Anomalous phenomena occurring during permeation and sorption of C 1-C6 alcohol vapors in Teflon AF 2400

This paper reports on the anomalous diffusion of C1-C 6 alcohol vapors in the high-free-volume, amorphous glassy perfluoropolymer Teflon AF 2400. In this material, during time lag measurements, the transient in the pressure increase curve of aliphatic alcohols exhibits a quite unusual behavior, different from the previously reported diffusion of permanent gases and that of other vapors (such as hydrocarbons, acetone, dichloromethane, etc.). Such unusual behavior can be explained by the simultaneous diffusion of dynamic molecular aggregates: clusters, made of at least two or three molecules with different diffusion coefficients, differing in orders of magnitude from those of single molecules. The probability of cluster formation increases at higher C1-C6 alcohol vapor activities. Moreover, the average cluster size decreases from methanol to pentanol with increasing molecular size, because of the longer hydrocarbon chain and, consequently, the lower polarity. Weak (butanol, pentanol) and almost zero (hexanol) clustering ability in Teflon AF 2400 was confirmed independently by the ENSIC model and through evaluation of diffusion from gravimetric sorption experiments in the vapor activity range (0.15-0.95). In addition, we present a novel method for the treatment of raw sorption kinetic data recorded by a gravimetric sorption apparatus equipped with a calibrated quartz (McBain's) spiral balance. The mentioned treatment allows one to eliminate overlapping mechanical oscillations of the spiral balance (caused by the initial charging of the sample gas/vapor into the evacuated measuring chamber) and successfully reconstruct the real elongation of the spiral caused by gas/vapor sorption into a polymer material. This allows to calculate accurate values of the gas/vapor diffusion coefficients, even from highly noisy sorption data.