Fabrication Of Ion Conducting Membranes With Bicontinuous Structures By Self-assembly Of Zwitterionic Amphiphiles

Liquid crystal（LC）templating has been demonstrated as a promising method for fabricating materials with highly ordered nanostructures.For instance,polymeric materials derived from polymerization/crosslinking of polymerizable LCs can replicate the original orders from the mesophase templates.Various ordered nanostructures can be formed,such as columns（cylinders）,layers（lamellae）and bicontinuous networks.The produced polymers have shown applications as drug delivery vehicles,separations membranes,heterogeneous catalysis,and materials for energy storage.Among the variety of ordered morphologies,bicontinuous cubic（Q）LCs have attracted considerable interest because of their unique geometries with interconnected networks.In particular,these bicontinuous morphologies can by highly useful in efficient membrane separation and ionic conductance.However,due to the presence of non-constant interfacial curvature in this class of LCs,access to Q mesophases is challenging compared with their lamellar or cylindrical counterparts.This challenge has been even more remarkable when it comes to polymerizable Q mesophases where the presence of reactive groups significantly affects the mesophase stability.Furthermore,structural retention by polymerization of reactive Q mesophases can be exceptionally difficult since polymerization often results in structural destruction.Although successful cases of structural lock-in of Q mesophases by polymerization have been reported previously,the syntheses of the constituent polymerizable mesogens or amphiphilic molecules have been extremely complicated.Therefore,scalable fabrication of the polymeric materials with bicontinuous morphologies have been hindered.In this thesis,we propose a simple method to prepare polymer materials with bicontinuous network structures using a readily synthesized polymerizable zwitterionic amphiphile.Lyotropic Q mesophases were obtained by mixing the zwitterionic amphiphile with proper amounts of aqueous solutions phosphorous acid.A significant advantage exhibited by the lyotropic LCs is the capability to take up large amounts of common crosslinkers,which facilitates structural preservation by photo-curing.Polymer films with bicontinuous cubic structures were produced,which display efficient proton conducting.The thesis is mainly organized as follows:1.A polymerizable zwitterionic amphiphilic monomer（named as MAC11-APS）and a similar crosslinker（named as DMAC11-APS）were synthesized.Temperature-composition phase diagrams of lyotropic LCs derived from mixing MAC11-APS or DMAC11-APS with water were obtained on the basis of characterizations by polarized optical microscopy（POM）or synchrotron small angle X-ray scattering（SAXS）.2.Influence of the additional acids in the lyotropic LC formation has been investigated.MAC11-APS was mixed with aqueous solutions of common acids or salts,such as benzenesulfonic acid,phosphoric acid（PA）,lithium bis（trifluoromethylsulfonyl）imide（Li TFSI）.Lyotropic double gyroid（G）LCs can be formed by dissolving MAC11-APS in aqueous PA solutions.The phase window of the G LCs has found the broadest for the system prepared with aqueous PA solutions where the original PA concentration is 50 wt%.3.Structural retention of the G morphologies have been targeted by introducing commercial crosslinkers such as such as 1,6-hexanediol dimethacrylate（HDDMA）,ethylene glycol dimethacrylate（EGDMA）,divinyl benzene（DVB）.A correlation between the degree of structural preservation and the crosslinker content was found by a systematic variation of the crosslinker content in the lyotropic LCs and subsequent studies on the ordered structures in the polymers by synchrotron SAXS.High fidelity preservation of the G₁ structures was realized as confirmed by synchrotron SAXS and transmission electron microscopy（TEM）imaging.Moreover,atomic force microscopy（AFM）imaging provides similar evidence for the excellent structural lock-in.4.Polymer films with preserved G morphologies were fabricated for ionic conductivity measurements.Using AC impedance tests,the proton conductivity of the polymer films with bicontinuous cubic structures reaches 0.096 S/cm under 90%relative humidity at 50℃,close to that of Nafion117 membranes under the same conditions of humidity and temperature.