Influence of thermal treatment on the pervaporation separation properties of polyamide-6 membranes

Integrally-skinned asymmetric polyamide-6 membranes were prepared by castin g 15 wt.% solution of polymer in 85 wt.% formic acid onto a glass plate and precipitation in water at 4 degrees C. The obtained membranes were dried f or 1 h at different temperatures ranging from 25 to 140 degrees C in air an d from 25 to 200 degrees C in vacuum, respectively. The pervaporation separ ation properties of such treated membranes were studied with binary water-2 -propanol mixtures consisting of 5 to 95 wt.% water. The membranes showed w ater permselectivity over a wide range of the feed composition. Thereby the water permselectivity reached a maximum at low water contents in the feed mixture. The selectivity was improved by thermal treatment of the membranes at temperatures above the glass transition temperature T-g of polyamide-6 (approximate to 60 degrees C). However, the permeate flux decreased with in creasing drying temperature due to the removal of entrapped water from amor phous domains, the formation of new crystalline domains and the decrease of the pore size and of the specific surface area during the thermal treatmen t. These effects were confirmed by dsc measurements, X-ray scattering exper iments and BET measurements. (C) 2000 Elsevier Science B.V. All rights rese rved.