Radiation initiated transformations

The basic mechanisms of radiation-initiated transformations in polymers are far from being completely understood. Most reactions are commonly interpreted on the basis of free radical processes, but other species—e.g., ions, and other reactive intermediates—may play a significant role. A better understanding of the basic reactions occurring in irradiated polymers is required, and this would undoubtedly spur further developments and industrial applications. This paper is devoted to a survey of the different species found in irradiated polymers. Atten-... 

Chojnowski and co-workers have studied the polymerization of octamethyltetrasila-l,4-dioxane, a monomer more basic than cyclosiloxanes, which is capable of forming more stable oxonium ions, and thus being a useful model to study the role of silyloxonium ions.150-152 In recent work, these authors used Olah s initiating system and observed the formation of oxonium ion and its transformation to the corresponding tertiary silyloxonium ion at the chain ends.153 The 29Si NMR spectroscopic data and theoretical calculations were consistent with the postulated mechanism. Stannett and co-workers studied an unconventional process of radiation-initiated polymerization of cyclic siloxanes and proposed a mechanism involving the intermediate formation of silicenium ions solvated by the siloxane... 

The passage of a charged particle through a medium results in the formation of disturbance areas along the particle s trajectory that contain excited molecules, positive ions, and knocked out electrons and atoms. These disturbances areas make up the track of the particle (see Section VIII). An important role in the process of formation of the track and in the following radiation-chemical transformations is played by the degree of delocalization of the initially absorbed energy. 

The great variation in the types of active centres generated in the irradiated monomer makes it possible to initiate polymerization by different mechanisms. In each specific case, the nature of the monomer determining the formation of a certain type of active centre which ensures effective initiation and the polymerization conditions, mainly the temperature and the medium (solvents), are of the greatest importance. Hence, the polymerization process usually occurs by a certain definite mechanism. Since in the course of secondary radiation-chemical transformations, in practice, particles with a longer lifetime form free radicals, the free-radical mechanism is the simplest process of radiation-induced initiation. 

UV-radiation curing has become a well-accepted technology which has found numerous industrial applications because of its distinct advantages 1-3. One of its main characteristics is the rapidity of the process which transforms quasi-instantly the liquid resin into a solid polymer under intense illumination by a UV-source or a laser beam4. The polymerization rate can be finely controlled by acting on the initiation rate through the intensity of the UV radiation. It is... 

The incidence of ovarian tumors in mice, guinea pigs, and rabbits increased after 3 years of chronic irradiation at doses as low as 1.1 mGy daily (Lorenz et al. 1954). Unlike other tumors, the induction of ovarian tumors depended on a minimum total dose and seemed to be independent of a daily dose (Lorenz et al. 1954). Radiation-induced neoplastic transformation of hamster cells may be associated initially with changes in expression of the genes modifying cytoskeletal elements (Woloschak et al. 1990b). 

In terms of nonlinear dynamical systems, the second waveguide of the junction can be considered as a system that is initially more or less far from its stable point. The global dynamics of the system is directly related to the spatial transfomation of the total field behind the plane of junction. In structure A, the initial linear mode transforms into a nonlinear mode of the waveguide with the same width and refractive index. In the structure B, the initial filed distribution corresponds to a nonlinear mode of the first waveguide it differs from nonlinear mode of the second waveguide, however. The dynamics in both cases is complicated and involves nonlinear modes as well as radiation. Global dynamics of a non-integrable system usually requires numerical simulations. For the junctions, the Cauchy problem also cannot be solved analytically. 

In Ref. 13, we have proved that the A transformation constructed is invertible for the classical model discussed in the previous section. Here, using the same system discussed in the previous section, we demonstrate the invertiblity of our transformation by a numerical calculation of the time evolution of the action variable J (f) for an initial condition where all the field actions are zero [20]. Due to radiation damping, J t) follows an approximately exponential decay. However, there are deviations from exponential in the exact evolution both at short and long time scales as compared with the relaxation time scale. In Fig. 1, we present numerical results. 

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