The temperature-dependent transition of the crystal phases of poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) was investigated in the electrospinning process. A solution of PVDF-HFP in N,N-dimethyl acetamide (DMAc) produced only the β-phase-dominant crystal up to 70 °C, irrespective of the spinneret temperature. In a mixed solvent of DMAc and acetone, however, the crystal phase of the electrospun fibers was dependent on temperature: β-phase-dominant at 30 and 50 °C and α-phase-dominant at 70 °C. The transition was related to a change of the coagulation rate during electrospinning, because the less perfect α phase is preferable to the β phase at a higher coagulation rate. The temperature-dependent increase of the coagulation rate was more drastic in the presence of acetone, so the transition took place only in the mixed solvent. At elevated temperature, acetone not only raised the evaporation rate of the solvent but promoted the phase separation of the polymer resulting from the lower critical solution temperature behavior, which was rheologically traced.
- crystal phase
- poly(vinylidene fluoride)