Structure And Properties Changes Of Aromatic Copolysulfonamide Under Different Treatment Conditions

Aromatic polysulfonamide(PSA) fiber has been applied in the field of heat-resistant filter, flame-resistant material and insulating material thanks to its unique properties. At present, the study of PSA fiber mainly focused on its spinning process, structure and properties. However, research on its application characteristic and structure as well as properties changes under application conditions is relatively lacking, which does not benefit to exploring its application. Therefore, PSA fiber samples taken from the wet-spinning line were analyzed to determine the relationship between its structure and application characteristic. Based on the results that structure and properties of heat-drawn PSA fiber varied greatly from that of heat-setting fiber, the effect of heat setting process on the morphology and property of PSA fiber was further studied. Then on this basis, changes of structure and properties of PSA fiber during acid and alkali treatment were investigated.Firstly, structure and properties of PSA fiber samples taken from the wet-spinning line were compared systematically and relationship between the structure and application characteristic was discussed. Results showed that the crystal structure of PSA fiber only formed after heat drawing process. After heat setting process, fraction of crystal phase increased significantly and the crystalline structure improved, which was conductive to the development of the tenacity and initial modulus of the fiber. The long period and the amorphous orientation of heat-drawn PSA fiber was relatively high as the molecular chains of straightened under drawing stress. Correspondingly, the elongation at break, creep strain and rate of stress relaxation of the heat-drawn fiber was low due to the lowered flexibility of molecular chains after drawing. After heat setting process, the long period and amorphous orientation decreased because the molecular chain in amorphous region crimped during heat setting process, resulting in the decreased intermolecular interaction and enhanced flexibility of molecular chains. Consequently, the elongation at break, creep strain and rate of stress relaxation of the heat-set PSA fiber increased.In accordance with the drawback of tenacity and the elongation at break of heat-drawn PSA fiber, we designed a heat-setting experiment to investigate the changes of structure and properties at different heat-setting temperatures and time. Results showed that the tenacity of PSA fiber did not increase monotonously with heat setting temperature and heat setting time. At higher heat setting temperature, the elastic recovery rate was lower and the fiber was more likely to creep while the heat setting time had no significant influence on the viscoelasticity of PSA fiber. Combined with the results of microstructure changes, a simple model was proposed to describe the morphology changes of PSA fiber with different heat setting conditions. More perfect structure with larger crystalline, mesophase and crystal phase, as well as the long period and orientation could be formed with favorable heat setting temperature and time, which was corresponding to the maximus of strength of the fibers while the creep strain remained a relatively low level.Finally, experiments of acid and alkali treatment for PSA fiber were designed in order to reveal the failure mechanism of PSA fiber in the application process as high temperature filter material and determine the suitable application conditions. Results showed that the mechanical properties and thermal stability of PSA fiber declined gradually during the acid and alkali treatment. The molecular chain broke in the acidic and alkaline corrosion conditions, which weakened the intermolecular interaction. The fraction of mesophase phase, crystal orientation, fibril length and orientation decreased, resulting in the decline of mechanical properties. It indicated that the corrosion resistance of PSA fiber in acid condition was superior to that in alkali condition. PSA fiber could be used in acidic conditions at high temperature, especially suitable for the working environment with less acid gas. However, anti-corrosion processing must be done when the fiber is used in alkaline environment for long-term service.