Ferroelastic ionic organic crystals that self-heal to 95%

Many materials suffer wear and fracture over time, motivating the development of self-healing architectures that can autonomously restore integrity. While amorphous polymers often heal efficiently due to facile chain mobility and interfacial contact, ordered molecular crystals are limited by slow interfacial mass transport and the need for precise alignment of fractured faces, typically requiring long contact times and near-perfect registry. Prior demonstrations of self-healing molecular crystals have relied on mechanisms such as dynamic covalent chemistry, ionic diffusion, and crystal–polymer networks, but practical translation has been hindered by slow kinetics, alignment challenges, and inadequate quantitative methodologies, which often depend only on optical observations. This study asks whether ferroelastic detwinning in an ionic organic crystal (anilinium bromide) can provide rapid, robust, and highly efficient self-healing by mechanically ensuring interfacial alignment and leveraging nondirectional ionic bonding. The purpose is to measure, visualize, and quantify the healing process and mechanical recovery, and to assess its relevance for durable crystal-based optoelectronics.

Early self-healing in molecular crystals showed modest efficiencies (~6.7%) using dynamic covalent bonds. Subsequent work improved efficiencies to ~67% via dynamic covalent chemistry and up to ~82% by ionic diffusion in hybrid perovskite single crystals. Additional strategies included crystal–polymer interactions and ferroelastic effects in MOFs. However, characterization methods lagged behind; healing propensity was largely tested mechanically with occasional optical quantification, and many systems required ≥24 h contact times and near-perfect fragment alignment. Technical issues such as misalignment and debris accumulation further limited practical application. The present work positions ferroelastic detwinning as a route to overcome alignment and kinetics barriers in an organic ionic crystal.

Materials and crystal growth: Monoclinic (P21/m) anilinium bromide (AniHBr) crystals were grown by mixing freshly distilled aniline with >48% HBr in methanol, slow evaporation yielding rectangular plates up to ~10×4×1 mm3. Crystals can degrade under humidity or repeated thermal cycling. Trace phenazine (P) forms by air oxidation of aniline and is occluded at low concentration; it was used as a fluorescent probe. Acridine orange (AO) was also doped for polarization-sensitive fluorescence. Identity and orientation of guest molecules were confirmed by fluorescence spectra, lifetimes, mass spectrometry, and polarized microscopy. Ferroelastic manipulation and self-healing protocol: Crystals undergo ferroelastic twinning upon pressure on (100)/(100) facets; detwinning occurs with force on (010)/(010). Additional force on a twinned crystal creates a crack along a || [100]. Healing is induced by compressing along b || [010], bringing faces into registry and closing the crack via detwinning. Completely separated pieces could not be reattached. Imaging and structural characterization: Optical/fluorescence microscopy documented twinning/healing. SEM (FEI Quanta 450, 2 kV, spot size 3) with in situ tensile tester enabled controlled tension/compression and real-time imaging under high vacuum to verify healing without atmospheric water. AFM (Bruker Dimension Icon, tapping mode) measured topography across partially healed cracks to confirm continuous healed surfaces. Micro-CT (Bruker Skyscan 1272; 55 kV, 80 µA, 0.25 mm Al filter; voxel 1.3–4.7 µm) reconstructed bulk density in pristine, cracked, and healed states to visualize crack closure. Single-crystal XRD (Bruker APEX III; MoKα or CuKα) confirmed crystallinity, face indexing, and mosaicity changes. Digital image correlation (DIC) with mechanical testing: Crystals were randomly speckled with black/white paint and mounted on a universal testing machine via UV-curable resin clamps. A displacement-controlled triangular profile (0.3 mm min−1; tension to 25 µm then symmetric compression) was applied along [010]. High-definition video was processed with Ncorr to compute von Mises strain maps, tracking twin band nucleation, growth, and detwinning; a sensitive load cell recorded force simultaneously. Cyclic tests probed repeatability and onset of fracture. Tensile testing for healing quantification: Pristine crystals’ ultimate tensile strength was measured (rate 0.3 mm min−1). For self-healing, crystals were twinned, fractured, detwinned to heal, then either tested immediately or after 100 min rest. Failure modes were categorized as brittle rupture along a (010) or twin rupture along (110) with a ferroelastic plateau preceding failure. Healing efficiency was quantified by comparing healed strength to pristine strength. Fluorescence and sFCS: Spectra were recorded on a Jasco FP8500; lifetimes and confocal images on Leica TCS SP8 with pulsed 405/488 nm excitation. Scanning FCS using two-photon excitation quantified phenazine concentration in crystals (circular scans; ALV hardware correlator; fits to autocorrelation provide N and D), yielding an average ~3.32 µM; fluorescence lifetime τ ~1.4 ns in both parent and twinned domains. Molecular dynamics simulations: LAMMPS with GAFF force field and AM1-BCC charges simulated uniaxial tension (0→20%) along [010] followed by compression to zero, repeated for three cycles on a periodic cell (~12×42×5 nm). Simulations revealed ion rotations forming a twin-like domain under load and relaxation/reorientation during detwinning; energy–strain profiles and equilibration (e.g., at 325 K) examined time-dependent reduction in potential energy and extent of reordering.

• AniHBr crystals self-heal rapidly and efficiently via ferroelastic detwinning: mechanical recovery reached ~49% within seconds and ~95% after 100 min rest.
• Healing mechanism relies on reversed twinning to ensure interfacial alignment and strong, nondirectional ionic bonding across the interface.
• DIC strain maps showed nucleation/growth of a twin band under tension and full detwinning under compression; maximum von Mises strain εv ~0.1 at 24 µm displacement; force plateau ~0.33 N during twin propagation.
• Crystals sustained two full twinning–detwinning cycles with negligible loss in peak force; fracture typically occurred in the third cycle.
• Pristine ultimate tensile strength: 4.19 ± 2.30 MPa (n=11).
• Immediately tested after healing (brittle rupture mode): 49% recovery (n=7). After 100 min rest: 95% recovery (n=10).
• Twin-rupture mode (failure along (110) after ferroelastic plateau): immediate tests showed ~20% recovery (n=12); after 100 min, ~10% (n=2).
• AFM across partially healed cracks confirmed continuous, scar-free surfaces in healed regions; micro-CT showed restoration from near-zero-density crack to full density upon healing.
• Healing occurred under SEM high vacuum, indicating no requirement for atmospheric moisture.
• sFCS quantified phenazine guest concentration ~3.32 µM; fluorescence lifetime τ = 1.4 ± 0.1 ns in both parent and twinned domains; polarized fluorescence indicated guest orientation along b.
• XRD showed only marginal increase in mosaicity after healing, indicating minor crystallinity loss.
• Interfacial ionic separations on fracture planes: N(H)–Br distances ~3.3256 Å on (010) and ~3.3400 Å on (110), consistent with ionic interactions governing both rupture modes.
• Healed regions did not act as the weakest link during reloading; new cracks often formed in pristine regions, indicating substantial restoration of mechanical integrity.
• MD and fluorescence reorientation experiments supported time-dependent structural reordering in detwinned regions, explaining improved recovery after 100 min.

The work addresses whether ordered molecular crystals can achieve rapid, high-efficiency self-healing comparable to polymers. By leveraging ferroelastic detwinning, AniHBr aligns fracture faces precisely and allows ionic, nondirectional interactions to re-bond the interface, overcoming typical alignment and mass-transport limitations. Real-time DIC maps and force–displacement profiles show that twinning-driven deformation is reversible under compression, enabling crack closure and structural restoration. Mechanical testing establishes that apparent optical crack disappearance does not guarantee full strength recovery; instead, time-dependent molecular reorientation in the detwinned region is crucial to reach near-pristine strength. The observation that re-fracture occurs away from the healed site demonstrates that the healed interface regains robustness comparable to or exceeding surrounding regions. Combined AFM, CT, and SEM in situ tests corroborate healing at the surface and in the bulk and confirm independence from environmental water. MD simulations and fluorescence polarization studies support a microscopic mechanism wherein locally disordered ions formed during twinning/detwinning gradually reorient to lower-energy configurations, accelerating at elevated temperatures. Together, the results validate ferroelastic detwinning as a practical route to efficient, fast self-healing in ionic organic crystals and highlight the importance of mechanical measurements for quantitative assessment.

This study demonstrates that an ionic organic crystal, anilinium bromide, can self-heal efficiently through ferroelastic detwinning, achieving ~49% recovery within seconds and up to ~95% after 100 min. The mechanism combines mechanically enforced interfacial alignment with strong, nondirectional ionic bonding, yielding robust restoration confirmed by DIC, AFM, CT, SEM, mechanical testing, and supported by molecular simulations and fluorescence reorientation experiments. Healed regions are not the weakest sites upon reloading, and crystallinity loss is minimal. These findings elevate organic molecular crystals closer to best-in-class self-healing polymers, opening avenues for durable, crystal-based optoelectronic and photonic devices. Future work could aim to standardize healing kinetics measurements, explore temperature-assisted reordering to reduce equilibration times, engineer crystal chemistries and dopants to optimize ferroelastic response and ionic interface strength, mitigate humidity/thermal degradation, and integrate such materials into device-relevant architectures.

• Environmental sensitivity: crystals degrade under extended humidity exposure or repeated thermal cycling, limiting detailed temperature-dependent studies.
• Reattachment limit: completely separated crystal pieces could not be rejoined, likely due to poor interfacial alignment.
• Cyclic durability: twinning–detwinning was reversible for two cycles; fracture typically occurred in the third cycle, suggesting defect accumulation and limited fatigue life.
• Partial structural disorder: detwinned regions initially contain locally disordered ions; full recovery requires time for reorientation, influencing immediate mechanical recovery.
• Characterization comparability: healing kinetics across studies are not standardized, complicating direct rate comparisons.
• Minor crystallinity changes: small increases in mosaicity after healing indicate marginal but nonzero crystallinity degradation.