3D fluorescent mapping of invisible molecular damage after cavitation in hydrogen exposed elastomers

Visualizing fiber end geometry effects on stress distribution in composites using mechanophores

Localized stress concentrations at fiber ends in short fiber-reinforced polymer composites (SFRCs) significantly affect their mechanical properties. Our research targets these stress concentrations by embedding nitro-spiropyran (SPN) mechanophores into the polymer matrix. SPN mechanophores change color under mechanical stress, allowing us to visualize and quantify stress distributions at the fiber ends. We utilize glass fibers as the reinforcing material and employ confocal fluorescence microscopy to detect color changes in the SPN mechanophores, providing real-time insights into the stress distribution. By combining this mechanophore-based stress sensing with finite element analysis (FEA), we evaluate localized stresses that develop during a single fiber pull-out test near different fiber end geometries-flat, cone, round, and sharp. This method precisely quantifies stress distributions for each fiber end geometry. The mechanophore activation intensity varies with fiber end geometry and pull-out displacement. Our results indicate that round fiber ends exhibit more gradual stress transfer into the matrix, promoting effective stress distribution. Also, different fiber end geometries lead to distinct failure mechanisms. These findings demonstrate that fiber end geometry plays a crucial role in stress distribution management, critical for optimizing composite design and enhancing the reliability of SFRCs in practical applications. By integrating mechanophores for real-time stress visualization, we can accurately map quantified stress distributions that arise during loading and identify failure mechanisms in polymer composites, offering a comprehensive approach to enhancing their durability and performance.

Role of molecular damage in crack initiation mechanisms of tough elastomers

Tough soft materials such as multiple network elastomers (MNE) or filled elastomers are typically stretchable and include significant energy dissipation mechanisms that prevent or delay crack growth. Yet most studies and fracture models focus on steady-state propagation and damage is assumed to be decoupled from the local stress and strain fields near the crack tip. We report an in situ spatial-temporally resolved 3D measurement of molecular damage in mechanophore-labeled MNE just before a crack propagates. This technique, complemented by digital image correlation, allows us to compare the spatial distribution of both damage and deformation in single network (SN) elastomers and in MNE. Compared to SN, MNE have a wide-spread damage in front of the crack and, surprisingly, delocalize strain concentration. A continuum model, where damage distribution is fully coupled to the crack tip fields, is proposed to explain these results. Additional measurements of time-dependent molecular damage during fixed grips relaxation in the presence of a crack reveal that the less localized damage distribution delays fracture initiation. The observations and exploratory modeling reveal the dynamic fracture mechanism of MNE, providing guidance for rational design of high-performance tough elastomers.

Hierarchical channel morphology in O-rings after two cycling exposures to 70 MPa hydrogen gas: a case study of sealing failure

This study investigates the impact of high-pressure hydrogen gas exposure on the structural and morphological characteristics of O-ring materials. O-ring specimens undergo two cycles of sealing under 70 MPa hydrogen gas, and their resulting variations are examined using advanced characterization techniques, including powder X-ray diffraction (PXRD), small-angle X-ray scattering (SAXS), scanning electron microscopy (SEM) and atomic force microscopy (AFM). Our findings reveal that the lattice parameters of the O-ring material show no significant changes when exposed to 70 MPa hydrogen gas. However, in the micrometre range, the formation of a hierarchical channel morphology becomes evident. This morphology is accompanied by the separation of carbon black filler from the rubber matrix, contributing to mechanical weakening of the O-ring. These observations can be attributed to the pressure gradient that develops between the inner and outer radii of the O-ring, resulting from compression forces acting perpendicularly to the radial direction due to clamp locking.

Fracture Detection in Bio-Glues with Fluorescent-Protein-Based Optical Force Probes

Glues are being used to bond, seal, and repair in industry and biomedicine. The improvement of gluing performance is hence important for the development of new glues with better and balanced property spaces, which in turn necessitates a mechanistic understanding of their mechanical failure. Optical force probes (OFPs) allow the observation of mechanical material damage in polymers from the macro- down to the microscale, yet have never been employed in glues. Here, the development of a series of ratiometric OFPs based on fluorescent-protein-dye and protein-protein conjugates and their incorporation into genetically engineered bio-glues is reported. The OFPs are designed to efficiently modulate Förster resonance energy transfer upon force application thereby reporting on force-induced molecular alterations independent of concentration and fluorescence intensity both spectrally and through their fluorescence lifetime. By fluorescence spectroscopy in solution and in the solid state and by fluorescence lifetime imaging microscopy, stress concentrations are visualized and adhesive and cohesive failure in the fracture zone is differentiated.

The Mechanochemical Synthesis and Activation of Carbon-Rich π-Conjugated Materials

Mechanochemistry uses mechanical force to break, form, and manipulate chemical bonds to achieve functional transformations and syntheses. Over the last years, many innovative applications of mechanochemistry have been developed. Specifically for the synthesis and activation of carbon-rich π-conjugated materials, mechanochemistry offers reaction pathways that either are inaccessible with other stimuli, such as light and heat, or improve reaction yields, energy consumption, and substrate scope. Therefore, this review summarizes the recent advances in this research field combining the viewpoints of polymer and trituration mechanochemistry. The highlighted mechanochemical transformations include π-conjugated materials as optical force probes, the force-induced release of small dye molecules, and the mechanochemical synthesis of polyacetylene, carbon allotropes, and other π-conjugated materials.

Tailoring the Properties of Optical Force Probes for Polymer Mechanochemistry

The correlation of mechanical properties of polymer materials with those of their molecular constituents is the foundation for their holistic comprehension and eventually for improved material designs and syntheses. Over the last decade, optical force probes (OFPs) were developed, shedding light on various unique mechanical behaviors of materials. The properties of polymers are diverse, ranging from soft hydrogels to ultra-tough composites, from purely elastic rubbers to viscous colloidal solutions, and from transparent glasses to super black dyed coatings. Only very recently, researchers started to develop tailored OFP solutions that account for such material requirements in energy (both light and force), in time, and in their spatially detectable resolution. We here highlight notable recent examples and identify future challenges in this emergent field.

Why is mechanical fatigue different from toughness in elastomers? The role of damage by polymer chain scission

Although elastomers often experience 10 to 100 million cycles before failure, there is now a limited understanding of their resistance to fatigue crack propagation. We tagged soft and tough double-network elastomers with mechanofluorescent probes and quantified damage by sacrificial bond scission after crack propagation under cyclic and monotonic loading. Damage along fracture surfaces and its spatial localization depend on the elastomer design, as well as on the applied load (i.e., cyclic or monotonic). The key result is that reversible elasticity and strain hardening at low and intermediate strains dictates fatigue resistance, whereas energy dissipation at high strains controls toughness. This information serves to engineer fatigue-resistant elastomers, understand fracture mechanisms, and reduce the environmental footprint of the polymer industry.

Triazole-Extended Anthracenes as Optical Force Probes

Optical force probes (OFPs) are force-responsive molecules that report on mechanically induced transformations by the alteration of their optical properties. Yet, their modular design and incorporation into polymer architectures at desired positions is challenging. Here we report triazole-extended anthracene OFPs that combine two modular ‘click’ reactions in their synthesis potentially allowing their incorporation at desirable positions in complex polymer materials. Importantly, these retain the excellent optical properties of their parent 9-π-extended anthracene OFP counterparts.