Photoactivated Cyclic Polyphthalaldehyde Microcapsules for Payload Delivery

Microcapsules with a cyclic polyphthalaldehyde (cPPA) shell and oil core were fabricated by an emulsification process. The low ceiling temperature cPPA shell was made phototriggerable by incorporating a photoacid generator (PAG). Photoactivation of the PAG created a strong acid which catalyzed cPPA depolymerization, resulting in the release of the core payload, as quantified by ¹H NMR. The high molecular weight cPPA (197 kDa) yielded uniform spherical microcapsules. The core diameter was 24.8 times greater than the cPPA shell thickness (2.4 to 21.6 μm). Nonionic bis(cyclohexylsulfonyl)diazomethane (BCSD) and N-hydroxynaphthalimide triflate (HNT) PAGs were used as the PAG in the microcapsule shells. BCSD required dual stimuli of UV radiation and post-exposure baking at 60 °C to activate cPPA depolymerization while room temperature irradiation of HNT resulted in instantaneous core release. A 300 s UV exposure (365 nm, 10.8 J/cm²) of the cPPA/HNT microcapsules resulted in 66.5 ± 9.4% core release. Faster core release was achieved by replacing cPPA with a phthalaldehyde/propanal copolymer. A 30 s UV exposure (365 nm, 1.08 J/cm²) resulted in 82 ± 13% core release for the 75 mol % phthalaldehyde/25 mol % propanal copolymer microcapsules. The photoresponsive shell provides a versatile polymer microcapsule technology for on-demand, controlled release of hydrophobic core payloads.