Ultralow Coefficient of Thermal Expansion and a High Colorless Transparent Polyimide Film Realized Through a Reinforced Hydrogen-Bond Network by In Situ Polymerization of Aromatic Polyamide in Colorless Polyimide

Abstract

Colorless polyimides (CPIs) are a key substrate material for flexible organic light-emitting diode (OLED) displays and have attracted worldwide attention. Here, in this paper, the dispersion and interfacial interaction of aromatic polyamide (PA) in CPI (synthesized from 4,4′-(hexafluoroisopropylidene) diphthalic anhydride (6FDA) and 2,2′-bis(trifluoromethyl)benzidine (TFMB)) were significantly improved by in situ polymerization, and colorless transparent macromolecular polyimide composites (CPI-PA x ) were successfully prepared by PA and CPI. By adjusting the ratio of PA to CPI, a high-performance engineering plastic with excellent film-forming properties was obtained. Molecular simulations confirmed the uniform distribution of PA in CPI and its interaction in polymers. In CPI-PA x , the CPI was locked by the PA chain, and numerous molecular chains were mutually entangled to form a hydrogen-bond network structure. Due to the strong interaction between the chains imparted by the hydrogen bonds of the PA, they do not slide under external forces and heating. In addition, the additive PA has excellent dimensional stability, thermal, and mechanical properties, and CPI has outstanding optical properties, so the synthesized CPI-PA x combines the comprehensive properties of PA and CPI. The CPI-PA x has excellent thermal and mechanical properties, with a thermal decomposition temperature of 499 °C, a glass transition temperature of 385 °C, a coefficient of thermal expansion of 0.8 ppm K −1 , a tensile strength of 50.9 MPa, and an elastic modulus of 3.9 GPa. Particularly, CPI-PA x has a 90% transmittance in the visible region. These data prove that the strategy of combining PA and CPI by in situ polymerization is an effective method to circumvent the bottleneck of CPI in the current flexible window application, and this design strategy is universal.

Synthesis and comparative study of high Tg and colorless polyamide‐imides with different amide‐to‐imide ratio

“…Since 2 θ is inversely proportional to the interchain spacing distances ( d ), high amide content decreased the d ‐spacing (Table 5). PI‐1 without amide linkage had the largest d ‐spacing of 6.04 Å, and among PI‐(2‐3) with the increase of amide groups, the hydrogen bond formation increased and resulted in more tightly stacked molecular chains which lowered the d ‐spacing 11,12,32 . With the same content of imides and amides, d values of PI‐(2‐3) were lower than PI‐(4‐5) .…”

“…The PI‐(2‐5) films exhibited tensile modulus ( T M ) of 4.9–5.4 GPa, tensile strength ( T S ) of 107–128 MPa, and ( E B ) elongation at break of 2.7%–8.8% (Table 5). The introduction of rigid amide groups and an increase of hydrogen bonds derived from the amide group could result into a high tensile modulus, and the higher the number of the amide groups, the higher the tensile modulus of the PAI 11–13,37 . Therefore, PI‐1 without rigid amide linkage in the polymer chain displayed a low T M of 3.8 GPa and T S of 103 MPa.…”

“…The introduction of rigid amide groups and an increase of hydrogen bonds derived from the amide group could result into a high tensile modulus, and the higher the number of the amide groups, the higher the tensile modulus of the PAI. [11][12][13]37 Therefore, PI-1 without rigid amide linkage in the polymer chain displayed a low T M of 3.8 GPa and T S of 103 MPa. It is also noted that PI-(2-3) synthesized from amide diamine AB-TFDB had superior mechanical properties than PI-(4-5) from TPC F I G U R E 4 TGA curves of PAIs in nitrogen and air atmosphere.…”

“…[5][6][7][8][9][10] The first two methods involve multistep synthesis of monomers, while the third method is a facile synthesis of PAIs by copolymerization of diacyl chlorides such as terephthaloyl chloride (TPC) with different dianhydrides, and has been recently studied to tailor the excellent properties of PAIs. [11][12][13] For achieving the present target properties, some approaches such as the incorporation of electronwithdrawing bulky trifluoromethyl group (CF 3 ) and alicyclic unit into CPI chains to afford polyimides with high transparency and thermal stability are reported. [14][15][16][17] However, most of the CPIs incorporating alicyclic units do not have high T g or low-CTE characteristics because of their nonlinear or nonplanar structures which distorted PI chain alignment.…”

“…Polyamide‐imides are usually prepared by three methods; (1) polycondensation of dianhydrides or diamines monomers containing amide group; (2) diacids containing imide group with diamines; (3) copolymerization of dianhydrides and diacyl chlorides with diamines for simultaneous formation of imide and amide groups in polymer chains 5–10 . The first two methods involve multi‐step synthesis of monomers, while the third method is a facile synthesis of PAIs by copolymerization of diacyl chlorides such as terephthaloyl chloride (TPC) with different dianhydrides, and has been recently studied to tailor the excellent properties of PAIs 11–13 …”