Monomer Determination of Polyamic Acids by Hydrolysis and Methanolysis, and Gas Chromatography–Mass Spectrometry

Abstract

Polyimides (PIs) have attracted attention because of their excellent thermal stability, high mechanical properties, and good chemical resistance. 1 Hence, they have been widely used in many applications such as aerospace, microelectronics, and optoelectronics. [2][3][4][5] As most PIs are insoluble except soluble PIs, they are often formed by two-step processes involving the synthesis of polyamic acid (PAA) as a precursor of PI, followed by thermal dehydration of the amide-acid to form PI. In addition, PAA are formed by the condensation reaction of a diamine and a dianhydride in a polar aprotic solvent. 4,6 Many articles have been reported for the thermal behavior of PI film, 4,7 the surface modification of PI, 3,8 the fabrication analysis, 9,10 and the degree of imidization, 11,12 respectively. In general, PIs provide wide variations in technical properties according to their chemical structures. Therefore, it is important to achieve an adequate analytical method for the compositional analysis in manufacturing and benchmarking of PAAs.The analytical method has been studied and reported for the pyrolytic methylation of organic acids, 13,14 the hydrazinolysis for the cleavage of PI and the derivatization of monomers, 15,16 and the hydrolysis of PIs. 17 In this study, we describe a simple method for the determination of the monomer composition of PAAs. A gas chromatography/mass spectrometry (GC/MS) method using hydrolysis and methanolysis as the sample pre-treatment procedure was used to analyze diamine and dianhydride components of PAAs. In addition, results obtained from pyrolysis-GC/MC with tetramethylammonium salt as a derivatization reagent (React Py-GC/MS) were compared with the data from GC/MS method.To analyze the chemical composition of PAAs by GC/MS, hydrolysis and methanolysis were first performed for the cleavage of polymer chain and derivatization of the monomer. The overlaid total ion chromatogram (TIC) derived from GC/MS is shown in Figure 1, and the chemical structures identified are given in Table 1. A PAA can decompose into small molecules by treatment with sodium hydroxide for hydrolysis and acid-catalyzed methanol for methanolysis. Consequently, diamines and tetracarboxylic acid sodium salts were produced by hydrolysis, and ammonium sulfates of corresponding diamines and methyl esters of tetracarboxylic acid were produced by methanolysis. p-Phenylenediamine (PDA) and 4,4-oxydianiline (ODA) were detected as the diamine compounds from the hydrolysates of PAA-1 and PAA-2, and PDA was only detected in the hydrolysates of PAA-3 and PAA-4. After hydrolysis, all diamines detected were in the free form as monomers of PAA preparation. Hence, the analysis of hydrolysates was useful for the determination of the diamine composition of PAAs. In addition, any interfering components were absent in the GC/MS analysis. The tetracarboxylic acid compounds hydrolyzed from the polymer chain were converted to the corresponding carboxylic acid sodium salts in the basic hydrolysis solution. Thus, they were not extracte...