摘要:

AbstractThe stereospecificity of acrylonitrile polymerization initiated by organometallic compounds of alkali and alkaline earth metals was investigated. The triad tacticities of the polymers obtained was analyzed by13C NMR and the molecular weights and their distributions were measured by size exclusion chromatography. A decisive influence of temperature on acrylonitrile polymerization initiated by dialkylmagnesium (di‐n‐hexyl‐ and di‐n‐butylmagnesium) in mesitylene was found. Products polymerized at temperatures above 100°C contain high molecular weight fractions with predominantly isotactic configuration (isotactic triad content 50—55%). The stereospecificity of polymerization was observed only in a medium which is a non‐solvent for the product. All the other investigated metalorganics do not initiate a stereospecific polymerization on these conditions. In order to elucidate the stereospecific polymerization mechanism we examined NMR data by model calculations. The triad tacticity of some predominant isotactic polyacrylonitrile samples is described by the enantiomorphic sites (e.m.s.) model. This means that the propagation takes place at enantiomeric catalyst sites which determine the configuration of the incoming monomer unit. Summarizing the experimental observations and the interpretation of the polymer structure, a model of the active centers involving associated dialkylmagnesium structur

ISSN:0323-7648    

DOI:10.1002/actp.1996.010470201

出版商:Akademie Verlag GmbH    

年代:1996

数据来源: WILEY

摘要:

AbstractThermal and microwave‐induced free‐radical non‐aqueous dispersion polymerization of methyl methacrylate (MMA) in the presence of poly(styrene)‐block‐poly(ethene‐alt‐propene) dispersing agent were compared. For controlled polymerization in the microwave field, a new microwave polymerization reactor was built to afford uniform heating and to control temperature via microwave power variation. At identical MMA concentration and polymerization temperature of 70°C no special effect of the microwave field on conversions, molecular weights and particle sizes of PMMA was detected with respect to conventional thermal free radical dispersion

ISSN:0323-7648    

DOI:10.1002/actp.1996.010470202

出版商:Akademie Verlag GmbH    

年代:1996

数据来源: WILEY

摘要:

AbstractSynthesis of the hydroxyethyl‐substituted poly(para‐phenylene)2bemploying Suzuki polycondensation is described. This PPP derivative is used as a matrix for the attachment of first‐ and second‐generation dendrons of the Fréchet‐type with isocyanate functions at the focal point. The degree of coverage is determined from NMR integration to be virtually 100% in

ISSN:0323-7648    

DOI:10.1002/actp.1996.010470203

出版商:Akademie Verlag GmbH    

年代:1996

数据来源: WILEY

摘要:

AbstractTrianthrylene compounds with film‐forming capabilities were synthesized in a three‐step procedure by means of transition‐metal catalyzed Grignard and Suzuki coupling reactions. Upon UV exposure, photopolymerization of the model compound 1,11‐di‐(9‐anthryl)undecane was observed in solution as well in the film due to [4+4] cycloaddition of the anthracene moieties. The soluble photopolymers were characterized by NMR spectroscopy and GPC analysis. As expected, irradiation of solutions of the trichromophoric hydrocarbons afforded crosslinked photoproducts which precipitated from solution. In the case of an analogous trianthryl derivative with tetraoxyethylene spacers, formation of a gel was observed due to cycloaddition of the anthracene groups. The film‐forming capabilities of trianthryl compounds in conjunction with UV‐induced photocrosslinking can be useful for photoresist applications and will be discussed in the foll

ISSN:0323-7648    

DOI:10.1002/actp.1996.010470204

出版商:Akademie Verlag GmbH    

年代:1996

数据来源: WILEY

摘要:

AbstractFilms of oligoanthrylenes are readily available by spin coating. Upon UV exposure, the properties (refractive index, fluorescence and solubility) of these films are changed due to [4+4] cycloaddition of the anthracene groups. This behavior appears to be useful for information recording purposes, which is demonstrated by surface plasmon and fluorescence microscopy. Although complete photobleaching of the films is not possible, effective photocrosslinking occurs for trichromophoric systems affording relief textures after development in ethanol/methylene chloride mixtures (negative tone photoresist). Furthermore, the reversibility of the solid state photodimerization of the anthracene moieties is discussed.

ISSN:0323-7648    

DOI:10.1002/actp.1996.010470205

出版商:Akademie Verlag GmbH    

年代:1996

数据来源: WILEY

摘要:

AbstractThe general crystallization behavior of amorphous polyamides during heating treatment was measured by wide angle X‐ray scattering (WAXS) and differential scanning calorimetry (DSC) to get information on (1) the temperature range where the crystallization in an anisothermic process takes place and (2) the type of the crystalline phases (α or γ) considering the Brill transition and the state of order in the α‐crystal. Various polyamides (PA 6, 6.10 and 6.6) were quenched and cooled to 200 K and then heated until they melted. During heating from the glassy state the crystallization starts rapidly at the glass transition temperature and then the crystallinity slowly increases until the melting point. The crystallinity was calculated on the basis of the separation of the amorphous and crystalline scattering by profile fitting. The angular position of the amorphous halo and so the density of the amorphous phase is higher in the presence of the crystalline phase compared to the completely amorphous state. The crystalline structure (α or γ), which is difficult to distinguish at higher temperatures due to the Brill transition, was determined by quenching the samples from elevated temperatures to room temperature after heating. The perfection of the α‐crystal, i.e. the crystal perfection index, increases with temperature during continu

ISSN:0323-7648    

DOI:10.1002/actp.1996.010470206

出版商:Akademie Verlag GmbH    

年代:1996

数据来源: WILEY

摘要:

AbstractLow‐density polyethylene films, 80 μm thick, were exposed to microwave radiation and the following parameters were characterized: (i) the monoclinic to orthorhombic phase transformation, crystallinity and crystallite size using wide‐angle X‐ray diffraction, (ii) mechanical properties, and (iii) light absorption in the wavelength range of 200—800 nm. The following effects have been detected after microwave treatment: The major orthorhombic phase component increased, the monoclinic component decreased, the orthorhombic crystallite size decreased, the crystallinity, which was taken as the ratio of the scatter from the orthorhombic phase to the total scatter under the whole diffraction scan, increased. The mechanical properties improved. No change in the absorption of light was

ISSN:0323-7648    

DOI:10.1002/actp.1996.010470207

出版商:Akademie Verlag GmbH    

年代:1996

数据来源: WILEY

摘要:

AbstractThe surface modification of a commercial polyethylene fiber treated in an oxygen low temperature microwave plasma was investigated. The change in surface chemistry was analyzed with electron spectroscopy for chemical analysis. A controlled state of oxidation of the fiber surface was reached by variation of gas flux, process pressure and microwave energy. The oxygen was incorporated into the fiber surface in the form of functional groups. In the determined treatment range, a change in surface morphology as a result of the plasma treatment was not observed with electron microscopy. The higher oxygen content in the fiber surface increased the fiber‐matrix adhesion as shown by mechanical test

ISSN:0323-7648    

DOI:10.1002/actp.1996.010470208

出版商:Akademie Verlag GmbH    

年代:1996

数据来源: WILEY

摘要:

AbstractIt has been demonstrated that radiation‐induced conductivity (RIC) is strongly dependent on the chemical structure of the macromolecules, this dependence being influenced by the position of the active molecular group acting as a hopping center for charge carriers: main chain vs pendant case. Both RIC magnitude and its nature (free ion vs geminate) are affected. Annealing of the radiation‐induced conductivity (RIC) in four common polymers (PS, PET, LDPE and PTFE) continuously irradiated in vacuum to doses of about 5 × 104Gy has been studied. While substantial recovery (up to 75%) has been observed in PS and PET, only steady degradation of RIC has been encountered in LDPE and especially in PTFE (annealing at elevated temperatures has been fulfilled only in air). Unlike other polymers tested, PS shows recovery that depends critically on the surrounding atmosphere (air or vacuum). To describe these results one relies heavily on the radiation‐induced oxidation as a chain radical process with degenerate branching (a case of LDPE and PTFE) and catalytic recombination of macroradicals (as

ISSN:0323-7648    

DOI:10.1002/actp.1996.010470209

出版商:Akademie Verlag GmbH    

年代:1996

数据来源: WILEY

摘要:

AbstractIn the literature, an estimate of the degree of orientation in drawn polyacrylonitrile (PAN) fibers is often inferred from the arc‐width of the strongest equatorial peak at 2ϑ = 16.8° in the wide angle X‐ray diffraction pattern. The absence of sharp meridional peaks in the diffraction pattern of oriented PAN means that chain orientation‐factors cannot be directly calculated. In this work, Wilchinsky's method for calculating the degree of chain orientation from the azimuthal intensity distribution of a strong equatorial peak has been

ISSN:0323-7648    

DOI:10.1002/actp.1996.010470210

出版商:Akademie Verlag GmbH    

年代:1996

数据来源: WILEY