Three-dimensional morphology of PVDF single crystals forming banded spherulites

Abstract

We have examined the morphology of poly(vinylidene fluoride) (PVDF) single crystals grown from melt and from blends with poly(ethyl acrylate) (PEA), PVDF/PEA=0.5/99.5 and 30/70 by weight. The single crystals, of relatively higher molecular weight, were grown isothermally in the temperature range where banded spherulites are formed with sufficient crystallization time. The crystals were extracted by dissolving amorphous PEA and PVDF crystals formed on quenching. The three-dimensional morphology of the single crystals was examined by transmission electron microscopy (bright field, dark field and diffraction) with a tilting stage. For all cases, the tilting of chains (∼25–27°) to the fold surface has been confirmed. The three-dimensional shape of all the crystals was chair type for the 30/70 blend and pure PVDF. In chair crystals, spiral terraces keep the handedness in each growth direction. From these evidence, it is proposed that the chair crystals with consecutive creation of spiral terraces of the same sense are responsible for the twisting relationship between crystallites in the radial direction of the banded spherulites.