Preparation of a carboxymethylated bacterial cellulose/polyaniline composite gel membrane and its characterization

Abstract

Bacterial cellulose (BC) was modified by carboxymethylation (CM) to get carboxymethylation bacterial cellulose (CM-BC) with different degrees of substitution (DS). Then the novel conductive CM-BC/PANI nanocomposite membranes were successfully synthesized in situ by oxidative polymerization of aniline using CM-BC as the template. The influence of carboxymethylation on the structure, morphology, electrical/proton conductivity and mechanical property of the CM-BC/PANI composites has been investigated. The CM-BC/PANI networks display a combination of hybrid electronic/proton conductivity and better mechanical property compared to that of BC/PANI. The electrical conductivity can reach 1.69 × 10⁻² S cm⁻¹ when the DS is 0.146 with carboxymethylation for 13 hours. In addition, the proton conductivity of CM-BC/PANI shows an obvious improvement in comparison to that of BC/PANI, which could reach 2.86 × 10⁻⁴ S cm⁻¹. The CM-BC/PANI composite membranes exhibit certain flexibility and good mechanical properties with a tensile strength of 0.23 MPa.

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1. Introduction

Bacterial cellulose (BC), synthesized in abundance by Acetobacter xylinum, has a highly hydrophilic characteristic due to the many hydroxyl groups on the surface,¹ and its fibrils have diameters between 10 and 100 nm compared to cotton with a diameter of 10 mm, which forms a specific ultrafine three-dimensional (3-D) network structure.² Besides, BC has high crystallinity, sufficient porosity, high purity, high water holding capability, renewability and excellent biocompatibility.^1,3–6 In addition, the mechanical properties of BC are much better than those of conventional natural fibers and this biopolymer can maintain its structure and mechanical stability even in hydrogel form.^7,8 Therefore, BC could act as a supporting material to deposit the nanofillers in order to create advanced BC-based functional nanomaterials for various technological applications. For example, BC composites with introduction of silver nanoparticle,^9,10 platinum nanoparticle,¹¹ nano graphite,¹² zinc oxide¹³ and titanium dioxide^14,15 have been found to have high conductivity and these composites have potential applications in electrical instruments, batteries, proton exchange membranes and display devices. Moreover, BC has been also adopted to prepare the composites with electrically conductive polymer such as polyaniline (PANI) and polypyrrole.¹⁶

Among the available intrinsically conducting polymers, PANI is one of the most useful conjugated polymers because it is stable in both its doped, conducting form and de-doped, insulating form among the family of polymer.^17,18 Compared to other conducting polymers, PANI was endowed with the advantages of facile synthesis, low-cost, simple doping/dedoping chemistry, low synthesis costs and good environmental stability.^19–21 Therefor, PANI has been used in different applications including sensors, capacitor, nerve regeneration material, flexible electronics and fuel cells. However, PANI has poor processability and mechanical properties due to the rigid-rod macromolecular chains, which limits its application. To solve this problem, many researchers have done lots of investigations on BC reinforced polyaniline composites, which offers applications in biosensors, flexible electrodes, electromagnetic interference shielding material, flexible displays and other electronic device.^7,22–25

In Shi et al. study, PANI was synthesized on the surface of BC nano-fibers and assembled into a novel 3D network. The BC–PANI nanofiber composite is an electro-conductive hydrogel that combines the properties of hydrogels and conductive systems. The electroconductivity of composite hydrogels was enhanced from 10⁻⁸ to 10⁻² S cm⁻¹ (ref. 26). A BC membrane was also used as a template to deposit PANI uniformly and then flexible electrically conductive nanocomposite membranes have been synthesized in situ by oxidative polymerization of aniline with ammonium persulfate as an oxidant and BC as a template. Effect of different types of acids such as dodecylbenzene sulfonic acid (DBSA), HCl, H₂SO₄, p-methylbenzene sulfonic acid (p-TSA) on the electrical conductivity of PANI/BC composite membranes was studied. The electrical conductivity arrived to 5.0 × 10⁻² S cm⁻¹ when the reaction time was 90 min and HCl was used as proton acid.²⁷ Wang et al. got BC/PANI composites exhibited outstanding electrical conductivity. The morphology evolution of the composites is closely related to the reaction parameters including the feeding ratio of BC/AN, concentration of oxidant and dopant, reaction duration, temperature and media.²⁸

Most previous studies focused on the electric conductivity of BC/PANI composite but rarely considered the proton conductivity of the composites. Actually, proton conductivity also plays an important role which could expand the applications in a variety of electrochemical devices including fuel cells, batteries, capacitors, electrochromic displays, sensors and photo-electrochemical solar cells.²⁹ Our group has researched the proton conductivity of BC/PANI, but it was sill not high enough.³⁰ As a fact of that, BC has lots of hydroxyl groups that could be modified easily due to their activity, such as by acetylation,³¹ silylation,³² sulfation,³³ phosphorylation³⁴ and succinylation.³⁵ However, there were no reports of aniline polymerization on modified bacterial cellulose, which restricts the application of BC/PANI composite membrane.

In this paper, BC was carboxymethylated by reacting BC with sodium monochloroacetate in the presence of NaOH to get CM-BC with different degree of substitution (DS). The ionization of CM-BC in aqueous solution may increase the composites' proton conductivity. Then CM-BC/PANI composite membranes with hybrid electric/proton conductivity and good mechanical properties were developed with CM-BC nanofibers as the template, which would have more potential applications than BC/PANI in a variety of fields including fuel cell,³⁶ electronics,³⁷ and energy storage.^38,39 The influence of the carboxymethylation on the structure, morphology, electrical/proton conductivity and mechanical property of the CM-BC/PANI composites has been investigated. The resultant CM-BC/PANI nanocomposite membranes were characterized by Fourier transform infrared spectroscopy (FTIR), field-emission scanning electron microscopy (FESEM), electrical conductivity, AC impedance and mechanical measurements.

2. Experimental

2.1 Material

Gel-like cellulose pellicle formed by A. xylinum AGR 60, which was kindly supplied by the Hainan Yida Food Co., Ltd, was used in this study. As a pretreatment, BC specimens were immersed in a 0.1 mol L⁻¹ sodium hydroxide solution for 60 min in a 90 °C water bath to remove the bacterial cell debris, and then thoroughly washed with deionized water until got neutral. The aniline (AN) used in this study was purchased from Sinopharm Chemical Reagent Co., Ltd. The rest of the regents used in the experiments were all analytical reagents.

2.2 Carboxymethylation of the BC gel membranes

The CM-BCs were prepared as follows. Firstly, 7.5 g of sodium hydroxide was dissolved in a mixed solution with 150 mL deionized water and 200 mL ethyl alcohol. Then 107.5 g of wet BC membranes were immersed into the aqueous alkali over 40 min at room temperature. Afterwards, sodium chloroacetate (21.7 g) was added, and this mixed solution was stirred in an oil bath with reaction time varied from 1 h, 5 h, 9 h, 13 h, 17 h to 21 h respectively at 55 °C (CM-BC1, CM-BC5, CM-BC9, CM-BC13, CM-BC17 and CM-BC21). As cooled down, the specimens were rinsed repeatedly with deionized water until got neutral. Hence, the carboxymethylation was finished and the production of CM-BC membranes were obtained. Fig. 1 shows the chemical equations of BC to CM-BC with two steps.

Fig. 1 Chemical equations of BC to CM-BC.

The DS of CM-BC was estimated by acid–base titration as Lin's.⁴⁰ After vacuum drying at 110 °C, 0.1 g CM-BC was dissolved in 50 mL deionized water, and hydrochloric acid solution (30 mL, 0.105 mol L⁻¹) was added. Then, 1–2 drops of phenolphthalein were added after 0.5 h, and a 0.095 mol L⁻¹ sodium hydroxide solution was used as a titrant until the mixed solution turned pink. The volume of the sodium hydroxide solution consumed was recorded as V, and the DS of CM-BC was then calculated according to the following equation:

DS = [162B/(m − 58B)] (1)

where B represents the molar weight of sodium hydroxide reacted with carboxyl group is equal to (0.095V − 0.105 × 30) × 10⁻³ and m represents the weight of CM-BC.

2.3 Preparation of the CM-BC/PANI gel membranes

CM-BC gel membranes with various DS were placed in a 100 mL beaker containing a mixture of 50 mL HCl solution (1.0 mol L⁻¹) and 0.025 mol dissolved aniline monomers respectively. After the CM-BC membranes were fully immersed for 24 h, oxidant of 0.5 mol L⁻¹ ammonium peroxide sulfate (APS) was added into the solution dropwise with shaking in 200 rpm. The mixture was stirred in an ice bath for 90 min. At the end of each experiment, the obtained membranes, which were called CM-BC1/PANI, CM-BC5/PANI, CM-BC9/PANI, CM-BC13/PANI, CM-BC17/PANI and CM-BC21/PANI, were taken out and washed thoroughly with 75% (v/v) ethyl alcohol followed by deionized water to remove the byproducts and remaining reagents.

2.4 Membrane characterization

2.4.1 Fourier transform infrared (FT-IR) spectroscopy. ATR-FTIR spectroscopy was conducted to confirm the functional groups of the CM-BC and CM-BC/PANI composite gel membranes. The results were recorded on a Bruker TENSOR II Fourier Transform Infrared Spectrometer. For FT-IR, the specimens were analyzed over the range 600–4000 cm⁻¹, with a spectrum resolution of 4 cm⁻¹. All spectra were averaged over 32 scans.

2.4.2 Morphological characterization. The polymerization surface of the BC/PANI, CM-BC and CM-BC/PANI membranes was characterized with a field-emission gun scanning electron microscopy (FE-SEM) (AURIGA Cross Beam, 10 kV, Carl Zeiss, Germany). The acceleration voltage was 2 kV. Prior to analysis, the specimens were coated with a thin layer of gold to eliminate electron charging.

2.4.3 Electrical conductivity. Electrical conductivity (σ₁) of the membranes was measured with a conventional four-point probe technique (RTS-8, Probes Tech., China) at ambient temperature. According to the four point probe method, resistivity can be calculated with ρ = 2πS(V/I), where S is the probe spacing (mm), which was kept constant, I is the supplied current in microamperes, and the V is corresponding voltage measured in millivolts. Electrical conductivity can be computed using σ₁ = 1/ρ.

2.4.4 Proton conductivity. Proton conductivity was measured by an AC impedance method using an electrochemical workstation (CHI618D, China) over a frequency range from 0.01 Hz to 10⁵ Hz with AC amplitude of 5 mV. The membrane sample, with the size of Φ 15 mm, was sandwiched between two stainless steel screw thread electrode and held in a PTFE fixture. The complex impedance was reported in Bode plots with log frequency on the x-axis and both the absolute value of the impedance and phase-shift on the y-axis. The resistance of samples was obtained using the absolute value of the impedance in the frequency range where it was approximately constant and the phase angle was close to zero. The proton conductivity σ₂ was calculated using the equation:where R is the resistance value extracted by Bode plots as described above, l is the distance between the inner electrodes and A is the cross-sectional area of the membrane.

2.4.5 Mechanical property. The tensile properties of the CM-BC/PANI composite gel membranes were tested by a TA.XT Plus Texture Analyzer with the A/TG tensile grips. Tensile experiments were performed on rectangular strips (80 × 30 × 3 mm³) with a 5 kN load cell and a speed of 2 mm s⁻¹ at ambient temperature. Three identical specimens were tested for each sample and their average mechanical properties were reported.

3. Results and discussion

The flexible nanoporous BC membranes were firstly carboxymethylated to obtain CM-BC membranes. Then the CM-BC membranes were used to synthesize composite gel membranes. The aniline hydrochloride could permeate through the inner network of CM-BC after the process of soaking in aniline hydrochloride solution for 24 h. At the same time, hydroxyl groups of CM-BC could interact with amine groups of aniline to form the hydrogen bonds which ensure the uniform distribution of aniline on the surface of CM-BC nanofibers. After APS as oxidant was added into the aniline hydrochloride solution, the aniline monomer would in situ polymerize to produce polyaniline in the network of CM-BC, then the CM-BC/PANI composites would be got, and the colour of the membranes changed from ivory to dark green after polymerization was initiated, as illustrated in Fig. 2. The content of PANI was calculated based on the weight increment of the CM-BC membrane as shown in Table 1.

Fig. 2 Synthetic procedure of CM-BC/PANI.

Table 1 Effects of DS on the content of PANI and the electrical conductivity of CM-BC/PANI composite gel membranes

Samples	DS of CM-BC	Content of PANI (wt%)	Electrical conductivity (S cm^−1)
CM-BC1/PANI	0.074	76.9	7.21 × 10^−3 ± 0.00036
CM-BC5/PANI	0.096	78.6	8.49 × 10^−3 ± 0.00068
CM-BC9/PANI	0.125	82.5	1.29 × 10^−2 ± 0.0008
CM-BC13/PANI	0.146	85.7	1.69 × 10^−2 ± 0.0005
CM-BC17/PANI	0.162	79.3	9.75 × 10^−3 ± 0.00088
CM-BC21/PANI	0.178	78.2	8.42 × 10^−3 ± 0.00061

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3.1 Structure analysis

ATR-FTIR was preformed to characterize the structures of pure BC, CM-BC, PANI, CM-BC/PANI and BC/PANI samples (Fig. 3). For BC, there were three strong absorption peaks at 3370, 2900, and 1060 cm⁻¹, corresponding to the stretching vibrations of the ν(–OH), ν(–CH₂), and ν(C–O) groups respectively.⁴¹ In comparison, new absorption peaks appeared at 1597 and 1415 cm⁻¹ assigned to the C=O stretching vibrations of the carboxymethyl group in CM-BC spectrum, indicating the successful branch joint of the carboxymethyl group on the BC molecular chain. In PANI spectrum, the peak at 802 cm⁻¹ corresponding to the out-of plane bending vibration of the C–H band of p-disubstituted benzene ring appears. The peaks at 1560 cm⁻¹ and 1483 cm⁻¹, assigned to the stretching vibration of quinoid and benzoid structure, respectively. The spectrum of the CM-BC/PANI composite membrane exhibited overlapped absorption bonds of both components. Small peaks in transmittance at around 1552 and 1467 cm⁻¹ assigned to stretching vibration of N=Q=N and N–B–N (Q and B represent quinoid and benzenoid moieties in the PANI chains) structure of PANI were observed for CM-BC/PANI, which indicates that the CM-BC was coated with PANI. Further evidence of PANI coating onto CM-BC fibers can be found at the disappearance of the sharp peak at 1063 cm⁻¹ (C–O–C stretching vibrations) and the band at 2897 cm⁻¹, which is assigned to the aliphatic C–H stretching vibration. Similar spectrum was found for BC/PANI, which showed that both BC and CM-BC fibers were successfully coated by polyaniline, and the result could be confirmed from the images of SEM.

Fig. 3 FT-IR spectra of pure BC, CM-BC, CM-BC/PANI, BC/PANI and PANI samples.

3.2 Morphological characterization

Fig. 4 revealed the morphology evolution of pure BC, CM-BC, BC/PANI and CM-BC/PANI composite gel membranes. Pristine BC nanofibres boast an average diameter of about 30 nm and a length ranging from micrometers up to dozens of micrometers. With coating of PANI, the fiber-like morphology changed into a flake-structured morphology with high densification and aggregation of BC/PANI flakes (Fig. 4b), mainly due to strong hydrogen bonding between sequentially formed PANI layers.²⁸ As is shown in Fig. 4c, the CM-BC nanofibres became thicker compared with the morphology of pure BC, amounting to more than 60 nm after being carboxymethylated. Besides, many fiber bundles fractured, and 3-D structure of BC increased obviously. Fig. 4d is FE-SEM image of CM-BC/PANI composite, revealed that the CM-BC scaffold was fully coated with a PANI layer. In contrast to Fig. 4b, package polyaniline fiber bundle obviously got coarsen after carboxymethylated processing, reaching to more than 200 nm. At the experiment process, we found CM-BC became thicker and this phenomenon was confirmed by the SEM image as Fig. 4c. Before carboxymethyl modification, BC fibers were connected by amounts of hydroxyl groups with strong attractive force. As the carboxymethyl groups introduced, some hydroxyl group suffered a certain degree of damage, which decreased the amount of hydroxyl group between micro fibers. Besides, repulsive force between the carboxymethyl groups contributed to the separation of micro fiber and fiber bundles. That is why the CM-BC membrane is thicker and 3-D network of CM-BC is bigger than that of BC (Fig. 5).

Fig. 4 FE-SEM images of (a) BC, (b) BC/PANI, (c) CM-BC, (d) CM-BC/PANI composite gel membranes.

Fig. 5 Mechanism of microstructure evolution from BC to CM-BC.

3.3 Electrical conductivity

Polyaniline can conduct only after acid doped. It is confirmed that the electrical conductivity of BC/PANI composites is influenced by the type of dopant used, in which case HCl proves to be the best doped protonic acid.²⁷ In our previous work, conductivity of PANI was mainly affected by its concentration and content on the matrix material. In a certain range, with the gradually increase of polyaniline particles, electrical conductivity of BC/PANI composite membrane was improved.³⁰ Table 1 lists the effects of DSs on the content of PANI and the electrical conductivity of CM-BC/PANI composite membranes. From the table we could find that the content of PANI does not increase monotonously with the increase of DS, but rather has a threshold value. As we can see, the conductivity of CM-BC/PANI composites improves with the increase of DS in the range of 0.074–0.146, but it decreases as the DS is above 0.146 as shown in the Fig. 6. The initial increase may be due to the fact that CM-BC with higher DS could composite more PANI nanoparticles because of its larger 3-D network structure, leading to the optimum conductivity. The conductivity reached the maximum of 1.69 × 10⁻² S cm⁻¹ when the DS was 0.146. However, excessively high DS would damage the 3-D network structure, and may impede the polymerization of aniline on the fiber and fracture the PANI conjugated chain. At the same time, the doping system may be undermined to some extent, and the insoluble polyaniline may generate in the composite membranes, which results in the decrease of the conductive component and thus reduces the corresponding conductivity of composite membranes. The electrical conductivities as high as 10⁻² S cm⁻¹ were obtained for the CM-BC/PANI nanocomposite membranes, which were at least 1 order of magnitude higher than that of BC/PANI membranes in previously reported work.²⁴

Fig. 6 Conductivity and content of PANI of CM-BC/PANI composite gel membranes.

3.4 Proton conductivity

The Nyquist plots of BC/PANI and CM-BC/PANI composite gel membranes with different DS are displayed in Fig. 7. The Nyquist plots for all membranes are semicircular. The semicircular corresponds to the ion transfer limited process, and its diameter represents the resistance of membrane layer, with the letter R_ct. The intersection point of the high frequency band and the real axis on the left represents the resistance between the two stainless steel electrodes, with the letter R_s. Modification of BC/PANI with carboxymethylation leads to a smaller semicircle and decreased R_ct compared to BC/PANI, which reduces further in CM-BC5–17/PANI, this means the proton conductivity getting higher.⁴² Notably, the polymerization of polyaniline on CM-BC nanofibers gives rise to a considerable reduction in R_ct, indicative of the efficient mass and charge transport at the CM-BC/PANI composites. This can be attributed to the uniform distribution and high loading of polyaniline in CM-BC nanostructures, creating necessary conduction pathways in promoting the ion transfer on the resultant composites.

Fig. 7 Nyquist plots of BC/PANI and CM-BC/PANI composite gel membranes ((b) is magnified (a)).

The data of R_s, R_ct, CPE and proton conductivity are shown in Table 2. R_s changes little, basically in 0.8–20 Ω range. That is because R_s is mainly the series resistance between two stainless steel electrodes, the influence of compound gel electrolyte membrane is smaller. But charge transfer resistance change of gel electrolyte membranes with different DS is bigger. While the DS is low, CM-BC1/PANI composite gel membrane ionizes less H⁺, so the proton conductivity is as low as 1.99 × 10⁻⁴ S cm⁻¹. As the DS increases, carboxymethylation is enhanced, and the composite gel membranes ionize more H⁺, so the proton conductivity of CM-BC5/PANI and CM-BC9/PANI become higher, reaching to 2.10 × 10⁻⁴ and 2.86 × 10⁻⁴ S cm⁻¹. But when the DS is too high, the structure of membrane becomes loosen and polyaniline reunite badly, which prevents the membrane's ionization. That could explain the proton conductivity of gel membrane decreases gradually with increasing DS among the range of 0.127–0.184. It can be seen that the proton conductivity of CM-BC/PANI is higher than that of BC/PANI composite gel membrane in our previous work, and there is a several orders of magnitude improvement compared to that of BC/PANI composite in Marins's work.²² Besides, in Lim's work,⁴³ the proton conductivity of PES/SPEEK PEMs range from 9.63 × 10⁻⁶ to 6.935 × 10⁻⁵ S cm⁻¹ while the Nafion 117 and 112 membranes are better conductors with conductivity of 4.96 × 10⁻⁴ and 1.78 × 10⁻⁴ S cm⁻¹ respectively. The proton conductivity of CM-BC/PANI membranes range from 1.15 × 10⁻⁴ to 1.15 × 10⁻⁴ S cm⁻¹, which is much higher than that of PES/SPEEK proton exchange membranes, and approximately equal to Nafion membrane. The CPE factor values of CM-BC/PANI composite membrane correspond to about 0.88, indicating the composites has potential application in capacitor.

Table 2 Proton conductivity of CM-BC/PANI composite gel membranes

Samples	DS	Rs (Ω)	Rct (Ω)	CPE (n)	Proton conductivity (S cm^−1)
BC/PANI	0	0.8206	294.6	0.9415	3.76 × 10^−5 (ref. 30)
CM-BC1/PANI	0.074	5.144	213.1	0.887	1.99 × 10^−4
CM-BC5/PANI	0.096	4.768	201.7	0.8925	2.10 × 10^−4
CM-BC9/PANI	0.127	5.209	148.2	0.8802	1.15 × 10^−4
CM-BC13/PANI	0.146	18.67	168.3	0.8536	2.52 × 10^−4
CM-BC17/PANI	0.162	5.512	170.2	0.8935	2.49 × 10^−4
CM-BC21/PANI	0.184	3.318	369.8	0.8813	1.15 × 10^−4

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3.5 Mechanical property

The mechanical property of composite gel film is an important application parameter. Tensile stress–strain curves of CM-BC and CM-BC/PANI composite gel membranes with different DS are shown in Fig. 8. Due to the poor mechanical property of polyaniline, its introduction destroyed the strong interaction of CM-BC network and lowered the tensile strength of composite membrane. Therefore, the tensile strength of composite gel membrane has a reduction with different DS as compared to CM-BC membrane. Table 3 shows that the elongation at break of all composite samples are around 50%, which indicates that all of the membranes exhibited typical ductile fracture characteristic. The tensile strength of CM-BC gel membrane substrate is 0.41 MPa while those of all CM-BC composite gel membranes are above 0.15 MPa and the change of modulus is the same as tensile strength. The decreased mechanical behavior of CM-BC/PANI membranes might be associated with the weakened inter- and intramolecular hydrogen bonding of CM-BC caused by the introduction of PANI. But on the other hand, incorporation of PANI into CM-BC could reduce the porosity of CM-BC, which brought the increase in elongation of CM-BC/PANI membranes.²³ The maximum of the tensile strength of CM-BC/PANI membranes is 0.235 MPa (CM-BC13/PANI) and the tensile strength of other CM-BC composite gel membranes are all above 0.15 MPa, which are better than that of BC/PANI.³⁰ Compared with almost no forming ability of PANI, the mechanical properties of composites are improved significantly using CM-BC as a template, and the application scope is expanded obviously. It is obvious that the CM-BC/PANI nanocomposite gel membranes combine the good mechanical properties of the CM-BC membrane and the conductive properties of the PANI.

Fig. 8 Tensile stress–strain behaviors of CM-BC/PANI composite gel membranes.

Table 3 Mechanical property of CM-BC/PANI composite gel membranes

Samples	Tensile strength (MPa)	Modulus E (MPa)	Elongation (%)
CM-BC	0.413 ± 0.006	0.814 ± 0.009	35.8 ± 1.9
BC/PANI	0.145 ± 0.002	0.325 ± 0.005	46.4 ± 1.7
CM-BC1/PANI	0.152 ± 0.003	0.298 ± 0.005	51.0 ± 1.5
CM-BC5/PANI	0.151 ± 0.003	0.317 ± 0.007	50.0 ± 1.6
CM-BC9/PANI	0.162 ± 0.004	0.376 ± 0.009	45.3 ± 1.4
CM-BC13/PANI	0.235 ± 0.005	0.475 ± 0.008	51.5 ± 1.8
CM-BC17/PANI	0.178 ± 0.003	0.342 ± 0.006	53.7 ± 2.3
CM-BC21/PANI	0.206 ± 0.007	0.487 ± 0.012	49.7 ± 1.8

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4. Conclusions

BC was modified by carboxymethylation to get CM-BC with different DS, then the novel conductive CM-BC/PANI nanocomposite membranes were successfully synthesized in situ by oxidative polymerization of aniline onto CM-BC nanofibers. The structure, morphology, electrical/proton conductivity and mechanical property of the CM-BC/PANI composites with different degree of carboxymethylation have been tested and compared. The electrical conductivity can reach to 1.69 × 10⁻² S cm⁻¹ when the DS was 0.146 with carboxymethylation for 13 hours. In addition, the proton conductivity has an obvious improvement in comparison to BC/PANI, which can reach to 2.86 × 10⁻⁴ S cm⁻¹. The CM-BC/PANI composite membranes exhibit good flexibility and mechanical properties with the tensile strength of 0.23 MPa.

Acknowledgements

This study is financially supported by National Natural Science Foundation of China (Grant No. 51273021 and 51473019) and Beijing Municipal Science and Technology Plan Projects (No. Z161100000116003).

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