Molecular Layer Deposition of Thermally Stable Polybenzimidazole‐Like Thin Films and Nanostructures

Abstract

aliphatic or aromatic derivatives as monomers for optimizing application specific properties for bioactive materials and fuel cell membranes, for example. [17,18] In recent years, with the developments of using insulating polymer materials in electronic devices, needs for high-thermalresistance polymer dielectrics have been increasingly emphasized in order to ensure high reliability of devices operating at high temperatures. [19] While conventional polyethylene and polypropylene possess intrinsically excellent insulating properties, [20] they are facing great challenges in advanced electrical devices due to their limited service temperatures below 105 °C and increased leakage currents at elevated temperatures. [2,20,21] Another prevalent area for PBI lies in membrane applications such as H 2 separation membranes, fuel cells, and redox battery membranes. High intrinsic H 2 selectivity over larger gas molecules, such as CO 2 , N 2 , and CH 4 , was demonstrated with PBI film membranes prepared using flat and hollow polyamide P84 fibers as a support material. [8] PBI has gained great interest also as a membrane material in vanadium redox flow batteries where ion-selectivity for vanadium was obtained with a thin layer of PBI polymer. [22] Polybenzimidazoles and their more complex derivatives are often classified as expensive specialty plastics due to their outstanding properties but also difficulties in manufacturing. PBI plastics are known as the thermally most stable thermoplastics with a glass transition temperature (T g ) higher than 400 °C. [11] In a previous study by Iqbal et al. [19] PBI exhibited excellent thermal stability up to a temperature of 500 °C with a total weight loss of only 15%. The stability of PBI polymers drives from the high number of aromatic rings incorporated in the polymer chain. Aromatic rings are efficiently absorbing both thermal and radiation energy and aid in redistributing the excitation energy throughout the material. [18,19] Conventional PBI polymers are usually prepared via hightemperature polycondensation reactions in either solution or melt to achieve high degree of cyclization. [23][24][25] PBIs are typically synthesized from aromatic tetraamines (bis-o-diamines) and dicarboxylates (acids, esters, or amides) [26] (Scheme 2).PBI thin films are scarcely reported. Previous depositions were carried out either by a casting solution method from formic acid and dimethylsulfoxide (DMSO) solutions or dip-coating and blade-casting methods. [23] The deposition of polybenzimidazole (PBI)-like thin films by molecular layer deposition is reported here for the first time using isophthalic acid (IPA) and 3,3′-diaminobenzidine (DAB) as monomers and trimethylaluminum (TMA) as a linker precursor. Two precursor pulsing sequences are tested, the ABCB (TMA + IPA + DAB + IPA) and ABC (TMA + IPA + DAB) type MLD processes result in different types of PBI-like films. With the ABCB sequence thin film growth per cycle (GPC) of 6.0 Å is obtained at 225-280 °C, whereas GPC of 7.0 Å is obtained with the ABC s...