Patent ID: 7091295

Claim:
A method for preparing transparent films at 157 nm having a transmittance higher than 50%, comprising the step of preparing solutions in perfluorinated solvents of amorphous perfluorinated polymers, said perfluorinated polymers being free from unstable ionic end groups, said polymers formed of cyclic units deriving from perfluorodioxoles of formula: wherein R′ F is equal to R F or OR F wherein R F is a linear or branched perfluoroalkyl radical having 1–5 carbon atoms; X 1 and X 2 , equal to or different from each other, are F, CF 3 ; said polymers optionally containing units deriving from perfluorinated comonomers containing at least one ethylene type unsaturation, optionally containing oxygen atoms; in said polymers the cyclic units deriving from the perfluorodioxoles of formula (IA) being ≧95% by moles; said ionic end groups being determined by means of the following method by using the Fourier Transform IR spectroscopy by FT-IR equipment (256 scannings, resolution 2 cm −1 ): on a polymer sintered powder pellet having a diameter of 5 mm with a thickness from 50 to 300 micron (1.75–10.5 mg of polymer) a scanning between 4000 cm −1 and 400 cm −1 is initially carried out, then the pellet is transferred for 12 hours in an environment saturated with ammonia vapours; at the end of this period the IR spectrum is recorded under the same conditions of the initial IR spectrum; by elaborating the two spectra by subtracting from the signals of the spectrum related to the untreated specimen (starting spectrum) the corresponding signals of the specimen spectrum after exposure to the ammonia vapours, drawing the “difference” spectrum, which is normalized by the following equation: '' ⁢ Difference ⁢ ⁢ spectrum ⁢ '' [ pellet ⁢ ⁢ weight ⁢ ⁢ ( g ) ⁢ / ⁢ pellet ⁢ ⁢ area ⁢ ⁢ ( cm 2 ) ] the optical densities related to the COOH and COF end groups after the reaction with the ammonia vapours, end groups giving rise with this reactant to detectable peaks, are determined; the optical densities are converted into mmoles/kg of polymer obtaining the concentrations of the residual polar end groups as mmoles of polar end groups/kg of polymer: in the polymer spectrum in the correspondence of the wave length of the peaks related to COOH groups and/or COF groups are not shown peaks which are detectable from the irregularities of the base line.