Poly- [(benzo[ 1.2-d:4,5-d']bisthiazole-2,6-diyl)-1,4-phenylene], or poly-(p-phenylenebenzobisthiazole) (PPBT), is a very promising polymer with respect to its physicomechanical indexes (strength, modulus, thermal stability). Polyphosphoric acid (PPA), which can be used as reaction medium for synthesis of PPBT from 2,5-diamino-1,4-dimercaptobenzene (DADMB) and terephthalic acid (TPA), is used as the solvent for this polymer.The conditions of synthesis of PPBT from DADMB and TPA [ 1 ] require creating a dehydrating medium for binding the water ofpolycondensation. PPA with the corresponding phosphoric anhydride (PA) content fully satisfies this condition. The dehydrating properties (and water capacity) of PPA increase proportionally to the PA content [2]. An important drawback of PPA with a high PA content is the high dynamic viscosity in a relatively wide temperature range (Fig. 1).Processing of solutions of PPBT in PPA of a composition corresponding to the conditions of synthesis of the industrially acceptable concentration of 13-14 wt. % requires increasing the temperature of the spinning solution to 180-190~ which combined with the chemical aggressiveness of PPA, creates certain equipment difficulties.The formation of solvates in dissolution in proton-donor solvents, which include PPA, is a major characteristic of polymers with trivalent nitrogen in the polymer chain (azo, azoxy, thiazole, amide, and other groups). As a result ofprotonation, trivalent nitrogen equimolarly binds some of the solvent up to a certain temperature. For this reason, the molar ratio oftrivalent nitrogen groups in the polymer and the molar concentration of solvent become especially important for these solutions. For a molar ratio close to unity, the solution is a solid or highly viscous body in a relatively wide temperature range. Decreasing this ratio allows obtaining a fluid solution of solvated polymer in unbound solvent in the acceptable temperature range. We investigated the properties of solutions of PPBT in PPA of varying composition. The concentration of PA was decreased by adding water to the solution.Previously prepared solutions with a polymer content of 14.1-13.9 wt. % in PPA with 85% PA were used to prepare the solutions of PPBT in PPA with a different PA content. A calculated amount of water was added to the mixture of solutions of this composition at the softening point to reduce the PA content to the required content. It was arbitrarily assumed that during storage of the solutions, no more than 2% of the mass of PPA would be uncontrollably absorbed from atmospheric water. This 2% would be taken into account in changing the composition of PPA.The simplest calculation shows that changing the composition of the solvent by decreasing the PA content from 85 to 77% by addition of water decreases the concentration of the polymer to only 13%, so that a change in the concentration of polymer in the solutions was considered unimportant and was not taken into consideration in examining the results.The viscosity of the solutions was...

A method was developed for synthesis ofpoly-p-phenylenebenzobisthiazole (PPBT)from 2, The thermostable polymer fibres with a high modulus of elasticity and low density used in composite materials for aerospace engineering are of great practical interest. Fibres based on aromatic heterocyclic polymers occupy a special position among them. The rod-shaped macromolecules of polyheterocycles have high equilibrium rigidity quantitatively estimated by the value of the Kulm segment. The Kuhn segment of poly[(benzo[l,2-d:4,5d']bisthiazole-2,6-diyl)-l,4-phenylene] (PPBT) is 106 nm, which is greater than the value of the Kuhn segment for other rigid-chain polymers [1]. The greater skeletal rigidity of macromolecules of PPBT and the ability of its solutions to pass into the liquid-crystalline state in a certain concentration are responsible for the high mechanical properties of the fibre. Fibres spun from a liquid-crystalline solution of PPBT have a unique modulus of elasticity (300-350 GPa) close to the modulus of elasticity of several types of carbon fibres together with excellent thermal stability and strength (3000-3600 MPa) [2,3].PPBT is synthesized [4-6] in a one-stage reaction of cyclopolycondensation of terephthalic acid (TPA) with 2,5-diamino-1,4-dimercaptobenzene (DADMB) in a solution of polyphosphoric acid (PPA). Due to the instability of DADMB, it is used in the form of DADMB dihydrochloride [4][5][6]. The reaction takes place according to the scheme All-Russian Scientific-Research Institute of Polymer Fibres, Mytishchi.

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