Banded spherulites and twisting lamellae in poly–ε–caprolactone

By crystallization of poly–ε–caprolactone (PCL) at temperatures (55 ^∘C ≥ T_c ≥ 50 ^∘C) close to the melting point (T_m = 61 ^∘C), banded spherulites form. These structures show a periodicity of P = 32 µm 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 × 50 µm²) polarized infrared-spectroscopy measurements. From the oscillation of the order parameters of the crystalline \(\underline {a}\)-, \(\underline {b}\)-, and \(\underline {c}\)-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^∘). 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.