Visible Spectrum Sensitization of Vaporizable Polyaldehydes for Transient Devices

Materials which disappear or vaporize upon triggering are classified as “transient technology,” an emerging field of research with applications in electronic devices and small delivery vehicles. In these applications, a controlled, triggered vaporization of the device is desired when device recovery is not possible. In this project, low ceiling temperature polymers are used to construct transient devices. The ceiling temperature (T_C) of a polymer is the temperature at which the rates of polymerization and depolymerization are in equilibrium; at a temperature below T_C, the polymer form is preferred. The cationic polymerization of poly(ortho-phthalaldehyde) (p(PHA)) forms a cyclic polymer that kinetically stabilizes the macromolecule at ambient conditions, meaning that it will maintain its polymeric form at temperatures well above T_C. Low T_C polyaldehydes, such as p(PHA), can depolymerize and vaporize at ambient conditions by acid-catalyzed depolymerization, making them suitable for transient devices. A strong acid can catalyze depolymerization of the polymer at temperatures above T_C. The irradiation of a p(PHA) film loaded with a photoacid generator (PAG) can create such an acid. The photoacid cleaves the polymer backbone and decomposes the polymer into its monomer form. Most PAG molecules are only sensitive to UV light, making them near-useless in a sunlight exposure. Polycyclic aromatic hydrocarbons (PAHs) absorb visible light and sensitize the PAG to visible light through photoinduced electron-transfer (PET). In this project, the molar ratio of the PAG to PAH was investigated in terms of absorption coefficient, wavelength, energy transfer, and residue remaining after polymer depolymerization. The thermodynamic driving force of the PET reaction with various PAHs is characterized through the Rehm-Weller equation. Results show anthracene to be the most practical PAH due to the fast depolymerization of p(PHA), minimal residue formation, and a colorless polymer film. A 1.2:1 PAH to PAG molar ratio at 1 wt% PAG was shown to be the most efficient mixture for decomposition. The optimized ratio and PAH were used to examine dose linearity (i.e. the tradeoff between intensity and time) and to evaluate the evaporation rates of sensitized copolymer films. Absorbance spectroscopy was used to determine the path length through a polymer film with a given PAH. Utilizing the 1.2:1 ratio at 1 wt% concentration of PAG, the maximum path length of absorbance is 8500 µm in an anthracene-loaded polymer film.