Leveling effects of ammonium salts on thermal stabilities of polyethylene glycols

Abstract

In this work, the thermal stabilities of a series of polyethylene glycols (PEG 4000, 6000 and 10000) were investigated after compositing with different kinds of inorganic salts, such as ammonium molybdate tetrahydrate (AMT), NH₄VO₃, (NH₄)₂SO₄, NH₄NO₃, Na₂SO₄, Na₂MoO₄. It was first observed that all the ammonium salts exerted leveling effects for the thermal stabilities of the PEGs. In other words, the presence of the ammonium salts caused the occurrence of the maximum decomposition rates of the PEGs with the same repeat sequence but different chain lengths at almost the same temperatures. Leveling effects were defined by three parameters: leveling spans, leveling degrees and dispersion degrees of leveling. Further experiments revealed that leveling effects also occur in similar types of polymers: polypropylene glycols (PPG 2000, 3000 and 4000). A series of independent experiments including Fourier transformation infrared spectroscopy, Raman spectroscopy, differential scanning calorimetry, time-of-flight mass spectrometry, conductivity and field-emission scanning electron microscopy were performed to explore the origin of leveling effects. We consider that the interaction between inorganic ions and polymer molecules and the Hofmeister effect of ions in solution are two important factors affecting the stability of salt–polymer composites, because they can contribute to decrease the interaction between the polymer chains, leading to changes in the conformation and pyrolysis mode of polymers. We believe that the finding of leveling effects would be significant for both basic and applied research of soft matter.