Additive manufacturable polyimide vitrimer sensors for soft structural materials

Abstract

This study introduces a novel polyimide vitrimer ink specifically formulated for additive manufacturing on non-planar textile surface (i.e., Kevlar) substrates, aiming to advance the integration of strain sensors in aerospace and defense applications. The ink incorporates disulfide exchanges that enable reversible covalent adaptability, facilitating post-processing modifications and enhancing material recyclability. The synthesis involved a two-step process under nitrogen atmosphere, starting with the formation of polyamic acid (PAA) from 4-amino phenyl disulfide and pyromellitic dianhydride, followed by making the polymer UV-sensitive through the addition of a DMAEMA salt solution and a photoinitiator. Optimal printing parameters were established through experimental tuning of viscosity and modulus, enabling effective direct ink writing (DIW) on Kevlar. The printed sensors exhibited high sensitivity and durability under mechanical stress. Notably, the vitrimer’s disulfide linkages allowed for straightforward removal from substrates using a tailored solvent mixture, underscoring the potential for reusability and recycling in practical applications. This research demonstrates that polyimide vitrimers can be effectively used to create high-performance, adaptable, and recyclable sensors via additive manufacturing, providing a significant advancement in the field of smart materials for dynamic and harsh environments.

Keywords

Polyimide vitrimer Self-healing ink formulation

Introduction

In the dynamic field of advanced materials science, the demand for adaptable, high-performance materials is continually increasing. Such materials are essential for applications in dynamic environments, such as aerospace and wearable technologies. Monitoring systems for real-time structural changes and material safety have been required in all situations.^1,2 That has been successfully rendered with the help of structural health monitoring sensors.³ It is helpful to detect the damages quickly before the breakdown of soft structure material, which has enriched space industry applications like space habitats, decelerator systems, and parachutes.⁴ To enhance this system, several sensor-related studies have been performed in recent times^5,6; however, the inclusion of these non-invasive and non-destructive sensors demonstrates some of the drawbacks, that is, low flexibility, low-temperature stability, and low strain retaining capability.^7–9 Additionally, adhesives are required to paste the prominent sensor in soft materials; in the case of complex construction, special adhesives are needed to attach the dynamic sensors.^10,11 However, the adhesives partially affect the outcome of strain measurement.¹² Altogether, the sensor substrate and adhering medium should withstand high strain to analyze the soft materials under several conditions.¹³

Polyimide (PI) materials have been used as sensor substrates with different adhesive mediums¹⁴; also, PI has been prepared as printable ink in recent times.¹⁵ As a result, multiple conductive inks have been formed as sensors on the polyimide substrates.^1,16 However, the high strain restricts the sensor’s performance, and removing the adhered sensor damages the soft material surface.¹⁷ Thus, design, integration, and removable material are more critical when using sensors in soft materials.^18–20 Due to that, various sensor designs have recently been formed via additive manufacturing, enhancing sensor capability.²¹ Specifically, structural health monitoring of complex structures in real-time is challenging due to their integration; thus, demonstrating the integration of sensor substrates via additive manufacturing maximizes the possibility of sensing the strain at a micro level.^21–23 Apart from all this, if the sensor cracks (in the conductive path), it must be removed from the soft material without any damage or impact on the surface.²⁴ To achieve that, recyclable or reusable material is recommended.²⁵ Significantly, emerging covalent exchanges promoted “vitrimer” materials are perfect for this, where they can be easily removed based on their recycling conditions.^26,27 Polyimide vitrimers, known for their thermal stability, chemical resistance, and reversible bond exchange capabilities, stand out as a promising class of materials. It incorporates disulfide exchanges within their molecular structure, offering a unique reversible covalency that permits post-processing modifications. This feature is crucial for materials used in extreme or harsh environments where they may be subjected to fluctuating conditions. Additionally, the robust properties of reinforcements, such as Kevlar, are renowned for its high tensile strength and resilience make it an ideal substrate for integrating sensors and electronic components used in protective gear assemblies and complex structural applications.²⁸

Altogether, direct ink writing additive manufacturing of ultraviolet (UV) vitrimer inks is suitable for constructing complex structures as a high-toughness soft material.²⁹ In a recent study, the Estrada research group,³⁰ investigated soft structure material Kevlar webbing force sensors, which could be often used in defense and aero applications; however, adhesive usage hindered their performance. Thus, in contrast, this work reports the additive manufacturable direct ink writable (DIW) polyimide vitrimer material with exclusive properties and integrated into the Kevlar surface structure without facile adhesive elements. The additive printable polyimide ink is formulated based on rheological viscosities and was made UV-sensitive through additional chemical processing steps, with corollary optimized printing parameters. Significantly, the designed polyimide vitrimer demonstrates partial curing with UV exposure for ∼60 seconds, where the inclusion of UV agents forms them into organogel. The vitrimer disulfide exchanges provide removability and exchangeability, due to the acid-based recycling behavior without any damage to the sensor. Finally, silver ink (i.e., Ag-inks) was printed in the configuration of sensing structures and has been tested for force-sensing efficiency. Through systematic experimentation and detailed analysis, this research not only elucidates the complex interactions within the polyimide vitrimer system but also demonstrates the practical implications of these materials in high-performance applications. This sensor is particularly suited for dynamic environments where robustness and adaptability are required, such as in aerospace and defense applications.

Experimental Section

Materials

The components for the synthesis of PI-vitrimer structures are 4-amino phenyl disulfide (4-AFD), 4,4′-oxydianiline (ODA), pyromellitic dianhydride (PMDA), 2-(dimethyl amino) ethyl methacrylate (DMAEMA), diphenyl (2,4,6-trimethyl benzoyl) phosphine oxide (TPO), N-methyl-2-pyrrolidone (NMP), and sulfuric acid were purchased from Sigma-Aldrich. Ag-Ink was purchased from Sigma-Aldrich. Kevlar fabric-textile was purchased from FibreGlast. All reagents and solvents were used without further purification.

Characterization

Different characterization techniques were used to track the material formation, the material stability, and the performance of the fabricated sensor. Initially, the viscosity was measured with rheology (ARES-G2), and then thermogravimetric analysis (TGA, TA Instrument, Q50) was used to estimate the polyamic acid (PAA) content after the first step of the reaction. Curing of the materials was performed with dielectric cure monitoring analysis (Lambient cure monitor system) and followed by Fourier Transformed Infrared spectroscopy (FTIR) (Cary 630 FTIR Agilent) to track the curing of the material regarding UV exposure and temperature. The DELOLUX pilot S UV illuminator (365 nm, ≥1350 mW/cm²) is used for organogel formation (UV curing). For estimating glass transition temperature (T_g), differential scanning calorimetry (DSC, Q100, TA Instrument) was carried out at the temperature range of 40°C– 200°C. Vitrimer behavior is determined via stress-relaxation studies with a three-point bending test (1% strain), which is helpful for extrapolating topology freezing point temperature (T_v). After the successful evaluations of the above, additive manufacturing was performed via direct ink writing (DIW; nScrypt-300)—viscosity and modulus-based optimal printing conditions were observed for flow speed, valve speed, and pressure parameters. Regarding the optimized conditions, the material was printed on Kevlar, and the print quality was tested morphologically through SEM (Phenom XL Desktop SEM at 10 kV). Finally, a stress-based sensor study was conducted in a universal tensile tester (AGS-X and Keithley SMU) with a force of up to 300 N. High-resolution imaging and Energy-Dispersive X-ray (EDX) analysis were performed using a Tescan MIRA 4 field emission scanning electron microscope (FE-SEM) equipped with Oxford Instruments Ultim Max 170 mm² detectors. All imaging used an accelerating voltage of 20 kV and a beam current of 300pA. Data processing and analysis were conducted using Oxford Instruments Nano-Analysis AZtec software. X-ray fluorescence (XRF) data were collected using a Bruker Mettler Toledo M4 Plus Micro-XRF spectrometer, equipped with a Rh X-ray source operating at 50 kV and 600 µA. The spot size was 20 µm.

Ink formulation

Thermoset polyimide vitrimer with disulfide exchanges has been developed under N₂ using a two-step synthesis process (detailed in the supplemental information (SI)). In the first step, 4-AFD was dissolved in NMP, and then PMDA was added to form polyamic acid (PAA). For the second step, a DMAEMA salt solution and TPO mixture were added to the PAA solution to make them UV-sensitive, as depicted in Figure 1. The chemical reactions are shown in Scheme 1. Subsequently, the prepared polyimide inks were dispensed using the DIW printing system, specifically the nScrypt 3D-300. Finally, the printed samples were cured via UV and thermal treatment at 100°C for 5 hours. After that, the prepared polyimide ink samples were ready for further studies, especially dispensing material through the DIW printing system. For DIW purposes, the nScrypt 3D-300 was utilized.

It is worth noting that polymer viscosity has an important role in attaining complex printability. As a result, optimization of polymer viscosity was performed by changing the amount of NMP solvent used through the step 1 reaction. The prepared samples were labeled as PVN_x:y (where x and y are denoted as AFD and the NMP, respectively), the ratio between the curing agent and solvent. The as-prepared polymer viscosity was tracked via oscillation-based rheology studies, as shown in Figure S1. The polymer viscosity was found to be 0.9, 3.8, 49, 347, and 1028 Pa.s for PVN_1:10, PVN_1:5, PVN_1:3, PVN_1:2, and PVN_1:1, respectively. Furthermore, thermogravimetric analysis (TGA) was performed to estimate the concentration of the PAA polymer formed through the first-step reaction. Through TGA decomposition profiles, after NMP solvent evaporation, the retained weight percentage of PAA was found to be 18.7, 19.76, 23.4, 20.2, and 18.7% for PVN_1:10, PVN_1:5, PVN_1:3, PVN_1:2, and PVN_1:1, respectively, as depicted in Figure S2.1. Different concentrations of PAA upon different solvent concentrations (Table 1) confirm the solvent effect on the chemical reactivity. Moreover, the first step of PAA formation endured the addition of UV agents in the second step, where less PAA requires a lower amount of UV agent (Table 2). This processing addition directly impacted UV sensitivity and organogel formation; thus, optimizing PAA% and UV curing agent addition is vital for additive manufacturing. After the successful preparation of the samples, the polyimide curing was tested via a dielectric cure monitor, where ion viscosity and slope ion viscosity demonstrated the curing time after UV and thermal effects (started testing after 3 minutes). As shown in Table 1 (Figure S3), the UV curing time was found to be 180, 150, 45, 30, and 75 seconds for PVN1:10, PVN1:5, PVN1:3, PVN1:2, and PVN1:1, respectively.

Table 1.

Sample concentration and UV curing time.

Sample name	NMP (ml)	Viscosity (Pa.s)	PAA (wt%)	Curing with UV (Sec)
PVN_1:10	10	0.9	18.78	180
PVN_1:5	5	3.8	19.26	150
PVN_1:3	3	49	23.14	45
PVN_1:2	2	347	21.76	30
PVN_1:1	1	1028	20.22	75

Table 2.

Stoichiometric calculation table (based on 100 g first-step sample) DMAEMA MW (157.23 g/mol); TPO MW (256.3 g/mol).

Sample	PAA (g)	DMAEMA (g)	DMAEMA (moles)	(PAA + DMAEMA) (g)	TPO (2.5% of PAA + DMAEMA) (g)	TPO (moles)
PVN1:10	18.78	9.39	0.0597	28.17	0.7	0.0028
PVN1:5	19.26	9.63	0.0613	28.89	0.72	0.00288
PVN1:3	23.14	11.57	0.0736	34.71	0.87	0.00348
PVN1:2	21.76	10.88	0.0692	32.64	0.82	0.00328
PVN1:1	20.22	10.11	0.0643	30.33	0.76	0.00304

The completion of the curing and imidization was confirmed via FTIR analysis (Figure 2), where the C = O (810 cm⁻¹) stretching was denoted as complete curing after the thermal conditions. Altogether, approx. 21 % of the PAA particle-containing sample demonstrated high UV curing capability, rapid curing (less than 60 sec) and making them an organogel. Due to that, the optimal viscosity and PAA containing sample PVN_1:2 have been taken for further evaluation, and its decomposition temperature was observed at 342°C (Figure S2.2).

Vitrimer characterization

After that, glass transition temperature (T_g) was analyzed via DSC and reported at 81°C for the sample (Figure S4). In aromatic polyimides (PIs), T_g can decrease or increase with temperature, depending on the balance between backbone rigidity and intermolecular forces. A rigid backbone may initially lower T_g by increasing chain mobility, but at higher temperatures, stronger molecular packing or crosslinking can raise T_g. For this material, DSC values increase with temperature, showing a similar trend. The backbone structure mainly controls thermal behavior, while intermolecular interactions strengthen at higher temperatures, leading to the observed T_g rise.³¹ Further, the vitrimer materials contain an additional temperature range, known as topological freezing point temperature (Tv), which has been extrapolated with the help of stress-relaxation analysis. In the stress-relaxation study, a 1% strain was applied to the cured material, and the time-dependent relaxation modulus was analyzed. The relaxation time was determined as the time at which the modulus decreased to 1/e (approximately 37% of the initial stress value), indicating the transition from the initial stress state to the relaxation state. Based on the Tg values, stress-relaxation studies were performed at 65°C, 75°C, and 85°C, resulting in different relaxation times of 140.1 s, 66.6 s, and 18.2 s, respectively (Figure 3). In particular, the relaxation was rapid above the Tg, due to their William-Landel-Ferry time-temperature superposition. Furthermore, the obtained relaxation times were plotted via the Arrhenius equation to estimate the activation energy and Tv (detailed in SI), and the extrapolated values were 104 KJ/mol and −31.9°C, respectively. The obtained topology freezing point temperature is far below the observed Tg, and this behavior determines the silica like glass former. Significantly, this behavior renders to follow the William-Landel-Ferry equation after Tg and demonstrates the immediate viscosity change in the materials; however, after a certain point, it follows the Arrhenius equation and reduce the viscosity uniformly.^26,32,33 For that calculation, a three-point bending dynamical analysis was performed to determine the rubbery modulus for the calculation (shown in Figure S5).

DIW-processing

In addition, parallel plate oscillation frequency-based storage modulus was analyzed to predict the optimal printing condition based on the correlation of the yield strength. The observed frequency strain resulted in the modulus determining the critical viscoelastic yield point at 7.7 pa (Figure 4(a)). It has exhibited a greater loss modulus than the storage modulus (G”>G^'); also, the tan delta $δ$ (damping factor) > 1 described the printability of the material (Figure 4(b)).³⁴ After the analysis of yield strength, the material was taken for printing via an advanced DIW additive manufacturing system (nScrypt 3Dn-300), and a UV illuminator was attached to the printing head, as shown in Figure 5, to make them organogel after each printing line formation.

Initially, the printing was performed on a glass slide to estimate the ink extrusion pressure and speed for continuous flow; also, the construction design was demonstrated via the G-code. The figure shows the pyramid-like construction with low (PVN_1:10) and high (PVN_1:2) viscosities (Figure 5(a)). Based on this evaluation and the yield strength of the PVN_1:2, the pressure of 10 lbf/in² and the speed of 5 mm/sec were set for further formations at any substrate (while performing with a 200-micron tip). The state of fiber-wetting across these resultant viscosities was not observed during this study. Significantly, the nScrypt operation requires special valve conditions, and it has maintained less than 0.1 mm for the open and closed positions. After this parameter selection, the ink writing tip was placed 0.01 mm above the substrate, and the G-code program was run for head movement; additional codes were also included to pass the UV illuminator on the same. Then several repeated print jobs were effectively performed with the same parameters and multiple samples were prepared to form the strain sensor on it. The ink was formulated and effectively printed on the Kevlar substrate shown in Figure 5(b); at the same time, without the UV result, the infiltration in the Kevlar substrate resulted in a poor surface finish. Before and after printing, the performed SEM analysis depicted images, and roughness mapping effectively described the changes in the surface (Figure 5(c)–(f)). Further analysis was conducted to examine the presence of polyimide (PI) polymer before and after hydrochloric acid (HCl) etching, utilizing energy-dispersive spectroscopy (EDS), and X-ray fluorescence (XRF) measurements. As shown in Figure S6, the EDS peaks for copper (Cu) and aluminum (Al) present in the original Kevlar substrate disappeared after the application of the PI polymer. These peaks reappeared following HCl etching, confirming the complete removal of the PI polymer by the HCl solution. Additionally, the EDS peak for sulfur (S) increased significantly after the printing of the PI polymer, indicating its presence in the Kevlar. This S peak decreased sharply after HCl etching. This finding was further supported by XRF mapping, illustrated in Figure S7, which displays the elemental mapping of Cu and S. Overall, a UV-cured polyimide layer formed by Kevlar was used for sensor formation.

Strain sensor fabrication and testing

Smooth, uniform, and non-breaking polyimide surface printing helped the formation of conductive ink traces without breaking (Figure 6(a)). To enrich that and avoid the sensor ink damage during the printing at the time of formation, another polyimide surface layer (double layer) was printed and used for sensor tests (Figure 6(a)). For estimating, the direct integration of sensor effects simple conductive line was printed and tested with a tensile testing system. The variation in force-sensing behavior between printed and pasted sensors is shown in Figure 6(b), which has determined the difference in strain behavior. On the applied force (300N), the sensing of changes (Strain%) through the pasted sensor was comparatively lower than that through the printed sensor integrated system; this might be due to the adhesion strength and adhesive force-bearing capability. Substantially, printed sensor integration is directly indulged with Kevlar, allowing for effective measurement of micro changes. Overall, the tests were performed at room temperature (i.e. 24°C) and the humidity level was maintained around 30%–50%.

Removal of sensor

Polyimide vitrimer removal was performed based on acid-based recycling steps. These removal steps extended the further vitrimer usage (with some more processes) and resulted in significant removal from the Kevlar without damaging its surface. Conventional pasting technique would adhere to the surface, while removal from the surface impacts it. Removal of the sensor was achieved through complete immersion of Kevlar in the solvent mixture of NMP, DCM, and concentrated HCl (volume ratio of 1:1:0.16) at room temperature for 3h.³⁵ The final polyimide dissolved solution and sensor removed Kevlar is shown in Figure 7. Furthermore, the solution would be processed based on recycling studies and utilize to reform for PI film.

Conclusion

The additive-manufactured polyimide vitrimer sensor fabrication of soft woven material like Kevlar substrates is prominent for effectively measuring the microstrain of the material. Disulfide-promoted vitrimer covalent exchanges helped to withstand high temperatures and harsh chemicals; thus, the polyimide vitrimer, printed on the Kevlar fabric via DIW, has demonstrated efficient formation and environmental properties. In addition, the removal process was performed with a solvent mixture, which was removed clearly without any traces. Moreover, the sensitivity of the force sensor was increased due to the direct printed integration rather than the pasted sensor system. This work provides the potential to estimate the minor changes and strain in soft toughness materials, which would protect the material and application purpose without fail. Furthermore, printing vitrimer inks, external variables (humidity, temperature, and sizing agent of fiber), and materials exploration will help to execute the complex structure sensor integration effectively, potentially providing an ink sensor with high sensitivity, durability, reusability, and recyclability.

Supplemental Material

Supplemental Material - Additive manufacturable polyimide vitrimer sensors for soft structural materials

Supplemental Material for Additive manufacturable polyimide vitrimer sensors for soft structural materials by Balaji Krishna Kumar, Amr Elattar, Abdullah Al Noman, Austin Williams, Adrienn Maria Szucs, and Tarik Dickens in Journal of Reinforced Plastics and Composites.

Footnotes

Acknowledgments

The authors acknowledge the Florida A&M University NSF CREST Center for Additive Manufacturing (Award#1735968) and the NNSA MSIPP I-AM EMPOWER'D (Grant No. DE-NA0004004) for providing support and contributing to the research review reported in this publication. EDS and XRF measurements were performed at the Center for Rare Earths, Critical Minerals, and Industrial Byproducts at the National High Magnetic Field Laboratory, Florida State University, supported by the State of Florida through Contract # 0000071627. The National High Magnetic Field Laboratory is supported by the National Science Foundation under Grants DMR-1644779 and DMR-2128556, as well as by the State of Florida.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the NNSA MSIPP I-AM EMPOWER'D (DE-NA0004004), Florida A&M University NSF CREST Center for Additive Manufacturing (1735968), and National Science Foundation (DMR-2128556) and (DMR-1644779).

ORCID iDs

Amr Elattar

Tarik Dickens

Data Availability Statement

The data supporting this article has been included as part of the Supplementary Information.*

Supplemental Material

Supplemental material for this article is available online.

References

Herbert

Lim

H-R

Yeo

W-H

. Printed, soft, nanostructured strain sensors for monitoring of structural health and human physiology. ACS Appl Mater Interfaces 2020; 12: 25020–25030.

Farrar

Worden

. An introduction to structural health monitoring. Philos Trans A Math Phys Eng Sci 2007; 365: 303–315.

Laflamme

Kollosche

Connor

, et al. Soft capacitive sensor for structural health monitoring of large-scale systems. Struct Control Health Monit 2012; 19: 70–81.

Ranganatha

Novich

Phero

, et al. IEEE sensors. Piscataway: IEEE, 2021, pp. 1–4.

Zhang

. Materials, structures, and functions for flexible and stretchable biomimetic sensors. Acc Chem Res 2019; 52: 288–296.

Guo

Saleh

. 2021 IEEE Aerospace Conference (50100). Piscataway: IEEE, 2021, pp. 1–12.

Senthilkumar

Sreekanth

Reddy

. Nondestructive health monitoring techniques for composite materials: A review. Polym Polym Compos. 2021; 29: 528–540.

Hassani

Dackermann

. A systematic review of advanced sensor technologies for non-destructive testing and structural health monitoring. Sensors 2023; 23: 2204. DOI: 10.3390/s23042204.

Towsyfyan

Biguri

Boardman

, et al. Successes and challenges in non-destructive testing of aircraft composite structures. Chin J Aeronaut 2020; 33: 771–791.

10.

Chen

Han

Dong

, et al. Biomimetic self-adhesive structures for wearable sensors. Biosensors (Basel) 2022; 12: 431. DOI: 10.3390/bios12060431.

11.

Yin

Pan

, et al. A self-adhesive wearable strain sensor based on a highly stretchable, tough, self-healing and ultra-sensitive ionic hydrogel. J Mater Chem C 2020; 8: 17349–17364.

12.

Zhang

Wan

Liu

, et al. 2010 11th International Conference on Electronic Packaging Technology & High Density Packaging. Piscataway: IEEE, 2010, pp. 657–660.

13.

Liu

Niu

, et al. Flexible miniaturized sensor technologies for long-term physiological monitoring. Npj Flex Electron 2022; 6: 20.

14.

Rau

Herzberger

Long

, et al. Ultraviolet-assisted direct ink write to additively manufacture all-aromatic polyimides. ACS Appl Mater Interfaces 2018; 10: 34828–34833.

15.

Hong

Yan

. UV curable conductive ink for the fabrication of textile-based conductive circuits and wearable UHF RFID tags. ACS Appl Mater Interfaces 2019; 11: 27318–27326.

16.

Dobrzynska

Gijs

MAM

. Flexible polyimide-based force sensor. Sens Actuators A Phys 2012; 173: 127–135.

17.

Tibaduiza

Anaya

Forero

, et al. A Sensor Fault Detection Methodology applied to Piezoelectric Active Systems in Structural Health Monitoring Applications. IOP Conf Ser Mater Sci Eng. 2016; 138: 12016.

18.

Tang

KPM

Yick

, et al. Effect of contacting surface on the performance of thin-film force and pressure sensors. Sensors 2020; 20: 6863. DOI: 10.3390/s20236863.

19.

Banos

Toth

Damas

, et al. Dealing with the effects of sensor displacement in wearable activity recognition. Sensors 2014; 14: 9995–10023.

20.

Blazquez-Bello

Campos-Roca

Bangert

, et al. Impact of substrate and bending angle on the performance of microwave PCB sensors for permittivity measurements. Measurement 2021; 175: 109114.

21.

Lehmhus

Aumund-Kopp

Petzoldt

, et al. Customized smartness: a survey on links between additive manufacturing and sensor integration. Procedia Technol 2016; 26: 284–301.

22.

Montazerian

Rashidi

Milani

, et al. Integrated sensors in advanced composites: a critical review. Crit Rev Solid State Mater Sci 2020; 45: 187–238.

23.

Wang

Lasn

. Integration of optical fibre sensors by material extrusion 3-D printing – the effect of bottom interlayer thickness. Mater Des 2022; 221: 110914.

24.

Nishikawa

Yamane

Matsuhisa

, et al. Stretchable strain sensor with small but sufficient adhesion to skin. Sensors 2023; 23: 1774. DOI: 10.3390/s23041774.

25.

Tao

Liao

Wang

. Polymer-assisted fully recyclable flexible sensors. EcoMat. 2021; 3: 1–15.

26.

Krishnakumar

Sanka

RVSP

Binder

, et al. Vitrimers: associative dynamic covalent adaptive networks in thermoset polymers. Chemical Engineering Journal 2020; 385: 123820.

27.

Capelot

Unterlass

Tournilhac

, et al. Catalytic control of the vitrimer glass transition. ACS Macro Lett 2012; 1: 789–792.

28.

Mohammed

Al-Zubaidi

Bahrain

SHK

, et al. In: Nawab

Sapuan

Shaker

(eds). Composite Solutions for Ballistics. Sawston: Woodhead Publishing, 2021, pp. 3–54.

29.

Krishna Kumar

Dickens

. Dynamic bond exchangeable thermoset vitrimers in 3D-printing. J Appl Polym Sci. 2022; 1–13.

30.

Fujimoto

Watkins

Phero

, et al. Aerosol jet printed capacitive strain gauge for soft structural materials. Npj Flex Electron 2020; 4: 32. DOI: 10.1038/s41528-020-00095-4.

31.

Kumar

Lee

, et al. Preparation of robust, flexible, transparent films from partially aliphatic copolyimides. Macromol Res 2015; 23: 566–573.

32.

Azcune

Odriozola

. Aromatic disulfide crosslinks in polymer systems: self-healing, reprocessability, recyclability and more. Eur Polym J 2016; 84: 147–160.

33.

Denissen

Winne

Du Prez

. Vitrimers: permanent organic networks with glass-like fluidity. Chem Sci 2016; 7: 30–38.

34.

Jackson

Dickens

. Rheological and structural characterization of 3D-printable polymer electrolyte inks. Polym Test 2021; 104: 107377.

35.

Yang

Zhang

, et al. Fully closed-loop recyclable thermosetting shape memory polyimide. ACS Sustain Chem Eng 2020; 8: 18869–18878.

Supplementary Material

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