron-microscopy results, it is expected that the PCL blocks may be folded once in the longer copolymer and not at all in the shorter. As a result of their differing PCL lengths, the former crystallizes as regular PCL spherulites (at a growth rate reduced with respect to PCL homopolymer), whereas the latter yields only defective, immature axialites of low overall crystallinity. Electron diffraction showed that these spherulites grow preferentially along b crystallographic axis and that the PCL crystalline stems are arranged perpendicularly to their lamellae. © 1993 John Wiley & Sons, Inc.     

Spherulitic morphology in polyethylene and isotactic polystyrene: Influence of diffusion of segregated species

Morphological consequences of a localized diffusion of segregated species at crystal growth fronts have been studied in two specific contexts: (1) variation of texture in spherulites grown in unfractionated polyethylene over a range of crystallization temperatures mostly in regime II, and (2) development of elongated lamellar habits in spherulites of a polymer (isotactic polystyrene) whose native crystal habit is regularly polygonal. In relation to (1) it is shown that, as crystallization temperature is varied, there is a correlation between mean thickness of stacks of lamellae and an averaged diffusion range of segregated molecules of lower molecular weight. It is noted that lamellar organization appears to be significantly different in polyethylene fractions. In relation to (2) it is shown that principal contributors to the evolution of spherulitic texture from hedritic precursors are fragmentation of lamellae by screw dislocations and radially biased growth under the influence of concentration gradients of segregated species.