No abstract

The synthesis of original fluorinated (co)telomers containing vinylidene fluoride (VDF) or VDF and hexafluoropropene (HFP) was achieved by radical telomerizations and (co)telomerizations of VDF (or VDF and HFP) in the presence of 1, 6-diiodoperfluorohexane via a semisuspension process. tert-Butyl peroxypivalate (TBPPi) was used as an efficient thermal initiator. The numbers of VDF and VDF/HFP base units in the (co)telomers were determined by 19 F and 1 H NMR spectroscopy. They ranged from 10 to 190 VDF base units. Fluorinated telomers of various molecular weights (1200-12,600 g/mol) were obtained by the alteration of the initial [1,6-diiodoperfluorohexane] 0 /[fluoroalkenes] 0 and [TBPPi] 0 /[fluoroalkenes] 0 molar ratios. The thermal properties of these fluorinated (co)telomers, such as the glass-transition temperature and melting temperature, were examined. As expected, these telomers exhibited good thermal stability. They were stable at least up to 350 8C. The compounds containing more than 30 VDF units were crystalline, whereas all those containing VDF-co-HFP were amorphous with elastomeric properties, whatever the number was of the fluorinated base units. The structures of I-(VDF) n -R F -(VDF) m -I and I-(HFP) x (VDF) n -R F -(VDF) m (HFP) y -I (co)telomers were obtained, and the defects of the VDF chain and the À ÀCH 2 CF 2 I and À ÀCF 2 CH 2 I functionalities were studied successfully (where R F ¼ C 6 F 12 ). The functionality in the iodine atoms was modified: the higher the VDF content in the telomers, the lower the normal end functionality (À ÀCH 2 CF 2 I) and the higher the reversed extremity (À ÀCF 2 CH 2 I). In addition, the percentage of defects increased when the number of VDF units increased. The molecular weights and molecular weight distributions of different telomers and cotelomers were also studied.

Electron beam welding (EBW) shows certain problems with the control of focus regime. The electron beam focus can be controlled in electron-beam welding based on the parameters of a secondary signal. In this case, the parameters like secondary emissions and focus coil current have extreme relationships. There are two values of focus coil current which provide equal value signal parameters. Therefore, adaptive systems of electron beam focus control use low-frequency scanning of focus, which substantially limits the operation speed of these systems and has a negative effect on weld joint quality. The purpose of this study is to develop a method for operational control of the electron beam focus during welding in the deep penetration mode. The method uses the plasma charge current signal as an additional informational parameter. This parameter allows identification of the electron beam focus regime in electron-beam welding without application of additional low-frequency scanning of focus. It can be used for working out operational electron beam control methods focusing exactly on the welding. In addition, use of this parameter allows one to observe the shape of the keyhole during the welding process.