Polymer processing irradiation influence

Sonnier, R., Massardier, V., Clerc, L., Lopez-Cuesta, J. M., Bei eret, A., Reactive compatibilization of polymer blends by gamma-irradiation Influence of the order of processing steps. Journal of Applied Polymer Science 2010, 115(3), 1710-1717. 

It has to be pointed out that substantial numbers of free macroradicals remain in the polymer after irradiation. These macroradicals, formed in crystalline region, contribute substantially to a long term degradation of mechanical properties. The rate of decay of the residual radicals was found to depend strongly on the crystalline portion, i.e. it can be influenced by processing and molding procedures. 

A second important reaction observed in irradiated polyethylene and certain other polymers as well as paraffins is the formation of trans-vinylene. This product is far less influenced by such factors as temperature or radical scavengers than is crosslinking, and it is therefore often assumed to be formed directly by a molecular process—i.e., molecular detachment of hydrogen. Even this simple explanation cannot be fully sustained. Long after radiation has ceased, the frarw-vinylene concentration continues to rise or fall, whether the specimen is exposed to oxygen or not. This behavior may possibly be correlated with the reaction of trapped alkyl and allylic radicals, which, being slightly mobile, can add to frarw-vinylene or each other over a period of days. 

Although the first publications concerned with the possibilities of use of microwave irradiation in organic synthesis appeared in 1980 s [12,13] and in polymer chemistry even earlier at the end of 1960 s [14], the sudden growth of interest in the application of microwave irradiation in almost all fields of chemistry took place at the end of 1980 s. Nowadays, there is hardly find any reaction that has not been attempted under microwave conditions. The application of microwaves in chemistry is therefore so attractive that from the very beginning it was realized that a number of chemical processes can be carried out with a substantial reduction in the reaction time in comparison to conventional processes. Reactions that usually take many hours or days, under influence of microwave irradiation can be run in considerably shorter time of several minutes or even seconds [15]. These phenomena are not fully understood yet however, there are two groups of theories that are proposed to explain the reduction of the reaction time under microwave conditions in comparison with processes under conventional conditions. 

H.P. Brack, H.G. Buhrer, L. Bonorand, and G.G. Scherer. Grafting of pre-irradiated poly(ethylene-alt-tetrafluoroethylene) films with styrene Influence of base polymer film properties and processing parameters. Journal of Materials Chemistry 10, 1795-1803 2000. 

A synthesis of substituted phenanthrenes has been reported using the bicyclooctadienes (61) as starting material. The process makes use of the nucleofugal group on C-8 and follows the path outlined in Scheme 3. This involves a di-rr-methane bridging process followed by the collapse of the intermediate biradical (62). Normal di-Ti-methane behaviour is reported in the acetophenone-sensitized irradiation of the isoquinolinone derivative (63a). This yields the two products (64) and (65) as a 3 1 mixture in a total yield of 75%. An N-oxide derivative gave a brown polymer with little evidence for the formation of di-n-methane products. The influence of ring substituents was also studied for the derivatives (63b, d) and the results of this are shown in Scheme 4. The authors conclude that the cyclization process is under LUMO control. Irradiation of the dihydropyridine (66) affords the oxidised pyridine (67) as the major product. A minor product (68) is also formed by a di-ir- methane process. ... 

Also, the method how the ablation parameters are acquired can have a pronounced influence on the results. The ablation rate can be defined either as the depth of the ablation crater after one pulse at a given fluence, or as the slope of a linear fit of a plot of the ablation depth versus the pulse number for a given fluence. Very different ablation rates can result from the two different measurement methods. This is especially the case for materials where ablation does not start with the first pulse, but after multiple pulses, or if the ablation crater depth after one pulse is too small to be measured. The process that occurs if ablation does not start with the first laser pulse is called incubation. It is related to physical or chemical modifications of the material by the first few laser pulses, which often results in an increase of the absorption at the irradiation wavelength [32,33], for example, the formation of double bonds in poly (methylmethacrylate) (PMMA). Incubation is normally observed only for polymers with low absorption coefficients at the irradiation wavelength. 

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