Surface Crystallization of Molecular Crystals by Exsolution from Poly(ethylene)

The properties of synthetic polymers are often tuned with additives. Whilst most additives, like plasticizers, are non-migratory, some are migratory, like lubricants. Blooming and/or bleeding refers to phenomena in which additives in a polymer migrate to its surface; this is a common phenomenon that is important for several industries including food packaging and pharmaceutics, medicine, art, and the production of long-lasting insecticidal bed nets.<br/>Many long-lasting insecticidal bed nets for protection against disease vectors are made of poly(ethylene) fibers in which the insecticide is embedded at the time of manufacture. Insecticide molecules diffuse from within the supersaturated polymers to surfaces and become bioavailable to insects, in most cases eventually crystallizing. Recent studies have revealed that contact insecticides can be highly polymorphic. Moreover, insecticidal activity is polymorph dependent, with forms having higher crystal free energy yielding faster insect knockdown and mortality. Consequently, the crystallographic characterization of insecticide crystals that emerge from fibers is critical to understanding net function and improving net performance. Structural characterization of insecticide crystals on bed net fiber surfaces, let alone their polymorphs, has been elusive owing to the minute size of the crystals that by necessity grow in and on threads, however.<br/>Here we describe surface crystallization of several molecules by phase separation from extruded poly(ethylene) fibers including ROY, paracetamol, flufenamic acid, benzamide, benzil, deltamethrin, alpha-cypermethrin, lambda-cyhalothrin and chlorfenapyr. In general, solutes diffuse from the polymer matrix and form amorphous droplets on the surface. With time the system may undergo substantial evolution from droplet coalescence to metastable crystal growth, to recrystallizing as more thermodynamically stable forms, a demonstration of Ostwald’s Rule of Stages. In general, crystallization on the surface of poly(ethylene) resembles growth from vapor phase and is often limited by diffusion mass transport. With the highly polymorphous compound ROY (5-methyl-2-[(2-nitrophenyl)-amino]thiophene-3-carbonitrile) as a proxy for insecticide crystallization, we investigated blooming kinetics and crystallization on the surface of melt-extruded, and cold-drawn poly(ethylene) fibers that had been impregnated with ROY. The blooming rates, tracked from the time of extrusion, were determined by UV-vis spectroscopy after successive washes. Six crystalline polymorphs (of the 14 known) were observed on poly(ethylene) fiber surfaces, and they were identified and characterized by Raman microscopy, scanning electron microscopy, and 3D electron diffraction (3D ED). We show both polymorph control and particle size/morphology control. These observations reveal that the crystallization and phase behavior of polymorphs emerging from poly(ethylene) fibers is complex, dynamic, and tuneable. Furthermore, the demonstration that 3D ED can characterize the minute crystals grown on fibers promises a pathway to bed net crystallography and subsequent optimization of bed net performance.