Breaking the mutual restraint between low permittivity and low thermal expansion in polyimide films via a branched crosslink structure

Currently, it remains challengeable to establish a balance between low permittivity and low coefficient of thermal expansion (CTE) without incorporating nanoparticles. Polyimide (PI) films of relatively low permittivity, such as fluorinated PI or PI with bulky pendent groups attached to the backbone, may demonstrate undesirable CTE and mechanical properties. Herein, a series of PI films were prepared by employing 3,3',4,4'-biphenyltetracarboxylic dianhidride (BPDA) and 1,4-phenylenediamine (PDA) as constitutional units and 1,3,5-tris(4-aminophenoxy)benzene (TAPOB) as the crosslinker. The well-designed micro-branched structure led to enlarged free volume in PI films, which enabled accordingly a decrease of ∼8.8% in dielectric constant. Meanwhile, their CTE could be reduced to about 2.88 ppm/K in the absence of draft or nano-hybrid. Moreover, excellent tensile strength and modules were determined to be 362.2 MPa and 8.11 GPa, respectively, for these crosslink PI films.