A matrix EPR spectroscopy study of the low temperature γ radiolysis of precipitated (Zeosil) and mesoporous high surface silica has afforded evidence of the formation of trapped H-atoms, H-atom centers, siloxy radicals ≡Si–O•, anomalous silyl peroxy radicals ≡Si-OO• with reduced g tensor anisotropy, siloxy radical-cations (≡Si–O–Si≡)+•, E′ centers, and two species from Ge impurity. Coordination of peroxyl radicals with diamagnetic ≡Si+ centers is proposed and tested by DFT computations in order to justify the observed g tensor. Coordination of H-atoms to ≡Si+ centers is also proposed for the structure of the H-atom centers as an alternative model not requiring the intervention of Ge, Sn, or CO impurities. The DFT method has been employed to assess the electronic structure of siloxy radical-cations and its similarity with that of the carbon radical-cation analogues; the results have prompted a revision of the structures proposed in the literature for ST1 and ST2 centers. The comparison between the two type...

Reaction mechanisms in irradiated, precipitated, and mesoporous silica.

The role of silica in radiation induced grafting and crosslinking of silica/elastomers blends

The behavior of different nanoporous grafted silicas (−Si(CH3)2R) under ionizing radiation is investigated as a function of the chemical nature of the grafting. The structure and reactivity of the grafting after irradiation is characterized by solid-state NMR spectroscopy. The chemical reactivity is also investigated by mass spectrometry gas analysis . The major gases detected after irradiation are H2 and/or CH4. Different types of behaviors under ionizing radiation are depicted as a function of R. When R contains an aromatic ring or is the cyclopentadienyl moiety, then the overall measured radiolytic yields are low and CH4 is the major gas detected. The aromatic ring acts as an efficient energy trap. When R contains an alkyl chain with a −CN ending, the major gas detected is dihydrogen due to the lysis of the −CH(H) bond, but it is also shown that the −CN group acts as an energy trap, even if it is of course less efficient than an aromatic ring. When R consists of a long alkyl chain, an efficient energy ...

Influence of the Nature of Self-Assembled Monolayers on Their Reactivity under Ionizing Radiation: A Solid-State NMR Study

Towards controlled cationic polymer growth from inorganic oxide defects: Directing the mechanism of polystyrene grafting from γ-irradiated silica

Ionizing radiations from a 60-Co source have been employed for inducing radical reactions finally leading to the grafting of unsaturated olygomers and vinyl monomers onto the surface and micropores of precipitated silica Zeosil 116 5 (150- 170 m 2 /g). The target was to obtain modified silica with enhanced polar compatibility with respect to polybutadiene and styrene-butadiene copolymers rubber matrices and suited to favour the formation of chemically bound rubber in a vulcanization process. The latter property was expected to follow from the characteristics of free radical reactivity arising from the unsaturations present in the organic coating. Three classes of modified silica have been prepared: A)silica coated with polybutadiene olygomers with M n in the range 1000-5000 and having a different relative content of 1,4-cis, 1 , 4 -trans and vinyl double bonds [1,2] ; B)Silica coated with polybutadiene olygomers further modified by subsequent grafting of styrene monomer. The type A samples were obtained by γ irradiation under vacuum of silica samples preimpregnated with the oligomers in the dose range up to 30-200 kGy. Type B samples were obtained via three different steps as outlined in the scheme below: SiO 2 (PB) γ vacuum→/298K SiO 2 -PB γ air→/298K SiO 2 -PB-OOH Fe 2+ →/Sty SiO 2 -PB-O-PS The characterization of the modified silica was carried on by elemental and thermogravimetric analysis, FTIR (diffuse reflectance), Raman spectroscopy, inverse gas chromatography (IGC), CP/MAS 29-Si NMR, granulometry and TEM microscopy.

Marco Nahmias

Papers

Radiation Induced Double Grafting of Polybutadiene Olygomers and Styrene onto Silica: Characterization of the Materials and Mechanistic Studies