Transitional Ring Bands Constructed by Discrete Positive- and Negative-Birefringence Lamellae Packed in Poly(1,6-hexamethylene adipate) Spherulites

Abstract

Neat poly(1,6-hexamethylene adipate) (PHA) is one polyester that never forms ring banded spherulite when crystallized at any crystallization temperatures (Tcs). However, PHA spherulites readily transform from negative-type at low Tcs (32–38 °C) to positive-type at high Tcs (46–48 °C). By utilizing such morphological traits, “artificial” ring-banded spherulites can be constructed by step-crystallization cycles at 38 and 46 °C, respectively for controlled times. Such artificially constructed ring bands are analyzed in details using polarized-light optical microscopy (POM), atomic force microscopy (AFM), and electron diffraction (ED) in transmission electron microscopy. For the classical continuous spiral twisting to work for ring-banded spherulites, it requires two assumptions that may be far from reality and hard to prove: (I) lamellae in the spherulites remain single-crystal-like plates with no secondary/ternary branches at all from nucleation to completion of crystallization; and (II) strong forces are required for holding continuous spirals of all thousands of single-crystal lamellae in precise synchronizing paces of twists. On the other hand, as proven in this novel approach in constructing artificial ring-banded PHA spherulites, the ridge band and valley band are two discrete layers of lamellae species with interfacing discontinuity. No forces are required to hold the crystals in these two discrete layers of lamellae that have different morphology and optical axes. As a result of opposed crystal axes (radial vs tangential), they show optically alternating positive–negative birefringence rings in the artificial PHA spherulites just like the natural ring-banded spherulites in other polymers. Furthermore, AFM and ED patterns provide clear evidence that the crystals in two discrete layers of opposed optical birefringence colors are all polycrystals, with multiple branches as spherulites evolving from nucleation to growth completion, which describes the real assembly in polymer spherulites.