(187c) Salt Transport Structure/Property Relationships in Polymer Membranes for Water Purification and Power Generation

Providing
sustainable supplies of water and energy is a critical global challenge. Polymer
membranes dominate desalination and could be crucial to power generation
applications. These desalination and power generation applications include
reverse osmosis (RO), nanofiltration (NF), forward osmosis (FO),
pressure-retarded osmosis (PRO), electrodialysis (ED), membrane capacitive
deionization (MCDI), capacitive reverse electrodialysis (CRED), and reverse
electrodialysis (RED). Improved membranes with tailored water and salt
transport properties are required to extend and optimize these technologies. Water
and salt transport structure/property relationships must be used to provide the
fundamental framework for optimizing polymer materials for water and salt
transport property-critical applications.

A
tradeoff relationship between water/salt selectivity (i.e., the polymer's
ability to separate water and salt) and water permeability has been reported
where polymers that have high water/salt selectivity tend to have low water
permeability and vice versa. This tradeoff relationship is consistent with free
volume theory suggesting that attempts to prepare materials with favorable
combinations of both high selectivity and high water permeability should rely
on approaches that do more than simply vary the polymer's free volume. One
strategy is to functionalize an otherwise hydrophobic hydrocarbon polymer with
fixed charge groups that can ionize upon exposure to water. Sulfonated
hydrocarbon polymers are an example of such an approach. These polymers offer
an additional advantage from a desalination perspective because they have been
reported to resist degradation by oxidizers such as the chlorine-based
chemicals that are widely used in water treatment applications.

The
presence of fixed charge (e.g., sulfonate) groups in a polymer influences that
polymer's salt transport properties. Salt permeability, sorption, and diffusion
properties are sensitive to the uncharged or charged (e.g., sulfonated) nature
of the polymer. Poly(ethylene glycol) (PEG) hydrogels, sulfonated poly(arylene
ether sulfone) random copolymers, and sulfonated styrenic pentablock copolymers
were studied to probe the influence of polymer charge, water content, polymer
structure, and micro- as well as nano-scale structural heterogeneity on salt
transport properties. Salt sorption can be modeled using a combination of
Donnan exclusion theory and salt partitioning to further describe the influence
of fixed charge concentration, water content, and micro- as well as nano-scale
polymer structure heterogeneity on salt sorption and transport properties.