Soluble polyimides with trifluoromethyl pendent groups

Abstract

Unsymmetrical and symmetrical diamine monomers containing trifluoromethyl groups, 2-trifluoromethyl-4,4′-diaminodiphenyl sulfide and 2,2′-bis(trifluoromethyl)-4,4′-diamino-diphenyl sulfide, were synthesized from 2-chloro-5-nitrobenzotrifluoride as a starting material in two steps, respectively. Diamine monomers were polymerized with PMDA, BPDA, BTDA, and ODPA using a solution imidization method with N-methyl-2-pyrrolidone as a solvent at 190 °C to obtain the corresponding polyimides. They had inherent viscosities that ranged from 0.54 to 0.71 dL/g in N-methyl-2-pyrrolidone at 30 °C. All of the synthesized polyimides showed good solubility in polar aprotic solvents and phenolic solvents regardless of the number of trifluoromethyl groups. The 5% weight loss temperatures of the polyimides are in the range of 534-561 °C in nitrogen, and 505-542 °C in air. The Tg values and the thermal expansion coefficients of these polymers are in the range of 234-325 °C and in the range of 47.4-63.2 ppm/°C, respectively. Also, all of the synthesized polyimides have relatively low refractive indices (around 1.6) and birefringence (below 0.36).