Banded spherulites and twisting lamellae in poly-ε-caprolactone

By crystallization of poly-epsilon-caprolactone (PCL) at temperatures (55 degrees C T-c 50 degrees C) close to the melting point (T-m =61 degrees C), banded spherulites form. These structures show a periodicity of P =32 mu m as determined by polarized optical microscopy (POM). In weakly-to-non-banded spherulites formed under similar conditions, a less sharp distribution of characteristic length scales is observed, that exhibits the same mean value. Within bright and dark regions of the banded spherulites, the two-dimensional molecular order parameter of different crystal directions of the lamellae is deduced from tightly focused (15 x 50 mu m(2)) polarized infrared-spectroscopy measurements. From the oscillation of the order parameters of the crystalline -, and -axes, banding in pure PCL is proven to result from lamellae growing in a helicoidal fashion along the spherulites radius. No deviation of lamellar growth and radius is determined within experimental uncertainty (+/- 4 degrees). Furthermore, spatially averaged IR-microscopy results in the same characteristic polarization dependence in banded and weakly-to-non-banded spherulites. In conjunction with the mentioned characteristic length scales, we conclude that the mechanism, which results in banding, is also active in non-banded spherulites of PCL.