Structures and Properties of Poly(vinylidene fluoride-co-hexafluoropropylene) Fibers

In order to promote the development of poly(vinylidene fluoride-co-hexafluoropropylene) [P(VDFHFP)] fibers in industry, P(VDF-HFP) fibers were fabricated via single screw melt spinning machine. The fibers were drawn by thermal stretching process, and the polyvinylidene fluoride (PVDF) fibers were prepared as a reference. The crystal structure, melting behavior and mechanical properties of the fibers were investigated by X-ray diffraction, Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), thermogravimetric analysis (TG) and fiber strength test. The results revealed that the crystal morphology of fibers had a P(VDF-HFP) and beta polymorphic forms, similar to PVDF fibers contributing to the regularly stacking of VDF moiety. The addition of HFP had an effect on crystallization, resulting in lower crystallinity in P(VDF-HFP) fibers. The increase of draw ratio not only facilitated the transition of crystalline phase from a to beta phase, but also improved the crystallinity of the fibers. When the tensile strength was equal, a higher transformation of the crystalline form from a to beta phase was observed in P(VDF-HFP) fibers than that in PVDF fibers, due to the steric effect in HFP structural unit. The thermal stability of the fibers was also improved and their crystallization consummated with stretching. The melting temperature and thermal decomposition temperature of P(VDF-HFP) fibers under draw ratio of 6 reached 126.9 and 452.3 degrees C, respectively. With the increase of the draw ratio, the tensile strength increased owing to the regular rearrangement of the molecular chains along with the axial direction, and the optimal tensile strength of P(VDF-HFP) fibers was as high as 502.6 MPa at draw raio of 6. After repeatly stretched for 50 times under constant elongation of 20%, the elastic recovery rate of P(VDF-HFP) fibers under the draw ratio of 6 was 81%, which was much higher than that of PVDF fibers, indicating that the incorporation of the HFP segments into the PVDF molecular chains was beneficial to improvement of the flexibility of the fibers.