Introduction

Developing next-generation conductive, strong and tough eco-friendly fibers is urgent given their application potential in the areas of flexible smart wearable devices, such as sensors (Wan et al. 2019; Chen et al. 2018; Dichiara et al. 2017; Cho et al. 2019), electrical heaters (Zhao et al. 2020; Lee et al. 2016a, b) and electromagnetic interference (EMI) shielding fabric (Zhang et al. 2018; Lee et al. 2016a, b). Cellulose, as a typical natural fiber for garments, possesses biodegradability, chemical stability and skin-friendly properties compared with petroleum-based materials (Kontturi et al. 2018; Klemm et al. 2005). However, single cellulose fibers suffer from unbalanced mechanical performance (e.g., low toughness) and electrical insulating property, which significantly hinder its application in the latest functional fabric fields. To this end, considerable effort has been attempted to incorporate nanofillers into the cellulose matrix, including metal powders (Bober et al. 2014; Wang et al. 2017), carbon nanoparticles (Wan et al. 2019; Zhang et al. 2007; Hardelin and Hagstrom 2015) or conductive polymers (Shi et al. 2014), which endow the composite fiber with a certain electrical conductivity. Unfortunately, the aggregation of functional particles, as well as poor interfacial characteristics to the cellulose matrix, made it impossible to get adequate mechanical and electrical conductivity in the composite fiber at the same time. (Lee et al. 2016a, b; Hamedi et al. 2014; Huang et al. 2015).

Among the functional fillers, carbon nanotubes (CNTs) show a unique potential application value, as the long-range π-conjugation and large length: diameter ratio endow superior mechanical and electrical properties, making it valuable to prepare reinforced and conductive cellulose nanocomposites. Much work so far has focused on mixing CNTs with cellulose in a specific solution, such as ionic liquid (Zhang et al. 2007; Hardelin and Hagstrom 2015), N-methylmorpholin-N-oxide (NMMO) (Mahmoudian et al. 2017), or tetrabutylammonium acetate (TBAA)/dimethyl sulfoxide (DMSO) mixture solvent (Sun et al. 2015; Jiang et al. 2017a, b). Unfortunately, the solvent economy, environmental friendliness as well as the dispersion state of CNTs all will affect the preparation process of cellulose/CNT composite fibers. As an excellent water-soluble material, 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) oxidized cellulose nanofibrils were usually used to prepared cellulose/CNT composite films or fibers (Wan et al. 2019; Zhang et al. 2018; Koga et al. 2013). However, this material is hard to apply in the field of wearable textiles owing to its intrinsic water-solubility characteristic. Although there are reports that cellulose nanofibrils can act as a dispersion agent to assist CNT dispersal in aqueous solution, it is still hard to obtain a spinning dope with a high content of CNTs with excellent dispersion in cellulose solution (Carrasco et al. 2014; Ferreira et al. 2017). For instance, Zhang et al. (2007) reported a CNT-reinforced regenerated cellulose fiber via ionic liquid as the solvent, the resulting fiber containing 4 wt% CNTs exhibited excellent mechanical properties and a high electrical conductivity of 8.3 × 103 S/cm, whereas further increasing CNT content did not increase the electrical conductivity. Qi et al. (2015) studied CNT/cellulose composite fibers and demonstrated that the conductivity of the fiber increased with the CNT loading from 2 to 8 wt%, however, a slight decrease in both tensile strength and elongation at break is observed with CNT loading increase. In spite of efforts and resulting progress, simultaneously achieving strong, tough and conductive cellulose fiber is still an open issue.

In recent years, Zhang’s team developed a series of novel green solvents, which include aqueous NaOH/urea (Cai et al. 2008; Jiang et al. 2014), NaOH/thiourea (Jiang et al. 2017a, b), and LiOH/urea solutions (Cai et al. 2007). When precooled to low temperature (between − 5 and − 12 °C), the cellulose can be dissolved rapidly to obtain a transparent cellulose solution. Owing to its low cost, recyclable, and environmentally benign features, this kind of solvent shows a good application prospect (Ye et al. 2019; Zhu et al. 2018; Qi et al. 2013). For CNTs, the hydrophilicity can be obtained through acid-treatment to introduce carboxyl functional groups (Wepasnick et al. 2011). Combining the water solubility characteristics of carboxylic CNT as well as the co-alkali system for cellulose dissolution, it is possible to prepare a composite fiber with high CNT filling. In the current study, to simultaneously realize conductivity, reinforcing and toughening cellulose-based nanocomposite fibers, we devised a convenient strategy including solution dispersion and wet spinning technology to produce cellulose/CNT filaments based on the NaOH/urea dissolving system. Carboxylic CNTs can not only disperse efficiently in alkaline aqueous solution, but also forms a tight bond with the hydroxyl group on the cellulose macromolecular surface. After the spinning dope is extruded through the limited space of the spinneret, the nanocellulose networks wrapping the CNTs formed a unidirectional arrangement and thus achieving a high-performance structural material. The composite fiber with the diameter of about 50 μm exhibited a high mechanical strength of 185 MPa and electrical conductivity value up to 1274 S/m for 20% CNT loading. Encouraged by this balanced performance, we further explored the application of joule heating and humidity sensing for the composite fiber. Such application expanded the potential value for these cellulose/CNT fibers in the smart wearable field.

Experimental section

Materials

Cellulose powder (particle diameter: 50 μm) was supplied from Aladdin Co., Ltd (Shanghai, China). Sodium hydroxide (NaOH), urea, concentrated sulfuric acid (H2SO4, 98%), nitric acid (HNO3, 65%) and sodium sulfate (Na2SO4) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Multiwalled carbon nanotubes (MWCNTs, diameter ranging 10–20 nm) were purchased from Nanjing XFNANO Materials Tech Co., Ltd, China. All the chemicals were in analytical grade and used without further purification.

Preparation of cellulose/cnt composite fibers

Carboxylic CNTs were synthesized by mixed acid treatment according to previous reports (Wepasnick et al. 2011; Blanchard et al. 2007). Typically, 1 g pristine MWCNTs and 80 mL mixed acid (H2SO4/HNO3 = 3:1 in volume) were added into a round-bottom flask and dispersed in an ultrasonic bath for 30 min. Then the flask equipped with reflux condenser and magnetic stirrer was transferred to a water bath of 70 °C for 8 h. After reaction, the carboxylic CNTs were obtained after being washed by de-ionized water to neutral and freeze-dried.

A certain amount of carboxylic CNTs (0.25 g, 0.5 g, 0.75 g and 1 g) were well dispersed in 100 ml aqueous solution with 7 wt% NaOH/12 wt% urea via high-speed shear (20,000 rpm) mixing at room temperature. After that the mixed solution was precooled to − 12 °C, and 5 g cellulose powder was added to the precooled mixture with high-speed shear mixing at 0 °C for 2 h to obtain a homogeneous cellulose/CNT composite solution. The samples with the weight ratio of CNTs to cellulose was 5:100, 10:100, 15:100 and 20:100, which were denoted as C/CNT-5%, C/CNT-10%, C/CNT-15% and C/CNT-20%, respectively. For comparison, pure cellulose solution was also fabricated under the same conditions and coded as Cellulose.

Set-spinning process was performed at room temperature on a modified wet‐spinning set‐up. The spinning dope was loaded into the syringe and then extruded through a syringe needle (diameter, 160 μm) into a coagulation bath of 10 wt % H2SO4/15 wt % sodium sulfate aqueous solution at 5 °C. The flow rate of the spinning dope through the syringe needle was 7 m/min. Then, the gel-state fibers were washed to remove the residual salts and acids, dried at room temperature until the weight stays the same, and finally obtained the cellulose/CNT composite fiber.

Characterization

The morphology of the prepared fibers was characterized by a field emission scanning electron microscope (FESEM) (Quanta-450) operating at an accelerating voltage of 10 kV, as the samples for the SEM investigations were treated by spray-gold. Rheological characterization of the spinning dope was conducted with a rotary rheometer (MCR302, Anton Paar). A shear rate sweep was performed at 25 °C from 10–1 to 103 s−1 to measure their apparent viscosity. A stress sweep was performed at 25 °C from 10–1 to 103 Pa with a constant frequency of 1 Hz. FTIR spectra of the composite fabric was obtained with a Spotlight 400 spectrometer in ambient condition. An average 32 scans with a resolution of 4 cm−1 in the range of wavelength 650–4000 cm−1 was employed in attenuated total reflectance (ATR) mode. Raman spectra were recorded using a Raman spectroscope (XploRA, HORIBA) with a 532 nm laser. Contact angle was measured on an OCA-20 contact angle system (Data-physics, Germany) by dropping water (4 μL) onto the samples at ambient temperature. Average contact angles were obtained by measuring the same sample at five different positions. Thermogravimetric analysis (TGA, TA Instruments) was performed from 50 to 800 °C at a heating rate of 10 °C/min under air atmosphere. The mechanical and electrical properties of the fibers were measured with a universal testing machine (RWT-10, Reger) and a digital multi-meter (OWON B35T). The temperature change and thermal imaging feature of fabric were measured via an infrared thermal camera (FLIR ONE Pro).

Results and discussion

Preparation process of conductive cellulose/CNT fiber

To construct a CNT reinforced cellulose fiber, we firstly prepared the composite spinning dope followed by the implementation of the wet spinning process. Here we employed the precooled 7 wt% NaOH/12 wt% urea aqueous solution (− 12 °C) as the co-solvent to disperse the carboxylic CNT and dissolve microcrystalline cellulose, as the schematic diagram shown in Fig. 1a. Here, carboxylic CNT can be well dispersed in NaOH/urea aqueous solution, as there is no stratification or precipitation occur over a long time for the mixture. When solid cellulose is immersed into the precooled NaOH/urea solvent, NaOH hydrate interacts with hydroxyl groups on cellulose macromolecules will break the intra- and intermolecular hydrogen bonds of cellulose. Meanwhile, the dissolved cellulose can form a wormlike inclusion complex with urea as the shell packaged cellulose-NaOH hydrogen-bonded ligand in the alkaline aqueous solution (Cai et al. 2007). Based on the characteristic of co-solvents between carboxylic CNT and cellulose dissolution, a stable and uniform cellulose/CNT composite spinning dope can be acquired via high-speed mixing (Fig. 1a). Next, this spinning dope was extruded into a coagulation bath of H2SO4/NaSO4, where solvent exchange and neutralization reaction were performed to form a stable gel fiber (Fig. 1b, c). The gel-state fiber was then dried under tension at room temperature and finally wound onto a spool for later use (Fig. 1d). Thanks to the good dispersion and tight bonding of carbon nanotubes in cellulose solution, the composite fiber with 20 wt% CNT still exhibited excellent flexibility, as this composite fiber can be knotted easily (Fig. 1e). Moreover, the efficiently filling of conductive material lead the composite fiber with outstanding conductivity, as shown in Fig. 1f that an LED can be lighted when using the composite fiber as a conductor under only 3 V.

Fig. 1
figure 1

a Schematic diagram for the preparation process of cellulose/CNT spinning solution as well as wet spinning process of cellulose/CNT composite fiber; b, c Digital image of the cellulose/CNT composite fiber with wet and dry state; d Digital image of dry cellulose/CNT composite fiber wound around a spool; e SEM image of knotted C/CNT-20% composite fiber filament; f C/CNT-20% composite fiber as a conductive line to light an LED

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Properties and characterization of the spinning dope

To obtain cellulose/CNT fibers through the wet spinning process, firstly the spinning dope should possess a proper rheological property for smooth extrusion. Here the diameter and length of the syringe needle was 160 μm and 12.7 mm (Fig. 2d), as this high aspect ratio (≈80) of the needle guarantees the effective alignment for cellulose molecules and CNTs during extrusion. Figure 2a shows the curves of viscosity as a function of shear rate for pure cellulose and CNT mixture. We can see that all samples displayed a shear-thinning behavior, which is vital for extrusion process. Figure 2b presents the corresponding storage modulus (G′) and loss modulus (G′′) as a function of the shear stress. For cellulose and C/CNT-10% solution, G′′ is always higher than G′ which means the solution maintains the property of viscous fluid state under test condition (Li et al. 2017; Siqueira et al. 2017), while an elastic viscous crossover located around 200 Pa appears for the C/CNT-20% sample, which indicate that the solution of C/CNT-20% can flow smoothly through the nozzle and partially preserve shape integrity after leaving the nozzle (Ma et al. 2019). This rheological characteristic also can be seen from the flowing state for the spinning dope, as shown in Fig. 2c. When the freely distributed CNTs enter the restricted space of the injection nozzle, this ultra-large aspect ratio material will be aligned with the flow direction of spinning solution. Meanwhile this oriented structure for CNTs can be effectively maintained to the composite fiber through the instant filament formation process in the coagulation bath. Benefiting from the shear thinning characteristics as well as the maintenance of the aligned structure for CNTs, we can always obtain a flexible composite fiber via solvent exchange in the coagulation bath, no matter whether for gel or liquid spinning dope.

Fig. 2
figure 2

a Viscosity versus shear rate curves of cellulose and C/CNT composite spinning dope; b Storage modulus (G′) and loss modulus (G′′) versus shear stress curves of cellulose and C/CNT composite spinning dope; c Digital image of the cellulose spinning solution, C/CNT-10% and C/CNT-20% composite spinning dope; d Digital images of the wet spinning apparatus assembled in laboratory as well as the schematic diagram for the state of cellulose molecules and CNTs during extrusion

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Microstructure and properties characterization of the composite fiber

The successful implementation of the above strategy embeds CNTs into the cellulose network with favorable orientation, effectively enhancing the interaction between CNTs and cellulose fibrils. SEM images of both surface and cross section of cellulose composite fiber were shown in Fig. 3. The pristine cellulose fiber shows a smooth surface without visible grooves (Fig. 3a). After CNTs were added, significant grooves along the radial direction of the filament appeared on the surface of the composite fiber (Fig. 3c, d), which demonstrated that the CNTs embedded into cellulose network with favorable orientation. The cross section of the filament is almost a circle, no matter for cellulose or C/CNT composite fibers. For the investigated composite filaments, their diameters are always kept to about 50–60 μm which was attributed to the dense structure formed between CNTs and the cellulose matrix. Benefiting from this oriented arrangement as well as dense embedding structure, the composite fiber exhibited excellent physical and chemical properties, as can be seen in the following investigation.

Fig. 3
figure 3

a, b SEM images for the surface (a) and cross section (b) of pure cellulose fiber; c, d SEM images for the surface (c) and cross section (d) of cellulose/CNT-10% composite fiber; e, f SEM images for the surface (e) and cross section (f) of cellulose/CNT-20% composite fiber

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The reason for this oriented arrangement as well as dense embedding structure of C/CNT fiber is attributed to the adequate interfacial interactions between CNTs and the cellulose matrix, which originated from hydrogen bonding, as deduction can be further demonstrated by Raman spectra and FTIR measurements. The Raman spectra of cellulose, C/CNT composite fibers as well as carboxylic CNTs are shown in Fig. 4a.Two characteristic peaks (ID and IG) of CNTs, 1337 and 1577 cm−1, are attributed to disorder induced by defects in the nanotube lattice and in-plane vibration of the C−C bonds, respectively. No significant change of these two peaks was found for composite fibers compared to those of CNTs, which indicated that there is no obvious destruction of CNTs during the dissolution-regeneration process (Qi et al. 2015). As shown in Fig. 4b, the stretching vibration band of hydroxyl groups at around 3352 cm−1 in the pure cellulose fiber, substantially shifts to a lower wavenumber (3260 cm−1) for the C/CNT-20% sample. This result can be ascribed to the hydrogen bonding between the residual hydroxyls on the CNT surfaces and the hydroxyl groups in the cellulose chains (Huang et al. 2015; Yan et al. 2008). TGA was carried out to verify the component proportions of the fiber (Fig. 4c); the weight loss around 100 °C mainly originated from elimination of the water molecules of the composite fiber. Meanwhile, the residual weight percentage of composite fiber kept a good agreement with the filling amount of CNTs, which indicated that the CNTs will not be lost during the solidification process. Contact angle was employed to elucidate the relationship between water molecules and C/CNT composite fiber. Figure 4d shows that the contact angle between water and tested filaments increases from 27 ± 2 to 76 ± 4° when the content of CNT varies from 0 to 20 wt%.

Fig. 4
figure 4

a Raman spectra of regenerated cellulose and C/CNT composite fiber; b FTIR spectra of regenerated cellulose and C/CNT composite fiber. For clarity, a featureless region is omitted in the range of 2250–1750 cm−1; c TGA curves of regenerated cellulose and C/CNT nanocomposite fiber. d Contact angle with water of regenerated cellulose and C/CNT composite fiber

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Typical stress–strain curves of cellulose and CNT filled composite fibers are shown in Fig. 5a. Notably, all the fibers fracture in a brittle manner with limited elongation at break and no distinct yielding, which should be attributed to the rigid nature of cellulose and CNTs. Compared with cellulose, adding carbon nanotubes will significantly enhance the mechanical properties of composite fiber. As shown in Fig. 5c, the cellulose fiber exhibited minimum tensile strength and Young’s modulus, i.e., 114 ± 5 MPa and 6.1 ± 1 GPa, respectively. When the CNT concentration rises to 20 wt%, the tensile strength and Young’s modulus values reached to 185 ± 9 MPa and 10.6 ± 1 GPa, respectively, corresponding to the increases of 62% and 73% relative to the pure cellulose fiber. Here, C/CNT-20 fiber with a diameter of about 45 μm can withstand a weight of 50 g, whereas only 20 g for cellulose fiber with the diameter of 35 μm (Fig. 5d). This phenomenon can be ascribed to the strong interfacial hydrogen bonding between cellulose and carboxylic CNTs, which has been demonstrated by FTIR results (Fig. 4b). Figure 5e illustrates that the hydrogen bonds as well as slip and release between the aligned CNTs will affect the stress transfer of the composite fiber. This eventually led to a greatly improved of the mechanical strength and Young’s modulus of these C/CNT fibers. However, owing to the restrictive effect of CNTs on the movement of the cellulose molecules, it also leads to a decrease in the elongation at break of the composite fiber which can be seen in Fig. 5b.

Fig. 5
figure 5

a Stress–strain curves for regenerated cellulose fiber and C/CNT composite fiber; b Comparation of the toughness and elongation at break of regenerated cellulose fiber and C/CNT composite fiber; c Comparison of the Young’s modulus and tension strength of regenerated cellulose fiber and C/CNT composite fiber; d Optical image of a weight weighing 50 g suspended by a 45 μm C/CNT-20% fiber as well as a weight weighing 20 g suspended by a 35 μm regenerated cellulose fiber; e Schematic to show the fracture process of the CNT reinforced cellulose fiber under tension

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Joule heating and moisture sensing application of C/CNT composite fiber

Owing to the formation of a conductive path via CNTs embedded (Fig. 6a), the composite fiber possessed an electrical conductivity value in the range of 64–1274 S/m for 5–20 wt% CNT loading (Fig. 6b). According to the Joule heating principle, the conductive fiber can be used as a potential wearable heater (Wang et al. 2019). An electric heating experiment with the C/CNT-20% fiber sample was carried out by monitoring temperature changes at constant applied voltages of 3 V, 6 V and 9 V, as can be seen in Fig. 6c. It can be seen that the steady-state maximum temperatures were increased with the applied voltage increasing (Lee et al. 2016a, b). For the test sample, the temperature increased rapidly when a certain voltage of 9 V was applied at 10 s, reached a maximum value within 20 s, and decreased quickly to room temperature when the applied voltage was off at 170 s. The electric heating test sample and the associated infrared thermal images for C/CNT-20% fiber are shown in Fig. 6d, which shows the heating process more clearly.

Fig. 6
figure 6

a Schematic diagram of the conductive structure of cellulose/CNT composite fiber; b Current–voltage tests of C/CNT composite fiber with different CNT contents; c Time-dependent temperature changes of C/CNT-20% composite fiber under different applied voltage; d Infrared images for the electric heater made of C/CNT-20% composite fiber under 9 V for electric heating experiments

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The humidity sensing measurements were carried out using C/CNT-20% fiber connected copper electrode as sensor module, as illustrated in Fig. 7a. The dependence of relative resistance changes (ΔR/R0) of C/CNT-20% fiber to humidity is demonstrated in Fig. 7b. Over the investigated relative humidity range between 30 and 100%, the value of ΔR/R0 increases monotonically as the humidity increases. In typical CNT composites for liquid sensing, the resistance change of the material can be attributed to (1) changes in carrier concentration due to adsorbed molecules on the surface of the CNTs, and (2) swelling of the polymer matrix, altering the electron transport between conducting CNT networks (Zhao et al. 2020; Qi et al. 2014). For the cellulose/CNT composite fibers, CNTs are tightly coated with cellulose microfibers, the swelling of the cellulose plays a major role in the water sensing mechanism of this fiber. When the water molecules enter or leave the amorphous region of cellulose, the molecular chains will move apart or draw closer together, which alters the electron transport by varying the inter-nanotube distance between neighbor CNTs above or below the tunneling distance, causing the unique reversibility of the composites’ electrical resistance. Benefitting from this property, the C/CNT composite fibers can be used as humidity sensors. As shown in Fig. 7c, d, the C/CNT-20% fiber was placed 100 cm from the human mouth to monitor the signal of resistance change for 10 breath cycles. The value of ΔR/R0 increases and decreases rapidly corresponding to breath exhalation and inhalation, which is attributed to a reversible hygroscopic swelling of the cellulose matrix. Drastic resistance change could be observed when the C/CNT-20% specimen was immersed in water, which can be seen in Fig. 7e-f. While during the drying process via hot air, ΔR/R0 values return to their initial base level. It’s worth noting that after 4 wetting–drying cycles, the resistance curve of the specimen exhibited excellent stability and reproducibility. Although systematic research is still needed to clarify the performance of C/CNT composite fibers, this fast and reversible water sensing property indicated a good application prospect in humidity sensing for these fibers, such as smart fabric for monitoring human sweat.

Fig. 7
figure 7

a Humidity sensing test device with commercial hygrometers as reference; b Dependence of relative resistance change (ΔR/R0) on relative humidity; c Schematic for in-situ monitoring electrical resistance changes of the C/CNT-20% fiber placed at a distance about 100 mm from one’s mouth; d Dependence of ΔR/R0 on test time for in-situ monitoring human breathing cycles; e Photograph of the liquid sensing test before and after being submerged in water; f ΔR/R0 of C/CNT-20% fiber sensor during successive wetting–drying cycles in water/air

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Conclusions

In summary, a series of cellulose/CNT composite fibers was fabricated via a simple wet spinning method using an aqueous NaOH/urea system as solvent. Benefiting from the strong interaction between cellulose and CNT, along with the directional arrangement of CNTs in fibers matrix, which make the prepared fibers, such as the C/CNT-20% sample, exhibit good mechanical and conductive properties, with the tensile strength of 185 ± 9 MPa and electrical conductivity value of 1274 S/m. This intrinsic conductivity combined with hygroscopic swelling of cellulose/CNT composite fiber to make a material that can be used either as an electric heater or a humidity sensor. Through the Joule heating and humidity sensing measurements, we can conclude that the C/CNT-20% fiber exhibited an outstanding electric heating performance, as the heating temperature reaches more than 55 °C within 15 s at 9 V. In addition, this composite fiber showed an excellent humidity sensitivity at ambient temperature, no matter for humidity change caused by breathing or immersing in water. Combined with its scalable production process, flexible fiber morphology as well as multifunction feature, this cellulose/CNT composite fiber may open new opportunity for the development of lightweight and flexible fabrics in the field of wearable electronics and for the design of various smart flexible fabrics.