Tuning the persistence lengths of main chain towards colorless and transparent polyimide with low dielectric loss and excellent general properties

Abstract

Colorless transparent polyimide (CPI) film simultaneously combining with low dielectric loss at high frequency, excellent thermal, mechanical properties and folding resistance is urgently required in nowadays flexible electronics industry. Inspired by liquid-crystal elastomers containing both soft and hard segments, one rigid and flexible single block copolymerization strategy was designed by introducing the hard segments of liquid-crystal-like PI into the flexible perfluorinated CPI, where it was found that tuning the persistence lengths of hard segments of liquid-crystal-like PI can significantly improve the general properties of CPI films. With the persistence lengths of main chain of CPI films increased from 2.17 ± 0.07 Å to 16.56 ± 0.12 Å, the transmittance of CPI films at 550 nm remained above 85%, while the dielectric loss at 10 GHz of CPI films reduced from 0.00479 to 0.00407 with decreasement of 15.1% due to the increased chain rigidity limits friction work. Besides, CPI film with the persistence length of 16.56 ± 0.12 Å also exhibited well mechanical properties like thermal expansion coefficients of 24.8 ppm/K, tensile strengths of 162.5 MPa and Young's moduli of 4.88 GPa, which were 30.1%, 25.8% and 49.7% better than that of the CPI film with the persistence length of 2.17 ± 0.07 Å. Furthermore, this type of CPI films with tunable elasticity also had better folding resistance properties. Therefore, tuning the persistence lengths of main chain of CPI films can meet the application of high frequency/speed communication, flexible display and other fields at the same time.