Stimuli-Responsive Epoxy Coatings

Abstract

The design, formulation, and characterization of new epoxy coatings with built-in chemical and threshold temperature sensors are reported. The materials were prepared by dissolving a chromogenic, fluorescent oligo(p-phenylenevinylene) dye into a cross-linked epoxy resin by reacting monomer/cross-linker/dye mixtures at 180-200 degrees C and quenching the cured polymer to below its glass transition temperature (T(g)). Subjecting these kinetically trapped, thermodynamically unstable molecularly mixed epoxy/dye blends to temperatures above T(g) leads to aggregation of the dye molecules and causes permanent and pronounced changes of their optical absorption and fluorescence properties. Exposure of the materials to selected chemical stimuli, e.g., water, acid, base, and several organic compounds, also causes plasticization of the polymer matrix and leads to irreversible aggregation of dye molecules, concomitant with the pronounced fluorescence and absorption color change. This response is well described by standard kinetic models and can be controlled via the chemical structure and cross-link density of the resin and the dye content.

Luminescent Mechanochromic Sensors Based on Poly(vinylidene fluoride) and Excimer‐Forming p‐Phenylene Vinylene Dyes

“…The approach exploits the excimer-forming properties of chromophores such as cyano-substituted oligo(phenylene vinylene)s [7][8][9][10][11][12] (cyano-OPVs, Figure 1) and further relies on the stimulus-driven self-assembly or dispersion of nano-scale aggregates of these sensor dyes in a range of host polymers. We have shown that this general concept of stimulus-triggered dye (dis)assembly in polymer matrices allows for the design of a broad range of sensor materials, which are useful for the detection of temperature history, [13][14][15][16][17] exposure to chemicals, [18,19] and mechanical deformation, [20][21][22][23][24] as well as more complex combinations of stimuli, such as seen in shape-memory materials. [25] Others have adapted the concept and extended it to a range of dyes including cyano containing poly(phenylene ethynylenes), [26] perylenes, [27] CdS nanoparticles, [28] and bis(benzoxazolyl)stilbenes.…”

“…[29] The temperature and humidity sensors based on such polymer/dye blends operate by kinetically trapping a thermodynamically unstable molecular mixture of the components by rapidly cooling a hot (and at this temperature miscible) mixture below its glass transition temperature (T g ). [13,19] If the material is subsequently heated above T g , the system becomes sufficiently mobile to equilibrate, resulting in aggregation of the dye molecules and the formation of excimers. This approach yields timetemperature indicating materials that exhibit a pronounced fluorescence color change, whose kinetics follow a predictable, Arrhenius-type behavior.…”

“…The same concept can be applied to detect other chemical stimuli. [19] The mechanically responsive systems based on this framework rely on the inverse mechanism, namely the dispersion of nano-scale dye aggregates upon deformation of the material. Systematically investigating several model systems, including polyolefins, [14,[20][21][22][23] polyurethanes, [22] and polyesters, [14,15,25] we explored how the nature of the polymer, dye concentration and solubility in the host polymer, dye aggregate size, and effectiveness of the dye aggregate break-up influence the mechanochromic response of such materials.…”

Mechanochemistry in Polymers with Supramolecular Mechanophores

“…While the study did also not involve well-defined supramolecular assemblies, the results paved the way for subsequent work on mechanochromic polymers comprising assemblies of aggregachromic, excimer-forming cyano-substituted oligo(p-phenylene vinylene)s (cyano-OPVs, Figures 2-4). [39][40][41][42][43][44][45][46][47][48][49][50][51][52][53][54] Scanning confocal microscopy images of blend films of ultra-high-molecular-weight polyethylene and 10 -4 % w/w EHO-OPPE (Figure 1). a) As prepared film.…”

Stimuli‐Driven Assembly of Chromogenic Dye Molecules: a Versatile Approach for the Design of Responsive Polymers

“…Initial kinetic experiments reflect a behavior that mirrors the response of the thermally triggered systems discussed above: as expected, the switching from monomer to excimer emission is much faster for samples exposed to higher humidity levels and the color changes appear again to be well described by single exponential functions. The framework was recently also adapted to create epoxy-based, thermosetting epoxy resins, as examples for thermosetting polymers with integrated sensing capabilities [74]. Such resins are widely used for decorative, protective, and/or functional coatings [75].…”