Smart sensor using self-healing material

Abstract

Disclosed is a smart sensor using a self-healing material. The self-healing material can be manufactured by a manufacturing method comprising: a first step of synthesizing an oligomer of a first self-healing polymer; a second step of hydrothermally synthesizing multi-walled carbon nanotubes functionalized with an amino group (NH4OH) by heating a solution in which multi-walled carbon nanotubes (MWCNT) and ammonium hydroxide (NH4OH) are mixed; and a third step of preparing a second self-healing polymer by molding and drying a solution in which the oligomer and the multi-walled carbon nanotubes are mixed.

Description

Smart sensor using self-healing material

[0001] The present invention relates to a method for manufacturing functionalized MWCNTs, a self-healing material comprising the same, and a smart sensor using the same.

[0002] Self-healing and stretchable polymer composites (SHS Polymer Composites) based on multi-walled carbon nanotubes (MWCNTs) are attracting the attention of many researchers due to their excellent properties, offering great potential particularly in applications such as sensing, actuation, prosthetics, implants, biomedical, energy harvesting, and storage. However, the surfaces of MWCNTs or other untreated carbon nanotubes (CNTs) remain chemically inert due to strong Van der Waals interactions and high structural rigidity. The high surface area between CNTs and the high surface energy resulting from Van der Waals interactions cause the CNTs to attract each other strongly, eventually leading to a tendency for CNTs or MWCNTs to aggregate in the form of entangled bundles. This aggregation phenomenon makes it difficult to uniformly disperse CNTs in the polymer matrix, ultimately degrading the performance of the composite. To solve this problem, the key is to modify the CNT surface with various functional groups.

[0003] Over the past few years, various functional groups used for the functionalization of carbon nanotubes (CNTs) have been reported. Representative examples include hydroxyl groups (-OH), carboxyl groups (-COOH), furan groups (-FA), and amino groups (-NH2). Surface modification of CNTs using these functional groups improves their mechanical, thermal, and electrical properties. Among the various functional groups reported to date, the amino group (-NH2) is considered the most promising. This is because the amino group possesses a high electron-donating effect and excellent nucleophilicity. Furthermore, the amino group acts as a useful link in forming bonds between CNTs and ions, epoxy, and polymer matrices. Therefore, amino-functionalized CNTs and multi-walled carbon nanotubes (MWCNTs), namely NH2-MWCNTs and their composites, offer diverse application possibilities. Additionally, NH2-MWCNTs have been reported to possess excellent biocompatibility, which implies greater suitability for medical and wearable human monitoring devices.

[0004] Various approaches for the amino functionalization of carbon nanotubes (CNTs) have been reported. For example, previous researchers have reported a method for amino functionalizing CNTs using oxidation. This method involves several complex steps. First, it requires functionalization with carboxyl (-COOH) functional groups, followed by reduction of the carboxyl groups to hydroxymethyl, and finally, the formation of amino functional groups. In addition, methods such as silanization, chemical vapor deposition (CVD), plasma, radicals, and layer-by-layer self-assembly have been used for the amino functionalization of CNTs. However, all of these reactions are difficult to control and require multi-step post-processing. Furthermore, following these methods may inevitably cause the CNTs to break into short fragments or alter their intrinsic shape. Amino-functionalized CNTs are rarely commercially available and are sold at very high prices ($500–$900 per 100 mg, Sigma Aldrich Co. Ltd.). On the other hand, other functionalized CNTs or pure CNTs are relatively inexpensive, costing between $50 and $100 per 100 mg. Therefore, to effectively utilize the surface modification of carbon nanotubes, it is essential to develop low-cost, safe, and simple synthesis methods for CNT amino-functionalization, but this remains a challenging task.

[0005] This specification proposes a low-cost, simple, yet effective method for performing amino (NH2) functionalization of multi-walled carbon nanotubes (MWCNTs) by reacting them with NH4OH using hydrothermal synthesis and a simple vacuum filtration process. The synthesized NH2-MWCNTs were successfully incorporated into a dimer acid (DA)-DETA-based supramolecular self-healing polymer matrix. The resulting NH2-MWCNTs / DA-DETA-based polymer composite possesses superior mechanical and electrical properties compared to composites reported in previous studies. A multi-mode sensing device was developed by leveraging the composite's excellent self-healing ability, high stretchability, and electrical conductivity. The developed device demonstrated potential for use as a strain sensor and temperature sensor with self-healing and stretchability (SHS), suggesting that it can play a significant role in emerging technology fields.

[0006] The problem that the present invention aims to solve is to provide a method for synthesizing functionalized multi-walled carbon nanotubes (NH2-MWCNT) at a lower cost, simply, and effectively. Furthermore, in the process of applying self-healing stretchable composites as smart sensors, it is necessary to improve the healing speed of the composite and maintain consistent performance even after repeated damage.

[0007] In one aspect, the present invention provides a method for manufacturing functionalized multi-walled carbon nanotubes, comprising the step of hydrothermally synthesizing multi-walled carbon nanotubes functionalized with amino groups (NH2) by heating a solution mixed with multi-walled carbon nanotubes (MWCNT) and ammonium hydroxide (NH4OH).

[0008] In one embodiment, the hydrothermal synthesis can be performed at about 150 to 200 ℃.

[0009] In one embodiment, the hydrothermal synthesis can be performed by heating for about 2 to 4 hours.

[0010] In another aspect, the present invention provides a functionalized multi-walled carbon nanotube manufactured by the method for manufacturing the functionalized multi-walled carbon nanotube described above.

[0011] In another aspect, the present invention provides a method for manufacturing a self-healing polymer, comprising: a first step of synthesizing an oligomer of a first self-healing polymer; a second step of hydrothermally synthesizing multiwalled carbon nanotubes functionalized with amino groups (NH2) by heating a solution mixed with multiwalled carbon nanotubes (MWCNT) and ammonium hydroxide (NH4OH); and a third step of manufacturing a second self-healing polymer by molding and drying a solution mixed with the oligomer and the multiwalled carbon nanotubes.

[0012] In one embodiment, the oligomer of the first self-healing polymer in the first step can be prepared by mixing dimer acid and diethylenetriamine (DETA).

[0013] In one embodiment, the hydrothermal synthesis of the second step can be performed at about 150 to 200 ℃.

[0014] In one embodiment, the hydrothermal synthesis of the second step can be performed by heating for about 2 to 4 hours.

[0015] In another aspect, the present invention provides a self-healing polymer manufactured by the method for manufacturing a self-healing polymer.

[0016] In another aspect, the present invention provides a self-healing strain sensor comprising: an elastic substrate; a strain sensing layer formed on the elastic substrate and comprising the self-healing polymer; a first electrode in contact with the strain sensing layer; and a second electrode spaced apart from the first electrode and in contact with the strain sensing layer.

[0017] In another aspect, the present invention provides a self-healing temperature sensor comprising: an elastic substrate; a temperature sensing layer formed on the elastic substrate and comprising the self-healing polymer; a first electrode in contact with the temperature sensing layer; and a second electrode spaced apart from the first electrode and in contact with the temperature sensing layer.

[0018] Amino (NH2)-functionalized multiwalled carbon nanotubes (MWCNTs) possess excellent electrical and mechanical properties, but their material costs are very high. The objective of the present invention is to synthesize NH2-MWCNTs using a low-cost, simple, and effective method. To this end, self-functionalized NH2 MWCNTs were prepared using a hydrothermal synthesis process involving reaction with NH4OH. The synthesized NH2-MWCNTs were confirmed to have performance equivalent to commercially available products and were subsequently utilized to develop self-healing and stretchable (SHS) conductive composite materials by mixing them with self-healing supramolecular polymers. The electromechanical properties and self-healing performance of this composite material can be adjusted by controlling the amount of NH2-MWCNTs included in the polymer matrix. The optimized composite material exhibits a maximum strength of 286.15 kPa at a maximum strain of 410% and displays high electrical conductivity of 8.23 ​​S / cm. Furthermore, an SHS strain sensor for biomechanical signal detection was fabricated using this composite material, demonstrating high sensitivity to extension and flexion movements of the human wrist. Moreover, this sensor exhibits an average Temperature Coefficient of Resistance (TCR) value of -5.2% ℃ over a wide temperature range of 25–100 ℃. -1It exhibits high sensitivity. Considering the performance of the low-cost in-house synthesis method developed in this invention and the fabricated sensor, this will contribute to presenting a new paradigm in the field of research.

[0019] The advantage of the present invention lies in the fact that it enables a process with lower cost and greater efficiency than existing methods through a simple synthesis method of NH2-MWCNT. This offers the potential to replace commercially expensive NH2-MWCNT and opens up possibilities for its application in various conductive composite materials.

[0020] Fig. 1a. Molecular structure and schematic synthesis process of amino (NH2) functionalized multiwalled carbon nanotubes (MWCNTs).

[0021] Fig. 1b. Schematic synthesis process of a supramolecular self-healing polymer.

[0022] Fig. 1c. Schematic synthesis process of NH2-MWCNTs-based self-healing conductive composite.

[0023] Fig. 1d. Conceptual schematic diagram of the self-healing mechanism: initial state (left), damaged or severed state (middle), healed state (right). Red beads represent the carboxylic acid (-COOH) functional group of dimer acid (DA), and green beads represent the amine (-NH2) functional group of diethylenetriamine (DETA) and NH2-MWCNTs.

[0024] FIG. 2a. FTIR (Fourier Transform Infrared Spectroscopy) spectra of pure MWCNTs, commercially available NH2-MWCNTs, and NH2-MWCNTs synthesized in the present invention.

[0025] Fig. 2b. Comparison of FTIR spectra of commercially available NH2-MWCNTs and NH2-MWCNTs synthesized in the present invention.

[0026] Fig. 2c. XRD (X-ray diffraction) patterns of NH2-MWCNTs synthesized in the present invention (left), supramolecular polymer (center), and NH2-MWCNTs-based self-healing composite (SHS polymer composite) (right).

[0027] Fig. 3a. Optical microscope (OM) image of a composite containing pure MWCNTs.

[0028] Fig. 3b. Optical microscope (OM) image of a composite containing NH2-functionalized MWCNTs.

[0029] Fig. 3c. Mechanical and electrical properties of the NH2-MWCNTs-based composite developed in the present invention: (Top left) Initial state, (Top right) Elongated state, (Bottom) Acting as a conductor in an LED circuit.

[0030] Fig. 3d. Optical microscope (OM) images of the self-healing process: (Left) A scratch of approximately 100 μm on the damaged (cut) sample; (Center) The scratch gradually disappearing within 10 minutes at room temperature, leaving only a bright trace; (Right) The trace completely disappearing after an additional 10 minutes. The scale bar of the optical microscope (OM) image is 200 μm.

[0031] Fig. 4a. Tensile stress-strain curves of composites prepared at different NH2-MWCNTs concentrations.

[0032] Fig. 4b. Maximum fracture strain values ​​of composites prepared with different NH2-MWCNTs concentrations.

[0033] Fig. 4c. Tensile stress-strain curves of composites cured at room temperature for various periods.

[0034] Fig. 4d. Mechanical healing efficiency of a composite containing 5% NH2-MWCNTs at different healing times (ηmec.).

[0035] Fig. 5a. Changes in electrical conductivity and sheet resistance according to changes in NH2-MWCNTs concentration.

[0036] FIG. 5b. The process in which the composite material developed in the present invention acts as a conductor connected to an LED in an electrical circuit and electrical healing takes place.

[0037] Fig. 5c. Real-time change in resistance during the repetitive cut-healing process of the composite.

[0038] Fig. 5d. Electrical healing efficiency (ηelec.) of a composite material achieved within 1 minute (60 seconds) during multiple cut-healing processes.

[0039] FIG. 6a. Schematic of the self-healing and stretchable (SHS) strain sensor developed in the present invention.

[0040] Fig. 6b. Photographs of the attached SHS strain sensor in various wrist movement states: (i) initial state, (ii) extension state, (iii) flexion state.

[0041] Fig. 6c. Change in relative resistance (ΔR / R0) of the sensor at different strains (0%, 100%, 400%) during 10 consecutive cycles.

[0042] Fig. 6d. Change in relative resistance (ΔR / R0) of the sensor at different bending angles (30°, 60°, 90°) during 5 consecutive cycles.

[0043] Fig. 6e. Comparison of kidney performance before and after healing.

[0044] Fig. 6f. Comparison of bending performance before and after healing.

[0045] Fig. 6g. Real-time strain detection response during wrist extension movement.

[0046] Fig. 6h. Real-time deformation detection response during wrist flexion movement.

[0047] FIG. 7a. Schematic of the self-healing and stretchable (SHS) temperature sensor developed in the present invention.

[0048] Fig. 7b. Change in relative resistance of the sensor at different temperatures (ΔR / R0).

[0049] Fig. 7c. Periodic temperature response of the sensor during 5 repeated cycles in two different temperature ranges (2540 °C and 25100 °C).

[0050] Fig. 7d. Response-recovery curve in the temperature range of 25-40-25 ℃.

[0051] Fig. 7e. Response-recovery curve in the temperature range of 25-100-25 ℃.

[0052] Hereinafter, embodiments of the present invention will be described in detail with reference to the attached drawings. Since the present invention is susceptible to various modifications and may take various forms, specific embodiments are illustrated in the drawings and described in detail in the text. However, this is not intended to limit the present invention to the specific disclosed forms, and it should be understood that it includes all modifications, equivalents, and substitutions that fall within the spirit and scope of the present invention. Similar reference numerals have been used for similar components in the description of each drawing. In the attached drawings, the dimensions of the structures are shown enlarged compared to the actual dimensions for the clarity of the present invention.

[0053] The terms used in this application are used merely to describe specific embodiments and are not intended to limit the invention. Singular expressions include plural expressions unless the context clearly indicates otherwise. In this application, terms such as “comprising” or “having” are intended to specify the presence of the features, numbers, steps, actions, components, or combinations thereof described in the specification, and should be understood as not precluding the existence or addition of one or more other features, numbers, steps, actions, components, or combinations thereof. In the context of this specification, terms such as “about” may mean about ± 1%, about ± 2%, about ± 3%, about ± 4%, about ± 5%, about ± 6%, about ± 7%, about ± 8%, about ± 9%, or about ± 10% of the figures described in the specification.

[0054] In addition, the description of one aspect of the present invention may be applied identically or similarly to the description of other aspects for identical or similar configurations or terms.

[0055] Unless otherwise defined, all terms used herein, including technical or scientific terms, have the same meaning as generally understood by those skilled in the art to which the present invention pertains. Terms such as those defined in commonly used dictionaries should be interpreted as having a meaning consistent with their meaning in the context of the relevant technology, and should not be interpreted in an ideal or overly formal sense unless explicitly defined in this application.

[0056] A method for manufacturing functionalized multi-walled carbon nanotubes according to an embodiment of the present invention may include the step of hydrothermally synthesizing multi-walled carbon nanotubes functionalized with amino groups (NH2) by heating a solution mixed with multi-walled carbon nanotubes (MWCNT) and ammonium hydroxide (NH4OH).

[0057] In the context of this specification, the term multi-walled carbon nanotube (MWCNT) refers to a nanometer-sized cylindrical structure in which multiple single-walled carbon nanotubes (SWCNT) are stacked in a concentric manner. Such structures can provide high mechanical strength, excellent electrical conductivity, and thermal stability, making them applicable in various industrial fields. In particular, MWCNTs possess a high specific surface area and excellent chemical stability, making them potentially suitable for use as reinforcing materials in composites and capable of playing an important role in electronic devices, sensors, and energy storage devices. The present invention aims to functionalize these MWCNTs to convert them into a more chemically reactive material and to ensure uniform dispersibility within the composite material.

[0058] The ammonium hydroxide mentioned above can play a key role in modifying the surface of MWCNTs to introduce amino groups (-NH2). Generally, MWCNTs tend to aggregate due to van der Waals interactions, and they are difficult to disperse uniformly within a polymer matrix without chemical functional groups being imparted to their surface. Ammonium hydroxide is strongly basic and has the potential to react with oxygen-containing functional groups present on the surface of MWCNTs to form amine functional groups. This functionalization process can not only increase the chemical reactivity of MWCNTs but also improve electrical and mechanical properties by enhancing compatibility with composites.

[0059] In the context of this specification, the term hydrothermal synthesis refers to a process of synthesizing specific materials by promoting chemical reactions in a high-temperature and high-pressure environment. Generally, hydrothermal synthesis can be advantageous for forming uniform nanostructures by increasing the solubility and reaction rate of reactants. In the case of the present invention, the main objective is to functionalize MWCNTs with amino groups by reacting them with ammonium hydroxide in a high-temperature and high-pressure environment. In this process, the surface modification of MWCNTs can be effectively achieved, and there is a possibility that this process will be relatively simpler and more environmentally friendly than conventional chemical treatment methods. Furthermore, since the degree of functionalization can be precisely controlled by adjusting the reaction conditions, hydrothermal synthesis can be useful for manufacturing MWCNTs suitable for specific applications.

[0060] The advantage of multi-walled carbon nanotubes functionalized with amino groups lies in their ability to provide excellent dispersibility within conductive composites. While MWCNTs themselves possess high electrical conductivity and mechanical strength, their chemical inertness can result in low compatibility with other materials. However, functionalization with amino groups increases the potential for MWCNTs to chemically bond with various polymer matrices. This reduces aggregation between CNTs within the composite and induces uniform dispersion, ultimately contributing to the improvement of the composite's mechanical and electrical performance. Furthermore, the action of amino groups facilitates the formation of hydrogen or covalent bonds, which can help enhance interactions with self-healing polymers.

[0061] The method for manufacturing multi-walled carbon nanotubes according to an embodiment of the present invention has the potential to achieve high functionalization efficiency while minimizing complex post-processing steps by adopting a hydrothermal synthesis method. Conventional MWCNT functionalization methods involve multi-step processes such as oxidation, silanization, and plasma treatment, which can lead to structural damage to the MWCNTs or make it difficult to uniformly control the degree of functionalization. In contrast, the hydrothermal synthesis method of the present invention allows for the introduction of amino groups through a single-step reaction, thereby simplifying the process. Furthermore, by optimizing reaction conditions, it has the advantage of allowing for more precise control of the desired level of functionalization. Additionally, applying this method allows for expansion into a form suitable for mass production, thereby increasing the potential for practical applications.

[0062] In one embodiment, the hydrothermal synthesis can be performed at approximately 150 to 200°C. If the hydrothermal synthesis temperature is excessively low, the reaction rate may slow down, and there is a possibility that the functionalization of the MWCNT surface may not be sufficiently achieved. Generally, in the process of introducing amino groups (-NH2), high temperatures provide reaction activation energy to promote surface modification. However, if the temperature is low, the reaction between ammonium hydroxide and MWCNT may not proceed sufficiently, which may reduce the efficiency of amino group introduction. As a result, the concentration of amino groups on the surface of the functionalized MWCNT decreases, which may lead to reduced dispersibility within the composite and a decrease in electrical and mechanical performance.

[0063] If the hydrothermal synthesis temperature is excessively high, there is a possibility of structural deformation or thermal damage to the MWCNTs. Although MWCNTs possess high thermal stability, at excessive temperatures, the carbon lattice may partially decompose or surface oxidation may occur, leading to the formation of unwanted functional groups. Furthermore, if the reaction proceeds at excessively high temperatures, amino groups may not be uniformly introduced onto the surface of the MWCNTs, potentially resulting in non-uniform functionalization. This can hinder the interaction between functionalized MWCNTs within the composite and degrade electrical properties and mechanical stability.

[0064] Functionalized MWCNTs with uniformly introduced amino groups (-NH2) can be obtained by hydrothermal synthesis at an appropriate temperature. When the reaction is carried out in the appropriate temperature range of 150 to 200 ℃, not only is surface modification effectively achieved, but the structural stability of the MWCNTs can also be maintained. In this temperature range, the reaction between ammonium hydroxide and MWCNTs proceeds smoothly, making it highly likely that amino groups will be evenly bonded to the surface. Furthermore, the dispersibility of the functionalized MWCNTs is improved, allowing for the formation of a more uniform network within the composite material and contributing to the improvement of electrical and mechanical performance.

[0065] In one embodiment, the hydrothermal synthesis may be performed by heating for about 2 to 4 hours. If the hydrothermal synthesis time is excessively short, there is a possibility that amino groups (-NH2) may not be sufficiently introduced onto the surface of multi-walled carbon nanotubes (MWCNT). If the reaction between MWCNT and ammonium hydroxide (NH4OH) does not take sufficient time during the functionalization process, surface modification may be incomplete. In particular, if the chemical introduction of amino groups through hydrogen bonding and covalent bonding is insufficient, the surface charge density of the functionalized MWCNT decreases, which may lead to reduced dispersibility within the composite material.

[0066] If the hydrothermal synthesis time is excessively long, excessive reaction may occur, leading to structural deformation or surface damage of the MWCNTs. Prolonged exposure to high-temperature environments can cause partial damage to the carbon lattice of the MWCNTs or lead to surface oxidation, which may result in the formation of unnecessary functional groups. This can degrade the electrical conductivity of the functionalized MWCNTs and potentially hinder network formation within the composite. Furthermore, excessive synthesis time can reduce process efficiency, lower productivity, and lead to unnecessary energy consumption.

[0067] Uniform and stable amino-functionalized MWCNTs can be obtained by hydrothermal synthesis for an appropriate amount of time. A synthesis time of 2 to 4 hours is considered the optimal range that allows the reaction to proceed sufficiently while maintaining the structural stability of the MWCNTs. If synthesis is performed within this range, amino groups are evenly introduced onto the surface of the MWCNTs, which increases the efficiency of functionalization. In addition, the dispersibility of the functionalized MWCNTs is improved, which can induce the formation of a uniform network within the composite material, and consequently, electrical and mechanical performance can be improved.

[0068] The advantage of the method for manufacturing multi-walled carbon nanotubes according to the embodiment of the present invention described above is that amino groups can be introduced stably while the process is simple. Conventional functionalization methods required complex steps or strong chemical treatment, which had a high probability of causing structural damage; however, this method utilizes hydrothermal synthesis to carry out the reaction under relatively mild conditions. Through this, surface modification is possible while maintaining the original structure of the MWCNT, and since the reaction can be carried out without additional reducing agents or catalysts, it has the potential to be applied as an economical and environmentally friendly process.

[0069] Meanwhile, the functionalized multiwalled carbon nanotubes according to the embodiments of the present invention can be manufactured by the method for manufacturing functionalized multiwalled carbon nanotubes. The advantage of the multiwalled carbon nanotubes manufactured in this way is that they can possess high chemical stability and excellent dispersibility compared to conventional methods. Amino groups introduced through hydrothermal synthesis are strongly bound to the surface of the MWCNTs, making it highly likely that they will not be easily lost even after long-term use, which can improve the reliability of the functionalized MWCNTs. In addition, the introduction of amino groups improves dispersibility in polar solvents or polymer matrices, which can help secure uniform physical properties when manufacturing composites. Compared to conventional oxidation process-based functionalization methods, the method of the present invention can minimize structural damage to the MWCNTs, thereby potentially maintaining their original electrical and mechanical properties more effectively.

[0070] Meanwhile, a method for manufacturing a self-healing polymer according to an embodiment of the present invention may include: a first step of synthesizing an oligomer of a first self-healing polymer; a second step of hydrothermally synthesizing a multi-walled carbon nanotube functionalized with an amino group (NH2) by heating a solution mixed with a multi-walled carbon nanotube (MWCNT) and ammonium hydroxide (NH4OH); and a third step of manufacturing a second self-healing polymer by molding and drying a solution mixed with the oligomer and the multi-walled carbon nanotube.

[0071] In the context of this specification, the term "self-healing" refers to the ability of a damaged material to restore its original physical and chemical properties on its own without external intervention. This can generally be achieved through reversible bonding mechanisms such as dynamic covalent bonding, supramolecular interactions, and hydrogen bonding, and is characterized by the ability of the material to naturally repair damage without the application of additional external heat, moisture, or light stimuli.

[0072] The role of the first step described above is to synthesize oligomers that impart self-healing capabilities; this is a critical process that determines the fundamental structural characteristics and physical properties of the self-healing polymer to be finally manufactured. Oligomers are monomers or low-molecular-weight compounds capable of forming polymer networks through various chemical interactions, and they serve as the basis for forming composites by subsequently combining with functionalized multi-walled carbon nanotubes.

[0073] In the first step described above, the oligomer plays a key role in realizing self-healing capabilities as a low-molecular-weight structure formed by the combination of a dimer acid and diethylenetriamine (DETA). The oligomer contains amine (-NH2) and carboxyl (-COOH) groups and possesses properties that induce self-healing within the material through hydrogen bonding and dynamic covalent bonding. Additionally, the oligomer helps to control mechanical strength and electrical properties through interaction with functionalized multi-walled carbon nanotubes. In this process, the viscosity, reactivity, and crosslinking ability of the oligomer can directly influence the performance of the final polymer.

[0074] The role of the second step described above is to functionalize the surface of multiwalled carbon nanotubes with amino groups, which is an important process for ensuring uniform dispersibility within the polymer matrix and improving mechanical and electrical properties. Conventional multiwalled carbon nanotubes (MWCNTs) have high surface energy and tend to aggregate; to overcome this, amino groups are introduced to increase the potential for chemical bonding within the polymer and enable the fabrication of functional composites. Furthermore, MWCNTs modified with amino groups can form cross-links or provide conductive pathways within the polymer network, which can ultimately help maximize the performance of conductive composites with self-healing capabilities.

[0075] The role of the third step described above is to manufacture a final self-healing polymer by mixing the previously synthesized oligomer with functionalized multi-walled carbon nanotubes, with the goal of forming a composite material having the desired shape and mechanical properties through molding and drying processes. During this process, the mixing ratio, solvent removal rate, and drying conditions can influence the physical properties of the composite material, thereby enabling the control of self-healing performance, mechanical strength, elasticity, and conductivity. In particular, if the bonding between the oligomer and the functionalized MWCNT is optimized, re-bonding at damaged sites can occur more quickly and effectively, resulting in properties suitable for practical sensor and wearable device applications.

[0076] In one embodiment, the oligomer of the first self-healing polymer in the first step can be prepared by mixing dimer acid and diethylenetriamine (DETA).

[0077] Dimer acids are aliphatic dicarboxylic acids, primarily possessing a C36 structure, that are compounds formed through the thermal polymerization of unsaturated fatty acids. Containing two carboxyl groups (-COOH), these compounds can be utilized as various polymer precursors, playing a role in providing flexibility and chemical resistance. Dimer acids can promote self-healing capabilities within polymer networks by inducing cross-link formation and can be applied to various polymer materials such as polyamides, polyesters, and polyurethanes.

[0078] Diethylenetriamine (DETA) is a low-molecular-weight organic compound of the ethyleneamine family, characterized by a triamine structure having three amine (-NH2) functional groups. This compound has high reactivity and can be used as a curing agent or crosslinking agent for various polymer materials, such as polyurethanes, polyamides, and epoxy resins. DETA can react with dimer acids to form amide bonds, and these chemical bonds can help form reversible dynamic bonds in self-healing polymers.

[0079] In one embodiment, the hydrothermal synthesis of the second step can be performed at about 150 to 200 ℃. In one embodiment, the hydrothermal synthesis of the second step can be performed by heating for about 2 to 4 hours.

[0080] The advantage of the method for manufacturing a self-healing polymer according to the embodiment of the present invention described above is that a polymer with self-healing capabilities can be manufactured through a relatively simple process. Unlike conventional multi-step synthesis processes, this method proceeds by forming an oligomer using a dimer acid and diethylenetriamine, followed by combining it with functionalized multi-walled carbon nanotubes (NH2-MWCNT); thus, a polymer with desired properties can be synthesized without a separate, complex post-processing step. Furthermore, the process of functionalizing NH2-MWCNT using hydrothermal synthesis is low-cost and has the advantage of enabling the production of functional fillers with high reactivity without complex processing involving high temperatures and organic solvents.

[0081] Meanwhile, the self-healing polymer according to the embodiment of the present invention can be manufactured by the self-healing polymer manufacturing method described above. The advantage of the self-healing polymer manufactured in this way is that it can self-recover with a high probability even if external physical damage occurs. In particular, the supramolecular network based on dimer acid and diethylenetriamine can form reversible amide bonds and hydrogen bonds, enabling rapid self-healing utilizing intermolecular interactions. Furthermore, the inclusion of NH2-MWCNT not only improves electrical properties but also increases durability and tensile strength due to strong interactions between the filler and the polymer.

[0082] Meanwhile, a self-healing strain sensor according to an embodiment of the present invention may comprise: an elastic substrate; a strain sensing layer formed on the elastic substrate and comprising the self-healing polymer; a first electrode in contact with the strain sensing layer; and a second electrode spaced apart from the first electrode and in contact with the strain sensing layer. The principle by which this sensor detects strain is based on utilizing the characteristic that the electrical properties of the strain sensing layer change according to external mechanical deformation. The strain sensing layer is composed of a conductive composite material comprising a self-healing polymer and functionalized multi-walled carbon nanotubes (NH2-MWCNT), and this composite material possesses elasticity and flexibility while its resistance can change in response to external stress.

[0083] When the sensor is subjected to deformation such as tension, compression, or bending, the electrical conduction path changes as the conductive network within the strain sensing layer deforms. It operates in such a way that electrical resistance increases as the conductive network is severed or stretched when deformation is applied, and decreases again as the network returns to its original state when the deformation is released. In particular, the inclusion of NH2-MWCNT can enhance electrical sensing sensitivity, and the long-term stability and reusability of the sensor can be increased by restoring damaged conduction paths using a polymer matrix with self-healing properties. Through this principle, the strain sensor of the present invention is capable of not only continuous strain detection but also the recovery of function even after damage, making it highly likely to be utilized in various application environments requiring durability.

[0084] The advantage of the self-healing strain sensor of the present invention described above is that it possesses high elasticity and the possibility of repeated use compared to conventional strain sensors. While general strain sensors are highly susceptible to performance degradation due to mechanical damage or fatigue, the sensor of the present invention includes a polymer matrix with self-healing capabilities, allowing it to repair itself even from external damage. Furthermore, by using an NH2-MWCNT-based conductive composite as the sensing layer, excellent electrical response characteristics can be maintained, and thanks to its high elasticity, it is possible to exhibit stable resistance changes even under deformation of more than 400%. Accordingly, the sensor can be used without performance degradation even under long-term repetitive deformation, thereby increasing reliability in wearable devices or biosignal detection fields.

[0085] Additionally, when applied to wearable devices, the PBS:Ecoflex-based elastic substrate can be used to provide greater flexibility and a better fit than conventional silicon-based sensors, and the simple structure of the sensor itself makes mass production easy.

[0086] Meanwhile, a self-healing temperature sensor according to an embodiment of the present invention may include: an elastic substrate; a temperature sensing layer formed on the elastic substrate and comprising the self-healing polymer; a first electrode in contact with the temperature sensing layer; and a second electrode spaced apart from the first electrode and in contact with the temperature sensing layer. The principle by which this sensor detects temperature is based on utilizing the characteristic that the electrical properties of the temperature sensing layer change according to temperature changes. The temperature sensing layer is composed of a conductive composite material comprising a self-healing polymer and functionalized multi-walled carbon nanotubes (NH2-MWCNT), and this composite material has the characteristic that its electrical conductivity changes according to temperature changes.

[0087] As the temperature rises, the contact resistance between NH2 and MWCNT may change as the polymer matrix expands due to heat or molecular mobility increases. Generally, as the temperature increases, the conductive network rearranges or the conduction paths change, causing resistance to tend to decrease; conversely, as the temperature decreases, the resistance may increase again as it returns to its original state. These characteristics can be quantified by the Temperature Coefficient of Resistance (TCR) value, and the temperature sensor of the present invention has the potential to sensitively detect even minute temperature changes by possessing a high TCR value. Furthermore, due to the inclusion of a self-healing polymer, the internal bonds can reform even if the sensor is damaged by the external environment, thereby maintaining the temperature sensing function.

[0088] The advantage of the self-healing temperature sensor of the present invention described above is that it possesses superior flexibility and durability compared to conventional temperature sensors, while also having the ability to self-heal against external damage. While general temperature sensors may experience performance degradation due to mechanical damage or repeated use, the sensor of the present invention can restore structural and electrical properties even after damage by including a self-healing polymer as the sensing layer. Furthermore, by applying an NH2-MWCNT-based conductive composite as the temperature sensing layer, it is possible to maintain high temperature sensitivity, and by utilizing the negative temperature coefficient (NTC) characteristics, it can exhibit stable and repetitive resistance changes in the range of 25 to 100°C. Accordingly, the sensor can respond sensitively to temperature changes and can have a high TCR (Temperature Coefficient of Resistance) value, enabling precise temperature detection.

[0089] The embodiments of the present invention are described below. However, the embodiments described below are merely partial embodiments of the present invention, and the scope of the present invention is not limited to the following embodiments.

[0090] [[Materials and Methods]]

[0091] [Materials and Reagents]

[0092] Multiwalled carbon nanotubes (MWCNTs-95%) were purchased from Graphene Supermarket Co. Ltd. in South Korea. Ammonium hydroxide (NH4OH, 28.0-30.0% NH3) ACS reagent solution, dimer acid (C36H68O4, hydrogenated, density: 0.95 g / ml), and boric acid (H3BO3) powder were obtained from Sigma-Aldrich Co. Ltd. in South Korea. Diethylenetriamine (C4H 13N3, DETA (99%, density: 0.952 g / ml) was purchased from Alfa Aesar Co. Ltd. in South Korea. Silanol-terminated polydimethylsiloxane (OH-PDMS, 700-1500, viscosity: 35-45 cSt) was obtained from Cymit Quimica SL. Ecoflex™ (Part A and Part B: silicone polymer, 00-30) was supplied by SMOOTH-ON in the United States. Silver nanowires (AgNW, R2R coating ink) were purchased from Novarials Corp. in South Korea. Ethanol (C2H5OH, 99.5%) and deionized water (DI water) were used as solvents and reagents, respectively, and were all supplied by Samchun Chemicals Co. Ltd. in South Korea. All materials and reagents were used as provided, without any separate pretreatment.

[0093] [Amino (NH2) Functionalization of Multiwalled Carbon Nanotubes (MWCNT)]

[0094] The functionalization process for introducing amino (NH2) functional groups into MWCNT follows a simple method. First, 1 g of pure MWCNT is added to 20 ml of ammonium hydroxide (NH4OH) solution. This MWCNT-NH4OH solution is vigorously stirred at 400 rpm for 30 minutes at room temperature, followed by sonication for an additional 10 minutes. The reaction-completed solution is transferred to a Teflon container and placed inside a stainless steel autoclave. After sealing the autoclave, the hydrothermal reaction between MWCNT and NH4OH is carried out by heating in a drying oven at 180 °C for 3 hours. Subsequently, the reaction-completed mixture is simultaneously washed with deionized water (DI water) and filtered using a vacuum filtration system with F1001 grade qualitative filter paper (Φ = 150 mm). The washing and filtration process is repeated several times until the pH of the filtered solution reaches neutral (pH = 7.0). The hydrated amino-functionalized MWCNT powder is dried in an oven at 100°C for 2 hours. Finally, the finely dried NH2-MWCNT powder is collected, stored in a glass vial, and used for further experiments.

[0095] [Synthesis of Self-Healing Polymers]

[0096] Self-healing polymers are synthesized by forming supramolecular oligomers using dimer acid and diethylenetriamine (DETA), as reported in previous studies. First, 27.66 g of dimer acid and 11.34 g of DETA are poured into a 250 ml clean beaker and mixed. Subsequently, the mixture is continuously stirred at 160 °C at a speed of 300 rpm for 24 hours under an argon atmosphere using a hot plate stirrer (DAIHAN Scientific). Afterward, the mixture is cooled to room temperature and dissolved in 100 ml of chloroform. The resulting solution is washed with 100 ml of deionized water (DI water) and 50 ml of methanol for 30 minutes each using a multi-purpose centrifuge (Combi-514R - Hanil Scientific). In the next step, the white oligomer is recovered from the washed solution and then chloroform is removed from it in a vacuum drying oven (VO-27, HYSC) at 50°C. After vacuum removal for 48 hours, the white oligomer turns into a yellow, viscous substance and is then stored at room temperature for further use.

[0097] [Synthesis of NH2-MWCNTs-based Self-Healing Stretchable Conductive Composites]

[0098] First, 1 g of the prepared oligomer is dissolved in 10 ml of chloroform to prepare an oligomer solution. This solution is stirred at 400 rpm and 50 °C for 1 hour. For the optimized sample, 5 wt% of NH2-MWCNTs relative to the oligomer is added to the oligomer solution. This mixture is continuously stirred using a hot plate stirrer at 1000 rpm and 140 °C for 40 minutes. After stirring, the suspension is first heated at 150 °C for 2 hours, and then heated further at 160 °C for 1 hour to completely remove the chloroform. Subsequently, the obtained final material is molded onto a glass substrate at 100 °C to fabricate an NH2-MWCNTs-based self-healing stretchable conductive composite having the desired size, shape, and thickness.

[0099] [Fabrication of Strain Healing and Stretchable (SHS) Strain Sensors]

[0100] A strain sensor was fabricated using the NH2-MWCNT-based conductive composite material developed in this invention as the SHS sensing layer and PBS:Ecoflex as the SHS elastic substrate. The fabrication method for the PBS:Ecoflex SHS substrate has been reported in prior studies. First, two PBS:Ecoflex substrates were prepared with a height × width × length = 1 × 10 × 20 mm 3 It was cut to a size of . One of the PBS:Ecoflex samples was used as the bottom substrate layer. Subsequently, the developed NH2-MWCNT-based conductive composite material (0.5 × 5 × 15 mm 3A sensing layer was formed by patterning ) onto a lower PBS:Ecoflex substrate. Next, copper wires were attached to both ends of the sensing layer. Finally, another PBS:Ecoflex substrate was attached to the sensing layer to serve as the upper substrate layer. Through this process, a wearable, complete SHS strain sensor with a sandwich structure was fabricated. The structure of the developed strain sensor is schematically illustrated in the strain sensing section.

[0101] [Fabrication of Self-Healing and Stretchable (SHS) Temperature Sensors]

[0102] Similar to the SHS strain sensor, an SHS temperature sensor is fabricated using PBS:Ecoflex as the SHS elastic substrate and an NH2-MWCNT-based conductive composite as the SHS sensing layer. First, a PBS:Ecoflex sample (0.5 × 10 × 10 mm) as the bottom substrate layer 3 Prepare ). Then, deposit an AgNW electrode pattern on the underlying substrate using an electron jet method. Subsequently, an NH2-MWCNT-based conductive composite (0.5 × 5 × 5 mm 3 A temperature sensing layer composed of ) is patterned onto the sensing area of ​​the AgNW electrode layer. Finally, the PBS:Ecoflex upper substrate layer (0.5 × 10 × 8 mm 3 A ) is attached to the electrode and sensing layer, with the electrode tip exposed to allow for the connection of a probe for temperature sensing measurement. The schematic of the developed temperature sensor is shown in the temperature sensing section.

[0103] [Characterization and Measurement]

[0104] Morphological analysis of the self-healing material was performed using an optical microscope (OM, OLYMPUS U-MSSP4). Electron microscope images, EDS spectra, and elemental mapping of the samples were observed using a field emission scanning electron microscope (FESEM, S-4800, Hitachi). Fourier transform infrared spectroscopy (FT-IR, Thermo NICOLET iS50) was used at 500–4000 cm⁻¹. -1 Infrared spectroscopic data were acquired within the specified range. X-ray powder diffraction (XRD) data were recorded using a MiniFlex II instrument (Rigaku). Changes in chemical bonds occurring during the sample damage and healing process were observed using X-ray photoelectron spectroscopy (XPS, K-alpha plus, Thermo Fisher Scientific). Electrical properties were measured using a Hall measurement system (HMS, Ecopia-AMP55), while electromechanical property analysis and sensing measurements were performed using a source meter (Keithley-2612A).

[0105] [[Results and Discussion]]

[0106] [Analysis of Molecular and Structural Characteristics]

[0107] Figure 1a shows the molecular structure and schematic synthesis process of NH2-functionalized MWCNTs. The surface of the MWCNTs was formed into NH2-MWCNTs by introducing amino (-NH2) functional groups from NH4OH. Next, the molecular structures of dimer acid (DA) [C36H68O4] and diethylenetriamine (DETA) [C4H13N3], and the process of forming the supramolecular self-healing polymer are shown in Figures 1b and 1c(i). An NH2-MWCNTs-based self-healing conductive composite was prepared by reacting the NH2-functionalized MWCNTs with the supramolecular self-healing polymer, as shown in Figures 1c(i-iii). The supramolecular polymer contains carboxylic acid (-COOH) groups derived from DA and amine (-NH2) groups derived from DETA, and the functionalized MWCNTs provide additional amine (-NH2) groups to the composite system. Carboxylic acid (-COOH) and amine (-NH2) groups are represented by red and green beads, respectively. Here, the R-COOH and R-NH2 groups can form covalent bonds and induce cross-linking reactions by forming hydrogen bonds with unreacted R-COOH and R-NH2 groups. When damage occurs, the bonds are reversibly broken and reformed, and the damaged bonds (radicals) can combine with unreacted groups to form new bonds. This can be observed in the healing reaction following damage and cleavage. In Figure 1d, which schematically illustrates the self-healing mechanism, dynamic reversible bonds or amine groups originating from supramolecular polymers act as key elements of the self-healing mechanism. In existing literature, amine groups and imino bonds are referred to as dynamic reversible bonds because they possess dynamic and reversible bond formation and dissolution characteristics. Such dynamic reversible mechanisms have been widely used to develop polymers with inherent self-healing capabilities. Meanwhile, the NH2 functional groups of NH2-MWCNTs increase covalently bonded cross-links and serve as effective reinforcing agents that improve the mechanical properties of composites.Furthermore, NH2-MWCNTs serve to impart electrical conductivity within the polymer matrix. Overall, NH2-MWCNTs significantly enhance the electrical and mechanical properties of the composite, a detailed explanation of which is covered in the analysis of electrical and mechanical properties.

[0108] To confirm the introduction of amine (-NH2) functional groups into MWCNT, Fourier Transform Infrared Spectroscopy (FTIR) analysis was performed on pure MWCNT, synthesized NH2-MWCNT, and commercially available NH2-MWCNT (purchased from US Research Nanomaterials, Inc., Houston, USA). As shown in Figure 2a, the FTIR spectrum of the pure MWCNT does not exhibit characteristic peaks of the specific functional group. However, after functionalizing the MWCNT, characteristic peaks of the specific functional group appeared in the spectrum of the synthesized NH2-MWCNT, and a significant change in the spectrum was confirmed. As can be seen in Figures 2a and 2b, the characteristic peaks and spectral changes of the synthesized NH2-MWCNT are identical to those of the commercial NH2-MWCNT. The most prominent changes in characteristic peaks in the spectra of the synthesized NH2-MWCNT and the commercial NH2-MWCNT were observed at 1126, 1572, and 3437 cm⁻¹, respectively. -1 It appeared at 1126 cm -1 The peak appearing at corresponds to the stretching vibration of the CN functional group, which is at 1000–1255 cm⁻¹. -1 May appear within the range. 1572 cm -1 The peak is related to the bending vibrations of CH and the stretching vibrations of the C=C functional group. Also, 1600–2200 cm⁻¹ -1 1972, 2029, and 2161 cm appearing in the range -1The three peaks are associated with the C=O, CN, and C=C stretching vibrations, respectively. These peaks also appear in the spectrum of pure MWCNT. Therefore, it can be confirmed that the basic structure remains intact even after surface modification or functionalization with amine functional groups of the MWCNT. Meanwhile, the most important peak associated with the NH stretching vibration of the amine (-NH2) functional group is generally at 3300–3500 cm⁻¹. -1 It appears in the range. In the spectra of synthesized NH2-MWCNT and commercial NH2-MWCNT, the NH stretching vibration peak is at 3437 cm⁻¹. -1 It was observed in the form of a broad band. FTIR spectroscopic analysis confirmed that the MWCNT was successfully functionalized with amine functional groups while maintaining its unique properties.

[0109] Field emission scanning electron microscopy (FE-SEM) analysis was performed to investigate the morphology of pure MWCNTs and synthesized NH2-MWCNTs for primary structural analysis. The morphology of NH2-MWCNTs is similar to that of pure MWCNTs, with the average diameter of both samples ranging from 30 to 40 nm. This confirms that the surface modification process using NH2 functional groups did not damage the original structure of the MWCNTs. Additionally, FE-SEM images of commercially available NH2-MWCNTs were analyzed and compared with those of the synthesized NH2-MWCNTs. While the commercial NH2-MWCNTs have a structure similar to the synthesized ones, their diameters differ slightly (20–25 nm), which appears to be due to the different original MWCNTs provided by different companies. In the results of energy dispersive X-ray spectroscopy (EDS) and elemental analysis of the measured samples, Si and Ni elements were detected in all samples, as the samples for FE-SEM / EDS analysis were prepared on p-type silicon wafers. However, aside from this, the EDS elemental analysis results of pure MWCNT show that it contains only carbon atoms. On the other hand, nitrogen atoms are detected, indicating the presence of NH2 functional groups in both synthesized and commercial NH2-MWCNT. The analysis results of the synthesized and commercial NH2-MWCNTs were nearly identical; however, trace amounts of calcium were detected in the commercial NH2-MWCNTs, which may be due to impurities.

[0110] Going a step further, the morphology of pure MWCNT and NH2-MWCNT (both synthetic and commercial) was analyzed using high-resolution transmission electron microscopy (TEM). The diameters of the samples measured by TEM analysis tended to be consistent with those measured in FE-SEM images. In particular, the sidewalls of pure MWCNT appeared smoother compared to the synthesized NH2-MWCNT. A similar trend of increasing surface roughness was observed in the commercial NH2-MWCNT as well. The surface roughness of the NH2-MWCNT samples demonstrates that the MWCNT was successfully modified with functional groups.

[0111] After the successful functionalization of MWCNTs, the synthesized NH2-MWCNTs were incorporated into a prepared supramolecular polymer to develop a self-healing and stretchable conductive polymer composite. Figure 2c shows the X-ray diffraction (XRD) patterns of the synthesized NH2-MWCNTs (left), the prepared supramolecular polymer (center), and the SHS polymer composite containing NH2-MWCNTs (right), respectively. In the XRD pattern of NH2-MWCNTs (Figure 2c (left)), the diffraction peaks appearing at 2θ = 26° and 43° correspond to the (002) and (100) planes of the hexagonal graphite structure derived from the MWCNTs, respectively. These diffraction peaks are consistent with the XRD data of NH2-MWCNTs reported in previous studies, which not only proves the successful synthesis of NH2-MWCNTs but also confirms that the graphene layer structure of the MWCNTs was not damaged during the surface treatment process. XRD data of the supramolecular polymer (Fig. 2c (center)) shows a small peak at 2θ = 36°, which reflects the short-range ordered structure of the polymer matrix. Additionally, no additional peaks appear in the high 2θ region, which indicates the amorphous behavior of the supramolecular polymer. The XRD pattern of the SHS polymer composite containing NH2-MWCNTs (Fig. 2c (right)) includes all characteristic peaks originating from both NH2-MWCNTs and the supramolecular polymer, confirming that NH2-MWCNTs were successfully incorporated into the supramolecular polymer matrix.

[0112] The reason NH2-MWCNTs could be successfully incorporated into a supramolecular polymer matrix is ​​that the MWCNTs were surface-modified with amino (-NH2) functional groups. This can be confirmed through optical microscope images of composites prepared using unmodified MWCNTs (Fig. 3a) and NH2-MWCNTs (Fig. 3b). In the composite containing unmodified MWCNTs, the formation of numerous entangled bundles can be observed. This is because strong van der Waals interactions between the MWCNTs attract each other, exhibiting a high tendency to aggregate and form bundles. Due to this phenomenon, unmodified MWCNTs become chemically inactive, making it difficult for them to react with other molecules and consequently difficult to incorporate into the polymer matrix. On the other hand, MWCNTs modified with -NH2 functional groups are chemically active and can selectively react with -NH2 and free hydrogen (-H) bonding sites in the supramolecular polymer matrix. The nitrogen atom of the amine (-NH2) functional group can form hydrogen bonds with hydrogen atoms of other amine functional groups, and the oxygen atom of the -COOH functional group can form hydrogen bonds with hydrogen atoms of other -COOH functional groups. However, these bonds are relatively weaker than the hydrogen bonds formed between the oxygen atom of the -CONH or -COOH functional group and the hydrogen atom of the -CONH or -NH2 functional group. These hydrogen bonds are reinforced by intermolecular polar-polar attraction. Additionally, the formation of multiple hydrogen bonds within the composite system plays a crucial role in ensuring that NH2-MWCNTs are uniformly dispersed within the polymer matrix; as a result, as shown in Fig. 3b, no aggregation or bundle formation of MWCNTs occurs. Consequently, the fabricated composite exhibits excellent mechanical and electrical properties.

[0113] Here, the DA / DETA polymer matrix is ​​a soft gel-like material that possesses the property of forming self-healing and stretchable elastomers or rubber-like composites when containing microparticles or nanoparticles. Loading conductive microparticles or nanoparticles into the polymer matrix can impart electrical conductivity to the composite. Therefore, composites without NH2-MWCNTs loading are electrically insulating and lack mechanical strength due to their gel form. However, the electrical and mechanical properties of the composite can be adjusted by controlling the loading amount of NH2-MWCNTs. To investigate the effect of NH2-MWCNTs content on the electromechanical properties and self-healing performance of the composite, four sample sets (5 samples per set) were prepared with NH2-MWCNTs content set to 1, 3, 5, and 7 wt%, respectively. The electromechanical and self-healing properties of all prepared samples were studied in detail in the mechanical and electrical properties analysis section and are summarized in Tables 1 and 2. The composite containing 5 wt% NH2-MWCNTs showed the best performance in all studied elements and was determined to be an optimized sample with high elasticity, self-healing ability, and electrical conductivity.

[0114] Mechanical properties and self-healing performance of composites according to the concentration of NH₂-MWCNTs.

[0115] NH2-MWCNT loading (%)Maximum strength (kPa)Maximum strain at break (%)Healing efficiency at 1 hr (%)174.21530983142.32480965286.15410937384.1229088

[0116] Electrical properties of composites according to the concentration ratio of NH₂-MWCNTs.

[0117] NH2-MWCNT loading (%)Conductivity, σ (S cm-1)Resistivity, ρ (Ω·cm)Sample thickness, t (cm)Sheet resistance, Rs (Ω sq-1)10.015464.9350.1649.3530.3173.1540.131.5458.230.1220.11.22717.210.0580.10.58

[0118] As shown in Fig. 3c, the optimized composite exhibits high elasticity (top left and top right) and provides electrical conductivity sufficient for use as a conductor in LED circuits (bottom). The physical self-healing performance of the optimized composite was investigated through optical microscopy (OM) analysis, and the results are shown in Fig. 3d. For this purpose, the sample was cut in half, and the cut sections were realigned. Subsequently, the sample was placed on a glass microscope slide to perform OM measurements. As can be seen in Fig. 3d (left), a scar approximately 100 μm in size remained immediately after the cut sections were realigned. As shown in Fig. 3d (center), this scar gradually disappeared within 10 minutes at room temperature, leaving only a bright mark indicating recent damage. After an additional 10 minutes, as shown in Fig. 3d (right), even this mark completely disappeared. This indicates that the sample underwent complete interfacial self-healing. Mechanical and electrical healing efficiencies are analyzed in subsequent sections. [Mechanical Properties and Self-Healing]

[0119] Mechanical performance was evaluated by tensile measurements using a tensile testing machine (Instron-3367), with the scan speed set to 20 mm / min. Figure 4a shows the tensile stress-strain curves of samples with NH2-MWCNT contents of 1, 3, 5, and 7 wt%, respectively. As the content increases, tensile stress tends to increase, while strain tends to decrease. Conversely, as the content decreases, strain increases, while tensile stress tends to decrease. As the amount of NH2-MWCNT increases, the maximum strength also tends to increase. The sample containing 7 wt% NH2-MWCNT exhibits the highest mechanical strength (384.12 kPa) among all tested samples. However, the maximum strain of this sample (290%) is lower than that of the other samples. On the other hand, the sample containing 1 wt% NH2-MWCNT has the highest maximum strain at 530%, but its strength is relatively low at 74.21 kPa. Meanwhile, the sample containing 5 wt% NH2-MWCNT simultaneously exhibits a relatively high strain (410%) and maximum strength (286.15 kPa). In addition, this sample shows relatively excellent mechanical self-healing performance.

[0120] To evaluate mechanical self-healing performance, the maximum fracture strain of all experimental samples was first recorded. Subsequently, the samples were cut in half and damaged, and self-healing was performed under the same conditions. After self-healing at room temperature for 1 hour, the maximum fracture strain of the restored samples was measured and compared with the value of the original samples (Fig. 4b). Based on the maximum fracture strain values ​​of the samples before and after self-healing, the mechanical self-healing efficiency (η) mech ) was calculated using the following formula.

[0121] η mech = (maximum strain_healed sample) / (maximum strain_prestine sameple) * 100%

[0122] The mechanical self-healing efficiency obtained from samples containing NH2-MWCNTs of different mass percentages (wt%) is consistent with the results of the mechanical property analysis, and the 5 wt% NH2-MWCNTs sample exhibited moderate performance in terms of mechanical self-healing. To further investigate the mechanical self-healing performance of the 5 wt% NH2-MWCNTs sample, the healing time and the maximum healing efficiency at those times were examined. To investigate the healing time, the 5 wt% NH2-MWCNTs sample was damaged and allowed to heal at room temperature for 30 seconds, 30 minutes, 1 hour, and 2 hours, and the results are shown in Figures 4c and 4d. The sample began self-healing within 30 seconds of healing time, and the maximum mechanical self-healing efficiency (η) mec .) represents 18%. η mec The value increases with increasing healing time, and 100% mechanical self-healing was achieved within 2 hours at room temperature. The excellent mechanical self-healing efficiency of this composite is repeatedly maintained even during multiple cut-healing processes.

[0123] Additionally, to investigate the state recovery of the optimized composite, iterative stretching-releasing experiments were performed, and periodic stress-strain curves were obtained at various strain values. Experiments were conducted at different strains of 100%, 200%, 300%, and 400%. At high strains, a clear hysteresis loop was observed, which implies that energy is dissipated during the stretching-releasing process at large deformations. Despite the observation of the hysteresis loop, the sample was able to return to its initial state after tension was released at any strain. These results confirm that the composite possesses excellent mechanical self-healing properties.

[0124] [Electrical Characteristics Analysis]

[0125] The electrical characteristics of samples prepared with various mass ratios (wt%) of NH2-MWCNTs were measured using a Hall measurement system (HMS, Ecopia-AMP55). Table 2 shows the electrical conductivity, resistivity, and sheet resistance values ​​of samples prepared with 1, 3, 5, and 7 wt% NH2-MWCNTs, respectively. It was confirmed that as the NH2-MWCNTs content increased from 1 wt% to 7 wt%, the electrical conductivity (σ) increased from 0.0154 to 17.21 S / cm, while the resistivity (ρ) and sheet resistance (Rs) decreased significantly from 64.935 to 0.058 Ω·cm and from 649.35 to 0.58 Ω·sq, respectively. Figure 5a shows the changes in electrical characteristics according to the NH2-MWCNTs content. Although the sample containing 7 wt% NH2-MWCNTs exhibited the highest electrical conductivity, it was not determined to be the optimal sample due to the relative degradation of mechanical and self-healing properties. Considering the balance between these conflicting characteristics, the sample containing 5 wt% NH2-MWCNTs was selected as the optimal composite material with the most balanced electrical properties and self-healing performance. Regarding the electrical properties of this sample, the sheet resistance is low at 1.22 Ω·sq, and the electrical conductivity is high at 8.23 ​​S / cm.

[0126] Figure 5b shows an experiment in which a self-healing conductor is connected to a light-emitting diode (LED) bulb in a simple circuit and operated. In Figure 5b (top left), the LED bulb was lit, but it was observed that the LED bulb turned off when the conductor was cut in Figure 5b (top right). Subsequently, when the two cut ends were brought together and joined by applying light pressure, it was observed that the LED bulb turned on again in Figures 5b (bottom left) and 5b (bottom right). These results indicate that the developed composite material exhibits rapid and effective electrical healing performance. In particular, this self-healing performance is maintained even after repeating the damage and cut-healing process multiple times. Figure 5c shows the real-time change in resistance while the sample undergoes the cut-healing process. Resistance values ​​were recorded during three repeated cut-healing cycles using a source meter (Kethely-2612A). It was observed that when the sample was cut in half, the gap between the ends increased, causing conductivity to disappear and the resistance value to immediately increase to an insulator level (>10 GΩ). Subsequently, when the severed ends were butted together, the resistance value gradually decreased from a high value to the original value. After each cut-healing cycle was completed, the resistance value was restored to nearly the original value after healing was performed at room temperature for 1 minute (60 seconds). Figure 5d shows the electrical healing efficiency (η) measured in each cycle after the cut-healing cycle was repeated. elec ....) is indicated. As a result of measurement, it was confirmed that the sample showed nearly 100% electrical healing efficiency within 1 minute (60 seconds), and showed only a small deviation of ±1.5% in repeated cut-healing experiments.

[0127] [Multi-mode SHS Sensing Implementation]

[0128] Utilizing the excellent electrical and mechanical properties as well as the self-healing capabilities of the developed composite material, various self-healing and stretchable (SHS) devices, such as SHS strain sensors and SHS temperature sensors, were developed. The device fabrication process is described in detail in the experimental section. Instead of conventional substrates (PDMS, PU, ​​etc.), a PBS:Ecoflex hybrid polymer was used as the substrate layer of the device. This allowed both the sensing layer and the substrate layer of the entire device to possess self-healing and stretchable (SHS) functions. The device was fabricated using a simple stacked self-adhesive method. Self-adhesion offers the advantage of allowing the developed device to be attached to any part of the body or various objects without additional support. As a result, the developed SHS devices are highly durable and possess robust application potential as wearable devices. The performance of each SHS device is discussed in the following section.

[0129] [Performance of SHS Strain Sensor]

[0130] Figure 6a shows a schematic of the developed SHS strain sensor. In practice, the strain sensor can operate in either bending mode or stretching mode depending on the usage environment. For example, the strain sensor can be attached to the wrist to detect wrist movement, as illustrated in Figure 6b(i). The sensor's response changes depending on the flexion or extension of the wrist. When the wrist is extended, the sensor operates in bending mode (Figure 6b(ii)), whereas when it is flexed, both stretching and bending modes occur simultaneously (Figure 6b(iii)). Therefore, it is important to investigate the sensor's response during both bending and stretching cycles. The sensor's response, specifically the change in resistance (ΔR / R0) during the stretching and bending cycles, was measured using a custom-made Arduino-based stretching and bending test device and a Keithley 2612A source meter. The change in resistance is defined as ΔR / R0 = (R1 - R0) / R0, where R0 is the initial resistance value and R1 is the resistance value when the sample is stretched or bent.

[0131] Figure 6c shows the change in resistance when the SHS strain sensor is stretched from 0% to 100% and to 400%. This experiment was performed for 10 repeated cycles. When the sensor is stretched, the resistance increases as the conduction path formed by NH2-MWCNT weakens. Therefore, when stretched from 0% to 100%, the relative resistance changed rapidly from ΔR / R0 = 0 to ΔR / R0 = 2.14, and when stretched to 400%, it increased to ΔR / R0 = 5.67. However, when the sensor is bent, the change in resistance was relatively small because the conduction path is not significantly affected. As shown in Fig. 6d, when the sensor was set to three bending angles of 30, 60, and 90 degrees and five repeated bending cycles were performed, the change in relative resistance was found to be ΔR / R0 = 0, ΔR / R0 = 0.05, 0.12, and 0.18. In addition, it was confirmed that continuous extension-relaxation and bend-straightening responses are repeatable over multiple cycles. These results demonstrate that the sensor can respond accurately with high sensitivity and repeatability over a wide detection range.

[0132] Going a step further, the bending and stretching performance before and after self-healing was investigated. As shown in Fig. 6f, the bending response of the sensor showed no significant change before and after self-healing (cured at room temperature for 2 minutes). On the other hand, the stretching response (Fig. 6e) showed a significant change after curing at room temperature for 2 minutes (first healing). This is because the mechanical healing of the sensing layer (NH2-MWCNT-based conductive composite) did not occur completely within 2 minutes. In other words, the cut edges were not properly bonded during the first healing, and the bonding began to separate when the sensor was stretched. When the sensor was stretched by 400% after the first healing, the relative resistance changed from ΔR / R0 = 0 to 12.39. Furthermore, the resistance did not immediately return to its initial value even after the stretching was released. This resulted in a slight baseline shift in the response after the first healing. After the stretching was released, the resistance was found to gradually recover to its initial value. After healing the sensor at room temperature for 2 hours (second healing), the response before and after self-healing was the same.

[0133] To demonstrate human motion detection as a practical application, a developed strain sensor equipped with elasticity and self-healing capabilities was attached to the wrist, and the sensor response was recorded during wrist extension and flexion states. As shown in Fig. 6g, when the wrist was repeatedly extended in two different states, the attached sensor flexed at different angles, and the relative resistance changed from ΔR / R0 = 0 to 0.08 and 0.18. Fig. 6h shows the sensor response during wrist flexion. During the first three repeated flexion cycles, the attached sensor was stretched by approximately 40%, and during the second flexion cycle, it was stretched by approximately 90%. As a result, the change in relative resistance was measured as ΔR / R0 = 0.77 and 1.9, which is nearly 10 times higher than the value measured during the extension cycle. Therefore, extension and flexion movements can be clearly distinguished through the sensor response, and the degree of wrist movement can be precisely monitored. These results demonstrate that the developed sensor maintains high sensitivity in detecting continuous human movement in real time and exhibits a stable and repetitive response.

[0134] [Performance of SHS Temperature Sensor]

[0135] Figure 7a shows a schematic of the developed SHS temperature sensor. To investigate the temperature sensing performance, experiments were conducted by adjusting the developed sensor to various temperatures inside an MFC-controlled sensing chamber. This chamber has a built-in heater to control the operating temperature. A Keithley 2612A source meter was used with two probes to measure the change in the sensor's resistance.

[0136] Figure 7b shows the change in resistance of the sensor according to temperature changes from 25 ℃ (room temperature) to 100 ℃. Experimental results showed that the resistance tended to decrease as the temperature increased, which means that the developed sensor has a negative temperature coefficient (TCR) characteristic. The TCR value is a factor that determines the sensitivity of the temperature sensor and is defined as the rate of change in resistance per unit temperature change. Therefore, temperature sensitivity can be calculated using the following equation.

[0137] TCR = {(R-R0) / R0} * (1 / ΔT) * 100 %

[0138] Here, R0 is the resistance value at 25 ℃ (room temperature), R is the resistance value at the measurement temperature, and ΔT is the temperature change value between the measurement temperature and 25 ℃.

[0139] Based on the analysis of the temperature and resistance change data in Fig. 7b, the developed sensor showed an average TCR value of -5.2% ℃ in the temperature sensing range of 25–100 ℃. -1 It exhibited high sensitivity. Furthermore, the graphs of relative resistance change and temperature change displayed linear behavior across the entire measurement range, with a coefficient of determination R² of 0.996. The temperature sensing range was subdivided to investigate linearity and sensitivity in the 25–45 ℃ and 50–100 ℃ ranges, respectively. In both temperature ranges, the sensor demonstrated excellent linearity (low temperature range: R² = 0.999, high temperature range: R² = 0.998) and high sensitivity (low temperature range: TCR = -5.8 % ℃). -1 , high temperature range: TCR = -4.7 % ℃ -1 The sensitivity of the developed sensor is higher than that of several sensors reported in existing literature.

[0140] To investigate the repeatability of the sensor developed by the inventors, the dynamic response to heating-cooling cycles was recorded. The repeatability test was performed in two stages, as shown in Fig. 7c. In the first stage, five heating-cooling cycles were performed at temperature settings of 25-40-25 ℃, with a time interval of 6 minutes (360 seconds) for each stage. In this stage, a constant response-recovery curve was observed for the repetitive heating-cooling cycles.

[0141] In the next step, the repeatability test was continued for five additional cycles at the same time intervals at temperature settings of 25-100-25 ℃. The response at the heating temperature (100 ℃) was almost repeatable, but the relative resistance at the cooling temperature (25 ℃) did not fully recover to the initial value. This was because the time interval (6 minutes) set for the measurement was not sufficient for the resistance to fully recover from a very high temperature (100 ℃) to room temperature (25 ℃).

[0142] The response and recovery times of the developed sensor across various temperature sensing ranges were also analyzed. As shown in Fig. 7d, the response and recovery times for a temperature change of 25-40-25 ℃ were measured to be 3.3 minutes and 6 minutes, respectively. For a very high temperature change of 25-100-25 ℃, the response time was almost identical at 3.6 minutes, but a relatively long 9.3 minutes was required for complete recovery (see Fig. 7e).

[0143] This means that the sensor can be used more efficiently for temperature detection in the range of room temperature to 40°C, which is a characteristic most required in practical applications, particularly human body temperature monitoring. Overall, the self-healing and stretchable deformation sensor developed in this invention has demonstrated excellent multi-stimulus detection performance and has great potential for use in medical and human monitoring fields.

[0144] [[conclusion]]

[0145] The goal of this study is to develop a simple method for synthesizing amino (NH2) functionalized multi-walled carbon nanotubes (MWCNTs) and to validate this method in practical applications. The NH2-MWCNTs synthesized using the proposed method exhibit properties similar to commercially available amino-functionalized MWCNTs. For practical verification in smart sensing applications, the synthesized NH2-MWCNTs were incorporated into a previously developed self-healing composite. The self-healing performance of the final composite was evaluated using electrical and mechanical methods. Additionally, experiments were conducted by fabricating the composite in the form of strain sensors and temperature sensors for actual application. Both sensors demonstrated high sensitivity and repeatability, and maintained consistent performance before and after self-healing.

[0146] Although the present invention has been described above with reference to preferred embodiments, those skilled in the art will understand that various modifications and changes can be made to the invention without departing from the spirit and scope of the invention as set forth in the following claims.

Claims

1. A step of hydrothermally synthesizing multiwalled carbon nanotubes functionalized with amino groups (NH2) by heating a solution mixed with multiwalled carbon nanotubes (MWCNT) and ammonium hydroxide (NH4OH); comprising, Method for manufacturing functionalized multi-walled carbon nanotubes.

2. In Paragraph 1, The above hydrothermal synthesis is performed at 150 to 200 ℃, Method for manufacturing functionalized multi-walled carbon nanotubes.

3. In Paragraph 2, The above hydrothermal synthesis is performed by heating for 2 to 4 hours, Method for manufacturing functionalized multi-walled carbon nanotubes.

4. Pursuant to Paragraph 1 Manufactured by a method for manufacturing functionalized multiwalled carbon nanotubes, Functionalized multiwalled carbon nanotubes.

5. A first step of synthesizing an oligomer of a first self-healing polymer; A second step of hydrothermally synthesizing multiwalled carbon nanotubes functionalized with amino groups (NH2) by heating a solution mixed with multiwalled carbon nanotubes (MWCNT) and ammonium hydroxide (NH4OH); and A third step of preparing a second self-healing polymer by molding and drying a solution of the above oligomer and the above multi-walled carbon nanotube; comprising Method for manufacturing a self-healing polymer.

6. In Paragraph 5, In the first step above, the oligomer of the first self-healing polymer is prepared by mixing dimer acid and diethylenetriamine (DETA). Method for manufacturing a self-healing polymer.

7. In Paragraph 5, The hydrothermal synthesis of the second step is performed at 150 to 200 ℃, Method for manufacturing a self-healing polymer.

8. In Paragraph 5, The hydrothermal synthesis of the second step above is performed by heating for 2 to 4 hours, Method for manufacturing a self-healing polymer.

9. Manufactured by the method for manufacturing a self-healing polymer according to paragraph 5, Self-healing polymer.

10. Elastic substrate; A strain sensing layer formed on the above elastic substrate and comprising a self-healing polymer according to claim 9; A first electrode in contact with the strain sensing layer; and A second electrode spaced apart from the first electrode and in contact with the strain sensing layer; comprising Self-healing deformation sensor.

11. Elastic substrate; A temperature sensing layer formed on the above elastic substrate and comprising a self-healing polymer according to claim 9; A first electrode in contact with the temperature sensing layer; and A second electrode spaced apart from the first electrode and in contact with the temperature sensing layer; comprising Self-healing temperature sensor.

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