Construction of stable hydrogen bonds at high temperature for preparation of polyimide films with ultralow coefficient of thermal expansion and high Tg

• Published in: Polymer (Guilford) 2020-02, Vol.188, p.122100, Article 122100
• Main Authors: Tian, Yiyao, Luo, Longbo, Yang, Qiqi, Zhang, Lingjie, Wang, Ming, Wu, Difeng, Wang, Xu, Liu, Xiangyang
• Format: Article
• Language: English
• Subjects: Bonding strength; Chemical crosslinking; Correlation analysis; Crosslinking; Display devices; High temperature; Hydrogen bond; Hydrogen bonding; Hydrogen bonds; Low temperature; Polyimide films; Substrates; Thermal expansion; Thermal stability; Ultra low CTE
• ISSN: 0032-3861; 1873-2291
• DOI: 10.1016/j.polymer.2019.122100

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 300oC–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 40oC–400 °C, along with high Tg (>450 °C) exhibiting great potential for flexible-display substrates. Schematic diagram of effect of chemical crosslinking and hydrogen bonding interactions on dimensional stability is presented here. At low temperature (40–300 °C), hydrogen bonding dissociation is not so dramatic and group vibration is inhibited to some degree. Thus, CTE of PI-0 is very low (2.9 ppm/K) in this temperature range. At high temperature, dissociation of most of hydrogen bonds weakens interchain interaction and consequently intensifies group vibration and dimensional expansion, leading to high CTEs (33.8 ppm/K). Introducing a small portion of crosslinking sites, confinement of covalent bond suppresses group vibration and consequently inhibits extension of distance between hydrogen bonding donors and accepters at high temperature, leading to the enhance