Microphase-separated ionic polymers

There is a considerable body of experimental and theoretical evidence for two types of ionic aggregates termed multiplets and clusters (95). The multiplets are considered to consist of small numbers of associated contact ion-pairs that are dispersed in the matrix of low dielectric constant, but do not themselves constitute a second phase. The number of ion-pairs in a multiplet is sterically limited by the fact that the salt groups are bound to the polymer chain. On the other hand, clusters are considered to be small microphase separated regions (<5 nm) of aggregated multiplets. Thus, the clusters are rich in ion-pairs, but they also contain an amount of the organic polymer. 

These five sets of observations, plus knowledge of the phenomenon of microphase separation in block copolymers leads to a model of reverse osmosis or ion exchange membranes in which the hydrophobic portions of the polymer chains have come together to form one more or less continuous microphase, while the hydrophilic portions of the polymer chains (ionic groups, -OH groups, -NH2 or > NH groups) have "dissolved" in a small amount of water to form another more or less continuous microphase when the meni>rane is swollen in water. The hydrophilic groups, in most cases, probably form clusters but not continuous microphases in the dried membranes. 

Ionomers are polymers that are functionalized with ionic groups (usually anionic sites) attached at various points along polymeric backbones that are not extensively crosslinked (1-2). Such materials have a tendency to form ionic domains in which the anionic groups and their associated cations are microphase separated from the typically hydrophobic portions of the polymer. Thus, the ionic domains formed are isolated by a medium of low dielectric constant (i.e. the polymeric backbone) although, in some cases, hydrophilic channels have been reported to connect adjacent ionic domains (3). The size and structures of these domains vary with the nature of the cation, the stoichiometry of the polymer, the degree of solvation of the system and the method of preparation. They can be as small as ion-pairscor small multiplets, but in some cases they have been reported to be in the 20-100 A" diameter range. 

Neutralization of ethylene copolymers containing up to 5%-10% acrylic or methacrylic acid copolymer with a metal salt such as the acetate or oxide of zinc, magnesium, and barium yields products referred to as ionomers. (Commercial products may contain univalent as well as divalent metal salts.) lonomers are marked by Du Pont under the trade name Surlyn. These have interesting properties compared with the nonionized copolymer. Introduction of ions causes disordering of the semicrystalline structure, which makes the polymer transparent. lonomers act like reversibly cross-linked thermoplastics as a result of microphase separation between ionic metal carboxylate and nonpolar hydrocarbon segments. The... 

The unique properties of pH-responsive polymers arise from the facile pH adjustment, which induces ionic interaction and hydrogen bonding, resulting in a reversible microphase separation or self-organization phenomenon. Thus, pH-responsive polymeric systems provide the possibility of preparation of smart functional materials that can be used for potential therapeutic applications, for example, controlled dmg delivery based on pH-triggered release. 

Our interest is to develop MIEC block copolymers with microphase separated structures such that the electronic and ionic phases are in intimate contact at the few hundred angstrom level. We believe that such intimate contact between the two phases will result in functional polymers capable of very fast responses, e.g., in sensors and MEMS devices. A synthetic muscle functions by converting chemical energy into mechanical energy [5]. A biopolymer strip capable of carrying out this function consists of a polyethylene layer, a thin gold layer, and a polypyrrole layer immersed in an electrolyte solution. Oxidation of the polypyrrole results in... 

Li, J., Arnold, S., and Khan, I. M., Microphase separated mixed (ionic and electronic) or MIEC block copolymers. New concept in electroactive polymers, SPIE Proc., 2189, 126-133 (1994). 

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