Rapid Photo-Triggered Release Kinetics of PPA Microcapsules

Stimuli-responsive microcapsules have been investigated as a means to release special-purpose payloads such as catalysts inside solid polymer materials. These catalyst payloads have garnered recent interest in the recycling and upcycling of plastics. A microcapsule can be loaded into common commodity plastics and triggered to release a catalyst for depolymerization. Specific chemical conditions or structural changes have commonly been used to trigger microcapsules for controlled release, but these mechanisms tend to not be amplified or take substantially long times to induce release. Long times for release impact the cost of processing, especially for recycling that would otherwise require an outside-in mechanism. Unlike other stimuli, light has advantages in that it is readily available and convenient to use with low cost. This study utilized poly(phthalaldehyde) (PPA) microcapsules formulated with various UV-sensitive photo-acid generators (PAGs). Upon exposure to UV irradiation followed by heat treatment, PAGs generate and release strong acids that attack the PPA, which initiates rapid depolymerization back to the monomer. In the case of a microcapsule made with a PPA shell, the rapid depolymerization results in the immediate release of the capsule payload to its surroundings. PPA capsules were made using an oil-aqueous emulsification process in the presence of surfactants and their morphologies are a fraction of blueberry shapes (high loading of core) and a fraction of concave shapes (low loading of core). As UV dose increased, more capsules were depolymerized and more dodecane core was obtained, indicating core release. The percentage of dodecane release and depolymerized PPA were analyzed using ¹H NMR. The photo-triggered response was further tuned by varying PAG concentration, PAG type, and post-exposure heating time. The efficiencies of the PAGs were compared for their release kinetics and the energy required to fully decompose the shell. PPA microcapsules with UV-sensitive PAGs successfully released their core within a short time and with low UV doses.