Enhanced Mechanochemiluminescence from End-Functionalized Polyurethanes with Multiple Hydrogen Bonds

Multicolor Luminescence of a Polyurethane Derivative Driven by Heat/Light-Induced Aggregation

The study of aggregate formation and its controllable effect on luminescence behavior has a far-reaching influence in establishing a universal aggregation photophysical mechanism. In this paper, we obtained clusters with different extents of aggregation by heat-induced or light-triggered aggregation of a new polyurethane derivative (PUE). The controllable regulation of multicolor fluorescence of a single (nondoped) polymeric material is realized. The luminescence behavior of PUE varies with microscopic control of the aggregation structure. Compared with the powder state, the enhanced atom-atom and group-group interactions of PUE-gel effectively limit the nonradiative transitions in the excited state and result in a red-shift in emission. This work avoids complex organic synthesis and demonstrates a simple strategy to induce aggregation and regulate the emitting color of macromolecules, providing a template for developing new materials for multicolor fluorescence. In addition, a pattern was constructed with encryption, anticounterfeiting, and information transmission functions which provide a proof-of-concept demonstration of the practical potential of PUE as a smart material.

Mechanochemical Release of Fluorophores from a "Flex-activated" Mechanophore

Optical force probes that can release force-dependent and visualized signals with minimal changes in the polymer main chains under mechanical load are highly sought after but currently limited. In this study, we introduce a flex-activated mechanophore (FA) based on the Diels-Alder adduct of anthracene and dimethyl acetylenedicarboxylatea that exhibits turn-on mechanofluorescence. We demonstrate that when FA is incorporated into polymer networks or in its crystalline state, it can release fluorescent anthracenes through a retro-Diels-Alder mechanochemical reaction under compression or hydrostatic high pressure, respectively. The flex-activated mechanism of FA is successfully confirmed. Furthermore, we systematically modulate the force delivered to the mechanophore by varying the crosslinking density of the networks and the applied macroscopic pressures. This modulation leads to incremental increases in mechanophore activation, successive release of anthracenes, and quantitative enhancement of fluorescence intensity. The exceptional potential of FA as a sensitive force probe in different bulk states is highlighted, benefiting from its unique flex-activated mode with highly emissive fluorophore releasing. Overall, this report enriches our understanding of the structures and functions of flex-activated mechanophores and polymeric materials.

Thermally Stable Photomechanical Molecular Hinge: Sterically Hindered Stiff-Stilbene Photoswitch Mechanically Isomerizes

Molecular photoswitches are extensively used as molecular machines because of the small structures, simple motions, and advantages of light including high spatiotemporal resolution. Applications of photoswitches depend on the mechanical responses, in other words, whether they can generate motions against mechanical forces as actuators or can be activated and controlled by mechanical forces as mechanophores. Sterically hindered stiff stilbene (HSS) is a promising photoswitch offering large hinge-like motions in the E/Z isomerization, high thermal stability of the Z isomer, which is relatively unstable compared to the E isomer, with a half-life of ca. 1000 years at room temperature, and near-quantitative two-way photoisomerization. However, its mechanical response is entirely unexplored. Here, we elucidate the mechanochemical reactivity of HSS by incorporating one Z or E isomer into the center of polymer chains, ultrasonicating the polymer solutions, and stretching the polymer films to apply elongational forces to the embedded HSS. The present study demonstrated that HSS mechanically isomerizes only in the Z to E direction and reversibly isomerizes in combination with UV light, i.e., works as a photomechanical hinge. The photomechanically inducible but thermally irreversible hinge-like motions render HSS unique and promise unconventional applications differently from existing photoswitches, mechanophores, and hinges.

High-Performance Intrinsically Stretchable Polymer Solar Cell with Record Efficiency and Stretchability Enabled by Thymine-Functionalized Terpolymer

Designing new polymer semiconductors for intrinsically stretchable polymer solar cells (IS-PSCs) with high power conversion efficiency (PCE) and durability is critical for wearable electronics applications. Nearly all high-performance PSCs are constructed using fully conjugated polymer donors (P_D) and small-molecule acceptors (SMA). However, a successful molecular design of P_Ds for high-performance and mechanically durable IS-PSCs without sacrificing conjugation has not been realized. In this study, we design a novel thymine side chain terminated 6,7-difluoro-quinoxaline (Q-Thy) monomer and synthesize a series of fully conjugated P_Ds (PM7-Thy5, PM7-Thy10, PM7-Thy20) featuring Q-Thy. The Q-Thy units capable of inducing dimerizable hydrogen bonding enable strong intermolecular P_D assembly and highly efficient and mechanically robust PSCs. The PM7-Thy10:SMA blend demonstrates a combination of high PCE (>17%) in rigid devices and excellent stretchability (crack-onset value >13.5%). More importantly, PM7-Thy10-based IS-PSCs show an unprecedented combination of PCE (13.7%) and ultrahigh mechanical durability (maintaining 80% of initial PCE after 43% strain), illustrating the promising potential for commercialization in wearable applications.

Multi-material 3D Printing of Mechanochromic Double Network Hydrogels for On-Demand Patterning

Cephalopods can change their color and patterns by activating the skin chromatophores for camouflage. However, in the man-made soft material systems, it is greatly challenging to fabricate the color-change structure in the desired patterns and shapes. Herein, we employ a multi-material microgel direct ink writing (DIW) printing method to make mechanochromic double network hydrogels in arbitrary shapes. We prepare the microparticles by grinding the freeze-dried polyelectrolyte hydrogel and immobilize the microparticles in the precursor solution to produce the printing ink. The polyelectrolyte microgels contain mechanophores as the cross-linkers. We adjust the rheological and printing properties of the microgel ink by tailoring the grinding time of freeze-dried hydrogels and microgel concentration. The multi-material DIW 3D printing technique is utilized to fabricate various 3D hydrogel structures which could change into a colorful pattern in response to applied force. The microgel printing strategy shows great potential in the fabrication of the mechanochromic device with arbitrary patterns and shapes.

Hybrid phenol-rhodamine dye based mechanochromic double network hydrogels with tunable stress sensitivity

Mechanochromic hydrogels, which can switch their color in response to the applied external force, have shown great potential in the stress visualization and damage indication. However, the kinds of the colors in the reported mechanochromic hydrogels are limited. It is challenging to develop the mechanochromic hydrogels with new kinds of color change. Herein, we report a kind of mechanochromic double network (DN) hydrogel based on the hybrid phenol-rhodamine (HPR) mechanophore. The hydrogels turn into orange color with an emission wavelength of around 566/574 nm in response to tensile and compressive stress. The DN hydrogels show great reversibility. The color of DN hydrogels vanishes slowly after releasing the stress. The stress sensitivity can be tailored by the crosslinking density and the mechanophore concentration of the first network. In addition, the influence of the pH on the mechanochromic properties of DN hydrogels is also studied. This study provides an insightful study in tuning the stress sensitivity in the mechanochromic hydrogel, which would be beneficial for the development of the mechanochromic materials. This article is protected by copyright. All rights reserved.

Room-Temperature Solid-State UV Cross-Linkable Vitrimer-like Polymers for Additive Manufacturing

In this paper, a UV cross-linkable vitrimer-like polymer, ureidopyrimidinone functionalized telechelic polybutadiene, is reported. It is synthesized in two steps. First, 2(6-isocyanatohexylaminocarbonylamino)-6-methyl-4[1H]-pyrimidinone (UPy-NCO) reacts with hydroxy-functionalized polybutadiene (HTPB) to obtain UPy-HTPB-UPy, and then the resulted UPy-HTPB-UPy is cross-linked under 365 nm UV light (photo-initiator: bimethoxy-2-phenylacetophenone, DMPA). Further investigation reveals that the density of cross-linking and mechanical properties of the resulting polymers can be tailored via varying the amount of photo-initiator and UV exposure time. Before UV cross-linking, UPy-HTPB-UPy is found to be vitrimer-like due to the quadruple hydrogen-bonding interactions. The UPy groups at the end of the chain also enable for rapid solidification upon the evaporation of the solvent. The unsaturated double bonds in the HTPB chains enable UPy-HTPB-UPy to be UV cross-linkable in the solid state at room temperature. After cross-linking, the polymers have good shape memory effect (SME). Here, we demonstrate that this type of polymer can have many potential applications in additive manufacturing. In the cases of fused deposition modelling (FDM) and direct ink writing (DIW), not only the strength of the interlayer bonding but also the strength of the polymer itself can be enhanced via UV exposure (from thermoplastic to thermoset) either during printing or after printing. The SME after cross-linking further helps to achieve rapid volumetric additive manufacturing anytime and anywhere.

Mechanochromic cyclodextrins

Mechanochromic cyclodextrins (MCDs) that can generate blue radical species, which are exceptionally stable toward atmospheric oxygen and can thus be quantitatively characterized via electron paramagnetic resonance (EPR) spectroscopy, were synthesized. MCDs have a defined structure that consists of a diarylbibenzofuranone skeleton mechanophore sandwiched between two CDs. Grinding tests and EPR measurements of the MCDs revealed their high mechanoresponsiveness, reflecting the inherent rigidity of the CDs and the formation of a supramolecular structure in the bulk.

End-Terminated Poly(urethane-urea) Hybrid Approach toward Nanoporous/Microfilament Morphology

In the present work, the effect of heteroatomic hydrogen bonding on the properties of -OH/-NH-terminated soft-segment-free polymers, viz, polyurethane (P-UT), polyurea (P-UR), and their hybrid (P-UT-UR), is explored. P-UT was synthesized from phloroglucinol and P-UR was synthesized from 1,3,5-triazine-2,4,6-triamine by employing hexamethylene diisocyanate as a counterpart. P-UT exhibited a spherulitic structure with varying sizes, whereas P-UR displayed a fibrillar structure characteristic as that of crystalline hard segments. The P-UT-UR hybrid exhibited a fine nanospherulitic structure with a high order of interconnectivity. Negative surface skewness values of -0.47 and -0.18 were measured (by AFM) for P-UT and P-UT-UR, respectively, which revealed that the surface is not smooth and is covered with features. Due to the increased H-bonding (-N-H···O-H) in P-UT-UR, its transparency decreased. A block copolymer hybrid of urethane-urea was synthesized, which preferred homoatomic H-bonding, whereas random urethane/urea bridges favored hetreoheteroatom H-bonding. A pentafluorophenyl end-functional hybrid (PFI-P-UT-UR) was synthesized, which displayed filaments of ∼2-3 μm length in contrast to the interconnected nanospherulitic structure observed for P-UT-UR. The self-aggregation and end folding led to the formation of a filament structure. By altering the chemical structure slightly, nano-ordered polyurethanes or their hybrids can be achieved.

Mechanochemiluminescent hydrogels for real-time visualization of chemical bond scission

Quantitative and real-time characterization of mechanically induced bond scission events taken place in polymeric hydrogels is essential to uncover their fracture mechanics. Herein, a class of mechanochemiluminescent swelling hydrogels have been synthesized through a facile micellar copolymerization method using chemiluminescent bis(adamantyl)-1,2-dioxetane (Ad) as a crosslinker. This design and synthetic strategy ensure intense mechanochemiluminescence from Ad located in a hydrophobic network inside micelles. Moreover, the mechanochemiluminescent colors can be tailored from blue to red by mixing variant acceptors. Taking advantages of the transient nature of dioxetane chemiluminescence, the damage distribution and crack evolution of the hydrogels can be visualized and analyzed with high spatial and temporal resolution. The results demonstrate the strengths of the Ad mechanophore and micellar copolymerization method in the study of damage evolution and fracture mechanism of swelling hydrogels.