Carbon nanotube synergistically reinforced shape memory polymer composite actuating fiber and method of making same

Abstract

This invention relates to the fields of smart materials and soft actuation technology, and discloses a carbon nanotube-synergistically reinforced shape memory polymer composite actuation fiber and its preparation method. The fiber's structure consists of an axially continuous carbon nanotube network and a shape memory polymer matrix encapsulated within it, with reversible dynamic bonds (such as multi-level hydrogen bonds) within the matrix. The two components form a synergistically reinforced structure through interfacial entanglement. During stretching, the carbon nanotubes induce axial orientation of the polymer molecular chains and store conformational entropy; under thermal or electrical stimulation, changes in the dynamic bonds drive entropy release, causing the fiber to contract axially / recover its shape, outputting stress. This fiber possesses high stress (>10MPa), large strain (>300%), fast response, and integrated electro-thermal-mechanical actuation characteristics, making it suitable for soft robots, smart fabrics, and other fields.

Description

Technical Field

[0001] This invention relates to the field of smart materials and soft actuation technology, and in particular to a carbon nanotube-synergistically reinforced shape memory polymer composite actuation fiber and its preparation method. Background Technology

[0002] Shape memory polymers (SMPs) have shown great potential in actuators, deployable structures, and smart devices due to their recoverable deformation and good processability. However, the inherent defects of traditional SMP materials, including shape memory polyurethane (SMPU), fundamentally limit their development in high-performance applications. This is mainly reflected in the fundamental contradiction between actuation strain and actuation stress. To achieve high actuation strain, the material network needs long and flexible segments and sufficient free volume, but this leads to insufficient modulus and recovery force, resulting in low actuation stress. Conversely, high actuation stress depends on high crosslinking density or strong molecular forces, but this severely restricts the segment mobility, leading to a significant reduction in recoverable strain. In addition, traditional SMPs also suffer from problems such as reliance on external heat sources for passive heating (slow response), low mechanical strength (limited output force), and separation of actuation function from structural load-bearing function.

[0003] To overcome these contradictions, existing research mainly focuses on two approaches: improving the intrinsic properties of polymers and developing nanocomposites. On the one hand, the strength, toughness, and recovery stress of materials are enhanced by constructing physical / chemical cross-linking networks (especially by introducing reversible dynamic chemical bonds, such as transesterification, disulfide bonds, metal coordination bonds, and hydrogen bonds). On the other hand, the mechanical, electrical, and thermal properties of materials are improved by adding nanofillers (such as carbon nanotubes, CNTs). However, the effects of these improvement strategies are often localized. Simple nanofiller doping cannot simultaneously solve problems such as uneven dispersion and weak interfacial bonding, resulting in limited effectiveness in synergistically improving high strain and high stress. Numerous literature reports also confirm the dilemma of achieving both properties simultaneously. For example, a spin-type liquid crystal elastomer fiber actuator reported in the literature (DOI: 10.1016 / j.apmt.2022.101449) achieved a driving strain as high as 1750%, but its corresponding driving stress was less than 3.5 MPa. The carbon nanotube yarn actuator reported in the literature (DOI:10.1002 / adfm.202200591) achieved a driving stress of 11 MPa, but its driving strain was only 14.3%. Another literature (DOI: 10.1021 / am403071k) reported a sulfonated graphene / carbon nanotube / polyurethane composite infrared actuator with actuation strain and stress of 75% and 1.22 MPa, respectively. These data indicate that, in existing technologies, single-fiber actuators struggle to overcome the performance barrier of the trade-off between high strain (>300%) and high stress (>10 MPa).

[0004] At the same time, the driving methods of existing technologies also limit their rapid response and integrated capabilities. Most SMP composite materials rely on external heat sources (such as ovens and hot air) for passive heating, resulting in slow response speeds and separation of the actuation unit from the heat source, leading to system complexity.

[0005] In summary, existing technologies have not yet reported the ability to simultaneously achieve high output stress (>10MPa), large recovery strain (>300%), rapid response, and integrated electro-thermal-mechanical actuation within a single fiber scale. Developing fibrous actuators that combine these superior properties remains a key technological challenge that urgently needs to be overcome in this field. Summary of the Invention

[0006] To address the shortcomings of existing technologies, the present invention aims to provide a carbon nanotube-synergistically enhanced shape memory polymer composite actuation fiber and its preparation method. This composite fiber achieves a synergistic improvement in actuation performance and structural mechanical properties through a unique structural design.

[0007] The solution of the present invention is: A carbon nanotube-synergistically reinforced shape memory polymer composite actuating fiber includes a one-dimensional carbon nanotube network that is continuously penetrating along the fiber axis, and a shape memory polymer matrix that is coated or embedded in the carbon nanotube network. The shape memory polymer matrix contains a reversible dynamic bond cross-linking structure. The carbon nanotube network and the shape memory polymer matrix form a synergistic reinforcing interface through interfacial entanglement and non-covalent interactions. The composite fiber is constructed such that, under external tension, the carbon nanotube network induces the polymer molecular chains to be highly oriented along the fiber axis and stores conformational entropy; when subjected to heat and / or electrical stimulation, the reversible dynamic bond cross-linking structure dissociates or recombines, driving the oriented structure to release conformational entropy, causing the fiber to undergo axial contraction and / or shape recovery, thereby outputting actuating force or actuating stress.

[0008] The synergistic mechanism of this invention lies in the fact that the dynamic bond network of the shape memory polymer can undergo dissociation-slippage-regeneration during stretching, maintaining network integrity under high strain and exhibiting excellent stretchability. The continuous carbon nanotube network provides the fiber with a powerful and efficient stress and heat transfer pathway. Furthermore, it creates a spatial confinement effect on the polymer chains within the polymer matrix, inducing the polymer chains to align along the fiber axis, thereby forming a storageable conformational entropy within the fiber. This allows the composite fiber to simultaneously achieve high actuation stress and minimal dynamic strain. The carbon nanotube network enables integrated electro-thermal-mechanical actuation, eliminating the need for external heating elements.

[0009] As a preferred technical solution, the carbon nanotube network is composed of carbon nanotubes uniformly dispersed in the shape memory polymer matrix, and the carbon nanotubes overlap each other in the fiber axis to form a continuous mechanical reinforcement skeleton, electrical and / or thermal conduction pathway.

[0010] As a preferred technical solution, the reversible dynamic bond crosslinking structure is a multi-level hydrogen bond structure.

[0011] As a preferred technical solution, the shape memory polymer matrix is ​​shape memory polyurethane, which is synthesized by polycaprolactone diol, N,N-dimethylformamide, bis(dilaurate)dibutyltin, dicyclohexane-4,4'-diisocyanate, cyclo-2-urea-4-pyrimidinone, and ethylenediamine.

[0012] As a preferred technical solution, the shape memory polyurethane is prepared by the following steps: (a) Dissolve polycaprolactone diol in anhydrous N,N-dimethylformamide and add bis(dilauric acid)dibutyltin dropwise; (b) Add dicyclohexamethylene-4,4'-diisocyanate to the dissolved liquid to form a prepolymer; (c) Add cyclo-2-urea-4-pyrimidinone to the prepolymer and stir until completely dissolved; then lower the temperature of the system to 70°C, add ethylenediamine, and after the reaction is complete, dry in a vacuum oven to obtain solid polyurethane (i.e. shape memory polyurethane).

[0013] As a preferred technical solution, the mass fraction of carbon nanotubes is 3% to 25% of the total mass of the composite actuating fibers.

[0014] This invention also discloses a method for preparing carbon nanotube-reinforced shape memory polymer composite actuating fibers, comprising the following steps: S1. Prepare spinning solution by dispersing carbon nanotubes in a first solvent to obtain a carbon nanotube dispersion; dissolve a shape memory polymer in a second solvent to obtain a polymer solution; mix and homogenize the carbon nanotube dispersion and the polymer solution to obtain a uniform spinning solution. S2. Wet spinning and forming: The spinning solution is extruded through a spinneret into a coagulation bath, solidified and formed to obtain nascent fibers. S3. Post-densification treatment: The nascent fibers are heat-treated at a temperature of 50-120°C to obtain dense composite fibers.

[0015] As a preferred technical solution, in step S1, both the first solvent and the second solvent are N,N-dimethylformamide; the homogenization treatment includes probe ultrasound and / or mechanical stirring.

[0016] As a preferred technical solution, in step S2, the coagulation bath is deionized water.

[0017] As a preferred technical solution, in step S3, the heat treatment is carried out under a vacuum or inert atmosphere for 2 to 24 hours.

[0018] The present invention also discloses a soft robot, smart fabric or deployable structure comprising shape memory composite actuation fibers.

[0019] Compared with the prior art, the advantages of the present invention are: (1) High actuation performance synergy, dynamic bond network ensures high tensile strength, carbon nanotube network induces high orientation energy storage. This enables the composite fiber to simultaneously achieve a pre-strain of up to 300% and an actuation stress output of over 10 MPa, breaking through the performance contradiction of traditional materials.

[0020] (2) Rapid integrated drive: The continuous carbon nanotube network forms an efficient conductive and thermal conduction path, which enables the fiber to generate Joule heat through direct current to achieve electro-driven operation. The response speed is fast (up to the second level), realizing the integration of drive, sensing (resistance change can feedback deformation) and load-bearing structure.

[0021] (3) The structure is stable and reliable. Through wet spinning combined with thermal densification process, the carbon nanotubes are uniformly dispersed and axially connected in the matrix, as well as the tight interface between the two, which gives the fiber excellent mechanical strength and cycle stability. Attached Figure Description

[0022] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the provided drawings without creative effort.

[0023] Figure 1 The image shown is a scanning electron microscope image of the composite fiber containing 25% carbon nanotubes obtained according to the present invention. Figure 2 The mechanical curve of the composite fiber obtained in Example 2 of this invention; Figure 3 Diffraction images of the stretched orientation of the composite fibers obtained in Example 5 of this invention; Figure 4 Images of the thermal actuation process of the composite fiber obtained in Example 8 of this invention. Detailed Implementation

[0024] To make the technical means, creative features, objectives and effects of this invention easier to understand, the invention will be further described below with reference to specific embodiments.

[0025] The synthesis method of shape memory polyurethane is as follows: Weigh 7.5 g of polycaprolactone diol (PCL) and dissolve it in 20 mL of anhydrous N,N-dimethylformamide (DMF). Add 2 drops of the catalyst bis(dilauric acid)dibutyltin and stir until homogeneous.

[0026] Subsequently, 1.5 g of dicyclohexylmethane-4,4'-diisocyanate (HMDI) was added. The reaction was carried out at 75°C for 2 hours under nitrogen protection to obtain the prepolymer.

[0027] Maintain the system temperature at 75°C, add 0.3 g of cyclic 2-urea-4-pyrimidinone (UPy), and stir for at least 6 hours until the cyclic 2-urea-4-pyrimidinone is completely dissolved. Cool the reaction system to 70°C and add 0.1 g of ethylenediamine (EDA). Continue stirring the reaction for 8 hours.

[0028] After the reaction was completed, the product was dried in a vacuum oven at 80°C to obtain shape memory polyurethane.

[0029] Example 1: Preparation of composite actuation fibers This embodiment provides a method for preparing electro / thermal actuated composite fibers with synergistic structural reinforcement.

[0030] Multi-walled carbon nanotube (CNT, purchased from Tiannai Technology) powder was uniformly dispersed in 5 mL of organic solvent N,N-dimethylformamide (DMF) by probe ultrasonication to form a stable carbon nanotube dispersion system.

[0031] The synthesized shape memory polyurethane containing reversible dynamic bonds (multi-level hydrogen bonds) was dissolved in N,N-dimethylformamide (DMF) to prepare a shape memory polymer solution with a concentration of 3-25 wt%.

[0032] The carbon nanotube dispersion system is mixed with the shape memory polymer solution in a certain proportion, so that the mass fraction of carbon nanotubes in the mixture (mass percentage of the final solid composite) is adjustable between 0% and 25%, and the mixture is stirred thoroughly to form a uniform spinning solution.

[0033] The spinning solution is extruded into a coagulation bath (deionized water) through a spinneret using a wet spinning process. After solidification and molding, the solution is collected to obtain nascent composite fibers.

[0034] The nascent composite fibers were placed in an oven at 80°C for heat-assisted densification treatment for 6–12 hours to fully eliminate internal pores and enhance the interfacial bonding between the carbon nanotube network and the polymer matrix, ultimately producing dense composite fibers with a diameter of approximately 216 μm. Figure 1 The image shown is a scanning electron microscope (SEM) image of the composite fiber prepared in this embodiment, revealing its microstructure.

[0035] Example 2: Mechanical property testing of composite fibers This embodiment characterizes the mechanical properties of composite fibers with different carbon nanotube mass fractions prepared in Example 1.

[0036] The stress-strain curves of the composite fiber were tested using a universal testing machine according to the standard fiber tensile testing method. The results show that the tensile strength and modulus of the composite fiber continuously increase with the increase of carbon nanotube content. The maximum tensile fracture stress of the fiber can reach approximately 80 MPa, while maintaining good toughness. Figure 2 Representative stress-strain curves of composite fibers with different carbon nanotube contents are shown.

[0037] Example 3: Electrical and thermal property testing of composite fibers This embodiment provides a series of methods for preparing electro / thermal actuated composite fibers with synergistic structural enhancement and different carbon nanotube contents.

[0038] Taking a sample with a carbon nanotube mass fraction of 20% (referred to as PCF20 for ease of description) as an example, its preparation process is the same as in Example 1, as follows: multi-walled carbon nanotube powder is uniformly dispersed in DMF, mixed with shape memory polyurethane solution, the mass fraction of carbon nanotubes in the solid phase is adjusted to 20wt%, and after wet spinning and heat treatment at 80℃ for 12 hours, sample PCF20 is obtained.

[0039] A series of composite fiber samples with carbon nanotube mass fractions of 0 wt% (pure polyurethane fiber), 3 wt%, 12 wt%, and 25 wt% (which can be denoted as Upy-PU, PCF3, PCF12, and PCF25, respectively) were prepared by the same method as a comparison.

[0040] Tests revealed that the electrical conductivity threshold of the composite fiber was above 20 wt% carbon nanotube content. Thermal conductivity increased with increasing carbon nanotube content.

[0041] Example 4: Verification of the electrothermal conversion performance of composite fibers This embodiment verifies the electrothermal capability of the composite fiber.

[0042] A section of a composite fiber sample prepared in Example 2, containing 20 wt% carbon nanotubes (PCF20), with a diameter of 250 μm and a length of 2 cm, was taken. Electrodes were connected to both ends of the fiber sample, and a constant DC power supply was provided using a KEITHLEY 2400 series source meter. When a driving current of 3 mA was applied, monitoring with an infrared thermal imager or thermocouple showed that the surface temperature of the fiber rose rapidly due to the Joule heating effect, reaching a steady-state temperature of 54 °C. This demonstrates that the composite fiber can achieve efficient electro-thermal conversion through its own conductive network.

[0043] Example 5: Characterization of the tensile orientation structure of composite fibers This embodiment characterizes the molecular chain orientation behavior of composite fibers during the stretching process.

[0044] A section of composite fiber prepared in Example 3, with a carbon nanotube content of 20 wt% (i.e., PCF20), 2 cm in length and 250 μm in diameter, was cut. In-situ testing of the fiber under different tensile strains (e.g., 0%, 100%, 200%, 300%) was performed using a small-angle / wide-angle X-ray scattering (SAXS / WAXS) instrument equipped with a copper target X-ray source (wavelength λ = 1.542 Å). The resulting two-dimensional diffraction pattern is shown below. Figure 3 As shown in the figure, the analysis results indicate that a significant orientation feature is observed when the tensile strain reaches 300%. This confirms that under external force, the carbon nanotube network effectively induces the polymer molecular chains to align along the fiber axis, forming an orientation structure that can store conformational entropy.

[0045] Example 6: Thermal Actuation Performance Test of Composite Fibers This embodiment tests the thermally actuated stress output capability of composite fibers.

[0046] A section of composite fiber prepared in Example 3, with a carbon nanotube content of 20 wt% (i.e., PCF20), 20 mm in length and 250 μm in diameter, was cut. It was tested using an in-situ tensile testing instrument equipped with a force sensor and a thermal stimulation module. The fiber was clamped at both ends on a fixed clamp and a movable clamp, respectively, with an initial clamping distance (gauge length) of 15 mm. The fiber was pre-stretched to 300% strain at a tensile speed of 15 mm / min and held. Subsequently, thermal stimulation was applied to the fiber using a hot air blower set to a specific temperature (above the polymer soft segment transition temperature). The force sensor recording showed that the fiber rapidly (within approximately 1 second) contracted upon heating, generating an axial contractile stress of up to 15 MPa, i.e., thermally actuated stress.

[0047] Example 7: Electro-actuation performance test of composite fibers This embodiment tests the electro-actuation properties of composite fibers.

[0048] A section of composite fiber prepared in Example 3, with a carbon nanotube content of 25 wt% (i.e., PCF25), 20 mm in length and 216 μm in diameter, was cut. Fine silver wires were connected to both ends of the fiber as electrodes via conductive silver paste and connected to a KEITHLEY 2400 source meter. Using the same mechanical testing apparatus and clamping method as in Example 6, the fiber was pre-stretched to 120% strain and fixed. Subsequently, a constant driving current (e.g., 3 mA) was applied to the fiber through the source meter. The fiber heated up due to the Joule heating effect, triggering shape recovery, and the resulting contractile force was monitored in real time by a force sensor. Test results showed that the composite fiber could generate an electro-actuated stress of up to approximately 13 MPa under electrical stimulation.

[0049] Example 8: Demonstration of the macroscopic load-bearing capacity of composite fibers This embodiment visually demonstrates the load-carrying capacity of composite fibers.

[0050] A section of composite fiber prepared in Example 3, containing 20 wt% carbon nanotubes (PCF20), weighing 0.0005 g, and with a diameter of 250 μm, was taken. The composite fiber was stretched to 300% pre-strain on a tensile testing machine, with the upper end fixed and a 1 g weight suspended from the lower end. A heat gun was then used to apply thermal stimulation to the fiber under tension and load. It was observed that the fiber underwent axial contraction upon heating, successfully lifting the 1 g weight. This load weight is equivalent to 2000 times the fiber's own weight, strongly demonstrating the powerful work capacity and practical application potential of this composite actuation fiber.

[0051] Conclusion: The above embodiments detail the preparation method of the composite actuation fiber of the present invention and systematically characterize its excellent mechanical, electrical, and thermal properties, as well as its outstanding thermal and electrical actuation properties. The results show that this fiber successfully integrates high output stress, large recovery strain, and rapid response capability, making it suitable for applications such as soft robots, smart fabrics, and deployable structures.

[0052] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed. The scope of protection of this invention is defined by the appended claims and their equivalents.

Claims

1. A carbon nanotube-synergistically reinforced shape memory polymer composite actuating fiber, characterized in that: It includes a one-dimensional carbon nanotube network that runs continuously along the fiber axis, and a shape memory polymer matrix that is coated or embedded in the carbon nanotube network; The shape memory polymer matrix contains a reversible dynamic bond cross-linking structure. The carbon nanotube network and the shape memory polymer matrix form a synergistic reinforcing interface through interfacial entanglement and non-covalent interactions. The composite fiber is configured such that, under external tension, the carbon nanotube network induces the polymer molecular chains to be highly oriented along the fiber axis and stores conformational entropy; when subjected to heat and / or electrical stimulation, the reversible dynamic bond cross-linking structure dissociates or recombines, driving the oriented structure to release conformational entropy, causing the fiber to undergo axial contraction and / or shape recovery, thereby outputting actuating force or actuating stress.

2. The carbon nanotube-synergistically reinforced shape memory polymer composite actuating fiber as described in claim 1, characterized in that: The carbon nanotube network is composed of carbon nanotubes uniformly dispersed in the shape memory polymer matrix, and the carbon nanotubes overlap each other in the fiber axis to form a continuous mechanical reinforcement skeleton, electrical and / or thermal conduction pathway.

3. The carbon nanotube-synergistically reinforced shape memory polymer composite actuating fiber as described in claim 1, characterized in that: The reversible dynamic bond crosslinking structure is a multi-level hydrogen bond structure and / or a reversible non-covalent interaction structure.

4. The carbon nanotube-synergistically reinforced shape memory polymer composite actuating fiber as described in claim 1, characterized in that: The shape memory polymer matrix is ​​shape memory polyurethane, which is synthesized by polycaprolactone diol, N,N-dimethylformamide, bis(dilaurate)dibutyltin, dicyclohexylmethane-4,4'-diisocyanate, cyclo-2-urea-4-pyrimidinone, and ethylenediamine.

5. The carbon nanotube-synergistically reinforced shape memory polymer composite actuating fiber as described in claim 1, characterized in that: The mass fraction of carbon nanotubes is 3% to 25% of the total mass of the composite actuation fiber.

6. A method for preparing carbon nanotube-synergistically reinforced shape memory polymer composite actuating fibers as described in any one of claims 1 to 5, characterized in that, Includes the following steps: S1. Prepare spinning solution by dispersing carbon nanotubes in a first solvent to obtain a carbon nanotube dispersion; dissolve a shape memory polymer in a second solvent to obtain a polymer solution; mix and homogenize the carbon nanotube dispersion and the polymer solution to obtain a uniform spinning solution. S2. Wet spinning and forming: The spinning solution is extruded through a spinneret into a coagulation bath, solidified and formed to obtain nascent fibers. S3. Post-densification treatment: The nascent fibers are heat-treated at a temperature of 50-120°C to obtain dense composite fibers.

7. The method as described in claim 6, characterized in that: In step S1, both the first solvent and the second solvent are N,N-dimethylformamide; the homogenization treatment includes probe sonication and / or mechanical stirring.

8. The method as described in claim 6, characterized in that: In step S2, the coagulation bath is deionized water.

9. The method as described in claim 6, characterized in that: In step S3, the heat treatment is carried out under a vacuum or inert atmosphere for 2 to 24 hours.

10. A soft robot, smart fabric, or deployable structure, characterized in that, The composite actuated fiber comprises a shape memory polymer composite reinforced with carbon nanotubes as described in any one of claims 1-5.

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