Development and characterization of polyphthalaldehyde for transient applications

Metastable polymers that depolymerize in response to environmental stimuli to change shape, form, or function have garnered increased research interest in recent years. Demonstrated applications include signal amplification, temporary electronics, and drug delivery. These transient applications take advantage of the large transformation in properties that accompany the depolymerization of high molecular weight polymers into their monomeric components. Depolymerization of metastable polymers can be tailored by incorporating stimuli responsive agents that destabilize the host polymer in reaction to a variety of environmental stimuli. This thesis reports on a detailed investigation of the metastable polymer cyclic polyphthalaldehyde (cPPA). cPPA can be triggered to depolymerize into its monomeric component by exposure to heat or acid. Its degradation in response to these stimuli was characterized along with its aging-dependent mechanical properties. Significant effort was invested in on processing (at scale) of cPPA in both solvent/tape casting as well as thermoforming via hot press molding. A photoacid generator (PAG) was incorporated into cPPA films to generate acid within the film in response to exposure to UV light. Acid cleaved the polymer backbone of cPPA and resulted in solid state depolymerization into the monomer oPA. The depolymerization kinetics in response to UV light were characterized by dynamic mechanical analysis and FTIR. These polymer films were then used as substrates for the fabrication of UV triggered transient electronics that were destroyed by exposure to UV light. Transience rates were tuned by modifying the PAG concentration and the irradiance of the UV source. A thermoacid generator (TAG) was also incorporated into cPPA films to generate acid in response to thermal heating. The TAG poly(vinyl tert-butyl carbonate sulfone) (PVtBCS) forms acid at moderate temperatures (ca. 85 °C) in the presence of water and also depolymerizes into purely volatile products. Upon heating to 85 °C, the PVtBCS/cPPA films generated acid and began to depolymerize and evaporate due to volatility of the monomeric byproducts. Thermally-triggered films evaporate to leave < 2 wt% residual mass and depolymerize rapidly with tailorable depolymerization kinetics. The depolymerization kinetics can be accelerated by increasing the triggering temperature or increasing the concentration of PVtBCS in the PVtBCS/cPPA film. It is demonstrated that the total time for depolymerization is much less than the time required for complete evaporation. Solvent-based approaches were developed for the processing of cPPA. Uniform cPPA films were fabricated with both solvent casting and tape casting methods. It was discovered that a considerable amount of residual solvent remained in the polymer films after processing. The mechanical properties of the cPPA films and their dependence on several processing parameters were assessed. As expected, the parameters that resulted in a reduction of the residual solvent concentration led to an increase in the Tg of the cPPA. The volatility of the solvent played a large role in the plasticization of the films and an inverse relation between Tg and boiling point of the solvent was demonstrated. The elastic modulus, ultimate tensile strength, and Tg all increased as the residual solvent concentration decreased. The thermally-triggered depolymerization of neat cPPA was investigated and tailored in order to enable thermal processing routes towards the thermoforming and molding of cPPA. Stabilization of cPPA at elevated temperature was accomplished by removal of the latent Lewis acid catalyst BF3 and by addition of radical inhibitors and a Lewis base. Addition of a plasticizer to the stabilized cPPA significantly depressed the thermal transitions of the stabilized cPPA below the onset temperature of depolymerization, opening a thermal processing route for cPPA. A monolithic solid cPPA polymer was fabricated via hot press molding at 100 °C without initiating thermal depolymerization of cPPA