Abstract
Small-molecule phthalic acid (PA), confined in micrometer thin films, was crystallized in the presence of strongly interacting tannic acid (TA) to investigate crystal assembly and correlation between banded patterns and branching structures. Several compositions of the mixture of ethanol/water solutions and evaporation temperatures were also manipulated to investigate the kinetic effects on the morphology of PA crystals. With increasing evaporation rate, the morphology of PA crystals systematically changes from circular-banded spherulites to highly ordered grating-banded patterns. A unique periodic fractal-branch pattern with contrasted birefringent bands exists at intermediate evaporation rate, and this unique grating architecture has never been found in other banded crystals. Crystal assembly of these three periodic morphologies was analyzed by utilizing atomic-force microscopy (AFM) and scanning electron microscopy (SEM) to reveal the mechanisms of formation of hierarchical structures of PA. The detailed growth mechanisms of the novel fractal-branching assembly into circular- or grating-banded patterns are analyzed in this work.
Highlights
Cooling of a matter in liquid state causes it to crystallize into either 2D or 3D ordered states
With increasing the composition of Tannic acid (TA), the morphology of poly(ethylene succinate) (PESu) transformed from well-defined round spherulites into seaweed-like dendritic spherulites composed of lenticular-shaped single crystals, indicating that the strong H-bonding interaction between TA and PESu was capable of inducing dendritic morphology consisting of ordered and regular single crystal assembly[12,13]
It is obvious that the Phthalic acid (PA) crystals display three categories of diversified grating morphologies depending on the evaporation temperatures (40–100 °C)
Summary
Cooling of a matter in liquid state causes it to crystallize into either 2D or 3D ordered states. Periodic ring patterns are commonly observed in some long-chain polymers[16,17,18,19,20,21,22,23,24] and many small-molecule compounds[25,26,27,28,29,30] Both dendritic pattern (grating structure) and ring-banded pattern are highly ordered self-organization in crystallization. Phthalic acid (PA), a small-molecule organic compound, has been reported having the ability to form diversified morphologies via rapid evaporation-induced crystallization from PA solutions (usually in water/ethanol mixtures), which is governed by the evaporation conditions including the type of solvent, composition of the solvent mixture, and evaporation temperature[29]. PA, blended with a strongly-interacting diluent TA (via inter-molecular –OH hydrogen bonding), was crystallized under controlled evaporation conditions to induce diversified morphology, including ring-banded, periodic branching and grating patterns. The correlation between the banded patterns and grating assembly via fractal growth can be established
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