Date of this Version
Wiley Encyclopedia of Composites, Second Edition. Edited by Luigi Nicolais and Assunta Borzacchiello
Polyquinoxalines (PQs) are heterocyclic polymers that contain the heteroaromatic benzo [α] pyrazine (quinoxaline) ring along the main chain. The polymers may be unsubstituted or contain pendent substituents such as phenyl groups on the quinoxaline ring. Quinoxaline polymers are generally synthesized from the step-growth polymerization of aromatic bis(o-diamines) and bis(α-dicarbonyl) compounds. This synthetic route to PQs was proposed in 1961 , and the first synthesis was reported in 1964 [2,3]. Polyphenylquinoxalines (PPQs) were first reported in 1967 . Recently, PPQs have been prepared by an aromatic nucleophilic displacement route [5,6]. Review articles concerning the synthesis, characterization, and properties of PQs are available [7–9].
The properties of the PQs can be altered by changing the chemical structure of the polymer backbone and also the chemical nature of the pendent group on the quinoxaline ring. This unique feature provides versatility in designing PQs with an attractive combination of properties for use in specific applications. Variations of quinoxaline polymers include copolymers and oligomers end-capped with reactive groups such as vinyl, ethynyl, and phenylethynyl.
A unique feature of linear PQs is their excellent solubility in the fully cyclized form. Unlike most other aromatic heterocyclic polymers, high molecular weight PQs form solutions that can be used in a variety of applications to form films, ultrafiltration membranes, bioreactor cells, and protective coatings, as well as to impregnate reinforcements. PQs are an important family of polymers that offer high chemical and thermal stability coupled with high mechanical properties. They are potentially useful as functional resins (e.g., films and coatings) and structural resins (e.g., adhesives and composite matrices) in many applications that demand stability in harsh environments. This article reviews some neat resin properties with emphasis on the use of PQs as matrix resins in fiber-reinforced composites.