Tailoring substrate adhesion via flexible chain architecture design in benzoheterocycle polyimide protective coatings

Benzoheterocycle polyimides (PIs) demonstrate exceptional metal substrate adhesion, positioning them as high-performance alternatives to conventional heterogeneous adhesives. These materials significantly enhance the weather resistance of flexible batteries and facilitate the development of thinner and lighter PI-based aluminum-plastic flexible packaging. Through molecular engineering of benzoheterocycle PI backbones, a series of ternary copolyimides (BIBOPIs) were synthesized utilizing four structurally distinct flexible diamine monomers. The thermodynamic properties, solvent resistance, water absorption, and coating adhesion of BIBOPIs were systematically evaluated. The distinctive molecular architectures of flexible diamines impart unique physicochemical properties to BIBOPIs. The light transmittance of BIBOPIs incorporating 4,4'-diaminodiphenylsulfone (DDS) exceeds 73%, with BIBOPI-0.5DDS achieving a transmittance of 85% at a wavelength of 800 nm. The light transmittance of BIBOPI-0.5RODA is reduced compared to that of BIBOPI-0.3RODA as the content of flexible ether groups in the molecular chain increases, consequently limiting its visible light transmittance. With the incorporation of a third monomer, the glass transition temperature (T _g) values of BIBOPIs demonstrate a consistent decrease, ranging from 332 to 410 °C. With the increase in 1,4-bis(4-aminophenoxy)benzene (RODA) ratio, the elongation at break of BIBOPIs exhibits a significant rise, from 6.8% for BIBOPI-0.1RODA to 33.8% for BIBOPI-0.5RODA. BIBOPIs demonstrate exceptional solvent resistance at both ambient and elevated temperatures. Additionally, the water absorption (W _A) of BIBOPIs decreases as the proportion of the third component increases. The adhesion grade of BIBOPI coatings is 0, with the exception of BIBOPI-0.1DDS, BIBOPI-0.1BPDA, and BIBOPI-0.1ODA. Pull-off experiments demonstrate that the incorporation of a flexible third monomer enhances the adhesion between BIBOPI coatings and substrates, with the adhesion strength surpassing 21.7 MPa. Unlike previous studies, the PI coating developed in this research enables the formation of a metal substrate protective layer with excellent adhesion, achieved through molecular design and structural optimization, without requiring complex surface treatment techniques. The optimized coating architecture enables molecular-level integration with dense PI protective layers, providing critical insights for developing advanced benzoheterocycle PI-based aluminum-plastic flexible packaging systems.