Analysis of crystal assembly in banded spherulites of phthalic acid upon solvent evaporation†
A low-Mw compound phthalic acid (PA) was chosen to investigate the packing mechanisms and lamellar assembly in ring-banded spherulites. Several solvents with PA were also investigated for evaluating the kinetic effects on ring bands brought about by solvents. The structural effects were examined by morphology analyses on two isomeric compounds of PA, isophthalic acid and terephthalic acid. Optical birefringence and variation in the PA ring-banded spherulites crystallized at a wide range of Tcs (25–110 °C) were correlated directly to the top-surface lamellar assembly via characterization using atomic force microscopy (AFM) and to the inner or lateral lamellar assembly using scanning electron microscopy (SEM). Amazingly, interesting features are seen in the differences in lamellar assembly in the two alternating banded regions (valley and ridge) of PA spherulites solvent-evaporation crystallized at higher temperatures (80 °C) other than the ambient temperature (28 °C). The “ridge” of the band is composed of discrete crystals pointing upward, like high-rise buildings viewed from high altitude. The valley crystals are also discrete and all laid flat-on parallel to the substrate surface. All crystals, in the valley or ridge, in the peripheral or inner regions, are discrete and no continuous spirals are formed from one to another. From the fact that the valley/ridge patterns are assembled like pieces of individual roof-shingle crystals pointing in two mutually intersecting directions, it is also consistent with the alternating optical bands of two birefringence shades. Each of the bands is not from the spiraling of a single continuous crystal but packed by polycrystals and their aggregates that discretely grow in two different orientations.