Due to the electrostatic ion interaction, ion-containing block copolymers display more complicated phase behaviors and generate morphologies beyond that of neutral copolymers (2). Among them, inverse-hexagonal (HEX) structures have been widely prepared (3-5). It has the major non-ion block form the ‘bulk-like’ cylinders residing in the ‘surface-like’ minor ion phase. Our experimental observation indicated that this inverse-HEX morphology may suffer from excessive water swelling when placed in liquid water environment.
In our work, the ion-containing block copolymer under study was supposed to form lamellar morphology according to the block copolymer phase diagram, however it generated a mixture of lamellar and inverse-HEX morphologies when casting from a polar solvent. The formation of inverse-HEX was suppressed when adding a little nonpolar solvent for casting. These two morphologies were imaged by TEM as shown in Figure 1. They are both in principle ‘percolating’ structures for ion transport. Although the membrane with a mixture of lamellar and HEX structures did display higher ion-domain connectivity than the pure-lamellar one as per our preliminary dynamic tension test, its ion conductivity under low water activity (in water vapor) and its water vapor transport flux are very close to the pure-lamellar one. This inconsistency may be because of the larger ion to non-ion interface within the inverse-HEX structure, resulting in more interface resistance. Only under high water activities (more than 0.95), the one with inverse-HEX displayed larger conductivity than the pure-lamellar one, probably due to more effective swelling of the hydrophilic phase for the former. And if the water vapor transport was measured with liquid water contacting one side of the membrane, the one with inverse-HEX had larger flux. Water sorption experiment showed that the one with HEX morphology swell more than the pure-lamellar one. And we actually observed the HEX membrane broke apart after placed in liquid water for 2 month while the pure-lamellar one was still intact. Thermodynamic swelling model considering both interface energy and elastic energy is being developed to explain the difference between inverse-HEX and lamellar morphologies.
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