Free volume characterization of sulfonated styrenic pentablock copolymers using positron annihilation lifetime spectroscopy

• Hydrophobic domains in phase separated block copolymers influence PALS data.
• Water sorption reduces o-Ps lifetime more than expected based on an additive model.
• Water permeability weakly correlates with PALS data.
• Salt permeability does not correlate with PALS data according to free volume theory.
• Transport property correlation with free volume is masked by morphological complexities.

A series of sulfonated styrenic pentablock copolymers was characterized using positron annihilation lifetime spectroscopy (PALS). Ortho-positronium (o-Ps) lifetime was measured as a function of the polymer's degree of sulfonation and the environment in which the polymer was equilibrated. Samples ranged from an un-sulfonated styrenic pentablock copolymer to one with an ion exchange capacity (IEC) of 2.0 meq/g(dry polymer), and the samples were equilibrated in either air having 50% relative humidity, de-ionized (DI) water, or 2000 mg L−1 aqueous NaCl solution environments. The o-Ps lifetime, and thus, the average free volume element size, decreased as the polymer's degree of sulfonation increased and as the amount of water sorbed into the polymer increased, similar to reported decreases in o-Ps lifetime accompanying increased water sorption in uncharged cross-linked poly(ethylene glycol) hydrogels. The o-Ps lifetimes of the DI water equilibrated acid counter-ion form polymers were quite similar to those of the salt solution equilibrated sodium counter-ion form polymers, suggesting that the PALS data are not particularly sensitive to the counter-ion form of these hydrated polymers. Previously reported water and salt permeability data in polymers were correlated with the PALS data. Correlating water and salt permeability with free volume data, estimated using o-Ps lifetime, was complicated by the micro-phase separated nature of the block copolymers. The water permeability of polymers whose hydrophilic micro-domains are believed to be well connected correlated with free volume data, based on o-Ps lifetime. Salt permeability, however, did not correlate with free volume data estimated from o-Ps lifetime, according to free volume theory. In general, contributions of the hydrophobic micro-domain free volume to the PALS measurement frustrate the correlation of water and salt transport properties with PALS parameters in a manner that is not observed in other sulfonated and uncharged polymers.

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