Direct investigations of deformation and yield induced structure transitions in polyamide 6 below glass transition temperature with WAXS and SAXS

Deformation and yield induced structure transitions of polyamide 6 (PA6) were detected with the combination of the wide- and small-angle X-ray scattering (WAXS and SAXS) at 30 °C below glass transition temperature (T_g) of PA6. During deformation, γ-α phase transition was found at elastic stage. The concentrated stress in crystals at elastic stage provided adequate energy for the direct γ-α phase transition under T_g. The force to promote the γ-phase into α phase directly is insufficient at the yield stage and a transient phase as a compromise was formed. The transient phase was confirmed by DSC measurements and assisted the γ-α phase transition indirectly. The γ-phase slips into incomplete fragments at yield point, and the parts along tensile direction are responsible for the formation of transient phase. The γ-fragments after yield is oriented by shear force following α-phase. The yield of PA6 was intimately related with the γ-transient phase transition and the slippage of γ-fragments. In addition, the increase of long spacing at elastic deformation along tensile direction was mainly contributed by the amorphous region. After yield, the phase transition and crystalline slippage affected the lamellar evolution. A simplified schematic diagram was provided to illustrate the structure evolution of PA6.