Mass spectrometric analysis of synthetic polymers using ultrasonic degradation

Abstract

Ultrasonic degradations of polyethylene oxide (PEO), polymethyl methacrylate (PMMA) and polyethylene oxide-block-propylene oxide (PPO) copolymers in aqueous media were studied using matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) and liquidchromatography atmospheric pressure chemical ionization mass spectrometry (LC-APCI-MS). The ultrasonic degradation of the polymers with a horn-type 28 kHz oscillator was monitored as a function of the ultrasonication duration to examine the structural details of ultrasonic degradation polymers. The ultrasonication of a PEO solution produced five types of oligomers (Mn= ca. 1000 Da) with different end groups, irrespective of the initial average molecular masses (Mn) 2, 6, 20, and 2000 kDa. Several degradation pathways with free radical reactions were suggested to explain these degradation products. On the other hand, ultrasonication of PMMA (Mn 1630 Da) and uniform PMMA (n =29) generated only one type of degradation oligomer. In addition to chemical reactions with OH· and H· radicals, the results indicated that primary degradation takes place near at the center of the polymer chains by mechanical forces generated around collapsing cavitation bubbles. Ultrasonic degradation of PEO-block -PPO copolymers consisting of a hydrophilic and a hydrophobic portion was studied to determine the location of bonds involved in the initial scission of the copolymers. A detailed structural analysis of the degradation products indicated that the initial bond scissions occurred principally at the boundary regions between the backbones of PEO and PPO chains. Further structural analysis revealed the presence of oxygen adducts in the degradation products. By a comparison with a thermal degradation carried out in a helium atmosphere, one can conclude that the oxygen adducts are formed by radical reactions with water or dissolving oxygen molecules. The study demonstrated that chemical reactions as well as physical bond stress scissions are involved in the ultrasonic degradation of the copolymers. © 2011 The Japan Society for Analytical Chemistry.