Phase-Separation Induced Lamellar Re-Assembly and Spherulite Optical Birefringence Reversion

Abstract

By using atomic-force and scanning electron microscopies (AFM, SEM), wide-angle and small-angle X-ray diffraction (WAXD and SAXS), and thermal and infrared spectroscopy characterization, mechanisms and correlations between lamellar reassembly, phase separation domains, and birefringence patterns in crystallized spherulites in poly(hydroxyl butyrate) (PHB) were probed in detail. Crystallization-induced phase separation in PHB blended with amorphous poly(methyl acrylate) (PMA) produces pebble-like PMA domains, which influence the birefringence types (positive vs negative types) of spherulites in crystallized blends with respect to composition and temperature. Extents of crystallization-induced phase separation were dependent on temperature and composition; for a fixed PHB/PMA (60/40) blend composition the phase separation extent was inversely proportional to temperature of crystallization (T c ). Correspondingly, depending on the extents of phase separation (counted as quantitative ratio of pebble-like PMA domains divided by entire area of the blend samples), the Maltese cross in spherulite patterns of PHB in the blend may rotate from the original positive-type spherulites at T c = 90°C to negative-type spherulites at T c = 60°C. The SAXS analyses showed the thickness variation of PHB lamellae (or PHB + amorphous polymer long periods) with T c was not an influencing factor, but that the formation of grains in the PMA domains from the crystallizationinduced phase separation could majorly influence the optical birefringence. Correlations among the phase domains, lamellae assembly, transitions from positive-type, unusual-type, and negative-types of crystallized spherulites in polymers have been expounded upon.

Synchrotron X‐Ray Analysis and Morphology Evidence for Stereo‐Assemblies of Periodic Aggregates in Poly(3‐hydroxybutyrate) with Unusual Photonic Iridescence

“…Previous studies [24,25] have adopted a novel approach via interior dissection into banded spherulites in correlation with the top-surface morphology of the PHB/PMA and PHB/PBA blends crystallized at a series of T c s with various domain sizes. Lovera, et. al.…”

Three types of banded structures in highly birefringent poly(trimethylene terephthalate) spherulites

“…Optical interference colors that appear along with the polymer spherulites under polarized‐light optical microscope (POM) are found interesting. Correlations between spherulite birefringence, interference colors, and lamellar arrangements within the spherulite of low birefringent polymers have been discussed in our previous publications . Nevertheless, there are only few studies that have ever probed and discussed the optical interference color of highly birefringent polymers like poly(trimethylene terephthalate) (PTT), and none so far are focused on exploiting how the crystal arrangement in spherulites may specifically affect the interference color patterns and/or distributions in the spherulites.…”

Reversible changes induced by temperature in the spherulitic birefringence of nylon 6 9

“…To the best of our knowledge, no explanations for reversible changes in birefringence of nylons induced by temperature have been reported. In some binary systems (e.g., poly(hydroxyl butyrate)/poly(methyl acrylate blend), reversibility of birefringence was accounted for by a crystallization-induced phase separation [43].…”