Unusual process-induced curl and shrinkage of electrospun PVDF membranes

A significant shrinkage of about 17% of poly(vinyliedene fluoride) (PVDF) fibrous membranes processed by electrospinning is reported and analyzed for the first time in this article. Such shrinkage leads to a time dependent curl of the membrane, probed by image analysis and tensile test measurements. Crystalline structure of PVDF is analyzed through X-ray diffraction and FTIR analyses while solvent evaporation is monitored by weight lost measurements. The electrospinning process induces a crystal structure change of PVDF from non-polar α phase to polar β phase exhibiting piezoelectric properties. The curl was modeled taking into account the bimorph structure of the polymer membrane deposited onto the aluminum substrate with the electric field acting on the PVDF through the piezoelectric effect. Besides, just after the processing, the membranes exhibited a nearly 15% weight loss ascribed to the evaporation of the solvents entrapped within the solid fibrous membranes. The piezoelectricity of the β-phase together with solvent evaporation may be responsible for the observed contraction inducing the curl, the second mechanism of evaporation being predominant.