Fabrication of Colorless and Transparent Semi‐Alicyclic and Fluoro‐Containing Polyimide Films From Environmentally Friend Solvent of γ‐Valerolactone

ABSTRACT The biomass‐derived green solvent of γ‐valerolactone (GVL) was endeavored to be used to fabricate the colorless and transparent polyimide (CPI) films derived from the alicyclic dianhydrides and fluoro‐containing diamine. For this purpose, two stereoisomeric alicyclic dianhydrides of 1S,2R,4S,5R‐hydrogenated pyromellitic dianhydride (ccHPMDA) and 1R,2S,4S,5R‐hydrogenated pyromellitic dianhydride (ctHPMDA) were polymerized with the fluoro‐containing diamine of 4,4′‐bis(trifluoromethyl)benzidine (TFMB) to afford the CPI‐1 (cc‐TFMB) and CPI‐2 (ct‐TFMB) resins, respectively via a one‐step high‐temperature solution polycondensation procedure using GVL as the main solvent. The same polymers CPI‐ref1‐NMP and CPI‐ref2‐NMP were prepared according to a similar procedure using N‐methyl‐2‐pyrrolidone (NMP) as the reaction medium. The PI resins with high molecular mass were obtained, which were soluble in GVL at a solid content of 15 wt% at room temperature. CPI films were prepared from the corresponding PI/GVL solution systems. The CPI films exhibited excellent optical transparency, which was a bit superior to that of the referenced films derived from PI/NMP systems. The ultraviolet cutoff wavelengths (λ cut ) of the CPI films were lower than 300 nm and the optical transmittance at the wavelength of 450 nm (T 450 ) was as high as 86.4%. The CIE LAB color parameters of the CPI films were also better than those of the referenced samples. The CPI films maintained good thermal resistance with the glass transition temperatures (T g ) higher than 360°C by differential scanning calorimetry (DSC) tests. The 5% weight loss temperatures (T 5% ) were higher than 450°C in nitrogen and the linear coefficients of thermal expansion (CTE) were 63.1 × 10 −6 /K for CPI‐1 and 57.9 × 10 −6 /K for CPI‐2 film, respectively from 50°C to 200°C.