Periodic crystal assembly of Poly(3-hydroxybutyric acid-co-3-hydroxyvaleric acid): From surface to interior microstructure

Abstract Poly(3-hydroxybutyric acid-co-3-hydroxyl-valeric acid) (PHBV) crystallized in the presence of a polymeric diluent impurity is analyzed at specific crystallization temperatures (Tc) from 60 °C to 80 °C. 3D-dissected interior analysis for ring-banded spherulites evidently reveals that the alternate bands exist in not only top surface but also 3D interiors composted of grating-assembled lamellae. Aggregation of the lamellae, initially from two ends of nucleus sheaf-bundles, packs via crystal-by-crystal mode into either Archimedean single-spirals or Fermat's double-spirals, which constitute the “ridge” bands. Then from the ridge-band lamellae, secondary lamellae then branch out at 60–80° oblique angles, with irregular bending, twisting, or scrolling to accommodate in the confined space, to form the “valley” bands. Each new growth cycle repeats the fractal patterns of previous one. The periodic cycles repeat themselves in a fractal fashion to form a cross-lamellar structure, with “crossbar pitch” equal to its optical band spacing. Periodicity of aggregation in top surface and interior is depicted in detailed 3D assembly, beyond and distinguished from conventional planar analytical approaches, for novel mechanisms.