Gas Permeability of Cellulose Acetate Films Treated with Fluorine in Perfluorodecalin

The surface of cellulose acetate (CA) films has been modified by gas-liquid fluorination. The treatment has been carried out with a fluorinating mixture (10 vol % F-2 + N-2) in a perfluorodecalin medium in a flow reactor for 30, 60, and 120 min. Film samples have been studied using X-ray diffraction and IR spectroscopy. It has been shown that the general appearance and position of peaks in the diffraction patterns of the initial and fluorinated CA samples are almost the same, indicating the absence of a significant effect of treatment on the phase composition of the film samples. On the contrary, according to IR data, there is an increase in the concentration of fluorine-containing and hydroxyl groups in the fluorinated CA layer. SEM images of cross sections of fluorinated films have confirmed their layered structure. The transport parameters of the films with respect to He, H-2, O-2, CO2, and CH4 have been also determined experimentally. The gas permeability has been found to monotonically decrease with an increase in the fluorination time and to increase with an increase in the size of the penetrant molecule. Direct fluorination also leads to an increase in O-2/N-2, CO2/CH4, N-2/CH4, He/CH4, H-2/CH4, and He/H-2 ideal permselectivities compared to those of the unmodified cellulose acetate film. The data points for fluorinated CA films on the Robeson plots for these gas pairs approach the 1991 upper bound, whereas those for the virgin polymer are located in the middle of the plots.