Formation of well-organized, concentric-ringed spherulites of four-arm star symmetric PEO-b-PCL via confined evaporative crystallization
Formation of well-organized, periodic spherulites of four-arm star symmetric poly(ε-caprolactone)-b-poly(ethylene oxide) (PEO-b-PCL) via confined slow evaporative crystallization of solution-cast films is reported, and the microstructures were characterized by optical microscopy, atomic force microscopy, and X-ray scattering and diffraction techniques. Having a Maltese cross pattern, the spherulites exhibit a unique concentric-ringed morphology under both polarized and unpolarized lights. The pattern regularity and ring periodicity are both controllable. The tunable periodicity is in the range of 10–45 μm, and the size of the spherulites even exceeds one millimeter. It shows that such birefringent concentric-structure is a reflection of periodic change of thickness caused by rhythmic stacking of lamellar crystals with varied orientation along the radial direction. The lamellae in valley bands are nearly flat-on, while the lamellar orientation becomes gradually inclined with normal thickening, leading to continuously enhanced birefringence in ridges. Finally, a coupling of the poor solubility of the PEO block and its inner geometry in the star topology is then presented to illustrate a possible solvent-assisted topology confined mechanism for inducing PCL-dominated rhythmic crystallization into concentric-ringed spherulites.