Poly(ether imide)/N‐methyl pyrrolidinone solutions: kinetics of water vapour absorption and surface layer formation

Abstract

AbstractBACKGROUND: Water sorption by polymer solutions during film fabrication may affect the film morphology and thereby the film properties.RESULTS: Water vapour sorption isotherms at 40 °C of poly(ether imide) (PEI)/N‐methyl pyrrolidinone (NMP) solutions were experimentally determined. Considering PEI concentrations ranging from 0 to 20 wt%, results obtained from gravimetric measurements and near‐infrared (NIR) spectroscopy, indicative of the total weight uptake and the local water concentration in the bulk solution, respectively, showed that a first stage occurred featuring a quasi‐linear time dependence of the weight fraction of absorbed water. Deviations from linearity dependent on the PEI concentration were afterwards observed, this behaviour being more marked for concentrations higher than 14 wt%. To achieve good predictions, the mathematical description of the sorption results must take into consideration both the classical approach using diffusion equations and the possible time dependence of the surface concentration of water at the air‐facing interface.CONCLUSION: This paper presents an analysis of water vapour sorption in PEI/NMP solutions combining local and global quantifications of the water content through NIR and gravimetric methods. The development of a mathematical model suggests a possible surface layer formation at the air‐facing interface. Copyright © 2008 Society of Chemical Industry