Tannin Induced Single Crystalline Morphology in Poly(ethylene succinate)

Polarized-light optical microscopy (POM), wide-angle X-ray diffraction (WAXD), and atomic-force microscopy (AFM) are used to analyze the crystalline morphology in poly(ethylene succinate) (PESu) interacting with hydrogen-bonding biodegradable tannin (TA). Upon interactions with TA via strong intermolecular H-bonding capacity with PESu, the regular Maltese-cross spherulites in PESu gradually transform to a pattern of maple leaves (10 wt.-% TA). At 20 wt.-% TA, the crystal patterns assume a highly dendritic seaweed shape (with the branches perpendicular to main stalks). Detailed characterization using AFM analyses showed that the minute crystal entities in the seaweed-like dendrite crystals are flat-on and take a multilayer diamond (lenticule) geometry approaching the shape of single crystals in PESu. The trend of variation with increasing TA is equivalent to decreasing the film thickness in the neat PESu. The diamond-shape flat-on crystals are packed into a terrace structure resembling a pyramid, indicating that multiple single crystals are stacked with partial overlapping. The strong H-bonding interaction between TA and PESu, assisted by a confined thickness, induces a single-crystal packing, which is equivalent to crystallization into discrete single crystals in dilute solutions.