190 research outputs found
Reis, Wasser und Palmen : aus dem Bildband "Vietnam" von Reinhold Wepf
No full textNo abstract available
Study and control of polymer blends morphology and related properties
No full textpeer reviewedIt will be shown how a combination of techniques allows to gain a rather precise idea of the (un)miscibility situation in polymer blends at different size scales (i.e. from ca.20 A up to lμ); typical examples include simultaneous use of TEM, SEM, NRET and ss. NMR. On these bases, interesting blends have been studied and tailored, in which both morphology and interfacial adhesion have been controlled (in particular by the use of diblock copolymers) to provide for a better spectrum of properties. A number of situations will be described, implying commodity and engineering polymers, but also different types of fillers; their optimization has led to interesting applications in the field of better emulsion dispersions, very high impact resins, economical engineering plastics, controlled migration, filled materials,. Copyright © 1988 Hüthig & Wepf Verla
From incompatible poly(aryl ether sulfone)/polyamide 4.6. blends to new impact resistant alloys by a synergistic combination of a block copolymer emulsifier and an impact modifier
No full textpeer reviewedThe compatibilization and impact modification of blends of a relatively new engineering plastic polyamide 4.6 (PA 4.6) and a poly(aryl ether sulfone) (PSU) are investigated. PSU‐b‐PA6 block copolymers, which can be easily synthesized by ring opening polymerization of ϵ‐caprolactam in the presence of a commercial PSU, were found to be very efficient emulsifiers for these incompatible blends. Small amounts (1–4%) of copolymer are sufficient to significantly reduce the particle size and to improve the tensile and impact properties. Combinations of the copolymer and an impact modifier (ethylene‐propylene rubber grafted with maleic anhydride) are synergistic and high impact PSU/PA 4.6 alloys are obtained in that way. Copyright © 1993 Hüthig & Wepf Verla
New prospects for “living” anionic polymerization of (meth) acrylic esters
No full textpeer reviewedThe living low temperature polymerization in polar solvents of most methacrylates, even functional ones, can now be performed by direct and experimentally convenient anionic methods, thanks to the use of α‐methylstyrene and of complex‐purified monomers. A diversified family of block copolymers has thus been generated, that are used as compatibilizing agents in the designing of interesting heterophase materials: i.e. blends of polymers and inorganic fillers, and liquid‐solid dispersions. The living polymerization of acrylates (or of methacrylates under more drastic conditions) raises additional requirements: they have been met to a large extent by the use of specific ligands of the growing ion‐pair. Resulting structures such as f.i. block copolymers and end‐functionalized oligomers of a low dispersity, should have a great interest in the molecular engineering of (meth)acrylate‐based products. Finally, these new initiators lend themselves to structural and mechanistic studies which should shed some light on the critical features of these living propagation processes. Copyright © 1990 Hüthig & Wepf Verla
Aluminium alkoxides: A family of versatile initiators for the ring‐opening polymerization of lactones and lactides
No full textpeer reviewedAluminium alkoxides, such as Al(OiPr)3 or Et3‐pAl(O (CH2)2X)p with 1≤p≤3, are very effective in initiating the polymerization of lactones, e.g.ϵ ‐ca‐prolactone and δ ‐valerolactone, and lactides (D,L‐ or L,L‐ isomers). The ring‐opening polymerization proceeds through a “coordination‐insertion” mechanism that involves the selective rupture of the acyl‐oxygen bond of the monomer and its insertion into an Al‐O bond of the initiator. Polymerization is typically “living” and allows block copolyesters with perfectly controlled molecular weight and composition to be prepared. Aluminium alkoxides carrying functional alkoxy groups (X = ‐Br, ‐CH2‐NEt2, ‐CH2‐CH=CH2, ‐CH2‐OC(O)‐C(Me)=CH2…) provide asymmetric telechelic polyesters (end‐groups being ‐X and ‐OH, respectively) and very interesting polyester macromonomers. Coupling the asymmetric telechelic chains via the hydroxyl end‐group ‐ or better its A1 alkoxide precursor ‐ is a straightforward way to the symmetric telechelic polymer bearing the X functional group. Copyright © 1991 Hüthig & Wepf Verla
A gadolinium-bridged polytungstoarsenate(III) nanocluster: [Gd8As12W124O432 (H2O)22]60-
No full textThis work was supported by the Swiss National Science Foundation (SNSF Professorship PP002-114711/1) and the University of Zurich. We thank Dr. B. Spingler, Institute of Inorganic Chemistry, University of Zurich, for helpful crystallographic discussions, and S. Weyeneth, Physics Institute, University of Zurich, for assistance with the magnetic measurements. We are grateful to Prof. A. Stemmer and R. Enning, Micro and Nano Science Platform, ETH Zurich, for help with AFM measurements. We thank Dr. R. Wepf, Electron Microscopy ETH Zurich, EMEZ, for Cryo-SEM investigations. The support of the Center for Microscopy and Image Analysis, University of Zurich, is gratefully acknowledged, and we thank Y. Zhou, UZH, for help with SEM measurements
The use of sodium birnessite as a curing agent for liquid polysulfide sealant
No full textThis thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University.Manganese (IV) oxide phases are widely used as curing agents for polysulfide sealants. The manganese based curing agents, however, do not give a reproducible cure and this results in material waste. As the market for insulating glass applications is increasing, efficient curing becomes more significant both commercially and environmentally. The research described in this portfolio focuses on using a laboratory synthesised manganese (IV) oxide phase, sodium birnessite, as a polysulfide curing agent. The synthesis and full characterisation of sodium birnessite is described and its curing ability measured in terms of the following properties of the cured polysulfide: rate of cure. surface condition, hardness build-up, stress-strain data, low and high temperature properties. and moisture transmission. The results obtained are compared with those of polysulfides cured with a good commercial product. Different concentrations of curing agents, and types and concentrations of accelerators, plasticisers and inert diluents were investigated to optimise the curing process, reduce material use and product wastage. Sodium birnessite is an active and reliable curing agent when used at half of the concentration of the commercial agent in the presence and absence of inert diluents. Diluted sodium birnessite (1: 1 with inert diluent) cures polysulfide to levels comparable in terms of stress-strain properties, moisture transmission rate, hardness, surface condition, low- and high- temperature properties, and degree of polysulfide cross linking to that achieved with commercial agent. The mechanism of the curing of polysulfide with sodium birnessite, as with all curing agents is conversion of mercaptan groups -S-H into disulfide linkages (-S-S-). Sodium birnessite is a mixed oxidation state compound and its synthesis results in the inclusion of Mn 2+ in addition to Mn 4+ ions in the manganese oxide based lattice. A rationale for the improved curing ability of sodium birnessite is presented. It is based on the ability of Mn 2+ to create vacancies in the Mn4+ sub-lattice thus increasing the mobility of the Mn4 in the structure and its transport to the surface of the curing agent where the oxidation reduction reaction responsible for the cure takes place.Financial support was obtained from Twinstar Chemicals Ltd, and the Engineering and Physical Sciences Research Council (EPSRC
Improved superabsorbent polymers
No full textThis thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University.This work is devoted to the synthesis, characterization and modification of a class of acrylic-based superabsorbent polymers. The techniques of inverse suspension and solution polymerisations were used for synthesis. Each absorbent was mainly characterised by its equilibrium capacity of water absorption and by the rate of absorption.
The swelling characteristics of the polymers were evaluated in terms of change in polymerisation variables which include, type and amount of crosslinker, monomer composition, process of polymerisation, temperature, initiator concentration, monomer concentration and particle size of the product. The swelling dependency on salinity was also examined. In some cases, the base polymer was blended with certain polymers in order to upgrade the swelling properties.
A swelling model was devised based on a simple viscoelastic model, i. e. Voigt model, to obtain a better understanding of the effect of the above-mentioned variables on the swelling behaviour.
Finally, our materials of choice were compared with some commercial analogues and some further comments have been made for continuing the work
Evidence of Polyphenols Nitrosation by Computer aided ESR Spectroscopy
No full textPolyphenols play an important role as model systems in transition metal derivatives for the preparation of macromolecular systems. Among the metal ions ironnitrosyl coordination chemistry has received much attention in the past because of its important role in inorganic and biological processes. In the case of Fe(I)(NO)2 complexes with polyphenols ligands in solution, difficulties in the interpretation of the ESR spectra arise from complicated patterns due to simultaneous presence of different nitrogen nuclei directly bound to the metal ion or due to the presence of equilibria between species under slow exchange conditions. In order to overcome these difficulties the investigations reported here were carried out using computer simulation of ESR spectra combined with selective isotopic substitution of 14NO with 15NO. Resorcinol displays an unexpected nine lines ESR pattern at g=2.018 which can be explained only by considering more than two nitrogen atoms interacting with the unpaired electron delocalized over the metal complex. Copyright © 1992 Hüthig & Wepf Verla
Prioritäre Arten für den Kanton Basel-Landschaft : die Verantwortung eines Kantons für den Schutz und die Förderung von Pflanzen- und Tierarten
No full text- …
