Die Lichtausbeute organischer Szintillatoren für Elektronen von 1 bis 12 keV

The article deals with a method of measuring the scintillation responce of anthracene and organic plastic scintillators for electrons of 1 to 12 keV. It results in stating that the scintillation response is a linear function of the electron energy down to energies of 4 or 5 keV. Scintillator thickness and surface preparation are of great importance for a linear scintillation response.     

Deformation and entanglement in semicrystalline polymers

Transmission electron microscopy (TEM) of amorphous and semicrystalline isotactic polystyrene (iPS) thin films deformed well below T _g, suggests the same crazing mechanism to operate in both cases. Therefore, by analogy with the amorphous case, highly entangled semicrystalline polymers, such as poly(ether ether ketone) (PEEK) should craze less readily in the glassy-semicrystalline state than iPS, which has a low degree of entanglement. Since this is confirmed by observation, it is reasonable to extend this analogy, and invoke entanglement in order to account for the high fracture resistance of highly entangled semicrystalline polymers, such as polyoxymethylene, using models previously applied to glassy amorphous polymers. There are nevertheless often significant decreases in fracture toughness in polyoxymethylene as the crystallization temperature is raised and/or the molecular weight is reduced, which we attribute to entanglement loss during lamellar folding.     

DSC non-isothermal crystallization curves in polyoxymethylene

Non-isothermal crystallization curves by differential scanning calorimetry for two-dimensional spherulite growth in polyoxymethylene have been modeled using the Avrami equation, taking into account heat dissipation in the sample holder during the phase transition. Good agreement is found for scanning rates up to 40 Kmin^–1 between experimental curves and predictions based on a knowledge of the isothermal crystallization kinetics of the same samples.     

An evaluation of the interlaminar fracture toughness of a thermoplastic composite with offset centre flies

Conclusion The offset DCB specimen has been used to characterize the influence of cooling rate and loading rate on the interlaminar fracture properties of carbon fibre reinforced PEEK. By offsetting the mid-plane fibres by several degrees, the amount of fibre bridging occurring during fracture has been reduced considerably. It has been shown that IM6 carbon fibre PEEK is quite sensitive to the cooling conditions employed after consolidation at 380 °C. Low rates of cooling yield a high level of crystallinity and a reduced fracture toughness. The modified DCB specimen has been successfully applied to highlight a distinct interlaminar fracture rate sensitivity. The high rate properties of this material still leave cause for concern and more work is required before these materials will find widespread use.Published in Mekhanika Kompozitnykh Materialov, No. 4, pp. 476–483, July–August, 1992.     

Stress-transmission through chain molecules

The stressing of chain molecules, the resulting conformational changes, the limiting phenomena (slip and chain scission) and their effect on ultimate sample properties are studied in this paper. It is pointed out that and how transitions from coiled to highly extended chain conformtions are obtained in liquid environment, in thermoplastic elastomers and in highly oriented fibers.Dedicated to Prof. Dr. R. Bonart on the occasion of his 60th birthday.     

Studies on a new craze phenomenon in bisphenol-A polycarbonate: Craze initiation by stress-activated molecular disentanglement

Summary It is demonstrated that under appropriate deformation conditions intrinsic crazes (crazes II) are initiated in unoriented and pre-oriented polycarbonate which is stretched to its natural draw ratio. Molecular disentanglement is proposed as the mechanism governing craze II initiation. The same molecular mechanism must be assumed to play an important role for the fracture of polycarbonate and for other instability phenomena such as those associated with the self-oscillations of polyethyleneterephthalate, polypropylene and polyamide and with the stress-whitening of polymethylmethacrylate.

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Zusammenfassung Es wird gezeigt, daß unter geeigneten Verstreckbedingungen innere Crazes (Crazes II) in unorientiertem und vororientiertem Polycarbonat initiiert werden, falls dieses bis zu seinem natürlichen Verstreckverhältnis gedehnt wird. Kettenentschlaufung wird als maßgebender Mechanismus der Craze II-Bildung vorgeschlagen. Es muß angenommen werden, daß der gleiche Mechanismus eine wichtige Rolle beim Bruch von Polycarbonat and bei anderen Instabilitäts-phänomenen wie etwa den Selbstoszillationen von Polyäthylenterephthalat, Polypropylen and Polyamid sowie dem Weißbruch von Polymethylmethacrylat spielt.

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Résumé Il a pu être démontré que sous des conditions d'étirage appropriées, des craquelures intrinsèques (craquelures II) sont initiées dans le polycarbonate non orienté et préorienté si celui-ci est étiré jusqu'â son allongement maximal. On suppose que le désenchevêtrement est le mécanisme moléculaire responsable de l'initiation des craquelures II. En outre, on estime que le même mécanisme joue un rôle important dans la rupture du polycarbonate et dans d'antres phénomènes d'instabilité tels qu'ils se produisent dans les oscillations propres au polyéthylèneterephthalate, au polypropylène et au polyamide ainsi qu'au blanchissement sous contrainte du polyméthylméthacrylate.

     

Influence of nozzle geometry on polystyrene degradation in convergent flow

Polymer degradation is readily observed in flows where the extensional component surpasses the rotational component of the velocity gradient. This type of flow is conveniently obtained by pushing a liquid into a convergent channel across an orifice. Kinetics of chain scission is sensitive to subtle modification of the coil conformation, which in turn depends on the details of the pervading flow field. By changing the orifice diameter and the conical angle of the inlet, it is possible to modify the spatial distribution of the velocity gradient, and hence, the residence time of a fluid element in the high strain-rate region. Degradation yields, measured under -conditions in decalin by Gel Permeation Chromatography, showed a strong dependence on the fluid velocity at the orifice, but not on the magnitude of the strain-rate. This result is contrary to the common belief that assumes viscous friction, proportional to the strain-rate, is the determining factor for the scission rate of a bond under stress. Rather, experimental findings tend to indicate that the driving force for chain scission was provided by the energy accumulated in the coil during the flow-induced deformation process. The sharp propensity for mid-chain scission was maintained regardless of the nozzle geometry.Dedicated to Prof. W. R. Pechhold on the occasion of his 60th birthday     

Isomeric structures of protonated ozone: A theoretical study

Ab initio molecular orbital calculations have been used to determine the structure of protonated ozone. Four stable minima were found on the O₃H⁺ singlet potential energy surface. Three forms correspond to ozone protonated at the central oxygen (C_2v) or at the terminal oxygen (two C_s isomers, E and Z). The fourth isomer (C_s) is a derivative of trioxirane. The most stable structure is the planar E form I. The proton affinity of ozone (to give I) is given as 123.6 kcal/mole (MP2/6-31G*//4-31G). The energy difference between I and protonated trioxirane VI is greater than that between ozone and trioxirane.     

Real-time image analysis and numerical simulation of isothermal spherulite nucleation and growth in polyoxymethylene

A method of analysing nucleation and crystallization kinetics, based on real time image analysis and hot stage optical microscopy, has been used to investigate the isothermal crystallization of different grades polyoxymethylene. The data were compared with results from differential scanning calorimetry (DSC), using a simple numerical simulation to model the effects of finite smaple thickness on the form of the isothermal DSC curves. This simulation was then used to predict the microstructural development in a bulk sample for different boundary conditions, taking into account latent heat evolution and diffusion during crystallization.