We demonstrate a single-phase, two-way shape actuator that, in the absence of an external load, elongates upon cooling and reversibly contracts upon heating. In a simple and straightforward process, a partially cross-linked, semicrystalline poly­(ε-caprolactone) (PCL) network is melted, stretched to several hundred percent strain, and further cross-linked. Upon removal of the applied load, the elastic double network adopts a “state-of-ease” that retains part of its former strain. When cooled, internal stress-induced crystallization causes further elongation of configurationally biased chains. When heated, crystallites melt, and the sample returns to its equilibrium state-of-ease. Under optimized conditions, reversible actuation >15% strain can be reproducibly achieved, and samples can be cycled multiple times with highly uniform actuation with no observable creep. The mechanism behind such actuation was further confirmed via calorimetry and X-ray scattering.

Shape‐memory polymers (SMPs) that respond near body temperature are attracting broad interest, especially in the biomedical fields. In this study, the triggering temperature of poly(caprolactone) SMP networks is precisely adjusted by inclusion of non‐crystallizable molecular linkers and by variation of prepolymer molecular weight. Longer, non‐crystalline linkers and lower molecular weight prepolymers interfere with crystallization, lowering the transition temperature. Networks are prepared with crystallization temperatures that are beneath the human body temperature and yet are above room temperature. Upon cooling such amorphous networks to room temperature, crystallization is sluggish. There, elastomers can be easily strained by several hundred‐percent to induce crystallization, thereby fixing strained states. If subsequently heated, programmed SMPs can release significant amounts of stored strain energy (∼3 MJ/m3). SMPs that combine elastic energy storage and exhibit triggering temperatures near the human body temperature could benefit emerging applications in the biomedical space. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 1397–1404

The study and development of transient devices is an emerging field where the disposal of a device after use is desired to avoid reverse engineering and minimize the environmental impact. Polyaldehydes with phototriggers have been investigated because the radiation wavelength can be adjusted to meet the transient application. Polynuclear aromatic hydrocarbons (PAHs) were used as the optical sensitizer for photoacid generators (PAGs). Photoinduced electron transfer (PET) with an iodonium-based PAG was used to expand the spectral sensitivity range. Anthracene, tetracene, and pentacene derivatives were synthesized with appended phenylethynyl groups to improve the solubility of the sensitizer and adjust the absorption wavelength. Sensitization of the iodonium-based PAG with the PAH derivatives was found to have thermodynamically favorable PET reactions for depolymerization of poly(propylene carbonate) and poly(phthalaldehyde) (PPHA). The Rehm-Weller equation and Stern-Volmer analysis were used to study the electron transfer and the fluorescence quenching rates of the PAHs with the iodonium salts, respectively. The photosensitivity, efficiency, and byproducts of the PET reactions in the decomposable polymer films are reported. A rapid photoreaction is reported for the depolymerization of PPHA exposed to a sunlight dose of <6 J cm −2 (i.e., 1 min of direct sunlight) with a pentacene-based sensitizer.

Fixing chain orientation within polymeric materials can impart anisotropic mechanical, optical, and electrical properties. Although macroscopic anisotropy in amorphous or liquid crystalline phases has been achieved by cross-linking or by thermoreversible bond shuffling under strain, these methods lack spatial and temporal resolution. Here, we demonstrate a method to controllably write chain anisotropy into polymer networks containing both permanent and light-sensitive bonds. While held under mechanical stress or strain, light initiates a cascade of addition–fragmentation chain transfer reactions, causing photosensitive functional groups to reshuffle, thereby stabilizing the deformed network. Photoinscription of chain anisotropy allows for simplified processing on fully cross-linked networks with spatial and temporal control over chain orientation, thus enabling a spectrum of anisotropic polymeric materials. As an example, we demonstrate how built-in anisotropy of a semicrystalline network encourages crystallization along a preferred direction, leading to fully reversible shape actuation.

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.

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.

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Epoxide functionalized poly(propylene carbonate) (ePPC) was included in an epoxy resin formulation and thermally decomposed to create nanoporous epoxy film. The dielectric constant of the porous epoxy was lower than the epoxy formulation control. The introduction of 30% porosity in the epoxy lowered the dielectric constant from 3.78 to 2.76. A postporosity chemical treatment further lowered the dielectric constant. Hexamethyldisilazane (HMDS) was used to terminate the pore walls with the hydrophobic silane layer and reduce both the dielectric constant and tangent loss of the porous epoxy. Two different styrene maleic anhydride crosslinking agents were used in the epoxy formulation, styrene maleic anhydride 2000 (SMA2000) and styrene maleic anhydride 4000 (SMA4000). The effect of the maleic anhydride concentration within SMA on the electrical, mechanical, and thermal properties of porous epoxy film was evaluated. Epoxy films crosslinked with SMA2000 resulted in films with a higher dielectric constant compared to films prepared with SMA4000 due to higher mole fraction of maleic anhydride within SMA2000. However, SMA2000 crosslinked films yielded films with better mechanical and thermal properties. SMA2000 crosslinked films with 30% porosity had a coefficient of thermal expansion (CTE) of 35.2 ppm/K and glass transition temperature of 143 °C.