1992 рік

Stress on particles in silica particulate epoxy composite under constant load was investigated by X-ray diffraction. Microdeformation of the crystal lattice of silica could be detected as a shift of the X-ray diffraction peak induced by the applied stress. When a tensile stress was applied to the particulate composite, incorporated particles were found to be subjected to a stress several times larger than the applied stress. The stress concentration onto particles in particulate composite material, which was considered to result in a mechanical reinforcement of the composite, depends on the volume fraction and size of particles. Quantitative relationships between the stress concentration coefficient, particle diameters and the increment of macroscopic Young's modulus with the incorporation of filler were stated.

Детальніше...  

Pyrene fluorescence spectra and lifetimes have been employed to study the pH-induced conformational transitions of maleic acid-ethyl vinyl ether copolymers that have been hydrophobically modified with varying amounts of 4-butylaniline (4-BA). Results indicate that those copolymers containing up to 50 mol% 4-BA (relative to the total number of maleic acid groups) undergo a transition from a highly collapsed 'hypercoil' conformation at low degrees of ionization (~) to an open, hydrated conformation at high ~. In contrast, a copolymer containing 70 mol% 4-BA moieties forms compact, hydrophobic microdomains over the entire range of ~. Fluorescence quenching of the pyrene probe by T1 ? and nitromethane indicates that the accessibility of these microdomains is related to ~ and/or the degree of hydrophobe incorporation. Diffusion coefficients and effective coil diameters of these polymers as determined via dynamic light scattering measurements indicate an increasing degree of intramolecular association with increasing hydrophobe content at high ~, while fluorescence measurements indicate no such interactions. It is suggested that, though the pyrene probe may associate with the hydrophobic regions of these polymers, it is not adequately 'protected' from the bulk aqueous solution for fluorescence enhancement to occur.

Детальніше...  

The erosion/corrosion properties of the advanced polymer composites, PMR-15, BMI (Matrimid 5292) and PMR-II-20 have been studied. Specimens of these materials were exposed to jets of an erosion/corrosion medium consisting of salt, iron oxide, quartz and coarse desert dust dispersed in water. The jets impinged on the specimens perpendicular to the plane of the reinforcements for a period of approximately 2 months. The velocity of the jets was measured to be 10 m s-r. The damaged surfaces were examined both visually and under a scanning electron microscope. Damage mechanisms which are associated with this phenomenon are identified. Matrix microcracking, matrix removal and blister formation are the principal damage mechanisms observed. Deep cuts, cutting lines and rust deposition are the main features of the damaged surface. It is observed that BMI matrix composites display a large amount of matrix removal and consequently, fibre exposure. A large amount of scales of deposits are also observed. On the other hand, PMR-15 shows cutting lines, with less matrix removal than BMI. Large scale deposits are also observed on the surface of PMR-15 composite. The PMR-II composite displays the least amount of matrix removal, with brittle matrix microcracking. Blister formation is a dominant corrosive feature of the PMR-II composite.

Детальніше...  

X-ray diffraction patterns of unoriented samples of a recently synthesized highly crystalline poly(3-methyl-l,3-pentadiene) show the presence of two polymorphic forms. For one of these forms a fibre pattern is presented and the identity period of the chain is determined. To obtain information on the type of tacticity of the polymer, geometric and energy analyses and Fourier transform calculations on isolated chain models have been performed. It is suggested that the experimentally observed chain axis repeat value is more easily accounted for by a 1,4-cis-syndiotactic structure

Детальніше...  

Intensity ratios of the bands due to CH 2 bending and rocking normal vibrations, used as a measure of the surface crystallinity in polyethylene, are examined by attenuated total reflectance Fourier transform infra-red (ATR FTi.r.) spectroscopy. This study shows that the ATR optical dispersions and the orientation of surface crystallites have a substantial effect on the crystallinity measurements. It appears that improper analysis of strong, overlapping i.r. bands in the ATR spectra may generate systematic errors. While the optical effects associated with ATR measurements can be accounted for and corrected by the use of the Kramers-Kronig transform and Fresnel's equation, determination of the crystallinity gradients and orientation of the crystallites requires detailed polarization studies and theoretical considerations.

Детальніше...  

I.r. spectra and temperature dependences of 1H n.m.r, linewidths were measured for poly(p-phenylene sulphide) (PPS) heavily doped with sulphur trioxide (SOa) in the form of vapour and liquid. X-ray diffraction studies confirmed the total destruction of the crystalline phase as a result of the doping process. I.r. spectra show that doping with SO3 leads to irreversible chemical modifications of the polymer by sulphonation. A comparison of the experimental values of the n.m.r, second moment determined for a rigid structure of the doped PPS with the poly(benzothiophene) excludes the formation of the latter as a result of doping. Doping with SO3 in the vapour form leads to structural changes in the polymer with its two-phase character preserved. Annealing of the doped sample at 450 K results in the removal of chemically unbonded SO3. However, the annealed polymer still has a different structure from the pristine polymer. PPS doped with liquid SO 3 shows a monophase structure indicated by a liquid-like resonance line.

Детальніше...  

The nature of epoxy resins of the diglycidyl ether of bisphenol A modified by incorporation of 10 to 20 g phr of various linear polysulphides has been examined by 13C n.m.r. spectroscopy (conventional and cross-polarization/magic angle spinning), transmission electron microscopy, scanning transmission electron microscopy and dynamic mechanical thermal analysis. Most, but not all, cured materials feature disperse rubber particles with diameters of < 1 pm. The materials properties of the modified resins have been measured by tensile and impact testing: modification entails improvement but not necessarily either in proportion to the linear polysulphide content or dependent on the demonstrable presence of disperse particles.

Детальніше...  

Time-resolved fluorescence emission spectra of isotactic polystyrene/benzyl alcohol (iPS/BA) gel annealed at 318 K are reported. Time-resolved spectra and analysis of the fluorescence decay parameters on fresh gel, crystalline isotactic polystyrene together with data from the gel allow identification of two excimer components. Good agreement between the steady-state fluorescence emission spectrum with a theoretically predicted excimer emission spectrum, constructed on the basis of a mechanism that assumes the existence of separate domains, was observed. The dependence of the emission intensity of components is discussed in terms of the effectiveness of photo energy transfer and energy migration.

Детальніше...  

The essential properties of polyimide films of importance in microelectronic applications are thermal and thermo-oxidative stability, dimensional stability, glass transition behaviour and the relative permittivity (dielectric constant e'). A segmented rigid-rod polyimide was synthesized from 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA) and 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl (PFMB) in order to develop new materials for microelectronic applications. The thermal and thermo-oxidative stability were investigated by determining the thermal degradation activation energy in air (210 kJ mol- 1 ) and in nitrogen (303 kJ mol 1 ). The thermal stability was further studied through thermogravimetry-mass spectroscopy. The coefficient of thermal expansion, which indicates the dimensional stability, was measured via a tension mode of a thermomechanical analyser and doubly extrapolated to zero stresses, and was 6.98 x 10 6 c C- 1 for the BPDA- PFMB films. The glass transition temperature, measured thermomechanically, was found to be 287°C. The dielectric constant for the films, measured after ageing at 50% relative humidity for 48 h at 23C, was between 2.8 and 2.9 in a frequency range from 0.1 kHz to I MHz. The temperature and frequency dependence of the dielectric behaviour is also discussed.

Детальніше...  

The stereochemical configuration of the regioirregular unit with racemic placement included in isotactic polypropylene, which was prepared with a rac-ethylene(1-indenyl)zirconium dichloride/methylaluminoxane catalyst system at 30°C, was studied by 13C_ 1H shift correlation two-dimensional nuclear magnetic resonance spectroscopy. The stereostructure was also studied for the regioirregular unit of propylene copolymer with a small amount of ethylene units. It is concluded that the 2,1-insertion, in addition to the ordinary 1,2-insertion, of propylene monomer is also stereospecifically controlled by the catalyst.

Детальніше...