Organo-soluble polyimides:: Synthesis and polymerization of 2,2′-bis(trifluoromethyl)-4,4′,5,5′-biphenyltetracarboxylic dianhydride

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 degrees C. The polymers underwent 5% weight losses when subjected to thermal gravimetric analysis between 440 and 570 degrees 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 (inplane) ranged from 327 to 345 degrees 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 x 10(-5) to 2.50 x 10(-5) degrees C-1. These films also showed linear optical anisotropy, which is characterized by the presence of larger, isotropic refractive index (n(parallel to)) in-plane, and a smaller refractive index (n(perpendicular to)) 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(perpendicular to) - n(parallel to)). Films having a thickness of 5 mu m were transparent above 330 nm, and their in-plane refractive indices (n(parallel to)) were 0.060 to 0.074 larger than their out-of-plane refractive indices (n(perpendicular to)). This UNB make the films candidates for use as retardation layers in liquid crystal displays.