Modified method for determining aromatic diamines during the synthesis of oligomeric para-aramids

A modified high-performance liquid chromatography method for determining the quantitative content of unreacted paraphenylenediamine and diaminophenylbenzimidazole in the presence of dimethylacetamide is proposed. The method includes acetylation of the reaction mixture after copolycondensation of the para-oligomeric copolyamide in the presence of an excess of the aromatic diamines. Kevlar, Twaron, and domestic fibers such as CBM, armos, rusar, etc. are known brands of para-aramid fibers. They are used to prepare high-strength high-modulus construction polymers [1-6] that assist the development of primarily aviation and space-rocket technology and machine building in addition to the fabrication of flak jackets and special clothing for use under harsh professional service conditions. A promising scientific and technical development in the area of para-aramids was the synthesis of copolyamides and their use to produce fibers with an elevated set of elastic and strength properties. The incorporation of various comonomers into the aromatic fiber-forming polymers helped to increase the flexibility of the macromolecules, the thermal stability, and the strength in the microplastic. Therefore, a determination of the copolymer macromolecule structure holds great scientific and practical significance. The reactivities of the diamines used for these purposes, e.g., paraphenylenediamine (p-PDA) and 5(6)-amino-2-(p-aminophenyl)benzimidazole (DAPBI) for armos and rusar fibers, are a priori different in the copolymerization reaction. It has been found that the copolymer properties depend not only on the chemical structure of the comonomers and the composite composition of the copolyamide but also to a large extent on the microstructure, i.e., on the nature of the distribution of the monomeric units throughout the macromolecule chain that is determined largely by the reaction rates of the amines with the acidic component. Compositional heterogeneity among the copolycondensation products should be expected as a result of the different reactivities of the comonomers. Keeping in mind that existing methods for determining reactivity do not consider the effects of several factors and give only an approximate representation of the processes occurring under actual conditions, the stated hypothesis required experimental confirmation. This was obtained by performing the copolyamide synthesis with varying amounts of terephthalic acid dichloride with excesses of the studied diamines p-PDA and DAPBI taken in equimolar amounts. Difficulties with identification of the aromatic diamines in the copolymers make it important in principle to develop a method for their determination in the reaction mixture after the synthesis. This would allow the amount of each reacted diamine to be calculated and, thereby, the chemical composition of the resulting oligomeric copolyamide to *A. N. Kosygin Moscow State Textile University; **Research Institute of Chemical Diversity, Moscow; ***Scientific-Production Company Termoteks, Mytishchi. Translated from Khimicheskie Volokna, No. 5, pp. 48-51, September-October, 2011.