Terminal Group Structural Characterization of Hydrolyzed, Thermally Oxidized and Alcoholyzed Polyamide 6 and Polyamide 66 by MALDI-TOF-MS

We attempted to determine the cause of the degradation of polyamide (PA), using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS), which enables a characterization of the terminal group structure of PA. First, optimization of the matrix for PA ionization was performed using the original and hydrolyzed PA6; 2’-(4-Hydroxyphenylazo)benzoic acid (HABA) and 4-Hydroxybenzylidenemalononitrile (HBMN) were found to be optimum matrices. We then studied the terminal group structure of the original PA6 and PA66, and their hydrolyzed, and thermally oxidized models, using MALDI-TOF-MS with HABA as a matrix. Although only cyclic oligomers were observed on the mass spectrum of the original PA, cyclic oligomers, as well as linear oligomers containing amino and carboxyl terminal groups at one or both ends, were observed on the mass spectrum of hydrolyzed PA. Also, cyclic oligomers, as well as linear oligomers having aldehyde, carboxyl, amide, N-formamide, and alkyl terminal groups at one or both ends, were observed on the mass spectrum of thermally oxidized PA. In addition, MALDI-TOF-MS with HBMN as a matrix was utilized in the terminal group structural characterization of PA66 heated at 120 °C for 1000 hours in a 50 wt% ethylene glycol aqueous solution; it was clear that the PA66 was decomposed by alcoholysis as well as hydrolysis of the amide linkage. Therefore, MALDI-TOF-MS was confirmed to be a powerful tool for a detailed analysis of the terminal group structure of PA, and for an easy determination of the causes of degradation, such as hydrolysis, thermal oxidation and alcoholysis.