Characterization of High Polymerization Degree Ammonium Polyphosphate and Its Chain Structure by ³¹P NMR

Ammonium polyphosphate(APP) of high polymerization degree is an kind of important inorganic polymer flame retardant. Characterization of APP with high polymerization degree and its solution property are a meaningful topic. In this paper, quantitative liquid ³¹P nuclear magnetic resonance(NMR) is used to characterize APP polymerization degree and chain structures. A method of preparing aqueous APP solution by adding NaCl in water to auxiliary dissolving was established without resulting in APP degradation. The effects of different dissolution and testing temperature, and different NMR parameters were studied on the characterization of APP polymerization degree. Finally, The method to accurately characterize the APP polymerization degree was suggested based on liquid ³¹P NMR, that is: dissolving APP could be in water at 36℃ with auxi-liary NaCl salt, setting the temperature as 26℃ in ³¹P NMR testing. In this case, the good solubility of APP could be achieved, at same time no degradation of APP happened, the measurement of APP polymerization degree can be stable and accurate. The relaxation time(T₁) of terminal phosphorus(P_end) and middle phosphorus(P_mid) at different temperatures were determined by liquid ³¹P NMR, and the relationship between T₁ and temperature was found to be linear. The relaxation time parameter(D₁) in ³¹P NMR has a great effect on the characterization of APP polymerization degree, and the polymerization degree of APP will be inaccurate when the setting of D₁ could not meet the T₁ required for phosphorus. Characterization of APP chain structures by ³¹P NMR showed that there are three kinds of chemical environments in the terminal phosphorus of APP, that is the P connecting with two NH₄ ⁺, one NH₄ ⁺ and one H⁺, and two H⁺, respectively. The NaCl concentration and testing temperature obviously influence the chemical shift of terminal phosphorus in the three chemical environments. It was verified that there was the middle phosphorus connecting with H⁺ in APP as well.