Synchronous biaxial stretching: An effective pathway to manipulate the structure-property relationship in polyimide films

Targeting the drawbacks of molecular chain slippage in thermal imidisation and solvent retention in chemical routes, this research utilises the monomers hexafluoroisopropyl phthalic anhydride (6FDA) and 2,2′-bis(trifluoromethyl)-4,4′-diaminobiphenyl (TFMB) to develop polyimides (PIs) with enhanced thermo-mechanical stability and reduced solvent affinity. The colourless and transparent PI films were fabricated via a ‘chemical imidisation–simultaneous biaxial stretching–thermal imidisation’ process. Initially, a polyamic acid (PAA) precursor solution was chemically imidised using an acetic anhydride/pyridine catalytic system. This step enabled the in-situ formation of rigid imide rings, yielding poly(amide-imide) copolymer films. Subsequently, the resulting film was subjected to simultaneous biaxial stretching for molecular orientation. Finally, high-temperature thermal imidisation ensures complete imidisation of the molecular chains, successfully producing PI films with excellent optical transparency. The enhanced performance is attributed to the synergistic effects among its molecular structure, the imidisation kinetics and the stretching-induced orientation. Through high-temperature, high-rate synchronous biaxial stretching, the molecular chains attained a pronounced in-plane bidirectional orientation, leading to the formation of a dense and ordered structure with reduced in-plane anisotropy. Furthermore, Pearson correlation analysis indicated a strong positive correlation between the imidisation degree and tensile strength ( r = 0.94), while the degree of order was also significantly correlated with the elastic modulus ( r = 0.89). Statistical analysis confirms the dominant role of structural parameters in determining the bulk properties. The synchronous biaxial stretching process further improves the macroscopic uniformity of the film, allowing it to maintain dimensional stability even at elevated temperatures. PI films stretched at 260 °C with a stretch ratio of 1.1:1.3 exhibited optimal overall performance, achieving a transmittance of 87.27% and a low yellowness index of 5.0. These films also displayed outstanding mechanical properties, including a tensile strength of 147.7 MPa and an elastic modulus of 3.661 GPa, along with excellent thermal stability.