Abstract
Low-melting methylammonium phosphate glasses are synthesized from crystalline starting agents. To this end crystalline tris(methylammonium) cyclotriphosphate [CH3NH3]3P3O9, was synthesized by a novel and simple synthesis route from P4O10 and N-methylformamide. It, undergoes an irreversible phase transition to methylammonium catena-polyphosphate [CH3NH3]PO3. The crystal structure of the catena-polyphosphate was solved and refined from X-ray powder diffraction data by the Rietveld method using constraints obtained by solid-state 31P and 1H NMR spectroscopy. This compound crystallizes in a triclinic space group with a = 13.2236(9), b = 7.8924(6), c = 4.6553(2) Å, α = 91.068(4), β = 87.840(5) and γ = 106.550(3)°. Quantum chemical calculations confirm that the obtained structure lies at an energetic minimum. Finally the reaction of tris(methylammonium) cyclotriphosphate and P4O10 into methylammonium phosphate glass is presented. The synthesized, water-free phosphate glass shows a very low glass transition temperature Tg of 33 °C, which was verified by dynamic scanning calorimetry and NMR. The chain-like crystal structure of the high-temperature methylammoniumphosphate [CH3NH3]PO3 serves as an approximation for the short-range order of the glass.
Highlights
For synthesis of crystalline trismethylammonium cyclotriphosphate 7 mL N-methylformamide (Alfa Aesar, 99%) was added drop-wise under ice cooling to 1.0 mmol (284 mg) P4O10 (Sigma Aldrich, 99%)
The reaction of N-methylformamide and P4O10 yielded a pale yellow powder which could be indexed within a monoclinic unit cell P21/n
Solution NMR spectra of N-methylformamide and P4O10 a er the reaction show additional signals compared to the spectra for pure N-methylformamide
Summary
Phosphate glasses nd wide application in industry and medicine, for example as implant coatings, for tissue engineering,[1,2,3,4] as optical materials[5,6] and ionic-conducting materials.[7,8] An application of glasses with low glass transition temperatures are glass seals.[9,10,11] Glasses with extremely low glass transition temperatures would open a much wider range of applications, for example enabling organic compounds as glass additives. Synthesis of crystalline methylammonium phosphates which are required as starting agents cannot proceed via the routine high-temperature pathway, because methyl ammonium ions decompose under these conditions Despite this complication ammonium phosphates including mono-, di-, tri- or tetramethylammoniumphosphate nd widespread application: ammonium polyphosphates are used as ame-retardant additives for organic polymers and for intumescent coatings in industry.[14,15] In polyphosphate fertilizers usually between 50 and 75% of the phosphorus content is present in chained polymers. Methylammonium hydrogenphosphate (254.2 C) and methylammonium formate (162.1 C) have low decomposition temperatures.[28] for their synthesis in general low synthesis temperature are required, for example making use of solvents like dimethyl sulfoxide[23,29] or water In this contribution the smaller but asymmetric methylammonium ion [CH3NH3]+ is explored as an alternative to the tetramethylammonium ion to produce low melting phosphate glasses. In this context the question, if N-methylformamide may act as source of the methylammonium ion in the synthesis, is tested.[30]
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