Organo-Soluble Polyimides: Synthesis and Polymerization of 2,2‘-Bis(trifluoromethyl)-4,4‘,5,5‘-Biphenyltetracarboxylic Dianhydride

Abstract

2,2‘-Bis(trifluoromethyl)-4,4‘,5,5‘-biphenyltetracarboxylic dianhydride (6FBPDA) was synthesized from 1-iodo-4,5-dimethyl-2-nitrobenzene via a six-step synthetic route. The dianhydride was polymerized with eight different substituted 4,4‘-diaminobiphenyls in refluxing m-cresol containing isoquinoline to afford a series of fluorinated aromatic polyimides. The polyimides were soluble in polar aprotic, ether and ketone solvents. They had intrinsic viscosities that ranged from 1.70 to 6.72 dL/g in N-methyl-2-pyrrolidinone or m-cresol at 30 °C. The polymers underwent 5% weight losses when subjected to thermal gravimetric analysis between 440 and 570 °C in air and nitrogen atmospheres. The polymers could be solution cast into water-white, flexible, tough films which exhibited “in-plane” structural anisotropy. Their glass transition temperatures along the directions parallel to the film surface (in-plane) ranged from 327 to 345 °C (thermal mechanical analysis). Multiple relaxation processes associated with segmental and subsegmental motions were also observed with dynamic mechanical analysis. The films had in-plane coefficients of thermal expansion (CTEs) that ranged from 1.58 × 10-5 to 2.50 × 10-5 °C-1. These films also showed linear optical anisotropy, which is characterized by the presence of larger, isotropic refractive index (n∥) in-plane, and a smaller refractive index (n⊥) perpendicular to the film surface (out-of-plane). The optical symmetry axis of the films is along the out-of-plane direction. This optical anisotropy is defined as uniaxial negative birefingence (NUB = n⊥ − n∥). Films having a thickness of 5 μm were transparent above 330 nm, and their in-plane refractive indices (n∥) were 0.060 to 0.074 larger than their out-of-plane refractive indices (n⊥). This UNB make the films candidates for use as retardation layers in liquid crystal displays.