Effect of Hydrothermal Treatment on Structure and Properties of Polyamide 6 Fibers

Hydrothermal treatment of polyamide 6 ( PA6) fibers at 150 degrees C was studied. Different characterization and measurement were used to investigate the structural transformation and mechanical properties of PA6 fibers after hydrothermal treatment, which included two dimensional wide angle X-ray diffraction ( 2D-WAXD), two dimensional small angle X-ray scattering ( 2D-SAXS), differential scanning calorimeter ( DSC), field emission scanning electron microscope ( FE-SEM) and single fiber strength test. Under hydrothermal condition, the movement of the molecular chains was enhanced due to permeability of water molecules, which led to structure changes in PA6 fibers. After hydrothermal treatment, the crystal structure of the fibers transformed from gamma into a phase, crystallinity degree increased from 25. 2% to 45. 9%, crystalline perfection was improved and crystallite size became bigger. Hence, the melting peak temperature of the fibers increased from 220. 6 degrees C to 227. 7 degrees C and the melting enthalpy also increased. In addition, the molecule chains in the amorphous region began to crystallize and grew along the radial direction of the fiber to form a crystal lamellae structure during hydrothermal treatment. At 150 degrees C, ellipsoidal micro. pores formed in the fiber, which could be monitored by 2D-SAXS test. The hydrothermal treatment also caused changes of the fiber surface morphology. After the hydrothermal treatment, the smooth surface of the original fiber became rougher and a large number of macro. pores were formed on the surface. However, the micro. and macro. pores in fibers almost did not affect the melting temperature of the fibers. The crystal lamellae in fibers showed very weak orientation, but tensile could promote the lamellae orientation. When tensile strain increased from 0 to 50. 7%, the average long period of lamellae increased from 10. 6 nm to 14. 8 nm. The hydrothermal treatment induced the reduction of amorphous region in the fibers and the formation of micro. and macro. pores, which resulted in the lower breaking strength and elongation of PA6 fibers.