Design and preparation of thermoplastic polyimides with high transmittance based on 4,4′-(4,4′-isopropyldiphenoxy)bis(phthalic anhydride) (BPADA)

[Display omitted] •CPI resins were prepared by copolycondensation of BPADA and TFMB with 6FDA or CBDA.•Chemical imidization was used to reduce the influence of the high-temperature process on optical transmittance.•CPI resins showed low melt viscosity due to the flexible structures and the introduction of the third monomer.•CPI resins showed higher transmittance and lighter color in contrast to commercial thermoplastic PI resins. In order to prepare polyimide resins with excellent melt processing properties and high transmittance, a series of thermoplastic fluorinated polyimide resins was prepared by copolycondensation of the flexible dianhydride monomer 4,4′-(4,4′-isopropyldiphenoxy)bis(phthalic anhydride) (BPADA) and the fluorinated diamine monomer 2,2′-bis(trifluoromethyl)benzidine (TFMB) with different contents of fluorinated dianhydride 4,4′-(hexafluoroisopropyl)diphthalic anhydride (6FDA) or cyclobutanetetracarboxylic dianhydride (CBDA) via chemical imidization. The effects of the third monomer content on the melt processing properties, optical properties, heat resistance, and mechanical properties of the polyimide resins were systematically investigated. The minimum melt viscosity of the PI resins reached 630 Pa·s below 400 °C. The transmittance at 450 nm of PI sheets prepared by compression molding reached 77%, and the color was significantly lighter than the commercial thermoplastic resin ULTEM, indicative of excellent processability and optical properties. Besides, the PI resins displayed outstanding thermal performance with glass transition temperatures ranging from 224 °C to 242 °C, 5% weight loss temperatures higher than 520 °C in a nitrogen atmosphere, and coefficients of thermal expansion of 51.45–68.97 ppm/K. Moreover, the PI sheets possessed tensile strengths of 88.4–104.7 MPa.