Influence of cyclodehydration on formation and properties of poly(p-phenylene-1,3,4-oxadiazole) fibre

A series of poly(p-phenylene-1,3,4-oxadiazoles) (PODs) were synthesised from terephthalic acid (TPA) and hydrazine sulphate in fuming sulphuric acid via a recently developed procedure, namely, one step with three stages. The influence of the reaction temperature and time, and molar ratio of SO3/TPA on the cyclisation degree of POD was investigated. The structure of PODs was characterised by a Fourier transform infrared spectrophotometer, and the cyclisation degree was calculated based on the results of elemental analysis. The stability and fluidity of POD solutions with different cyclisation degrees were determined by viscometry. The filaments of PODS were formed through wet spinning, in which the POD solutions were directly extruded into a coagulating bath containing 40% sulphuric acid solution. The thermal resistance and structure of POD fibres were studied by thermogravimetric analysis, SEM and wide angle X-ray diffraction. The relationship between the fibre's properties and the cyclisation degree was discussed. Results show that the POD solutions with high cyclisation degree are stable and spinnable. The thermal, mechanical and chemical properties of fibres depend greatly on the cyclisation degree of POD fibre and post-treatment conditions. The higher the cyclisation degree of POD fibre, the better the properties, except for those fibres which have undergone extensive heat-treatment. The resulting POD fibres have good thermal stability and moisture regain, as well as proper mechanical properties. These fibers can be used as heat clothes for firemen, metallurgists and welders. resistant filtering fabrics, and protective working clothes for firemen, metallurgists and welders.