Crystal Aggregation into Periodically Grating-Banded Assembly in Phthalic Acid Modulated by Molten Poly(ethylene oxide)
A small-molecule compound, phthalic acid (PA) crystallized in the presence of poly(ethylene oxide) (PEO) with various compositions was utilized as a model to investigate the morphology and crystal assembly of periodically ordered structures in banded spherulites. After etching off PEO from crystallized solids, detailed crystal assembly in PA was analyzed by utilizing atomic-force microscopy (AFM) and scanning electron microscopy (SEM) to reveal the mechanisms of the formation of the fractal-shape banded spherulites. A novel banded pattern with fractal-shape lamellar structures is found when the composition of PEO is at or above 20%, and this pattern significantly differs from ordinary extinction-bright banded morphology of neat PA. The diluent PEO is regarded as the main factor to transform the crystalline morphology from compact banded pattern to fractal-shape banded pattern. The fractal-shape banded spherulites are composed of numerous of fractal structures periodically branching out. Each fractal unit contains two portions: main stalk (ridge) whose discrete crystalline aggregations are arranged along the radial direction, and fern-like dendrite (valley) whose crystals are arranged along a tangential direction. In-situ monitoring of the growth process of the fractal-shape banded spherulite proves the fractal-branching growth mechanism for the formation of banded spherulites. The periodically perpendicular intersection of discrete crystals results in the contrasted birefringent bands of banded spherulites. A periodically grating structure with fractal branching leads to such a novel pattern of banded PA spherulite.