Post effect

It is to be anticipated, and this will be in fact confirmed experimentally, that post effects may be followed in the same way. 

When log I/I0 is plotted against D, a straight line is obtained in a rather large region (4x 106 to 4x 107 roentgens) as in all the cases which will be handled here. No post effect was observed. Such a phenomenon has only been detected for CHI3 3S8 which will be studied in a next paragraph. 

CH127I3- 3S8. The forty per cent dose is about 1.9 xlO8 roentgens13 (resonance line located at 303.435 Mc/sec). The intensity of the resonance line does not remain unaffected after irradiation, and this post effect is dependent upon dose and temperature. 

In the associates, polymerization occurs according to the zip-propagation mechanism, and auto-acceleration takes place. In the range of temperatures from 10 to GO C, the system gradually changes from the system dominated by occlusion to the system where template effect determines the kinetic behavior. In this range of temperatures, a significant post-effect occurs. ... 

In order to estimate kinetic constants for elementary processes in template polymerization two general approaches can be applied. The first is based on the generalized kinetic model for radical-initiated template polymerizations published by Tan and Alberda van Ekenstein. The second is based on the direct measurement of the polymerization rate in a non-stationary state by rotating sector procedure or by post-effect in photopolymerization. The first approach involves partial absorption of the monomer on the template. Polymerization proceeds according to zip mechanism (with propagation rate constant kp i) in the sequences filled with the monomer, and according to pick up mechanism (with rate constant kp n) at the sites in which monomer is outside the template and can be connected by the macroradical placed onto template. This mechanism can be illustrated by the following scheme ... 

Figure 8.8. Conversion during the post-effect for MAA with PDAMA template. Reprinted from J. Matuszewska-Czerwik and S. Polowinski, Eur. Polym. J., 28,1482, (1992) with kind permission from Elsevier Science Ltd.

Post effects have not been examined in detail in homogeneous systems. On yon (109) attempted to use the viscometric method, but the data did not vary in accordance with theory and did not allow accurate determination of individual rate constants. 

The ratio kjkp can also be obtained, from relation (55). This is a somewhat difficult, but relatively accurate method [34]. Reliable determination of the instantaneous rate during polymerization decay is not simple. The measurements are much easier when the pre- and post-effects are alternated in close order. An elegant way of producing such conditions is illumination of photo-initiated polymerization either with intermittent switching or by the motion of the polymerizing mixture in bands of light and darkness. 

The molecular weights were roughly constant at a given temperature and about equal to those calculated from the chain transfer to monomer values. Some post effects were observed. 

Some preliminary results were presented by Barriac et al. (1969) and followed by a more detailed report (Barriac et al., 1976) wherein two emulsifiers were used. Again at 25 C, for example, tbe degree of polymerization was essentially independent of tbe dose rate or the emulsifier concentration and similar to that obtained by Wang. A strong post effect... 

As it is seen from Figure 2.6 (curve 2) at the moment of light source switching off the rate of O2 absorption decreases up to stationary rate in dark process and oxidation reaction proceeds for some time. This effect is the effect of photochemical aftereffect or post-effect. It is explained [173] by lasting radical-chain process of oxidation, consisting of two stages reactions of continuation and breaks of chain of photooxidation. Kinetic curve of oxygen... 

As it is seen from Figure 2.6, experimental data according to post-effect are well described by the equation (1) at the values =5,3 Iobtained data with kinetic law of photochemical aftereffect at quadratic break of oxidation chains with equation (1) shows that chains break at CDA oxidation is quadratic. 

On the other hand, inhibiting effect of nitroxylic radical at irradiation is not observed owing to its strong action as photosensitizer. Photosensitization is absent after irradiation in dark reaction and nitroxylic radical displays its inhibiting action. That is why oxidation after irradiation (post-effect) is not observed (Figure 2.7, curve 2). 

Not fully understood but several sites of action at the neuromuscular junction (pre and/or post, effects on ion-channels or receptors) have been suggested. 

When a polymer is irradiated below the melting temperature of crystallites, the polyfunctional monomer remains in the amorphous regions. Gels of polypropylene crosslinked in the presence of butadiene are crystalline which is a manifestation of the formation of crosslinks at the lamellar surfaces [79]. The presence of hexadecane and hexadecene-1 in isotactic polyiroiylene increases the rate of termination of radicals formed in the irradiated polymer and the oxidative post-effect is reduced as well as the deterioration of mechanical properties is less than in a polymer without additives [80]. 

Quintas AF, Araujo M AJ, Bottino MA, Snelart N, Carbon fibre posts, effect of surface treatment on core retention. Journal Of Dental Research, 77(SIB), 344, 1998. 

Classical post-effect (including with EPR) Yes (difficult at low conversion) No Requires expertise Wide... 

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