Acid‐catalyzed degradation mechanism of poly(phthalaldehyde): Unzipping reaction of chemical amplification resist

Polymers for Advanced Techs

Phillips

Engler

et al. 2019

Metastable poly(phthalaldehyde) (PPHA) can be triggered to depolymerize under visible light by incorporation of photosensitive compounds, such as a photoacid generator (PAG), which can generate a strong acid in situ. However, photosensitive compounds can be thermally unstable and have limited shelf life, causing inadvertent device triggering. It can also be difficult to fabricate components that are photosensitive because special lighting conditions are needed. In this paper, nonphotosensitive PPHA films were formed and made photosensitive at the point of use. This improved the material shelf life and manufacturability by adding a second, PAG‐containing layer to the original nonphotosensitive layer at an optimal point before use. The catalytic photoacid was generated rapidly by exposure of the PAG‐containing layer to radiation. Depolymerization of PPHA via the acid catalyst was followed by diffusion of the acid into the nonphotosensitive layer causing it to depolymerize. Diffusion of the photoacid into the nonphotosensitive medium was quantified at various temperatures. Photoacid diffusion in a liquid, moving‐front caused depolymerization of the nonphotosensitive PPHA layer. The fabricated bilayer structure allowed for better stability of the structural material using PPHA while still achieving transience.

Section: Resultsmentioning

confidence: 99%

“…PPHA is a self‐immolative polymer where breaking an acetal linkage in the cyclic PPHA polymer backbone leads to cascading chain unzipping resulting in the polymer reverting back to its monomer form at temperatures above its ceiling temperature, ca. −42°C …”

Section: Introductionmentioning

confidence: 99%

Photodegradable transient bilayered poly(phthalaldehyde) with improved shelf life

Polymers for Advanced Techs

Phillips

Engler

et al. 2019

“…The reaction scheme is shown in Figure 2a. [25] Upon ring-opening of cyclic PPHA, the end-group unprotected PPHA would rapidly depolymerize back to oPA due to its low ceiling temperature nature. [24], ensured the formation of high molecular PPHA until the reaction equilibrium was achieved.…”

Section: Wwwadvelectronicmatdementioning

confidence: 99%

Sunlight‐Triggerable Transient Energy Harvester and Sensors Based on Triboelectric Nanogenerator Using Acid‐Sensitive Poly(phthalaldehyde)

Guo

et al. 2019

Adv Elect Materials

operation. [5] They have found potential applications as medical diagnostic and therapeutic devices that can resorb into the body, environmental sensors that do not require recovery after data collection, and consumer devices that can be easily disposed without hazards. [6] The potential feature of disappearance with minimal or non-traceable remains also enables transient electronics to find opportunities in privacy or security applications where stealth is required or reverse engineering needs to be avoided. The transience property is largely attributed to the triggerable degradation of device substrates and encapsulation, which are typically made of a group of materials called transient polymers. Depending on constituting material, the transience can be achieved either through material dissolution or depolymerization. Pioneering efforts on transient electronics focused on solution dissolution of polymer matrix. Bioresorbable polymers such as silk, polycaprolactone, poly(glycolic acid), and poly(L-lactide-co-glycolide), were used as substrates, [7][8][9] and biocompatible, implantable devices such as transistor, [1] ring oscillators, [10] and energy harvesters, [11] have been demonstrated. The lifetime of such devices is controlled by the dissolution rate of the materials in the surrounding aqueous solution, making them unsuitable for non-biological applications. Transient electronics that disintegrate via material dissolution or depolymerization under certain stimuli have great potential in biomedical and military applications. The triboelectric nanogenerator (TENG), an emerging mechanical energy harvesting technology with great flexibility in material choices, is promising in offering transient power sources. Previously reported transient energy harvesters using biodegradable polymers require solutionbased degradation and have limited applications in non-biological scenarios. A short time span, sunlight-triggered degradable TENG is developed. The main substrate includes an acid-sensitive poly(phthalaldehyde) (PPHA), a photoacid generator (PAG), and a photosensitizer (PS). Through photoinduced electron transfer, the ultraviolet radiation absorbed by the PS istransferred to the PAG to generate photoacids that trigger the depolymerization of PPHA. Transient TENG-based mechanical energy harvesters and touch/acoustic sensors are successfully demonstrated by embedding silver nanowires onto the PPHA-based films. The fabricated devices degrade rapidly under winter sunlight. The degradation rate can be further tuned via changing the ratio of photosensitive agents. This work not only broadens the applicability of TENG as transient power sources and sensors, but also extends the use of transient functional polymers toward advanced energy and sensing applications.

“…The removal of the kinetic‐trap can induce rapid, unzipping depolymerization at room temperature . The PPHA backbone is susceptible to cleavage via free‐acid protonation of the acetal linkage . A recent study used polymeric thermal acid generators, such as poly(vinyl t‐butyl carbonate sulfone), to increase the depolymerization kinetics at low temperature .…”

Section: Introductionmentioning

confidence: 99%

“…13,14 The PPHA backbone is susceptible to cleavage via free-acid protonation of the acetal linkage. 15 A recent study used polymeric thermal acid generators, such as poly(vinyl t-butyl carbonate sulfone), to increase the depolymerization kinetics at low temperature. 16 A specific-ion coactivation effect at the surface of cyclic PPHA microcapsules has been shown to accelerate depolymerization in acidic methanol solution.…”

Section: Introductionmentioning

confidence: 99%

Time‐delayed photo‐induced depolymerization of poly(phthalaldehyde) self‐immolative polymer via in situ formation of weak conjugate acid

Polymers for Advanced Techs

Phillips

Engler

et al. 2019

Poly(phthalaldehyde) (PPHA) can be used as a structural material in transient devices and photo‐catalytically depolymerized at the end of device life by the use of a photo‐acid generator (PAG). However, device degradation requires the presence of a radiation source at the end of device mission. It has been found that the onset of PPHA depolymerization after PAG photo‐exposure can be delayed by incorporation of a particular weak bases in the PPHA/PAG mixture. This method of delayed PPHA depolymerization allows for PAG activation prior to or during device deployment when the device is under full user control. The basicity of specific lactams and amides was found to slow the PPHA depolymerization, giving the transient device a longer but finite mission lifetime. The weak base reacts with the photo‐generated strong acid to form a weak conjugate acid, which reacts more slowly with PPHA to extend the onset of PPHA depolymerization. The addition of a molar excess of specific lactams or amides, with respect to PAG, maintains PPHA stability and mechanical properties for more than 80 minutes after photo‐exposure at room temperature. The amide or lactam mediated acid activation of PPHA follows first‐order kinetics. The time delay of PPHA depolymerization can allow for prelaunch photo‐exposure and eliminates the need for postmission photo‐exposure where reliable light‐sources may not be available.