Effect of nonsolvent additive on the surface morphology and the gas separation performance of poly(2,6-dimethyl-1,4-phenylene)oxide membranes.

Abstract

Integrally skinned asymmetric membranes were prepared from poly(2,6-dimethyl-1,4-phenylene)oxide (PPO) using different nonsolvent additives. These nonsolvent additives consisted of branched and linear alcohols ranging from C3 to C10. Permeation data of these membranes were obtained from a constant pressure permeation system for pure gases Of CO2, CH4, O 2 and N2. An empirical correlation relating the pure gas permeance ratio of CO2/CH4 and the structural components of the nonsolvent additives has been proposed. The surface of the membranes was characterized by atomic force microscopy (AFM). It was observed that there were two types of surface morphologies: merged nodules and discrete nodules. The appearance of the nodules were reflected in the mean roughness data, Ra. It was revealed that membranes with smaller and merged nodules resulted in higher pure gas permeance ratios for O2/N2 and CO2/CH4 with the exception of 3,5,5-trimethyl-1-hexanol and 3-ethyl-2,2-dimethyl-3-pentanol. The microscopic studies showed that the membranes containing discrete nodules resulted in lower pure gas permeance ratios for O2/N2 and CO2/CH4. It was observed that nonsolvent additives that possess a long straight hydrocarbon chain such as 2-ethyl-1-hexanol, 1-octanol and 2-decanol produced the highest pure gas permeance ratios. The polymer solutions from which the membranes were made were characterized through intrinsic viscosity and surface tension measurements. The intrinsic viscosity studies revealed that as the intrinsic viscosity increased, the chances of entanglement of the polymer molecules increased. As a result, the pure gas permeance for CO2/CH4 increased with the exception of 3,5,5-trimethyl-1-hexanol. The surface tension studies showed that as the surface tension of the solutions increased, the pure gas permeance for CO 2/CH4 and O2/N2 increased. This was due to the increase of the capillary forces within the spherical particles of the polymer-rich phases. Plasma-etching is a powerful tool for investigating the structure of the skinlayer of asymmetric membranes. This process involves the gradual etching of the skinned membrane surface by highly reactive particles from an oxygen gas plasma. The rate of etching was low at 0.1 x 10--6 m/min. The membranes were etched at 30, 60, 120 and 190 sec. Pure gas permeances using CO2, CH4, O2 and N2 were determined for the etched and unetched membranes. The morphology of these membranes was determined from scanning electron microscopy (SEM) and atomic force microscopy (AFM). SEM of the cross-section of the membranes showed that there is a transition in the membrane structure from the dense layer to the layer with visible pores. AFM images of the top surfaces of the membranes showed that mean roughness of the nodules increased linearly as the membrane is exposed to longer periods of etching time.