Lamellar Organization and Assembly-Induced Iridescence Features of Poly(Nonamethylene Terephthalate) Ring-Banded Spherulites

This study presents a detailed investigation into the crystallization behavior and hierarchical morphology of poly(nonamethylene terephthalate) ring-banded spherulites (PNT-RBS), a relatively unexplored system. Two distinct morphological types: type I and type II were identified, differing in optical band spacing, lamellar architecture, and birefringence patterns. Type I spherulites exhibit narrow zigzag ring bands with single extinction under polarized light, while type II features broader bands and dual-color birefringence arising from alternating flat-on and edge-on lamellae. Microbeam X-ray diffraction mapping reveals periodic lamellar transitions in type II spherulites, with SAXS signals confirming vertical alignment in ridges and horizontal alignment in valleys. Although crystals in both types share the same triclinic crystalline structure and comparable crystallinity, lamellae of type II spherulites exhibit a ∼1 nm shorter long period, indicating denser lamellar packing. This morphological variation is further supported by SEM imaging, which reveals a grating-like lamellar assembly. Additionally, iridescent features observed under varied crystallization conditions suggest tunable optical properties, positioning PNT-RBS as promising candidates for advanced photonic and birefringent materials.