Highly conductive and flexible chitosan based multi-wall carbon nanotube/polyurethane composite fibers

Abstract

Chitosan-based CNT composite fibers have received much attention due to their biocompatibility and biodegradability. However, further development and practical application of the fibers has been blocked due to their relatively low mechanical strength and electrical conductivity. In order to overcome these disadvantages and to extend the practical applicability of the composite fibers based on chitosan, a novel composite fiber based chitosan/MWNT composite and cross-linked polyurethane (PU) with 4,4′-methylenebis(phenyl isocyanate) (MDI) were newly developed herein using a wet-spinning technique. The experimental results confirmed that the proposed method was highly effective, allowing practical formation of the structures with enhancement of the mechanical strength and electrical conductivity by cross-linked PU as well as optimized MWNTs in the chitosan/MWNT composites fibers. Furthermore, the fabricated composite fibers with good mechanical strength (88.2 MPa) and excellent electrical conductivity (7.3 S cm⁻¹) can be fluidly spun on any substrate.

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Introduction

Chitosan is the second most abundant natural polymer on the earth, consisting of β-(1-4)-2-amino-D-glucose and β-(1-4)-2-acetamido-D-glucose. It has been extensively investigated for several decades for molecular separation, artificial skin, bone substitutes, and so on, because of its biocompatibility, biodegradability, and multiple functional groups. In addition, it can easily be shaped into fibers, films, hollow fibers, and membranes in many applications such as textiles. However, the mechanical properties of pure chitosan fibers are insufficient to meet the wide range of possible applications. Many attempts to improve chitosan fibers have been made, involving the blending of chitosan with other polymers¹ as well as CNTs² to enhance the mechanical, electrical, and thermal properties.

CNT fibers have been considered as fully reinforcing fillers for polymer matrixes for the development of high performance and multifunctional macrostructures because of the nanometer size, high aspect ratios, mechanical strength, and high electrical and thermal conductivity.^3–5 They are much more flexible and require higher energy to break than many commercial fibers. Especially, high concentrations of CNT single dispersions and stabilization can be achieved by a variety of polysaccharides with chitosan. Chitosan-based CNT composite fibers can easily be modified because of the hydroxyl and amino groups in chitosan, and are attractive because of the possibility of making continuous, microscopic structures with improved mechanical properties, while retaining the capacity to adsorb liquids.⁵

Recently, PU monomers have gained attention as a kind of cross-linker that is widely used in the chemical synthesis of aliphatic polyisocyanates and polyurethanes.^6,7 The chemical cross-linking modification of readily available polymers offers an attractive way to improve some of the inherent characteristics of such polymers.⁸

Herein, we report a facile approach for the production of macropores, neat and large-scale chitosan-based MWNT/PU composite fibers (CMPFs) by a simple and effective wet-spinning technique. The main objective of this study was to evaluate the possibility of developing high conductive CMPFs using cross-linked MDI, and demonstrated to have the ability to perform as perfect lead wires and sensing electrodes.

Experimental

Materials

Chitosan (>93% deacetylated chitin with an average molecular weight of 140 000–220 000), the multi-walled carbon nanotubes (>8% carboxylic acid functionalization with an average D:9.5 nm × L:1.5 μm), Triton X-100, tetrahydrofuran (THF), and acetic acid, 4,4′-methylenebis(phenyl isocyanate) (MDI) were purchased from Sigma-Aldrich (St. Louis, MO, USA).

Preparation of chitosan/MWNT composite solution

Aqueous dispersions of MWNTs were used for fiber spinning. 3 wt% MWNTs were dispersed in 0.1 M phosphate buffer solution (PBS) buffer (pH 7.4) solution using 1 wt% triton X-100 surfactant. And then, the mixture was ultrasonicated for 1 h. Chitosan (2 wt%) was completely dissolved in the MWNT/acetic acid dispersion at a 2 wt% concentration by stirring at room temperature for 4 hours. The chitosan/MWNT solution was centrifuged at 12 000 rpm at 20 °C for 20 min. After centrifugation, the supernatant was carefully decanted. The chitosan/MWNT solution was then stored at 4 °C until use.

Preparation of chitosan-based MWNT/PU composite fiber

CMPFs were prepared using a wet spinning method, in which THF/MDI solvent was used as a coagulation bath. For the coagulation solution, 0–40 mM of MDI was dissolved in THF solvent. The dispersed chitosan/MWNT solution was slowly injected at a flow rate of 15 mL h⁻¹ through a 21-gauge syringe needle using each of the different concentration of chitosan/MWNT mixtures into a THF/MDI coagulation bath rotating at 4 rpm. Continuous and uniform CMPFs were thereby fabricated. The fibers were thoroughly washed in deionized water and dried at room temperature for 2 hours.

Characterization

Thermogravimetry-differential thermalanalysis (TG-DTA) was done using a MAC Science TG-DTA 2000S. FTIR spectra were recorded on a Jasco 4600LE FTIR spectrometer from 4000 to 400 cm⁻¹. Scanning electron microscope (SEM) images were obtained on a field emission scanning electron microanalyzer (S-4700) at an acceleration voltage of 10 kV. The tensile strength test for the CMPFs was carried out with an Instron 5565A. The electrical conductivity of the CMPF was measured by a two-probe method with the PM5 Analytical Probe System at room temperature. For non-enzymatic glucose testing of the CMPFs, voltammetric measurements were performed using a CHI 660D electrochemical analyzer in a three-electrode arrangement. Glucose measurements were carried out in a 0.1 M NaOH-supporting electrolyte medium.

Results and discussion

Fig. 1 shows the preparation protocol of the CMPFs, in two steps: (A) formation of chitosan and MWNT composite, and (B) cross-linking of PU into the macroscopic fibers through the wet-spinning technique. MWNT was chosen for the carboxylic acid functionalization because of the covalent bonding interactions, as well as the dispersion and stabilization for the assembly of chitosan polymers on the MWNT surface. The MWNTs including long chain linear polymers of chitosan matrix initially require homogenous dispersion in a PBS buffer solution. Triton X-100 has extensively been used for the dispersion of MWNT in PBS buffer via a non-covalent approach. Both of them get adsorbed onto the surface of MWNT, rendering it soluble in aqueous solution. The phenyl ring in Triton X-100 also has an effective length of about three and a half carbon atoms, while carbon atoms on the branches contribute about one-half the effect of carbon atoms on straight alkyl chains.⁹

Fig. 1 Schematic illustration showing the mechanism of the formation of covalent bonding interactions between the amine functionalization of chitosan with long chain linear polymers and carboxylic acid functionalized MWNTs, and crosslinking of polyurethane as a result of wet-spinning.

The chitosan polymer matrix is well known for the formation of Schiff bases by the reaction between amines and aldehydes. Chitosan reacts rapidly with aldehyde in aqueous acid solutions. The modified chitosan contains primary and secondary amines and hydroxyl groups.¹⁰ The solubilized chitosan polymeric chains interacted through wrapping of the carboxyl functionalization on the MWNTs surface by the covalent bond with the chitosan/MWNT composite. The strong covalent bond could reinforce interfacial adhesion, and inevitably change the intrinsic electrical, mechanical and thermal properties between MWNTs and the chitosan composite matrix.

Wet-spinning technology, which is simple, efficient, scalable, economical, and green, has been widely used in industry, and has also been utilized in the fabrication of various fibers. The chitosan/MWNT composite was smoothly injected into a rotating coagulation bath of the THF/MDI solvent through the wet-spinning strategy. Upon immersion in the coagulation bath, the wet fiber in the gel state consisting of aligned chitosan and MWNT coagulated. The flow-induced alignment could be maintained by the cross-linking of MDI with PU, which allowed rapid stacking as they came out of the nozzle. Because of the NCO-terminated urethane prepolymer, chitosan or MWNT may reacting with diisocyanate and polyol at a specified NCO/OH equivalent ration for enhancement of the mechanical properties.¹¹ The steric hindrance of diisocyanate in the hard segment may also affect the crystallization of the urethane segment, which is related to the phenyl ring, and clearly hindered the hydrogen bonding of the urethane bonding.^12,13 In addition, the critical exponent of the CMPF, which is related to the electrical-activated hopping transfer mechanism,¹⁴ can lead to enhanced electrical conductivity.¹⁵

It is a facile and viable strategy to fluidly spin the macroscopic CMPF in a continuous and large-scale way. After injecting the chitosan/MWNT composite solution into the coagulation bath, a long CMPF (Fig. 2A) can be continuously drawn out from the coagulation solution and wound on a glass rod, as shown in the photographs (Fig. 2B and C). SEM measurements were carried out to determine the characteristics of the microstructures (Fig. 2D–F). The surface of the dried fibers had irregularly shaped bumps aligned along the fiber axis. The abundant porous and rough structures of the CMPF with coarse grains were also observed on the inner and outer surface of the fiber (Fig. 2D and E), providing benefit to the flexible properties and high surface area of the macroscopic fiber. Furthermore, the SEM image (Fig. 2F) clearly showed the MWNTs were achieved throughout the chitosan matrix. It is worth noting that, upon failure, most of the MWNTs were broken rather than just pulled out of the matrix. This phenomenon indicates strong interfacial adhesion between the MWNTs and the chitosan matrix.

Fig. 2 Photographs of (A) a long fiber drawn out from the 25 mM of THF/MDI solvent by wet spinning; (B) continuous fiber from rotation 5 rpm; (C) ∼100 cm-long fiber wound on a glass rod. SEM images of (D) a fiber with the diameter of about 50 μm; (E) the cross-sectional image; and (F) the fracture surface morphology of the chitosan-based MWNT/PU composite fiber.

The weight fraction of chitosan in the CMPF was estimated from the TGA curves using the results at 900 °C, shown in Fig. 3A. The initial weight loss of the MWNTs occurred from approximately 200 °C and continued to 900 °C. The weight loss of approximately 32% occurred, which was attributed to thermal decomposition of the carboxylic acid groups. Considering the weight losses of 32% and 81% for pure MWNT and chitosan, respectively, the fraction of chitosan was calculated to be 9 wt% and 20 wt% for the CMPFs. This means that after cross-linking with PU, the degree of infiltration was also remarkably improved. The degradation of chitosan also differed in the CMPF. Chitosan usually degrades in two stage.¹⁶ The first stage starts at about 80 °C with a weight loss of 8–10%, corresponding to moisture vaporization. The second stage begins at about 270 °C, reaching a maximum at about 340 °C with a weight loss of 55%. The CMPF had increase of the decomposition temperature by about 240 °C, reaching a maximum at about 340 °C with a weight loss of about 23%. Such an increase might provide an advantage to the application of the CMPF, especially for use at high temperatures.

Fig. 3 Characterization of pure chitosan, MWNT, and the chitosan based MWNT/PU composite fibers. (A) TGA curves; (B) FT-IR spectra; (C) comparison of noncross-linked and cross-linked polyurethane with composite fibers.

The FT-IR spectra of chitosan, MWNT, and the CMPF are shown in Fig. 3B. The FT-IR analysis of chitosan and the fiber depicted characteristic absorption bands at 3295 cm⁻¹, attributed to the –OH groups. Furthermore, bands were identified at 1606 cm⁻¹, typical of the N–H group bending vibration. The bands at 1343 and 1017 cm⁻¹ corresponded to the stretching of C–O–N and C–O groups. The bands appearing at 1192 and 666 cm⁻¹ were attributed to the glycosidic bondings, while the shoulder at 1651 cm⁻¹ represents the stretching of C=O. In the FT-IR spectra of MWNTs–COOH and fiber, the peaks at 3677 and 529 cm⁻¹ can be attributed to the stretching vibrations of O–H and C=O of carboxyl groups. In addition, the peak at 1069 cm⁻¹ was assigned to the C–O stretching vibration. Similar results were reported by G. D. Vuković et al.¹⁶ Absorbance at 1507 cm⁻¹ appeared in the spectrum of the CMPF, which indicates that the –COOH of MWNT reacted mainly with the –NH₂ of chitosan to turn into –NHCO–. The weak peak at 1713 cm⁻¹ indicates that some of the –OH groups of chitosan participate in the reaction to form ester (–COO–) bands. The FT-IR spectra of the cross-linked and non-cross-linked MDI were compared in Fig. 3C. The FT-IR spectrum of MDI with cross-linker showed a very strong peak at 2271 cm⁻¹, attributed to the attachment of an isocyanate (–NCO) group to 4,4′-diphenylmethane diisocyanate. A new C–H bending peak around 1510 cm⁻¹ appeared as the MDI content increased, which suggests that the C–H vibration mode was changed by the cross-linking.¹¹ Furthermore, as the cross-linking of MDI groups was sterically hindered by the phenyl groups, the hydrogen bonding of the urethane bonding was clearly hindered, causing a decrease in the degree of crystallinity and possibly, the aspect ratio of the crystallites.¹²

The effect of chitosan and MWNT on the tensile strength and electrical conductivity of the CMPFs are shown in Fig. 4. It can be seen that the addition of MWNTs significantly improved the tensile properties of the chitosan matrix, and the mechanical and electrical properties also increased with increasing MWNT concentration. As can be observed in Fig. 4A and B, the tensile strength of the CMPFs increased from 33 to 74.3 MPa by an increase of the chitosan concentration from 1.5 to 3 wt% with only 2 wt% of MWNTs filler, and from 34.2 to 79.4 MPa by increase of the MWNT concentration from 0.5 to 3 wt% with only 2 wt% of chitosan matrix. The increase in tensile strength was saturated at 2.5 wt% chitosan and 3 wt% MWNTs. The tensile strength of the CMPFs increased, enhancing the interfacial interactions of optimal relation between the MWNT and chitosan concentration.

Fig. 4 The variation of tensile strength and electrical conductivity with the increasing concentration of chitosan in fibers containing 2 wt% of MWNT (A and C), and with increase of MWNT when containing 2 wt% chitosan (B and D) from direct coagulation in the 10 mM THF/MDI solvent.

The change in the electrical conductivity of the CMPFs with increased chitosan and MWNT concentration were also measured. The CMPFs containing 1.5 wt% chitosan/2 wt% MWNTs and 2 wt% chitosan/3 wt% MWNTs showed the electrical conductivity of 2.1 S cm⁻¹ (Fig. 4C and D). This stepwise change of highly conductive MWNTs/chitosan resulted from the formation of an interconnected structure of MWNTs.¹⁷ The quality of such a network is ultimately defined by the MWNT concentration and relative extent of homogeneous to heterogeneous distribution in the chitosan matrix. These results clearly indicate that the highest performance with best electrical conductivity and tensile strength was achieved with 2.2 wt% chitosan and 2.5 wt% MWNT. However, the CMPFs did not increase and were not fabricated further with MWNT concentrations greater than 3 wt% due to the high viscosity and poor dispersion of the MWNTs.¹⁸

Examination of the tensile strength and electrical conductivity of the CMPFs was carried out for optimization of the cross-linker with PU, as shown in Fig. 5. The cross-linked composite fibers containing 2.2 wt% chitosan and 2.5 wt% MWNT were immersed into coagulation bath with different MDI concentrations. The tensile and electronic properties of the CMPF are influenced by the MDI concentration due to the stronger interactions prevailing at large isocyanate content.^14,15,19 The tensile strength of the cross-linked composite fibers increased gradually when the MDI concentration increased. A maximum tensile strength of 88.2 MPa was achieved at the MDI concentration of 25 mM, as shown in Fig. 5A. The as-fabricated cross-linked composite fiber shows similar or a little better tensile strength when compared with other chitosan-based MWNT composite fibers, as shown in Table 1.

Fig. 5 The variation of (A) tensile strength and (B) the electrical conductivity with increasing MDI concentration for the coagulation of fibers containing 2.2 wt% of chitosan and 2.5 wt% of MWNT.

Table 1 Mechanical and electrical properties of composite fibers compared to the carbon nanotube composites employed, by wet-spinning using cross-linker

Ref.	Structures	Cross-linker	Tensile strength (MPa)	Electrical conductivity
20	Chitosan	Sodium alginate	0.85	—
17	Chitosan/MWNT	—	52	0.27
21	Chitosan/MWNT	PMMA	22.54	1.1 × 10^−2
18	Chitosan/cellulose/MWNT	1-H-3-Methylimidazolium chloride	162	6.2 × 10^−3
This work	Chitosan/MWNT	MDI	2.6	7.3

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The electrical conductivity of the CMPFs was plotted as a function of the MDI concentration added to the matrix. It can be seen in Fig. 5B that all the cross-linked composite fibers had excellent electrical conductivity than the previous chitosan-based MWNT fibers shown in Table 1. The electrical conductivity of the CMPFs increased from 3.2 to 7.3 S cm⁻¹. The increase in conductivity was nearly proportional to the improvement in strength due to the decrease of contact resistance, which is dominated by the electron hopping mechanism.^22,23 The results indicated that increased concentration (2.5–40 mM) of MDI in the cross-linking structure of PU enhanced the tensile strength and electrical conductivity of the CMPFs.

To improve the mechanical properties of the CMPF, post-spin twisting was carried out for the as-spun fibers (Fig. 6A). This gave the CMPF a higher density and twist angle (about 10–15°). The post-spin twisting brought the CNTs in closer contact with each other, therefore enhancing the van der Waals forces and friction, which improved the load transfer between the CNTs.²⁴ The textile was woven by using the chitosan-based MWNT/PU composite fibers (CMPFs) with a size of 1 × 2 cm for the application of non-enzymatic glucose biosensor (Fig. 6B). To improve the electrical conductivity of the CMPF, an LED lamp can be lighted using the prepared CMPF as the leads, as shown in the photograph of Fig. 6C. In addition, the textile made from the CMPFs with Fig. 6B showed a typical amperometric response curve for the CMPF based non-enzymatic glucose biosensor (Fig. 6D). The textile based non-enzymatic glucose biosensor is operated through the glucose oxidation and reaction catalyzed by CMPFs in alkaline medium. The CMPF with electrode offers a marked decrease in the overvoltage for the glucose oxidation reaction to allow convenient low-potential amperometric detection.²⁵ Numbers in the chart represent the corresponding glucose concentrations in the 0.1 M NaOH solution at an applied potential of 0.2 V.²⁶ The near instantaneous rise in the sensor response current following the addition of 25 mM glucose stock solution to the final concentrations of 2, 4, 6, 8, 10, and 12 mM are evident from the stepwise increase in the sensor response curve. The non-enzymatic glucose sensor using the textile typed CMPFs showed better performance than other non-enzymatic glucose sensor systems using a chitosan/MWNT composite matrix, with sensitivity of 3.2 μA mM⁻¹.²⁷ The textile which was comprised of CMPFs provides several advantages such as high surface area and good electrocatalytic properties of the CNTs, while avoiding potential toxicity caused by asbestos-like CNTs when implanted in vivo.²⁸ The chitosan induced solubilization CNT permits a variety of manipulations, including modification of electrode surfaces and preparation of biosensors.

Fig. 6 Performances of the fabricated chitosan-based MWNT/PU composite fibers for various practical applications, demonstrating their good flexibility, mechanical, and electrical properties. (A) SEM image of the twisted fibers with yarn; (B) the textile type; (C) the lighted LED lamp using chitosan-based MWNT/PU composite fiber as the leads; (D) non-enzymatic glucose monitoring with the textile type of the chitosan-based MWNT/PU composite fibers.

Conclusions

Biocompatible, flexible, porous, and macroscopic CMPFs were successfully fabricated by a convenient wet-spinning technique followed with the cross-linking PU reaction. The optimal chitosan and MWNT concentrations for the CMPF fabrication were determined. The fabricated CMPF exhibited excellent mechanical properties of 88.2 MPa and electrical conductivity of 7.3 S cm⁻¹ through the cross-linking with PU. We expect that these macroscopic flexible CMPFs will be able to be woven into various objects alone or with other commercial threads together for various practical applications, such as sensors, energy harvesting, catalysts, lead wires for wearable electronics, etc.

Acknowledgements

This research was partially supported by the ICT R&D program of MSIP/IITP and Business for Cooperative R&D between Industry, Academy, and Research Institute funded by Korea Small and Medium Business Administration. The authors are grateful to MinDaP group members for their technical supports.

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