Ferroelastic ionic organic crystals that self-heal to 95%

The inherent tendency of materials to degrade over time due to wear and tear is a major limitation. Self-healing materials, capable of repairing themselves and restoring original properties, offer a promising solution. While amorphous materials, such as polymers, exhibit efficient self-healing due to favorable rheology, the self-healing of atomistically ordered materials like crystals is hindered by slower interfacial mass transport and the need for precise physical alignment. Previous research on self-healing molecular crystals has shown modest success, with healing efficiencies ranging from 6.7% to 82%, achieved using various mechanisms like dynamic covalent bonds, ionic diffusion, and crystal-polymer interactions. However, these methods often require long contact times (e.g., 24 hours) and perfect fragment alignment, limiting practical applications. This study aims to overcome these limitations by investigating the self-healing capabilities of an organic ionic crystal, anilinium bromide (AniHBr), leveraging its ferroelastic properties to achieve rapid and efficient self-repair.

The field of self-healing materials has seen considerable advancement, particularly in amorphous materials such as polymers. Early work focused on developing polymers that could autonomously heal cracks and fractures. More recently, the concept of dynamic crystals has emerged, expanding the possibilities of self-healing to the realm of ordered, solid entities. Several studies have demonstrated self-healing in molecular crystals using various interaction mechanisms. For example, dynamic covalent bonds, ionic diffusion, and crystal-polymer interactions have all been shown to contribute to self-healing. However, these previous efforts have been limited by low healing efficiencies and the requirement for long healing times and precise alignment of fractured surfaces. The lack of standardized characterization methods further complicates the comparison and advancement of self-healing molecular crystals.

Monoclinic crystals of AniHBr, intentionally doped with trace amounts of phenazine (P) for fluorescence visualization, were grown from a solution of aniline, hydrobromic acid, and methanol. The phenazine impurity served as a convenient probe for visualizing structural changes during twinning and healing. The concentration of P was determined using Scanning Fluorescence Correlation Spectroscopy (sFCS). The self-healing properties were investigated using a combination of techniques. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) provided high-resolution images of the crystal surfaces before, during, and after healing. Computed tomography (CT) scanning was used to reconstruct the three-dimensional density of the crystal and visualize the healing process in the bulk. Digital image correlation (DIC) analysis tracked the spatial evolution of strain in real-time during cracking and healing, revealing the ferroelastic mechanisms. Mechanical tensile testing was used to quantify the healing efficiency by measuring the ultimate tensile strength of pristine, fractured, and healed crystals. Molecular dynamics simulations provided atomistic-level insights into the detwinning process and ionic reorientation during self-healing. Additionally, confocal fluorescence microscopy was employed to monitor the reorientation of acridine orange (AO) molecules within the crystals, further supporting the proposed healing mechanism.

Anilinium bromide (AniHBr) crystals exhibited remarkable self-healing capabilities. Upon applying mechanical stress, the crystals underwent ferroelastic twinning and subsequently fractured. Mild compression along the b-axis reversed the twinning, resulting in efficient self-healing. The healing process was rapid, with up to 49% recovery observed within seconds and up to 95% recovery after 100 minutes. SEM, AFM, and CT scans confirmed the structural recovery after healing. DIC analysis visualized the strain distribution during twinning, fracture, and detwinning, providing insights into the ferroelastic mechanism. The mechanical testing demonstrated that the healed crystals recovered a substantial portion of their original tensile strength, achieving 95% healing efficiency after 100 min. Molecular dynamics simulations showed that the anilinium ions reoriented during the detwinning process, leading to the observed self-healing. Interestingly, the healed crack did not appear to be the weakest point in the crystal during subsequent testing, suggesting complete structural recovery. The study also showed that visual inspection of the crack's disappearance is insufficient to accurately assess the degree of healing, and mechanical testing is necessary for quantitative analysis.

The findings demonstrate that the rapid and efficient self-healing in AniHBr crystals is attributable to the interplay of ferroelasticity and strong ionic bonding. The ferroelastic twinning allows for precise alignment of the fractured surfaces upon compression, maximizing contact area and facilitating ionic interaction across the interface. The non-directional nature of ionic bonds further contributes to efficient self-healing. The high healing efficiency of AniHBr crystals surpasses that of previously reported self-healing molecular crystals, placing it among the best-performing self-healing materials, comparable to some high-performing polymers. The observation that the healed crack does not act as the weakest point underscores the remarkable recovery of the crystal's mechanical integrity. This research challenges the traditional view of organic crystals as mechanically inferior and opens new possibilities for their application in robust optoelectronic devices.

This study showcases the impressive self-healing capabilities of AniHBr crystals, achieving a high healing efficiency of 95% within a reasonable timeframe, leveraging the material's ferroelastic properties. This work emphasizes the importance of robust mechanical testing in addition to visual inspection to accurately quantify self-healing in crystals. Future research directions could explore other ferroelastic organic crystals with similar self-healing potential and investigate the long-term durability and stability of healed crystals under various environmental conditions. Exploring the scalability and applicability of this self-healing mechanism for practical applications in optoelectronics and other fields represents exciting future avenues.

While the study demonstrates high self-healing efficiency in AniHBr crystals, several limitations should be considered. The current study primarily focused on crystals with phenazine doping, and the influence of other dopants or impurities on the self-healing properties needs further investigation. The long-term stability of the healed crystals under various environmental conditions and the effect of repeated self-healing cycles on the material's overall performance need further exploration. The crystal growth and handling processes need to be optimized for scalability and industrial applications.