Polyamide-imides as novel high performance primary protective coatings of silica optical fibers: Influence of the structure and molecular weight

Polyimides are commonly used as primary protective optical fiber coatings for high-temperature applications. However, a number of significant drawbacks such as thermal and hydrolytic instability of polyamic acid, its incomplete transformation into polyimide, several deposition cycles of the required thickness and insufficient adhesion of polyimide coatings to the surface complicate the manufacture of special optical fibers and their widespread use. In this study we showed the principal possibility of utilizing polyamide-imides (PAIs) as primary protective coatings of high operational reliability. Two series of high-molecular-weight (M-w = 84-365 kDa) and thermostable (T-10% = 490-520 C-degrees) PAIs were synthesized by low temperature polycondensation of terephthaloyl chloride and various tetracarboxylic acid dianhydrides with 9,9-bis(4-aminophenyl)fluorene or 2,2 '-bis(tri-fluoromethyl)benzidine in NMP. Coatings of a thickness of 10-15 mu m with good adhesion to the optical fiber were formed from the PAIs solutions in NMP with no any additives and in a single application cycle. Apart from the influence of solution viscosity and drawing parameters on the coating quality, a particular attention was paid to the dependency of the strength and mechanical reliability of the fibers with resulting coatings on the structure and molecular weight of the polymer. It was established that the fibers coated with PAIs prepared from 2,2 ' -bis (trifluoromethyl)benzidine (M-w = 190-200 kDa) can withstand 3 and 72 h annealing at 350 and 300(degrees)C, respectively, retaining >80% of the original strength. Thus, PAIs are considered as a promising candidate for further studies, i.e., for the creation of optical fiber sensors for large-scale applications.