Preparation of Polyimides by Using Mixtures of Tetrahydrofuran and Methanol and Their Properties

Abstract

ABSTRACT:Reaction conditions of the preparation of solutions of polyamic acids in the tetrahydrofuran/methanol mixed solvent (THF/MeOH) were studied. The polyamic acids were based on pyromellitic dianhydride and 4, 4 -oxydianiline. Transparent solutions were prepared at 5 wt% and lower concentrations of polyamic acid in the THF/MeOH mixture with component weight ratio ranging from 4:1 to 1:1. The properties of the final polyimide films were compared with those prepared by using N-methyl-2-pyrrolidone as the solvent.KEY WORDS Methanol / Polyamic Acid / Polyimide / Tetrahydrofuran / Aromatic polyimides (PI) exhibit very good mechanical, dielectric and chemical resistance also at elevated temperatures. They are often employed in (micro)electronics, aviation industry and membrane technologies. 1 Polyimides are traditionally synthesized in two steps ( Figure 1). The final product is often prepared by thermal solid-state imidization of a polyimide precursor (polyamic acid (PAA)). 1 The first step -preparation of PAA -comprises the reaction of an aromatic diamine with an aromatic dianhydride in aprotic polar solvents (e.g., N-methyl-2-pyrrolidone (NMP), N, Ndimethylacetamide, N, N-dimethylformamide). These amidic high-boiling solvents however associate with the carboxyl groups of PAA and removing them from polyimide films can be troublesome. 1 Moreover, polyamic acid solutions in these hygroscopic solvents are unstable and their application using the spin-on technique can be thus complicated. 2 From this point of view the preparation of PAA in the tetrahydrofuran/methanol mixed solvent (THF/MeOH) appears to be promising. [3][4][5][6][7][8] However, in our preliminary tests we were unable to reproduce the preparation of 15 wt% solutions of PAA based on pyromellitic dianhydride (PMDA) and 4, 4 -oxydianiline (ODA) in THF/MeOH with component ratio 4:1 (w/w) as described by Echigo et al. 3,4 The reaction conditions of the preparation of PAA solutions (based on PMDA and ODA) in THF/MeOH mixtures and the transformation of the resulting PAA into PI are studied in this work. The properties of final films are compared with those prepared by using NMP as the solvent. The behaviour of the mixed solvent in dependence on its composition has been monitored. EXPERIMENTALPyromellitic dianhydride was dried overnight at 180 • C in vacuum before using. 4, 4 -Oxydianiline (both Aldrich) was used as received. Tetrahydrofuran (Lachema, Czech Republic) was distilled over sodium. Methanol (Penta, Czech Republic) was used without purification. N-Methyl-2-pyrrolidone (Merck) was distilled under vacuum over phosphorus pentoxide and stored under an inert gas.Polyamic acids were prepared in a three-necked flask equipped with a mechanical stirrer, a nitrogen inlet, and a condenser with a drying tube. The diamine (ODA) was dissolved in the solvent (THF/MeOH or NMP). An equimolar amount of the dianhydride (PMDA) was then added and the reaction was allowed to proceed at room temperature for 24 h.A polyamic acid solution in THF/MeOH or NMP was cast onto...

Replacement of Hazardous Chemicals Used in Engineering Plastics with Safe and Renewable Hydrogen-Bond Donor and Acceptor Solvent-Pair Mixtures

“…All presently known solvents for preparing homogeneous PAA solutions are hydrogen bond acceptors (filled circles, Figure ). Solvent-pairs such as water–tetrahydrofuran (THF), water–acetone, methanol (MeOH)–THF, and ethanol (EtOH)–THF have been proposed in the literature − and as shown in Figure , these allow coverage of the solubility parameter window of PMDA and ODA monomers and the PAA polymer. However, THF-containing solvent-pairs shown in Figure may have safety issues when used on a practical scale due to possible formation of peroxides associated with handling THF .…”