Ultraviolet and x-ray-induced phase change in poly(phthalaldehyde)

Poly(phthalaldehyde) (PPA) can be photo-catalytically depolymerized to o-phthalaldehyde ( o PA) to induce a solid-to-liquid phase change. In this study, thick PPA films (0.18 mm) containing photoacid generators (PAGs) were evaluated for transient applications such as phase change devices. Three diaryliodonium PAGs, 4-Isopropyl-4′-methyldiphenyliodonium tetrakis(pentafluorophenyl)borate (FABA), bis(4- tert -butylphenyl)iodonium triflate (BIT), and diphenyliodonium hexafluorophosphate (DIHP), were incorporated to enable radiation-triggered, acid-catalyzed PPA depolymerization. The PPA depolymerization was quantified by 1 H nuclear magnetic resonance (NMR) spectroscopy. The characteristic doses, D 0 (the highest dose before depolymerization), D 30 (the onset of liquid formation), D 70 (full liquification), and D 100 (the lowest dose for complete PPA depolymerization) were compared for the different PAGs for both UV and x-ray exposure. The PAG efficiency followed the following trend: DIHP > BIT > FABA. The addition of a base, pyridine, and pH indicator, methyl orange, corroborated the trends in acid availability. DIHP generated more acid followed by BIT and FABA, all other conditions remaining constant. The post-exposure time affected the PPA phase transition for partly converted solid films indicating that delayed acid creation or diffusion can affect depolymerization and solid-to-liquid conversion. The effect of delayed acid generation on phase transition was also shown by equal-dose, variable intensity exposure experiments. Finally, x-ray-triggered liquification was shown with a non-ionic halogen compound.