Self-healing materials, especially self-healing polyurea/polyurethane, to replace traditional coating has been of increasing interest in the past decade. The frequency of regular maintenance work can also be reduced as the coating is capable of forming bonds at ruptured sites. This reduces the cost of maintenance and the risk involved in workers engaging in maintenance work. The extremely short curing time of polyurea coating could potentially outweigh the cost due to its short down time. With a high self-healing efficiency, self-healing polyurea could be the ultimate choice of protective coating. This report aims to find the optimum formulation for fabrication of polyurea with a high self-healing efficiency. This is conducted by changing the composition of the components chosen for formulation of polyurea. The choice of isocyanate and amine is varied to explore its impact on chain mobility and microphase separation, which are important factors affecting self-healing efficiency. A series of characterizations, including ATR-FTIR, DSC, optical microscope and mechanical tester, is used to analyze the factors affecting the self-healing efficiency of fabricated polyurea and to eventually determine the best formulation. The ideal formulation of toluene 2,4 diisocyanate-amine (TDI-P1000) polyurea managed to achieve a self-healing of 42%. Further studies could be done to include multiple healing mechanisms after different area of polyurea to boost its self-healing efficiency after repeated healing.

https://www.mdpi.com/2073-4360/14/17/3520/pdf?version=1661911499

A Comparative Study on the Self-Healing Characterizations and Formulation Optimization of Polyurea Coating

The present work describes the synthesis of new versatile polyurea (PU) and polyurethane (PUR) matrices, including different chain extenders, which facilitate the design of distinct, tunable properties, and high-performance derivatives. These polymers can be used for various defense and security applications, such as coatings for ballistic protection, CBRN protection, binders for energetic formulations, etc. Combining aliphatic and aromatic molecules in PU or PUR structures enables the synthesis of polymers with improved and controllable thermo-mechanical properties. Thus, for polyurea synthesis, we utilized two types of polymeric aliphatic diamines and three types of aromatic chain extenders (1,1’-biphenyl-4,4’-diamine, benzene-1,2-diamine, and 1,2-diphenylhydrazine). An analogous method was used to synthesize polyurethane films by employing one polymeric aliphatic polyol and three types of aromatic chain extenders (benzene-1,3-diol, benzene-1,4-diol, and benzene-1,2,3-triol). Subsequently, various analytic techniques (Fourier transform infrared spectroscopy–attenuated total reflectance (FTIR-ATR), single cantilever dynamic mechanical analysis (DMA), thermogravimetric analysis (TGA), frequency-dependent shear modulus survey, tensile tests, water contact angle measurements, and scanning electron microscopy (SEM) with energy-dispersive X-ray analysis (EDX)) have been utilized to characterize the synthesized materials and to evaluate the influence of each chain extender on their final properties.

/pdf/effect-of-aromatic-chain-extenders-on-polyurea-and-1u8trj70.pdf

Effect of Aromatic Chain Extenders on Polyurea and Polyurethane Coatings Designed for Defense Applications

Self-healing properties, originating from the natural healing process, are highly desirable for the fitness-enhancing functionality of biomimetic materials. Herein, we fabricated the biomimetic recombinant spider silk by genetic engineering, in which Escherichia coli (E. coli) was employed as a heterologous expression host. The self-assembled recombinant spider silk hydrogel was obtained through the dialysis process (purity > 85%). The recombinant spider silk hydrogel with a storage modulus of ~250 Pa demonstrated autonomous self-healing and high strain-sensitive properties (critical strain ~50%) at 25 °C. The in situ small-angle X-ray scattering (in situ SAXS) analyses revealed that the self-healing mechanism was associated with the stick-slip behavior of the β-sheet nanocrystals (each of ~2–4 nm) based on the slope variation (i.e., ~−0.4 at 100%/200% strains, and ~−0.9 at 1% strain) of SAXS curves in the high q-range. The self-healing phenomenon may occur through the rupture and reformation of the reversible hydrogen bonding within the β-sheet nanocrystals. Furthermore, the recombinant spider silk as a dry coating material demonstrated self-healing under humidity as well as cell affinity. The electrical conductivity of the dry silk coating was ~0.4 mS/m. Neural stem cells (NSCs) proliferated on the coated surface and showed a 2.3-fold number expansion after 3 days of culture. The biomimetic self-healing recombinant spider silk gel and thinly coated surface may have good potential in biomedical applications.

/pdf/self-healing-of-recombinant-spider-silk-gel-and-coating-h2psdieq.pdf

Self-Healing of Recombinant Spider Silk Gel and Coating

Extrinsic Conditions for the Occurrence and Characterizations of Self-Healing Polyurea Coatings for Improved Medical Device Reliability: A Mini Review

The multifrequency formalism is generalized and exploited to quantify attractive forces, i.e., van der Waals interactions, with small amplitudes or gentle forces in bimodal and trimodal atomic force microscopy (AFM). The multifrequency force spectroscopy formalism with higher modes, including trimodal AFM, can outperform bimodal AFM for material property quantification. Bimodal AFM with the second mode is valid when the drive amplitude of the first mode is approximately an order of magnitude larger than that of the second mode. The error increases in the second mode but decreases in the third mode with a decreasing drive amplitude ratio. Externally driving with higher modes provides a means to extract information from higher force derivatives while enhancing the range of parameter space where the multifrequency formalism holds. Thus, the present approach is compatible with robustly quantifying weak long range forces while extending the number of channels available for high resolution.

Quantification of van der Waals forces in bimodal and trimodal AFM.

Self-healing polymers are categorized as smart materials that are capable of surface protection and prevention of structural failure. Polyurethane/polyurea, as one of the representative coatings, has also attracted attention for industrial applications. Compared with polyurethane, polyurea coating, with a similar formation process, provides higher tensile strength and requires shorter curing time. In this paper, extrinsic and intrinsic mechanisms are reviewed to address the efficiency of the self-healing process. Moreover, formulation optimization and strategic improvement to ensure self-healing within a shorter period of time with acceptable recovery of mechanical strength are also discussed. The choice and ratio of diisocyanates, as well as the choice of chain extender, are believed to have a crucial effect on the acceleration of the self-healing process and enhance self-healing efficiency during the preparation of polyurea coatings.

/pdf/mini-review-of-self-healing-mechanism-and-formulation-183ecf6y.pdf

Mini-Review of Self-Healing Mechanism and Formulation Optimization of Polyurea Coating

During the past decade, Scanning Probe Microscopy (SPM) based surface strain detection techniques have been extensively used in the characterization of functional materials, structures, and devices. Here, we refer these techniques as Surface Strain Force Microscopy (SSFM), which mainly includes the Piezoresponse Force Microscopy, Atomic Force Acoustic Microscopy, Atomic Force Microscopy-Infrared spectroscopy (or photothermal induced resonance), Piezomagnetic Force Microscopy, and Scanning Joule Expansion Microscopy. The inception of SSFM opens up a pathway to study the nanoscale physical properties by using a sharp tip to detect the local field-induced surface strain. Through measuring the signals of the surface strain, multiple physical properties, such as the electromechanical, mechanical, photothermal, magnetic, thermoelastic properties, can be characterized with an unprecedented spatial resolution. In order to further develop and overcome the fundamental issues and limitations of the SSFM, the multi-frequency SPM technology has been introduced to the SSFM-based techniques, leading to the emerging of multi-frequency SSFM (MF-SSFM). As a technical breakthrough of the SSFM, MF-SSFM has demonstrated substantial improvements in both performance and capability, resulting in increased attentions and numerous developments in recent years. This Perspective is, therefore, aimed at providing a preliminary summary and systematic understanding for the emerging MF-SSFM technology. We will first introduce the basic principles of conventional SSFM and multi-frequency SPM techniques, followed by a detailed discussion about the existing MF-SSFM techniques. MF-SSFM will play an increasingly important role in future nanoscale characterization of the physical properties. As a result, many more advanced and complex MF-SSFM systems are expected in the coming years.

https://aip.scitation.org/doi/pdf/10.1063/5.0131075

Emerging multi-frequency surface strain force microscopy

Publisher's Note: “Emerging multi-frequency surface strain force microscopy” [J. Appl. Phys. 133, 040901 (2023)]

Lithium niobate (LiNbO3, LN) thin films have been extensively studied for applications in acoustic and photonic devices, due to their outstanding piezoelectric, ferroelectric and electro-optical properties. With the increasing demand for high speed and low latency wireless communication, LN thin films with high electromechanical coupling coefficients are very attractive to improve the performance of acoustic resonators for radio frequency filters. The current bottleneck for LN-based devices is the synthesis of high-quality LN thin films, which is typically fabricated by expensive and inefficient process of ion slicing and layer transfer from bulk single crystals. This review paper focuses on the direct growth of high-quality LN thin films, which has the potential to scale up and lower the cost of LN thin films. We first introduce the crystal structure and piezoelectric properties of LN, followed by an overview of the state-of-the-art LN acoustic resonators. After a summary of the challenges in the fabrication of LN thin films, we review the direct growth of LN thin films by sputtering, pulsed laser deposition, metalorganic chemical vapor deposition and molecular beam epitaxy. With the progress in optimizing the crystallinity and surface roughness, the quality of the LN thin films synthesized by direct growth has been greatly improved. As a result of the fast-growing industrial interests, we believe that the research works in direct growth of LN thin films will increase exponentially to achieve the same quality of the LN thin films as the bulk single crystals.

Celine Sim

Papers

Mini-Review of Self-Healing Mechanism and Formulation Optimization of Polyurea Coating

A Comparative Study on the Self-Healing Characterizations and Formulation Optimization of Polyurea Coating

Emerging multi-frequency surface strain force microscopy

Publisher's Note: “Emerging multi-frequency surface strain force microscopy” [J. Appl. Phys. 133, 040901 (2023)]

Direct Growth of Lithium Niobate Thin Films for Acoustic Resonators