A study on the relationship between the crystal structure and hard elasticity of PVDF fibers

Poly(vinylidene fluoride) (PVDF) fibers were prepared by melt-spinning process. The crystal structure of annealed PVDF fibers was characterized by wide- and small-angle X-ray diffraction (WAXD and SAXD) and scanning electron microscopy (SEM). Crystalline reflections of c-axis orientation of annealed PVDF fibers were illustrated by WAXD pattern. The stacked lamellar structure aligned in the direction normal to the fiber axis was found in SAXD pattern and the d-spacing of the lamellae was 13.4 nm. Such lamellar structure was supported by SEM micrographs as well. The elastic recovery of annealed PVDF fibers was above 80% from 50% extension, which was much higher than that of unannealed fibers on the first cycle. The initial elastic modulus of annealed fibers reached to a value of 3.5 GPa. The morphological and mechanical properties, all indicated that the annealed PVDF fibers had the characteristic of hard elasticity. A typical stress-strain curve at a very low strain rate indicated the deformation of crystal lamellae in the fibers and a suggested structural deformation mechanism detailed the characteristics of hard elasticity.