Noncovalent Photochromic Polymer Adhesion

Recent Development of Photochromic Polymer Systems: Mechanism, Materials, and Applications

Photochromic polymer is defined as a series of materials based on photochromic units in polymer chains, which produces reversible color changes under irradiation with a particular wavelength. Currently, as the research progresses, it shows increasing potential applications in various fields, such as anti-counterfeiting, information storage, super-resolution imaging, and logic gates. However, there is a paucity of published reviews on the topic of photochromic polymers. Herein, this review discusses and summarizes the research progress and prospects of such materials, mainly summarizing the basic mechanisms, classification, and applications of azobenzene, spiropyran, and diarylethene photochromic polymers. Moreover, 3-dimensional (3D) printable photochromic polymers are worthy to be summarized specifically because of its innovative approach for practical application; meanwhile, the developing 3D printing technology has shown increasing potential opportunities for better applications. Finally, the current challenges and future directions of photochromic polymer materials are summarized.

Generating Stable Nitrogen Bubble Layers on Poly(methyl methacrylate) Films by Photolysis of 2-Azidoanthracene

2-Azidoanthracene (2N3-AN) can act as a photochemical source of N2 gas when dissolved in an optically transparent polymer such as poly(methyl methacrylate) (PMMA). Irradiation at 365 or 405 nm of a 150 μm-thick polymer film submerged in water causes the rapid appearance of a surface layer of bubbles. The rapid appearance of surface bubbles cannot be explained by normal diffusion of N2 through the polymer and likely results from internal gas pressure buildup during the reaction. For an azide concentration of 0.1 M and a light intensity of 140 mW/cm2, the yield of gas bubbles is calculated to be approximately 40%. The dynamics of bubble growth depend on the surface morphology, light intensity, and 2N3-AN concentration. A combination of nanoscale surface roughness, high azide concentration, and high light intensity is required to attain the threshold N2 gas density necessary for rapid, high-yield bubble formation. The N2 bubbles adhered to the PMMA surface and survived for days under water. The ability to generate stable gas bubbles "on demand" using light permits the demonstration of photoinduced flotation and patterned bubble arrays.

Disordered Low Molecular Weight Spiropyran Exhibiting Photoregulated Adhesion Ability

The functions of the materials composed of small molecules are highly dependent on their ordered molecular arrangements in both natural and artificial systems. Without ordered structure, small molecules hardly gain complicated functions, due to the absence of intermolecular covalent bond connection or strong network. Here, a low molecular weight spiropyran that could exhibit attractive photochromism and powerful adhesion property in disordered solid state is demonstrated. With maximum up to ∼8 MPa, the adhesion strength could be photoregulated in multiple levels, which also shows one-to-one correspondence to the specific color state. The working mechanism analysis on the photoregulated adhesion reveals that the isomer ratio of merocyanine form and the molecular packing density of spiropyran are the determining factors for the adhesion ability. The discovery of photoregulated adhesion from pure spiropyran provides a new strategy for developing functional materials by employing low molecular weight compounds.

Assessing Polymer-Surface Adhesion with a Polymer Collection

Polymer modification plays an important role in the construction of devices, but the lack of fundamental understanding on polymer-surface adhesion limits the development of miniaturized devices. In this work, a thermoplastic polymer collection was established using the combinatorial laser-induced forward transfer technique as a research platform, to assess the adhesion of polymers to substrates of different wettability. Furthermore, it also revealed the influence of adhesion on dewetting phenomena during the laser transfer and relaxation process, resulting in polymer spots of various morphologies. This gives a general insight into polymer-surface adhesion and connects it with the generation of defined polymer microstructures, which can be a valuable reference for the rational use of polymers.

Stimuli-responsive temporary adhesives: enabling debonding on demand through strategic molecular design

Stimuli-responsive temporary adhesives constitute a rapidly developing class of materials defined by the modulation of adhesion upon exposure to an external stimulus or stimuli. Engineering these materials to shift between two characteristic properties, strong adhesion and facile debonding, can be achieved through design strategies that target molecular functionalities. This perspective reviews the recent design and development of these materials, with a focus on the different stimuli that may initiate debonding. These stimuli include UV light, thermal energy, chemical triggers, and other potential triggers, such as mechanical force, sublimation, electromagnetism. The conclusion discusses the fundamental value of systematic investigations of the structure-property relationships within these materials and opportunities for unlocking novel functionalities in future versions of adhesives.

Structural and Electronic Characterization of a Photoresponsive Lanthanum(III) Complex Incorporated into Electrospun Fibers for Phosphate Ester Catalysis

Herein, we present the light-induced synthesis and characterization of a La³⁺/spiropyran derivative complex (LaMC) and its application as a catalyst when incorporated into electrospun polycaprolactone (PCL) fibers. In addition to experimental methods, computational calculations were also essential to better understand the structure and electronic characteristics of LaMC. The LaMC complex was identified as a 10-coordinated structure with the La³⁺ ion coordinated by four oxygens from the phenolate and the carbonyl of the carboxyl acid group from both MC ligands and by six oxygens from three nitrate ligands. In addition, LaMC was capable of getting reversibly isomerized by UV or visible light cycling. All PCL fibers were successively obtained, and their morphologies, surface properties, and catalytic behavior were studied. Results showed that PCL/LaMC fibers were capable of catalyzing bis(2,4-dinitrophenyl)phosphate degradation efficiently. Complete hydrolysis was accomplished in only 1.5 days relative to the half-life time of 35 days for the uncatalyzed hydrolysis at pH 8.1 and 25 °C.

Diarylethene Dyes

This article introduces the general characteristics of the diarylethene class of photochromic dye and the structural features that make photochromism possible. It touches on the methodologies employed to synthesize these compounds as well as the influences that typical substitution patterns exert on photocoloration. A demonstration is then given of the great diversity pertaining to the potential applications in which researchers are seeking to exploit them as functional colorants.

Spiropyran-based photoswitchable dimethylaminopyridine

A dimethylaminopyridine-embedded spiropyran compound switches its coordination capability under light.

Rewritable Anticounterfeiting Polymer Inks Based on Functionalized Stimuli-Responsive Latex Particles Containing Spiropyran Photoswitches: Reversible Photopatterning and Security Marking

Increase of safety in security documents by using anticounterfeiting inks based on fluorochromic and photochromic compounds has attracted a great deal of attention in the recent years. Herein, we developed novel functionalized stimuli-responsive latex particles containing spiropyran (1 wt %) by semicontinuous emulsifier-free emulsion polymerization, which are usable as anticounterfeiting inks for marking on security documents and also photopatterning on cellulosic papers. The size and morphology of the latex particles were characterized by scanning electron microscopy and dynamic light scattering and their functionality was characterized by Fourier-transform infrared spectroscopy. All the stimuli-responsive latexes are composed of spherical particles with different hydroxyl, epoxy, and carboxylic acid functional groups, and the size of the particles varies in the range of 400-900 nm. Additionally, the latex particles undergo a remarkable light-induced size variation (aggregation-disaggregation) upon UV illumination (365 nm), depending on the functional group type, as a result of π-π stacking interactions and also electrostatic attractions between the different particles. The photochromic behavior, kinetics of the SP ⇌ MC isomerization, photoswitchability, and photofatigue-resistant characteristics of the prepared latexes were extensively investigated. The results display that the photochromic behavior and SP ⇌ MC isomerization can significantly be influenced by the polar interactions between the functional groups and MC molecules. As a novel application, the prepared stimuli-responsive latexes were used as anticounterfeiting inks for writing on cellulosic paper and also security marking on several monies, where the written phrase displayed red fluorescence emission and coloration under and after UV illumination (365 nm), respectively. Additionally, the latexes were sprayed on cellulosic papers to prepare stimuli-responsive papers for investigation of their photopatterning ability under UV irradiation and different masking. The presence of functional groups and large particle sizes are the main effective factors for stabilization of the latex particles on cellulosic papers. This is the first report on application of functionalized stimuli-responsive latex particles containing spiropyran as anticounterfeiting inks for security marking and photopatterning on cellulosic papers, directly and without using further additives.