Crystal aggregation into periodically grating-banded assemblies 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 fractal-shaped banded spherulites. A novel banded pattern with fractal-shaped lamellar structures is found when the composition of PEO is at or above 20%, and this pattern significantly differs from the ordinary extinction-bright banded morphology of neat PA. The diluent PEO is regarded as the main factor in transforming the crystalline morphology from a compact banded pattern to a fractal-shaped banded pattern. The banded spherulites are composed of numerous fractal structures periodically branching out. Each fractal unit contains two portions: a main stalk (ridge) whose discrete crystalline aggregates are arranged along the radial direction and a fern-like dendrite (valley) whose crystals are arranged along the tangential direction. In situ monitoring of the growth process of the fractal-shaped banded spherulites proves the fractal-branching growth mechanism for the formation of banded spherulites. The periodically perpendicular intersection of discrete crystals results in the contrasting birefringence bands of banded spherulites. A periodic grating structure with fractal branching leads to such a novel pattern of banded PA spherulites.