Grating assembly in periodic crystal aggregates of aliphatic polyesters with potential iridescence photonics

Crystal assembly by crystallization and self-aggregation into periodicity, viewed as circular ring bands in optical birefringence patterns, in the family of aliphatic polyesters is surveyed. The periodic packing in all polyesters shares similarity of grating architectures in interiors and banding on top surfaces. More intereistingly, such grating structures in ring-banded polyesters can display interfacial photonics to cause optical light interference into colors. The unique periodic morphology and top-surface and interior architectures are explored in greater details using two polyesters in homologous series: poly(ethylene adipate) (PEA), poly(1,4-butylene adipate) (PBA), biodegrable poly(ε-caprolactone) (PCL), and poly(β-hydroxy butyrate) (PHB), which serve as ideal models for focused discussions on their photonics with periodic gratings. Analysis of the interior lamellae from ringless to periodically ordered aggregates is conducted for comparisons. Universally, the grating structure is composed of interior tangential lamellae intersecting with radial lamellae mutually at 60–90° angle, resembling shish-kebab or cross-hatch assembly. The cross-bar pitch of the interior gratings in all periodically banded polymer aggregates is invariably equal to the optical inter-band spacing as viewed in their optical birefringence patterns. 3D assembly mechanisms of periodically banded crystals are also responsible for the orderly ring arrays on top surfaces. The interior microstructures of alternate-layered assembly resemble the nature’s photonic crystals.