Fine characterization of the ethylene and ethane sorption in poly(amide 12-block-tetramethylenoxide) copolymer/AgBF 4 membranes

Abstract

Ethylene/ethane sorption characteristics were determined for dry Pebax™ (poly(amide 12-block-tetramethylenoxide) copolymer)/AgBF 4 membranes by using an electronic microbalance. The membranes containing 0.7 and 22 wt.% AgBF 4 showed a dual-mode sorption isotherm. The ethane isotherms for all the membranes were of the Henry-type, which is the normal sorption for gases in rubbery polymers. The abnormal presence of Langmuir sorption sites only for ethylene in the rubbery copolymer, never reported sofar, is attributed to the silver-based specific complexation sites. The silver salt which dissolved in limited amounts in the rubbery copolymer had a much smaller Langmuir sorption capacity than the salt that crystallized in the copolymer. The sorption kinetics indicate that the crystallized salt did adsorb slowly ethylene according to a zeroth-order kinetics, but not ethane. The gas uptake kinetics resulting from a step of the pressure surrounding the copolymer exhibited one stage for ethane but two stages for ethylene. For the latter, there was first a fast Fickian sorption stage, then a drift of the zeroth-order sorption of ethylene on salt crystals, which contributes for a large part to the total uptake. The zeroth-order sorption suggests that the sorbed ethylene amount in the second-stage is independent of the crystal-surface coverage. The value of the Fickian diffusion coefficient calculated by fitting the kinetics with a solution of the second Fick’s law was 5 × 10 −12 m 2/s for both ethylene (the first stage) and ethane, and is typical for small organic compounds in a rubbery material.