Nano-Assembly Architectures and Structural Iridescence in Poly(3-hydroxybutyric acid-co-3-hydroxyvaleric)

Detailed examinations were carried out using polarized-light optical microscopy (POM), scanning electron microscopy (SEM), photonic reflection, and laser-light diffraction to explore the periodically assembled microstructures of biodegradable polymer poly(3-hydroxybutyric acid-co-3-hydroxyvaleric) (PHBV). By manipulation of the ring-banded morphology, the surface topography and internal lamellar assembly are analyzed. The periodic crystals with the most regularity produced at crystallization temperature (T_c) = 70 °C was chosen for in-depth examination. In this study, advanced and schematic models were taken to gain insights. The interior dissection of the banded spherulites demonstrates that PHBV can form a highly ordered structure with grating-like periodicity. Periodicity of bands appears not only on the top surface but also in the interiors that are composed of cross-hatch micro- to nanolamellae. In alternate cycles, the inner lamellae are arranged perpendicular to the substrate underneath the ridge region, and they evolve at oblique angles to form daughter lamellae parallel to the substrate. Each of the subsequent growth cycles self-repeats and reproduces into a fractal pattern. The banded PHBV crystals, with a grating structure packed with micro- to nanosized crystal plates similar to those seen in nature’s iridescence species, have microstructures that mimic nature’s photonic crystals and are proven to display iridescence properties.