Sapegin, Denis Andzheevich (2022) Sulfonated polyimides as diffusion membrane materials and modifiers. (PhD thesis), Kingston University, .
Abstract
Polyimides are one of the most promising polymers for the development and manufacturing of diffusion membranes, however, sulfonated polyimides remain under-explored in this role. Despite this, most research efforts are directed towards their application as proton-exchange membranes and meanwhile, their other diffusion transport applications remain poorly studied. This work aims to investigate correlations between structural-morphological features of hydrolytically stable sulfonated polyimides and their influence on the realisation of their selective-transport properties with a focus on pervaporation and gas separation along with proton exchange applications. For these purposes, a series of novel hydrolytically stable sulfonated polyimides based on 3,3’,4,4’-(1,3-diphenoxy-benzene)tetracarboxylic dianhydride were synthesised and characterised in detail. Conformational aspects of the limitedly-flexible chain were established to play a key role in preventing film fragility. This allowed obtained polymers to form mechanically stable films by tuning their chemical structure to avoid undesirable structurisation. The relation between the chemical composition of the novel copolymers and their tendency to form films with micro-phase separated morphology was described by the application of the Flory-Huggins and Hansen solubility theories. The expansion of the solubility-theory approach on pervaporation and gas separation experiments allowed the development of a unique model for the prediction of real process selectivity values in terms of selective fixed-cite-carrier-facilitated transport observed for the studied systems. Based on discovered correlations, several high-performance pervaporation membranes were developed, including a first-of-its-kind asymmetric gradient-porous membrane with a defectless skin layer formed from a sulfonated polyimide. The utilisation of the facilitated penetrant transfer effect allowed to create membranes with 5 high values of both flux and separation factors for the separation of various binary mixtures of methanol, including one reaching a separation factor of 147 with an overall flux of 1.01 kg⋅m-2⋅h-1 when separating methanol/methyl-tertbutyl ether mixture at 52 oC. The hydrolytic stability of obtained polymers allowed their prolonged utilisation as proton-exchange membranes in lab-scale membrane electrode assemblies reaching proton conductivity values as high as 1.2⋅10-3 S⋅cm-1 at 25 oC. Discovered correlations allowed to increase the membranes’ swelling, dimension and proton conductivity stability by incorporation of blocks inclined to form highly crystalline regions.
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