Radiation induced contrast

The method of differential radiation induced contrast depends on enhancement of contrast in multicomponent polymers where the components have different electron beam-polymer interactions [173]. Contrast has been observed in sections of styrene-acrylonitrile/poly(methyl methacrylate) (SAN/PMMA) polymers where the PMMA exhibits a high rate of mass loss compared to SAN, creating contrast between the phases. It is well known that electron irradiation results in chain scission and crosslinking, loss of mass and crystallinity [75]. Polystyrene, polyacrylonitrile and SAN crosslink and thus are stable in the electron beam whereas polymers exhibiting chain scission, PMMA and poly(vinyl methyl ether), degrade in the beam. It is suggested that experiments be conducted on the homopolymers to determine the expected irradiation damage mechanism in the multi-component system [173]. 

The method of differential radiation induced contrast depends on enhancement of contrast in multicomponent polymers where the components have different electron beam-pol5aner interactions [140]. Contrast has been observed in sections of styrene-acrylonitrile/poly(methyl... 

Ablation of AP did not prevent radiation-induced emesis in cats, while abdominal vagotomy did [68]. In contrast, the cisplatin-induced emesis required the intact AP in the same species [69]. 

Deep-UV resists comprised of matrix polymers and a 2,6-dinitrobenzyl tosylate photoactive acid generator have been described and compared to previously reported onium salt systems. Although these resists exhibited lower sensitivity than onium salt-based materials, the contrast and processibility are superior. The use of a matrix polymer capable of radiation-induced chain scission improves the sensitivity and allows the 2,6-dinitrobenzyl tosylate acid generator to more nearly... 

Elastin-mimetic protein polymers have been fabricated into elastic networks primarily via y-radiation-induced, radical crosslinking of the material in the coacervate state [10]. Although effective, this method cannot produce polymers gels of defined molecular architecture, i.e., specific crosslink position and density, due to the lack of chemoselectivity in radical reactions. In addition, the ionizing radiation employed in this technique can cause material damage, and the reproducibility of specimen preparations may vary between different batches of material. In contrast, the e-amino groups of the lysine residues in polymers based on Lys-25 can be chemically crosslinked under controllable conditions into synthetic protein networks (vide infra). Elastic networks based on Lys-25 should contain crosslinks at well-defined position and density, determined by the sequence of the repeat, in the limit of complete substitution of the amino groups. 

The kinetics of radiation-induced polymerization of bulk nitroethylene was also studied at 10° C by the use of hydrogen bromide as an anion scavenger (27). The value of Gt (yield of the initiation by 100 eV energy absorbed) was found to be about 3, which was much larger than the value obtained for many radiation-induced cationic polymerizations. The propagation rate constant, kp, was estimated to be 4 x 107 M-1 sec-1. The large kp value was attributed to the concept that the propagating chain ends were free ions in contrast to the existence of counter ions in catalytic polymerization. 

Many causes of acute spinal cord infarction (of arterial and venous origin) have been reported (Table 17.2). They include diseases of the aorta and aortic surgery, thromboembolic events and cartilaginous disc embolism, vasculitis, coagulopathy, radiation-induced vasculopathy, toxic effects of contrast medium, epidural anesthesia, periradicu-lar nerve root therapy with crystalline corticoids, decompression illness, shock or cardiac arrest, lumbar artery compression and other etiologies... 

By contrast with the radiation-induced procedures, isotope exchange reactions catalyzed by Group VIII transition metals are applicable to both deuterium and tritium labeling of heterocyclic compounds. Because of recent mechanistic developments in this field, it is possible to predict with some degree of certainty the reactivity of a molecule for deuteration and also for moderate levels of tritiation. If compensation for additional radiation-induced interactions is made, then the theory also satisfactorily explains high specific activity tritiations. 

Acrylate resists such as polymethylmethacrylate (PMMA) have been used extensively in electron beam lithography because of their excellent resolution and contrast, despite their limited dry etch resistance and low sensitivity (1). Copolymers of PMMA, containing chemical groups more sensitive to radiation induced degradation, have also been studied and have shown up to a four-fold improvement in sensitivity (2). One approach has been to form a crosslinked gel, in-situ on the wafer, which contains radiation sensitive crosslinks and leads to improved sensitivity and improved contrast during development (3-7). 

From these results Okamura et al. [40] concluded that in toluene and methylene chloride the formaldehyde polymerization occurred by a cationic mechanism, but in diethyl ether by an anionic mechanism. This is in contrast to the earlier but less unlikely proposal by Chachaty et al. [42, 43] that the radiation induced polymerization of formaldehyde proceeded by a radical process. It was also shown by Okamura that the molecular weights of the formaldehyde polymers were independent of the monomer concentration, but that they were consistently higher when the radiation polymerization was carried out in ether as compared to those polymers obtained in toluene or methylene chloride. 

Alcock and coworkers studied the polymerization of butadiene (as well as of monoolefins, acetylene and aromatic olefins) trapped within the tunnel clathrate system of tris((9-phenylenedioxy)cyclotriphosphazene, induced by Co-y-radiation. The host was used in order to find if the concatenation and orientation of the monomer molecules under the steric forces generated within the host crystal lattice will lead to stereospecific polymerization. The clathrate was prepared by addition of liquid butadiene to the pure host at low temperature. The irradiation was conducted at low temperatures. Irradiation of pure butadiene (unclathrated bulk monomer) leads to formation of a mixture of three addition products f,2-adduct, cis- and trons-f,4-adducts. In contrast, the radiation-induced polymerization within the tunnel system of the host yielded almost pure trans-1,4-polybutadiene. A small percentage of f, 2-addition product was observed, but no evidence for the formation of c/s-f,4-adduct was found, confirming the earlier observation by Fin ter and Wegner. The average molecular weight was about 5000,... 

The temperature dependence of the photopolyraerlzation of [107] (201) and [109c] (203,205) Indicates the conditions are more favorable at temperatures far below the melting point of the monomer crystal so that the rigidity of the monomer crystal lattice is maintained. This is in contrast to many radiation-induced solid state polymerizations (222,223). 

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