Colorless Polyamide–Imide Films with Enhanced Thermal and Dimensional Stability and Their Application in Flexible OLED Devices

Abstract

In this study, we intend to solve the problem of the low glass transition temperature (T g ) and the high coefficient of thermal expansion (CTE) that colorless polyimide (PI) films always exhibit. First, we copolymerized 2,2′-bis(trifluoromethyl)-[1,1′-biphenyl]-4,4′-diamine (TFDB) with 4,4′-(hexafluoroisopropene) diphthalic anhydride (6FDA) and p-phthaloyl chloride (TPC) and obtained a series of transparent and colorless films (PAI-10). These PAI films showed a high T g of 346−362 °C and a low CTE of 25.2−62.9 ppm/K, which were better than those of PI 6FDA-TFDB . Subsequently, three rigid diamines, p-phenylenediamine (p-PDA), 5-amino-2-(4-aminophenyl)benzoxazole (DAPBO) and 5-amino-2-(4-aminophenyl)benzimidazole (DAPBI), were used to replace 10% of the TFDB (PAI-9P-5, PAI-9O-5, and PAI-9I-5, respectively) to investigate the effect of the rigid structure on the thermal and optical properties. The results showed that rigid structures can greatly improve the thermal properties but impair transparency to some extent. Among these, PAI-9O-5 possessed the best performance with T 430 of 81.9%, T g of 364 °C, and CTE of 19.8 ppm/K. Furthermore, we fabricated an organic light-emitting diode (OLED) with PAI-9O-5 as the substrate. The flexible and bendable OLED@PAI-9O-5 showed typical green emission at 520 nm, a turn-on voltage of 2.9 V, a luminance of 1000 cd/m 2 at 5 V, a current efficiency (CE) of 2.53 cd/A, and an external quantum efficiency (EQE) of 0.81%, which were comparable to those of devices fabricated on conventional glass substrates.

Comprehensive Performance of Polyimide Reinforced by Multiple Hydrogen Bonds for Flexible Electronics Application

“…The aggregation state of polymers is mainly related to the orientation of molecular chains and intermolecular chain interactions . Inter-HBs in PIs can not only greatly increase intermolecular interaction, bringing molecular chains closer together and enhancing crystallization, but also effectively improve the mutual orientation of molecular chains through their directional effect, resulting in more ordered packing. ,, In order to detect the aggregated state in the PI films, WAXD analysis was carried out at room temperature. It can be seen in Figure a that the 2θ peak gradually changed from a wide diffuse peak to a sharp peak with increasing DAPyBA content.…”

“…The thermal expansion behavior of polymers is mainly related to the rigidity and linearity of molecular chains and the interactions between the chains. Therefore, increasing the rigidity and linearity of molecular chains, as well as enhancing intermolecular interactions, can effectively improve the thermal dimensional stability of PI films. ,,− Here, TMA was used to test the CTEs of the PI films over the temperature range of 50–300 °C to evaluate their dimensional stability, and the results are shown in Figure and Table . Compared with PI-0 with a CTE of 31.9 ppm K –1 , when only 10% of ODA was replaced by DAPyBA, the CTE decreased by 26% to 23.7 ppm K –1 .…”

“…Multiple hydrogen bonds (multi-HBs) are often introduced in supramolecular polymers to enhance their mechanical properties. Although studies on the use of hydrogen bonds to improve the mechanical properties of PIs and to reduce their CTEs have been reported, − to the best of our knowledge, the performance of PIs containing multi-HBs has not been investigated to date. Pyrimidinone is a rigid heterocyclic compound and is an essential component of three of the four bases in DNA, i.e., thymine, cytosine, and guanine, which self-assemble to give a stable double-helix structure through multi-HBs.…”

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

“…The arrangement of hydrogen bonds formed by random copolymerization is poor, which greatly reduces the role of hydrogen bonds, so the overall performance of the film is poor, such as larger CTE, lower T g , and greater yellowness and haze. Block copolymerization reduces the imide ring density of PI and weakens the intramolecular CTC, which is not conducive to further improving the thermal performance of polyamide-imide. − In situ polymerization is a method of producing composite materials by adding reactive monomers to a dispersed phase and polymerizing them in the dispersed phase to produce well-dispersed nanoparticles. , Compared to the addition of preprepared nanoparticles, adding molecular-level precursors into the membranes inhibited agglomeration and leakage of nanoparticles through better dispersion. It has been considered by many research groups to develop new promising high-performance material systems by combining the advantages of polyimide and aromatic polyamide.…”

Colorless and transparent poly(amide imide) nanocomposites containing organically modified hectorite

“…Because of these advantages, PAI is used in various applications, including wire enamel, adhesives, membrane, injection, extrusion products, and other electronic devices. 18–20…”