Low coefficient of thermal expansion (CTE) in the range of 40–400 °C is a crucial factor for polyimide (PI) films serving as the flexible substrate for display devices. The introduction of hydrogen bonds (H-bonds) is an effective method to obtain high Tg and low in-plane CTE below polyimide (PI) film. However, these H-bonds commonly exhibit low thermal stability and would be ruptured at high temperature (>300 °C), which leads to a remarkable increase of CTE. Herein, we report a strategy to improve thermal stability of H-bonds at the high temperature via intermolecular crosslinking. For neat PI, its in-situ FTIR results confirm severe dissociation of H-bonds above 300 °C, While after the crosslinking reaction, the content of strong H-bonds at 400 °C is 110% higher than that of neat PI, which indicates this intermolecular crosslinking restrains dissociation of H-bonds. Correspondingly, CTE values of crosslinked PI films at 300^oC–400 °C are sharply decreased from 33.8 ppm/K to 6.0 ppm/K. Moreover, decrease of in-plane orientation accompanying lower CTEs in crosslinked PI deviates from common rule reported in linear PIs, implying the important roles of H-bonds and crosslinking. More over, the contribution of H-bonds and crosslink to the decrease of CTE are performed by Pearson correlation analysis and the results indicate that the decrease of CTE in crosslinked PIs at low temperatures is completely attributed to confinement effect of crosslinking, but contribution of stable H-bonds becomes more prominent above 300 °C. Consequently, the obtained PI films exhibit ultra low CTE (1.9 ppm/K) at the range of 40^oC–400 °C, along with high Tg (>450 °C) exhibiting great potential for flexible-display substrates.

在40-400°C的范围内，低热膨胀系数（CTE）是用作显示设备柔性基板的聚酰亚胺（PI）薄膜的关键因素。氢键（H键）的引入是一种在聚酰亚胺（PI）膜以下获得高Tg和低面内CTE的有效方法。但是，这些氢键通常表现出较低的热稳定性，并且会在高温（> 300°C）时破裂，从而导致CTE显着提高。本文中，我们报告了一种通过分子间交联提高高温下H键热稳定性的策略。对于纯净PI，其原位FTIR结果证实了300°C以上H键的严重离解，而在交联反应后，400°C时强H键的含量比纯PI高110％，这表明该分子间交联抑制了H键的离解。相应地，交联的PI薄膜的CTE值为300^o C–400°C从33.8 ppm / K急剧降低到6.0 ppm / K。此外，伴随着交联PI中较低CTE的面内取向的降低偏离了线性PI中报道的通用规则，这暗示了H键和交联的重要作用。此外，通过Pearson相关分析进行了H键和交联对CTE降低的贡献，结果表明，低温交联的PI中CTE的降低完全归因于交联的约束作用，但稳定的贡献在300°C以上，氢键变得更加突出。因此，所获得的PI膜在40 ^o C–400°C的范围内显示出超低的CTE（1.9 ppm / K），而高Tg（> 450°C）则在柔性显示基板上显示出巨大的潜力。