Effect of UV-irradiation on polyelectrolyte multilayered films and hollow capsules prepared by layer-by-layer assembly

Polymer multilayered films were assembled onto planar and particle substrates by the alternate adsorption of oppositely charged polyelectrolytes. Removal of the template from the coated melamine formaldehyde (MF) particles yields hollow polyelectrolyte capsules. The changes in morphology and chemical structure of the capsules after exposure to UV light over a range of intensities and time were characterized by transmission electron microscopy (TEM), Fourier transform infrared (FT-IR) spectroscopy, and quartz crystal microgravimetry (QCM). Pronounced shrinkage of the capsules was observed after UV irradiation for capsules formed with polyelectrolytes bearing aromatic groups, such as poly(sodium 4-styrenesulfonate) (PSS). Capsules composed of five bilayers of PSS and poly(diallyldimethylammonium chloride) (PDDA) shrunk to ca. 20% of their original diameter after UV irradiation (20 mW cm(-2) at 365 nm) for 120 min. In contrast, negligible shrinkage was observed for capsules formed from polyelectrolytes without aromatic groups (e.g., poly(potassium vinyl sulfate) (PVS)/ PDDA). FT-IR spectroscopy reveals that UV irradiation causes chemical changes in the polyelectrolytes with aromatic groups: the disappearance of bands assigned to aromatic groups and the formation of byproducts via chemical changes, such as SO42- ions, were observed. In addition, QCM frequency changes showed a decrease in film mass for polyelectrolyte multilayer films formed on planar surfaces that were subjected to UV irradiation, indicating changes in the film properties. UV irradiation causes changes in chemical composition of the capsules, which leads to structural rearrangement and capsule shrinkage. The role of UV irradiation on capsules could be used as a viable route to alter the chemical and physical properties of polyelectrolyte capsules after their formation. This strategy may be potentially useful in environmental applications, where it could be used to modulate the release of substances encapsulated in the capsules upon direct exposure to sunlight, or in UV-initiated chemical reactions in confined microreactor systems.