These polymers are best known as two component thermosetting adhesives, although linear polymers can be prepared. The term "epoxy" polymers is something of a misnomer, because the epoxy groups are in the monomer, not in the polymer. To form the actual polymer, one reacts a multifunctional epoxide with a multifunctional nucleophile. Epoxy monomers based on Bisphenol A are by far the most common substrates, although others can be used. The nucleophiles are most often amines or phenoxides. The number of reactive functional groups on the components governs whether the polymer is linear or crosslinked.
The basic adhesive chemistry involves a diepoxide and a polyfunctional amine (Parts A and B, respectively, of hardware store glue), leading to a crosslinked system. The diepoxide is almost always derived from Bisphenol A and epichlorohydrin. Control of the reaction conditions can produce either a simple diglycidyl derivative of Bisphenol A or oligomeric compounds with epoxide end groups.
The mechanism is rather interesting, as discerned by labeling studies on small molecules. Despite appearances, the initial attack of the phenolate nucleophile is not at the carbon bearing chlorine, but rather at the least substituted carbon of the epoxide. Internal ring closure reforms the epoxide ring.
The polyfunctional amine can be a simple small molecule or an oligomeric compound with primary amine end groups. To use as an adhesive, the two components are simply mixed together and allowed to react. The rate of curing depends on the reactivity of the nucleophile and the temperature.
Reaction at each of the NH's is possible, leading to a crosslinked matrix.
The resulting network will not dissolve in any solvents, and resists all but the strongest chemical reagents. The plurality of OH groups provides hydrogen bonding, useful for adhesion to polar surfaces like glass, wood, etc. Epoxy polymers are often used to form composite structures filled with glass or carbon fiber.
An interesting variation on epoxy polymers was published by workers at IBM during their research on nonlinear optical materials. Here, the diepoxide was based on aniline (as part of a dye), and Bisphenol A was used as the nucleophilic component. Both ingredients were difunctional, so a linear polymer resulted.
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