Morphology, polymorphism behavior and molecular orientation of electrospun poly(vinylidene fluoride) fibers

The morphology, polymorphism behavior and molecular orientation of electrospun poly(vinylidene fluoride) (PVDF) fibers have been investigated. We found that electrospinning of PVDF from its N,N-dimethylformamide/acetone solutions led to the formation of β-phase. In contrast, only α- and γ-phase was detected in the spin-coated samples from the same solutions. In the aligned electrospun PVDF fibers obtained using a rotating disk collector, the β-phase crystallites had a preferred orientation along the fiber axis. The degree of orientation did not, however, vary significantly with the speed of the rotation disk collector, and the β-phase was also not significantly enhanced with the increase in the rotation speed or the decrease in the size of spinnerets. These facts indicated that the orientation was likely to be caused by Columbic force rather than the mechanical and shear forces exerted by the rotating disk collector and spinnerets. The Columbic force may induce local conformational change to straighter TTTT conformation, and hence promote the β-phase. The addition of 3 wt.% of tetrabutylammonium chloride (TBAC) into the polymer solutions effectively improved the morphology of the electrospun fibers, and led to almost pure β-phase in the fibers. With spin coating, PVDF-TBAC did not, however, show any strong β-phase diffraction peak. The synergistic β-enhancement effect of TBAC and electrospinning is possibly due to the fact that while TBAC could induce more trans conformers, electrospinning promotes parallel packing, and hence inter-chain registration.