Stimuli-responsive poly(phthalaldehyde) (PPHA) polymer is a potential structural material for transient devices. It can be synthesized at high molecular weight, has long shelf-life, and undergoes rapid depolymerization when triggering by a strong acid. However, the rigid PPHA backbone results in poor mechanical flexibility. The high melting point of ortho-phthalaldehyde monomer (ca. 55 degrees C) causes the depolymerized polymer to crystalize and not flow. This paper addresses the need for improving the flexibility, toughness and melting point of the depolymerized products. This need is addressed by incorporating additives into the PPHA mixture. Ionic liquid and ether-ester plasticizers were added to make PPHA films more flexible by decreasing the storage modulus. The addition of both types of plasticizers broadened the storage modulus range to as low as 19 MPa from 2300 MPa. Films from the low-modulus mixtures were foldable and had excellent photo-transience at ambient and sub-ambient temperatures. The addition of both types of plasticizers also enhanced the photo-transient properties, as they helped to maintain the liquid state of depolymerized product for long times at lower temperatures so that the products can be absorbed into the environment. The discovery of ionic liquid and ether-ester plasticizers for PPHA expands and improves upon the applications for PPHA-based transient devices.