Hydrothermal treatment of polyamide 6 (PA6) fibers at 150℃ 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 γ into α 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℃ to 227.7℃ 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℃

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.