Rhythmic Growth-Induced Ring-Banded Spherulites with Radial Periodic Variation of Thicknesses Grown from Poly(ε-caprolactone) Solution with Constant Concentration

Birefringent ring-banded spherulites with radial periodic variation of thicknesses were grown from poly(ε-caprolactone) (PCL) solutions under conditions for which the solution concentration was held constant during the whole development of the morphology. The as-grown ring-banded spherulites were investigated by optical (OM) and atomic force (AFM) microscopies, by transmission electron microscopy (TEM) of samples sectioned parallel to the plane of film, and also by electron diffraction (ED) and grazing incidence X-ray diffraction (GIXD) techniques. The results indicate that the concentric rings in the birefringent ring-banded spherulites, as well as those in the nonbirefringent ring-banded spherulites, are a manifestation of periodic variation of thicknesses along the radius. The development of the ring-banded spherulites with radial periodic variation of thicknesses is due to periodic diffusion-induced rhythmic growth associated with periodical change in the concentration gradient in polymer solution with constant concentration. The morphological features reflect the competition between the diffusion flux of polymer chains in solution, J, and spherulitic growth with radial growth velocity, V, which can be characterized by the parameter δ = J/V. The effects of crystallization conditions, including polymer molecular weight, initial solution concentration, and solvent evaporation rate, on the formation of ring-banded spherulites with radial periodic variation of thicknesses were studied.