Effect of End-Group Ionization on Melt Stability of Poly(Phenylene Sulfide)

Poly(phenylene sulfide) (PPS) produced by the sodium sulfide method contains ionized end groups such as sodium mercaptide (-SNa) and sodium 4-(methylamino)butanoate​ (-SMAB), which can be exchanged to proton or other cations by ion-exchange process. Depending on the type of cation and the extent of replacement, PPS performances can vary significantly. In this study, PPS samples with different extents of sodium ionization are prepared by adjusting the p H of PPS aqueous dispersion, then the melt stability is systematically investigated by capillary rheometer. It is found that neutral PPS possesses the best melt stability, while melt viscosity increases with the p H. Moreover, molecular weights before and after melt stability test are compared, neutral and alkaline PPS demonstrate similar minor decrease in molecular weight, while acidic PPS suffers severe degradation. Furtherly, electron paramagnetic resonance spectroscopy (EPR), rotational rheometer, and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) are applied to reveal the mechanism behind. Loss of ionized end groups weakens ionic interactions in alkaline PPS, which accounts for reduction of melt viscosity, while protonated end groups in acidic PPS accelerate the formation of free radicals, leading to severe degradation of PPS chains.