Synergistic Effect in Moisture Sensing of Nylon-6 Polymer Films

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Synergistic Effect in Moisture Sensing of Nylon-6 Polymer Films through Molecular-Level Interfacial Interactions of Amide Linkages in the Presence of Graphene Muhammad Mohsin Hossain, Md. Akherul Islam, Hossain Shima, Mohd. Kafeela, Mudassir Hasan, and Moonyong Lee J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b05822 • Publication Date (Web): 16 Oct 2018 Downloaded from http://pubs.acs.org on October 18, 2018

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The Journal of Physical Chemistry

Synergistic Effect in Moisture Sensing of Nylon-6 Polymer Films through Molecular-Level Interfacial Interactions of Amide Linkages in the Presence of Graphene Muhammad Mohsin Hossain,‡ Md. Akherul Islam,§ Hossain Shima,⊥ Mohd. Kafeela,∥ Mudassir Hasan,∥,* Moonyong Lee±,* ‡Advanced

Institute of Convergence Technology, Seoul National University, Seoul 151-747, Republic of

Korea §Department

of Pharmacy, Atish Dipankar University of Science &Technology, Banani 1213, Dhaka,

Bangladesh ⊥Chemistry

∥College

Department, Rajshahi University, Rajshahi 6205, Bangladesh

of Engineering, Department of Chemical Engineering, King Khalid University, Abha 61411,

Kingdom of Saudi Arabia ±School

of Chemical Engineering, Yeungnam University, Gyeongsan 712-749, Republic of Korea

*Corresponding authors: [email protected] [email protected]

Abstract The highly conductive flexible graphene-nylon-6 composite film prepared in this study showed excellent response towards moisture sensing application for the first time. The mechanism of the excellent performance was explored and proposed using synergistic effects like (1) the interfacial linking between nylon-6 and graphene that played a major role in controlling electron movement, (2) the molecular-level interaction produced through the anchoring of water molecules by amide linkages, (3) the anchored moisture molecule creating an interfacial barrier for electron movement from one graphene surface to another through a graphene-nylon-6 bridge, and (4) graphene itself absorbing the moisture molecules, which collectively produced a significant resistance to obtain the sensing signals for water molecules. The demonstrated interface chemistry through the molecular-level interaction of moisture and nylon-6 polymer molecules was found to be highly useful in realizing a higher sensing performance. The sensing was 100 times faster than that observed with the current conventional sensors. The calculated Young’s 1 ACS Paragon Plus Environment

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modulus and electrical conductivity of the composite film showed excellent mechanical and electrical properties. Additionally, the reversibility data of the sensor showed outstanding performances. Therefore, excellent sensing performance based on a very short response time, quick reversibility, retained sensitivity, and flexibility made it a viable candidate for moisture sensing application. 1. Introduction Graphene has stimulated the great interest of scientists owing to its single-atom-thick two-dimensional conjugated structures, room-temperature stability, ballistic transport, large available specific surface areas, and excellent electrical conductivity. It can be used as an ideal platform for chemical sensors through the highly conductive and ballistic transport network of the sp2 conjugated system. Furthermore, it exhibits very low signal disturbance when used as a chemical sensor.1-2 These outstanding features make graphene an ideal material for gas/vapor sensors.3 In the past, several composites based on graphene thin films have been studied extensively for gas-sensing applications.

2, 4

However, graphene with nylon-6 has not been sufficiently considered for

sensing application. Among the few applications of gas sensing, nylon-6 is not used as a key matrix.5 In most of the cases, nylon-6 is used as a flexible robust substrate that can hold other materials either on its surface or inside the matrix. Actually, it is the materials that hold on to nylon-6 that play an important role in sensing, and not nylon-6 itself. Nevertheless, the moisture sensing application of graphene-nylon-6 has not been investigated yet, although nylon-6 is extensively utilized as an engineering plastic and textile fiber mostly owing to its outstanding properties. In particular, the excellent mechanical properties are attractive for many applications and remain unaffected over a wide range of temperatures. Nylon-6 is also easy to produce.6 The natural arrangement of nylon contains amide groups separated by a number of interconnected methylene groups. Consequently, a number of amide linkages are present in the nylon-6 backbone chain, which are really helpful in forming an interlink or anchoring an antenna for the water molecule being sensed.5 Practically, it is observed that nylon-6 has good water absorption capability because of its amide linkage.

6-10

Therefore, it has great potential to be utilized as a flexible device for

moisture sensing application by anchoring the moisture molecule using the amide linkage.7 However, the main problem is that nylon-6 is completely insulating and has no capability of conducting electrons for sensing the anchored moisture molecule. As a result, a conductor of electrons is needed in the nylon-6 polymer matrix that can transmit the signal from the absorbed moisture molecule. Since graphene has very high conductivity, is an excellent filler of a polymer matrix that improves the electrical conductivity of the polymer molecules, exhibits good absorption capability towards gas vapors,3 and by itself has good sensing capability,1-4 the combination of graphene with the

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polymer nylon-6 can be a great option for fabricating a moisture sensing device. Consequently, a combination of effects, such as (1) graphene working as an electron-conduction platform, (2) graphene working as the gas or vapor phase absorbent layer, (3) graphene itself showing good sensing capability, and (4) nylon-6 polymer also significantly working as an absorbent layer for the moisture molecules through the amide linkages, might play an important role in enhancing the sensing performance. Owing to these synergistic effects, the excellent sensing performance of this device can be harnessed. Thus, to prepare a highly flexible mechanically robust thin film of the nylon-6 sensing device, graphene is a good candidate as a filler in the nylon-6 matrix. In general, graphene has a very low dispersion concentration, less than 1 mg/ml11-17 (Table S1), although high dispersion concentration of graphene is very important in preparing graphene-polymer or other composite materials. This is because highly dispersed graphene facilitates the perfect mixing of graphene in polymers and prevents its self-aggregation, thereby enhancing the chances for better physical and chemical interactions of the polymer molecules with a higher number of graphene sheets to improve the mechanical and electrical properties. Hence, the higher dispersion concentration of graphene plays an important role in the preparation of graphene-based polymer composite films. In our previous study, we have already experienced that the high electrical conductivity and high dispersion stability both are required to prepare mechanically robust electrical conductive polymer composite film.18-21 However, to the best of our knowledge, there are few reports on significantly improving the electrical conductivity of nylon-6 and no report has yet been found that reveals its moisture sensing application. In this work, nanoparticles were utilized as an intercalating agent for the synthesis of highly dispersible graphene through a simple thrombolysis process. The produced graphene exhibited higher dispersion concentration (2.7 mg/ml) and electrical conductivity compared to those of other graphenes (Table S111-17, 22 and Table S211-12, 23-27). A highly conductive and mechanically robust graphene-nylon-6 composite film was prepared from a highly dispersed graphene solution using the in situ technique. The as-prepared flexible and mechanically robust film showed a remarkably high response towards the sensing of moisture molecules that was 100 times faster than that of the current conventional graphene sensors.3 The mechanism for this higher performance was investigated and it was suggested that the interface of polymer and graphene played a significant role in electron movement controlled by anchored moisture molecules. Indeed, molecular-level interactions are produced in the presence of water molecules with amide linkages, which generated an interfacial barrier that prevented electron movement from one graphene to another graphene surface through the graphene-nylon-6 bridge and enhanced the resistance for obtaining the sensing signals of water molecules. The proposed mechanism suggested that a synergistic effect of graphene and nylon-6 is responsible for the excellent sensing performance. The 3 ACS Paragon Plus Environment


suggested mechanism is helpful to understand the molecular-level interactions of moisture (water) molecules with the nylon-6 polymer. Moreover, Young’s modulus, tensile strength, and moisture sensing performance of the fabricated flexible composite film were measured. The composite film showed a 300% enhancement in Young’s modulus and a 10% enhancement in tensile strength in comparison to that of pure nylon-6. Such higher mechanical properties of the film suggested that the higher filler (graphene) content in the nylon-6 matrix did not reduce the interconnectivity of the polymer molecules due to the homogeneous dispersion of graphene in the polymer matrix. 2. Materials and methods 2.1. Chemicals and instruments Graphite (Alfa Aesar, graphite flakes, natural, 99.8%, 325 mesh, metal basis), zinc acetate dihydrate (99%), 1-methyl-2-pyrrolidinone (NMP; 99.5%, anhydrous), and HCl (37%) were procured from SigmaAldrich and used without additional purification. For nylon-6 polymer synthesis, 6-aminocaproic acid (99%, Sigma-Aldrich) and ε-caprolactam (99%, Sigma-Aldrich) were utilized as the initiator and monomer, respectively. For dissolving the polymer and its composite, formic acid (98%, Sigma-Aldrich) was used as the solvent. Sonication was used with a tapered microtip (VC-505, frequency 20 kHz, SONICS & MATERIALS. INC., U.S.A.) at 50% amplitude. Electrical conductivities were obtained by using a fourpoint probe (FPP-RS-8, Dasol Eng.). Atomic and structural characterizations were performed using fieldemission scanning electron microscopy (FESEM; S-4700, Hitachi), high-resolution transmission electron microscopy (HRTEM; JEM-2010, JEOL), atomic force microscopy (Dimension 3100, Digital Instrument, Veeco), and X-ray diffraction (XRD; X-Pert Powder). Raman spectroscopy (HORIBA Jobin Yvon, Lab RAM HR, Laser 514.54) and X-ray photoelectron spectroscopy (XPS; AXIS-NOVA, Kratos Inc.) were used to measure the defect and binding properties of graphene. The mechanical properties were characterized using a universal testing machine (UTM 5567A, INSTRON). Graphene disks were prepared with a CrushIR (Digital Hydraulic Press, PIKE Technology). Polymerization was achieved using a magnetic stirrer (Mighty Magnetic Stirrer, Heracles-20G, Koike Precision Instruments). Sensing performance was measured by constant current source meter (CCS-01, SES Instruments Pvt., Ltd.) and digital microvoltmeter (DMV-001, SES Instruments Pvt., Ltd.). The sensing device was reactivated by a PID-controlled oven (PID-200, SES Instruments Pvt., Ltd). 2.2. Preparation of graphene Graphene is prepared according to our previous work.28 Here, it is described in brief. Zinc acetate dihydrate (10.0 g) was dissolved in NMP (50 mL) using tip sonication for 10 min to prepare the zinc 4 ACS Paragon Plus Environment

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acetate solution (Figure S1b) in which graphite (15.0 g, Figure S1a) was dissolved through further 10 min sonication. A graphite-zinc acetate paste (Figure S1c) was prepared by vacuum drying the zinc acetate solution at 100 °C. Then, the paste was heated in a quartz glass crucible at 600 °C for 15 min to prepare a graphite-ZnO nanoparticle composite (Figure S1d). The graphene-ZnO nanoparticle (GZN) composite (Figure S1e) was prepared using tip sonication (for 2 h) with the GZN composite in NMP. GZN was treated with HCl (37%) in NMP and agitated for 5 min to remove ZnO as ZnCl2 (ZnO + HCl = ZnCl2 + H2O). The HCl-treated product was filtered to remove liquid ZnCl2, H2O, and NMP. The filtered graphene was collected and washed thoroughly with de-ionized (DI) water in order to maintain the pH at 6. The graphene were dried at 100 °C for 2 h. The dried graphene (Figure S1f) was tip-sonicated in NMP for 20 min at 50% amplitude to obtain the desired dispersion concentration (Figure S1g). The dispersion was allowed to settle down for two weeks to eliminate the sediment layer, if any. The physicochemical properties of the graphene sheets were tested by collecting the upper 80% of the dispersion.

To

understand the ZnO effect on dispersion concentration of graphene, the same procedure was done for the preparation of graphene dispersion solution without ZnO nanoparticles. 2.3. Preparation of nylon-6 A reflux system was used to melt ε-caprolactam (90 mmol) at 150 °C for 15 min in nitrogen atmosphere, before it was further heated for 10 min at the same temperature and condition after adding 6-aminocaproic acid (10 mmol). The mixture obtained was a thoroughly homogeneous solution that was polymerized at the set temperature of 260 °C for 7 h. The resulting product was very viscous after 7 h. As the supply of heat was stopped and the product cooled to room temperature, the polymer obtained was a white hard solid (nylon-6). Formic acid (75 ml) was used to dissolve the obtained hard nylon-6 sample and kept for 24 h before further stirring for 1 h. The residual monomer inside the polymer was discarded by pouring the formic acid solution of nylon-6 into hot DI water (4 L, 60 °C). The precipitate of nylon-6 started to form as the solution was poured into DI water, while the monomer remained as a liquid in DI water. For the complete removal of the monomer, the precipitated nylon-6 was further stirred for 3 h and filtered. The filtered nylon-6 was vacuum dried at 70 °C for 3 h. The nylon-6 (2.0 g) thus freed from the monomer was stirred further in formic acid (8 mL) for 10 h to synthesize a nylon-6 solution in formic acid. A bar coat film was synthesized by pouring the nylon-6 onto a commercial polyimide (PI) film. The solvent-free bar coat film of nylon-6 on the PI film was heated in a vacuum furnace at 70 °C for 3 h. Then, it was peeled off from the commercial PI film. 2.4. Synthesis of graphene-nylon-6 composite films



A magnetic stirrer equipped with a reflux system was employed for stirring ε-caprolactam (90 mmol) in nitrogen gas atmosphere at 150 °C for 15 min. Sonication for 30 min was performed for the solution of graphene sheets (40 mg) in dichloromethane (25 mL). This solution was then added to the ε-caprolactam solution to prepare a 0.5 wt% composite. This mixture was stirred further at 150 °C for 20 min, followed by the addition of 6-aminocaproic acid (10 mmol). To prepare a homogeneous solution of graphene sheets, ε-caprolactam, and 6-aminocaproic acid, the mixture was heated at 150 °C for 10 min. The temperature was increased to 260 °C and maintained for 7 h for promoting the polymerization process. Then, the graphene–nylon-6 composite film was prepared according to the synthesis method for a pure nylon-6 film. In this manner, 0.5, 1, 3, and 10 wt% graphene-nylon-6 composite films were prepared. 2.5. Sample preparation for electrical conductivity measurements 2.5.1. Graphene films The graphene solution in NMP was centrifuged at very high rpm (15000) for 3 h to separate the sediment layer. Then, the top layer of the centrifuged product was collected. It was used to prepare the graphene film by the drop-and-dry process on preheated silicon wafers at 170 °C. After dropping, it was dried at 300 °C for 3 h to remove the NMP solvent. 2.5.2. Graphene disk Acetone was used to wash the graphene powder. To remove acetone, the temperature was steadily increased to 300 °C at the rate of 5 °C/min and maintained there for 3 h. Then, it was decreased to ambient temperature at the same rate. Cleaned graphene (50-80 mg) was placed in a disk-making holder and pressed using a CrushIR instrument. A 14 US ton load was applied to the holder to prepare the graphene disk. 3. Results and discussion 3.1. Characterization of GZN composites and graphene Figure 1a reveals the FESEM image of the graphite-ZnO composite containing aggregated ZnO nanoparticles and graphite flakes (marked by arrows). Figure 1(b, c) shows the GZN composite fabricated using tip-sonication of the graphite-ZnO nanoparticle composite in NMP for 2 h. The transparent graphene (marked by arrows) is shown above and below the ZnO nanoparticles (Figure 1b). HRTEM imaging was employed to obtain the thicknesses of the graphene sheets. Figure 1e reveals four-layer graphene. On the other hand, Figure 1f indicates three- and one-layer graphene. Actually, the prepared graphene was a-few-layers graphene. Therefore, different positions show different thicknesses.


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Figure S2a reveals the XRD spectra of neat graphene and GZN composite. The pattern of GZN exhibits different plane peaks of the ZnO nanoparticles. To confirm the peaks of the ZnO nanoparticles in the GZN pattern (Figure S2a), the XRD pattern of pure ZnO nanoparticles (Figure S2b) is also shown, which exhibited different ZnO plane peaks. The positions of all these ZnO nanoparticle peaks matched those of the ZnO nanoparticle peaks of the GZN pattern (Figure S2a). It further suggests that the GZN material contains ZnO nanoparticles. Furthermore, energy-dispersive X-ray (EDX) spectroscopy patterns (Figure S2c) were obtained that show the presence of Zn, O, and C in the GZN composite, which are ascribed to ZnO and graphene. The graphene pattern displays peaks only corresponding to graphene (Figure S2a). Therefore, it confirms that the ZnO nanoparticles were completely removed from the graphene due to the addition of HCl. Bath sonication was used to dissolve ZnO (as aqueous ZnCl2) after treating the GZN composite with HCl. DI water was used for filtering this solution to ensure a pH of 6. Then, the filtered graphene sheets from the solution (Figure S1g) were dried at 100 °C for 3 h. The dried graphene was placed on a silicon wafer for FESEM imaging (Figure 1d) of the graphene sheets. The image obtained shows the thin layered structure of graphene. 3.2. Preparation of highly dispersed graphene solution The graphene sheets fabricated from GZN were dissolved in NMP using tip sonication for 20 min at the rate of 50% amplitude (Figure 2a). The precipitation of thicker graphene sheets occurred in two weeks. The concentration and thickness of the graphene sheets were analyzed by obtaining the upper 80% portion of the precipitated graphene sheets. The concentration was found to be 2.73 mg/mL, and remained stable even after three months (Figure 2b). Then, it was allowed to settle down for a further 1 year to observe the apparent solution color. It was apparently very dark. Subsequently, a centrifugation test was performed at 15000 rpm for 3 h and the solution still remained very dark following the test. UV-vis spectroscopy was performed in order to test the stability of the dispersed solution. The upper 80% portion (Figure 2b) was used to prepare solutions of various concentrations of graphene sheets. UV-vis spectroscopy (Figure 2c) was performed to also analyze their absorption characteristics. All the solutions consistently absorbed at ~270.5 nm, which might be ascribed to the π-π* transition of the C=C bond. The concentration versus absorbance intensity plot (Figure 2d) showed a straight line with absorption coefficient α = 2272, which is in good agreement with the Beer-Lambert law, suggesting that the dispersed graphene sheet was highly stable.29-30 The concentration obtained in this work was compared with other previously published methods (Table S1), and found to be higher.11-17, 22 This might be because of the settling of ZnO nanoparticles between the graphene layers during sonication, resulting in the formation of a barrier for the self-aggregation of the graphene sheets. In addition, the ZnO nanoparticles obtained by heating at 600 °C may lead to the thermal oxidation of carbon at the defect sites.31 The ZnO 7 ACS Paragon Plus Environment


nanoparticles might be playing the role of a catalyst in this thermal oxidation process, resulting in a more effective exfoliation of graphite. Hence, the number of graphene layers exfoliated in the NMP solvent increased considerably through the continuous exfoliation from graphite. To verify the role of ZnO nanoparticles for the exfoliation of graphite, graphite was exfoliated without ZnO nanoparticles in NMP solvent by maintaining same condition. For this case, the dispersion concentration (1.1 mg/ml) of the graphene was very lower compared to the graphene dispersion concentration (2.7 mg/ml) with ZnO nanoparticles. It confirms that the ZnO nanoparticles help for the exfoliation of graphene at high concentration. Actually ZnO nanoparticles work as exfoliating agent and it reside between the layers of graphene and prevent their parallel aggregation in the solution phase. ZnO nanoparticles work as barrier layer to prevent the aggregation of graphene. In the absence of ZnO nanoparticles, graphene were exfoliated and immediately many of them were aggregated by Van der Waals attraction force due to the absence of barrier layers between the exfoliated graphene. 3.3. Oxygen-containing functional groups in graphene sheets Figure 3a shows the C1s XPS patterns of graphene and graphite. The C1s spectrum of the graphene sheets at 284.6 eV is the characteristic peak of the C-C bond of sp2 carbon in carbon materials.32-34 The intensity of the circular marked area (around 290 eV) of graphene is higher than that of graphite, suggesting the existence of oxygen-containing functional groups on the graphene surfaces. To determine the types of the oxygen-containing functional groups, the C1s pattern of graphene was deconvoluted (Figure 3c) and the summation (sum) curve of the deconvoluted curves plotted with the C1s original pattern (nondeconvoluted curve), shown in Figure 3b. The sum curve was perfectly fitted or superimposed with the C1s pattern (original). It implies that deconvolution was carried out perfectly. The deconvoluted pattern in Figure 3c contains peaks at 287.2, 285.9, and 285.1 eV, which are related to the C=O, C-O-C, and C-OH functional groups, respectively.32-34 The peak at 288.8 eV arises from the O=C-O functional group. Furthermore, the peak at 290.3 eV (π-π*) is due to the delocalized π electrons of the sp2 C=C bonds of an aromatic system. These peaks (C=O, C-O-C, O=C-O, and C-OH) confirm that the graphene sheets have oxygen-containing functional groups. For further confirmation, the Raman spectrum (Figure 3d) of the graphene sheets was recorded, which contains a D band. This D band indicates the presence of defects in the graphene sheets. The graphene defects could be present in the form of discontinuous conjugated double bonds or functional groups such as –OH, C-O-C, –COOH, and C=O, which have already been confirmed using XPS. Therefore, both XPS and Raman spectra (Figure 3c, d) confirmed the presence of different types of oxygen-containing functional groups on the graphene surfaces.


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3.4. Electrical and mechanical properties A four-point probe was used to obtain the electrical conductivities of the graphene disk and a graphene film. They were measured to be 40,450 and 34,580 S/m for the graphene disk and film, respectively. These electrical conductivities were found to be higher than those of the graphene sheets fabricated using Table 1.. Mechanical and electrical properties of various composite materials of the nylon-6 polymer.

Young’s

Tensile Tensile

Electrical

modulus

strain% strength

conductivity

(GPa)

(MPa)

(S/m)

Filler materials (wt%)

1.79

12.5

95.9

-

GO (1)35

1.79

133

59

-

CNT-PBA (1)36

~1.60

~164

~65

-

CNT-Clay (1)37

0.85

125

40.3

-

MWCNT (1)38

0.79

350

108.8

-

CNT-CONH2 (4.2)39

0.77

249

68.8

-

H-MWCNT (1)40

0.72

269

123

-

Graphene (0.1)41

0.65

-

92.7

-

SWNT (0.2)42

0.9-2.4

-

-

-

Graphene (0.1-1.0)43

3.00

45

109.3

4.1 × 10-6

Graphene (3) (Current work)

4.00

3.4

75.8

0.24

Graphene (10) (Current work)

CNT: carbon nanotube, CNT-COOH: CNT-containing -COOH group (6.8%), CNT-CONH2: CNT-containing a CONH2 group (4.2%), CNT-PBA: polybutyl acrylate encapsulated CNT, H-MWCNT: hexadecylamine functionalized multiwalled CNT, MWCNT: multiwalled CNT. several other techniques (Table S2).11-12,

23-27

These highly conductive graphene sheets were combined

with pristine nylon-6 to prepare electrically conductive graphene-nylon-6 films (Figure 4). The pure nylon-6 film is shown in Figure 4a, while the other (0.5, 1.0, 3, and 10 wt%) graphene-nylon-6 composite films are shown in Figure 4(b-e), respectively. To inspect the interactions between graphene and nylon-6, the tensile strength and Young’s modulus are shown in Table S3, and the corresponding tensile strength versus strain % curves of the pure nylon-6 and composite films are displayed in Figure 4f. The mechanical properties of the composite films in this work are compared with those of the nylon-6 composites reported in the literature (Table 1).31-39 The low electrical conductivity (6.5 × 10-12 S/m) of nylon-6 suggested that it was practically an insulator (Table S3). Hence, making it electrically conductive remains a challenge. To the best of our knowledge, the improvement in the electrical conductivity of 9 ACS Paragon Plus Environment


nylon-6 has been reported poorly in the literature. In this work, we present a method to improve the electrical conductivity of nylon-6. It is observed that 3 wt% graphene in the composite enhanced the electrical conductivity of nylon-6 from 6.5 × 10-12 to 4.1 × 10-6 S/m (~6 orders of magnitude increase). Graphene up to 10 wt% was added for additional increments in the electrical conductivity of the nylon-6 composite film. The electrical conductivity of the 10 wt% graphene-nylon-6 composite film (Figure 4e) is found to be 0.25 S/m (Table S3). It shows 13 orders of magnitude higher electrical conductivity than that of the pure nylon-6 film, suggesting that graphene is well dispersed in the composite structure. The dispersion of graphene in nylon-6 matrix was also observed in the FESEM image of 10% graphene– nylon-6 composite films. The low resolution FESEM image (10 wt%) film (Figure S3a) showed smooth surface of the composite film. The comparative high resolution image (Figure S3b) also showed smooth film in which graphene is visualized. To see more clear images of graphene and nylon-6, Figure S3c and S3d were taken from the rectangular and circular area of the Figure S3b, respectively. Figure S3c exhibited the smooth dispersed surfaces of graphene. Similarly, Figure S3d displayed clearer dispersed graphene sheet like flat in which nylon-6 was also visualized (marked by arrow). So, 10% and 300% enhancements in the tensile strength and Young’s modulus, respectively, suggest a strong interfacial interaction between graphene and nylon-6. It can be observed that the 3 wt% nylon-6 composite film (Figure 4d) shows a tensile strength (109 MPa) (Table 1) that is remarkably high, with good Young’s modulus (3 GPa). The mechanical properties of the graphene-nylon-6 composite (3 wt%) presented in this work are found to be better than those of the nylon-6 composite films reported by other researchers (Table 1). However, high conductive graphene-nylon-6 is required for sensing application. So, to maintain high electrical conductivity, 10 wt% graphene-nylon-6 composite films were prepared. Even though 10% graphene was used in the nylon-6 matrix, the prepared composite film showed higher mechanical properties than that of the pristine nylon-6 film. 3.5. Reason for the enhancement in electrical and mechanical properties The enhanced mechanical properties of the graphene-nylon-6 composites can be attributed to the hydrogen bonding between the graphene and the terminal and backbone functional groups of nylon-6 (Figure 5b).44 XPS analysis clearly indicated that the graphene sheets contain oxygen-containing functional groups (C=O, C-O-C, COOH, and C-OH) (Figure 3c). The D band was observed in the Raman spectrum of the graphene sheets, suggesting the presence of defects in the sheets (Figure 3d). This possibly resulted in conjugated double bonds with discontinuities or functional groups such as –OH, – COOH, C-O-C, –COOH, and C=O. However, the oxygen-containing functional groups (–OH, –COOH) may result in the formation of hydrogen bonds (indicated by dotted lines) between the functional amide linkages and the terminal functional groups of nylon-6 (Figures 5b). In addition, new amide linkages 10 ACS Paragon Plus Environment

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(indicated by red arrows) between the terminal –NH2 functional groups of nylon-6 and the –COOH functional groups of graphene were formed. Possibly, owing to the formation of hydrogen bonds44and amide linkages, the nylon-6 molecules were tightly bounded, resulting in robustness against mechanical fracture under a tensile load and high modulus of elasticity. Consequently, Young’s modulus increased dramatically up to 10 wt% graphene and tensile strength also significantly increased up to 3 wt%. Pure nylon-6 is completely insulating. Its electrical conductivity (6.5 × 10-12 S/m) suggests that it has no capability of conducting electrons. Graphene works as the building blocks for electron movement in the nylon-6 polymer matrix, as shown in Figure 5c. Here, electrons move from one graphene sheet to another inside the nylon-6 polymer matrix. The red dotted arrows in the figure are shown to display the electron conduction platform. Up to 3 wt% graphene, the electrical conductivity improved to 4.1 × 10-6 S/m. It indicates that the amount of graphene is not sufficient to connect the sheets for the conduction of electrons in the nylon-6 matrix. When graphene was added up to 10%, the electrical conductivity improved to 0.25 S/m. This suggests that the amount of graphene is sufficient to overlap or connect the sheets for the conduction of electrons (Figure 5c). Figure 5(b, c) suggests a clear scheme of the interfacial interactions of nylon-6 molecules and graphene through the amide linkages and hydrogen bonds. The well-connected interfaces (Figure 5b) play a significant role in preventing fracture under a load through the hydrogen bonds and amide linkages, thereby increasing the mechanical properties. Similarly, the wellconnected interface (Figure 5c) helped the graphene sheets to maintain contact with each other so as to act as an electron-conduction bridge or channel through the amide linkages of the nylon-6 molecules and the hydrogen bonds inside the polymer matrix, which enhanced the electrical conductivity. 3.6. Moisture sensing application The moisture sensing device preparation circuit is shown in Figure 6a. The four probes, 1, 2, 3, and 4, were placed on the graphene-nylon-6 composite film to fabricate a complete sensing device (Figure 6a). A current (I) was applied to probe-4 and the output current (I) was collected through probe-1. The resulting output voltage (V) was measured between probes 2 and 3. The current was controlled by a direct current source meter and the voltage measured by a microvolt meter. The real device of the graphenenylon-6 composite is shown in Figure 6b, in which the four probes, 1, 2, 3, and 4, were connected to the 10 wt% graphene-nylon-6 film and the input and output current directions are shown. The sensing device was placed in a compact chamber, where a humidity meter was connected to control the artificially generated humidity (moisture). A humidifier was used to generate the moisture, and it was also connected to the chamber. A constant current was applied through probes 4 and 1 and humidity levels of 23, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, and 80% were generated artificially. Then, the corresponding voltages were measured. Finally, the different voltages obtained at the different humidity levels were converted to 11 ACS Paragon Plus Environment


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resistivities (Ω.cm) using equation (1) and the electrical conductivity values (Table 2). The measured resistivities and electrical conductivities were plotted as functions of humidity (Figure 6c). Resistivity= (V/I) × π × W × (1/ln2),

(1)

where I =Applied current, V= output voltage, π= 3.14, and W= thickness of the film. Table 2 and its corresponding three-dimensional plot (Figure 6c) indicate that the resistivity was significantly increased with increasing humidity. Alternatively, the electrical conductivity significantly decreased with increasing (%) humidity (Figure 6c). However, the pure nylon-6 polymer film does not show any sensing signal for the moisture molecules, which are not shown in this figure. Therefore, the excellent sensing response of the graphene-nylon-6 composite film suggests that graphene played a significant role in electron conduction inside the nylon-6 polymer matrix and that this electron conduction could be controlled using different humidity levels. Table 2. Moisture sensing performance.

Humidity (%)

Sensing parameter in

Corresponding

resistivity (ohm.cm)

conductivity (S/cm)x10-5 (second)

23

546.0

183

0

25

568.8

176

10

30

582.4

172

20

35

591.5

169

30

40

600.6

166

40

45

615.7

162

50

50

618.8

162

60

55

628.0

159

70

60

632.5

158

80

65

637.1

157

90

70

655.3

153

100

75

664.4

151

110

80

682.6

146

120


Response Time

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Figure 7 is used to explain the excellent sensing mechanism of the graphene-nylon-6 composite film in the presence of humidity. It reveals that the internal architecture of the graphene-nylon-6 composite film and the absorption mechanism of the moisture (water) molecules. We know that several Table 3. Reversibility test at 23% humidity. Applied

Retain sensing parameter in

Retain corresponding

Response Time

temperature

resistivity (ohm.cm)

conductivity (S/cm)x10-5

(second)

24

682.6

146.4

0

30

673.5

148.4

5

32

664.4

150.5

10

34

655.3

152.6

15

36

646.2

154.7

20

38

637.1

156.9

25

40

632.5

158.0

26

42

623.4

160.3

27

44

614.3

162.7

28

46

609.7

163.9

29

48

600.6

166.4

30

50

591.5

169.0

31

52

582.4

171.6

32

54

564.2

177.2

33

amide linkages are available in the backbone chain of nylon-6 (polyamide). It has already been described using Figure 5(a, b) that additional new amide linkages are formed with the –COOH functional groups of graphene and the terminal –NH2 functional groups of nylon-6. At the initial stage, the moisture (water) molecules are deposited on the surface of the graphene-nylon-6 composite film. Within 10 s (Table 2, last column), some moisture molecules were absorbed through the amide linkages (Figure 7b) and reached the junction point of the electron-conducting graphene platform, as shown in Figure 7c. As a result, electron movement from one graphene to another through the junction point was hindered by the moisture molecules lodged at the point that created a resistive signal. When the humidity was increased from 23 to 25, 25 to 30, 30 to 35, 35 to 40, 40 to 45, 45 to 50, 50 to 55, 55 to 60, 60 to 65, 65 to 70, 70 to 75, and 75 to 80% in consecutive 10 s intervals, the moisture molecules were continuously deposited on the composite film surfaces, and the deposited molecules penetrated the graphene-nylon-6 polymer matrix 13 ACS Paragon Plus Environment


through absorption involving the amide linkages; finally, they were lodged at the graphene junction point, thereby producing sensitive signals every 10 s. Therefore, only 10 s were required for the moisture molecules to produce signals in all the steps. Generally, graphene required a higher response time (400 s) for the absorption of vapor and producing the signal.3 In our case, the shortest time (10 s) indicates that the amide linkage works excellently towards the absorption of water molecules with the increase in humidity in each step. This behavior suggests a highly sensitive graphene-nylon-6 composite film for a moisture sensor. Thus, the sensing was 100 times faster than that of the current conventional sensors.3 Previous studies have reported the absorption of water molecules through amide linkages6-10 for measuring the absorption capability. Therefore, our novel sensing mechanism involving amide linkages is scientifically meaningful and has been well supported by previous research work. Actually, the interface of graphene and nylon-6 provides room for water molecule lodging through the amide linkage window, and the same windows are used to remove the water molecules, which is described below in the reversibility section. The observed sensing behavior indicates that the superb sensitivity of the nylon-6graphene composite ﬁlm might make it a potential candidate for moisture sensing application. 3.7. Interface chemistry of moisture molecule absorption How can absorption occur through the amide linkages of the nylon-6 polymer? To find this out, the interface chemistry of the moisture molecules and the amide linkages of the nylon-6 polymer were studied and two reaction mechanisms were proposed in Figure 9. In mechanism-1 (Figure 9a), the carbonyl oxygen of the amide linkage detached its (C=O) bonding electron (product-1), simultaneously forming negative and positive charges on the oxygen and carbon of the carbonyl group (product-2), respectively, in the presence of moisture molecules. Then, the lone pair electrons of the moisture molecules attract the positive charge of the carbonyl carbon (product-2) and product-3 is formed by bond formation between the oxygen of the moisture molecule and the carbonyl carbon of the amide linkage. In this case, the lone pair of electrons of the oxygen of the water molecule is used for bond formation. Therefore, a positive charge is developed on the oxygen of the water molecule, which is then neutralized by receiving the adjacent O-H bond electron (product-3) through oxygen. Since oxygen received the O-H bond electron, product-4 and a proton are produced. The proton produced during the formation of product-4 is attracted by the negative charge of the oxygen of the carbonyl carbon of product-4, and product-5 is formed. Finally, product-5 is converted to product-1 by releasing one molecule of water. The free water molecules are lodged at the graphene junction point, and this is shown in Figure 7c. In mechanism-2 (Figure 9b), in the presence of moisture molecules (water), the oxygen of the carbonyl carbon of the amide linkage detached its electron (product-1) and produced a negative charge on


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it and a positive charge on the nitrogen (product-6). The lone pair electron of the moisture molecule is attracted by the positive charge of the nitrogen of the amide linkage (product-6) and a new bond (N-O) is produced between the nitrogen and the oxygen of the water molecule in product-7. In this case, a positive charge developed on the oxygen of the water molecule is neutralized by detaching the electron of the adjacent OH bond to form product-8. Product-8 is converted to product-9 by releasing the –OH group. During the release of the –OH group, a positive charge is developed on the oxygen of the carbonyl group (product-9). Finally, product-9 is converted to product-1 by releasing one molecule of water. In this case, the positive charge of the oxygen of the carbonyl group is neutralized by receiving the adjacent O-H bond electron. Consequently, a proton is produced that is immediately attached to the –OH group to release one molecule of water. Now, the free water molecules lodged at the graphene junction point, which is also shown in Figure 7c, inside the nylon-6 polymer matrix generate a barrier for electron movement. This barrier gives rise to sensing signal in the form of resistivity. 3.8. Reversibility of the sensor To reactivate the sensing device, the heating and temperature-controlled systems were connected to it. Then, temperature was increased from 24 to 54 °C using a PID-controlled oven and monitored. With increasing temperature, the corresponding resistivities, conductivities, and required response times were calculated (Table 3). Only 33 s were required to return to the initial active state. The initial conductivity and resistivity were determined to be 183 × 10-5 (S/cm) and 546 Ω.cm, respectively (Table 3). After reactivation, they were 177 × 10-5 S/cm and 564 Ω.cm, respectively, which are very close to the initial conductivity (183 × 10-5 S/cm) and resistivity (546 Ω.cm) values. Figure 8a shows the three-dimensional curve between resistivity, conductivity, and the increase in temperature. It shows a decrease in resistivity with increasing temperature. Alternatively, it reveals an increase in conductivity with increasing temperature. Therefore, the behavior of the curve indicates that the electron movement barrier is reduced continuously up to 54 °C during the interval of increasing temperature. It also implies that the movement of electrons increased in the graphene-nylon-6 composite. There is a high possibility that the deposited water molecules lodged at the junction point of graphene through the amide linkage are removed from the composite film during the heating time. Therefore, the resistance is reduced continuously from 24 to 54 °C. If we consider the response time (Figure 8b), it indicates that the resistivity started to decrease dramatically at 25 s, and similarly, the electrical conductivity started to increase dramatically at 25 s. It is also observed that the critical temperature (38 °C) at which the response time was 25 s is highly effective for the noticeable commencement of the elimination of water (moisture) molecules that continued up to 54 °C (Table 3 and Figure 8c). Moreover, when exposed to high humidity levels (80%), the graphene15 ACS Paragon Plus Environment


nylon-6 film was found to be very much stable against the permanent loss of sensitivity, and was reactivated in a very short time (33 s), proving its excellence in terms of its reversibility as a moisture sensor. 4. Conclusion Highly dispersed and conductive graphene was synthesized and used to prepare mechanically robust graphene-nylon-6 films using an in situ process. The prepared polymer composite film showed very good electrical conductivity. The fabricated composite film also showed excellent sensing performance, quick response, as well as excellent reversibility. A novel mechanism was proposed to explain the excellent sensing performance in terms of the molecular interfacial interaction of the amide linkages. It suggested that a synergistic effect was responsible for the excellent sensing performance. Highly dispersed and stable graphene sheets were obtained from the GZN composites. Furthermore, it was used for the synthesis of graphene-nylon-6 composite films for sensing moisture under ambient conditions. Moreover, the film exhibited outstanding Young’s modulus and good tensile strength. The mechanism for the superb mechanical and electrical properties of the composite film was presented, which also suggested that the interfacial molecular interaction was the main cause for the enhanced mechanical properties.

AUTHOR INFORMATION Corresponding Author [email protected] [email protected] Supporting Information (SI) The Supporting Information is available free of charge on the ACS Publications website. Synthesis scheme of graphene (Figure S1), XRD spectra of graphene and graphene-ZnO nanoparticle composite (GZN) (Figure S2), FESEM images of the 10 wt% graphene-nylon-6 composite film (Figure S3), comparison of the dispersions of graphene sheets prepared from graphite by various methods (Table S1), electrical conductivities of various graphenes (Table S2), and electrical and mechanical properties of the graphene-nylon-6 polymer composite films (Table S3). Conflicts of Interest The authors declare no competing financial interest. Acknowledgements 16 ACS Paragon Plus Environment

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Figures

Figure 1. (a) SEM image of graphite-ZnO nanoparticle composite, (b, c) SEM images of graphene-ZnO nanoparticles, (d) graphene on silicon wafer substrate, which is prepared from the graphene-ZnO nanoparticle composite after the separation of ZnO nanoparticles, (e) HRTEM image of graphene, in which four-layers graphene is shown as the inset for the rectangular area, and (f) HRTEM images of mono- and three-layers graphene.


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Figure 2. Graphene solution and UV-visible spectra of graphene. (a) Graphene solution prepared by 20 min sonication, (b) the top 80% graphene solution collected from the original solution (a) after two weeks of settling down, (c) the UV-visible absorption curves of different known concentrations of the graphene solution shown in Figure a, and (d) absorbance vs concentration curve showing a straight line.



Figure 3. XPS of C1s core level, its deconvolution, and Raman curve. (a) Binding energies of the C1s core levels of graphene and graphite, (b, c) deconvolution curves of the C1s core level spectra of graphene, and (d) Raman spectrum of graphene.


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Figure 4. Polymer composite films and the curves of their mechanical properties. (a) Photograph of a pure nylon-6 film, (b, c, d, e) photographs of nylon-6-graphene composite films with 0.5, 1.0, 3.0, and 10.0 wt % graphene, respectively, and (f) tensile strength vs strain curves of pure nylon-6, 0.5, 1.0, 3, and 10 wt% graphene composite films.



Figure 5. Mechanism of enhancement of the mechanical and electrical properties of graphenenylon-6 composite films. (a) a 10 wt% graphene-nylon-6 composite film, (b) internal architecture of the composite film in which the hydrogen bonds (dotted lines) between graphene and nylon-6 molecules are formed through the amide linkages, and further new amide linkage (indicated by red arrows) formation with the –COOH functional groups of graphene and the terminal –NH2 functional groups of nylon-6, and (c) electron conduction inside the graphenenylon-6 composite film.


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Figure 6. Sensing device and its performance curve. (a) Scheme of the sensing device circuit using a four-point probe system, (b) photograph of a real sensing device in which the four probes are marked 1, 2, 3, and 4, and (c) sensing performances shown by three-dimensional plots of resistivity and electrical conductivity for different humidity levels.



Figure 7. Moisture sensing mechanism. (a) Graphene-nylon-6 polymer composite film in the presence of moisture (water) molecules, (b) moisture molecules absorbed by the amide linkages of the graphene-nylon-6 composite film, in which nylon-6 provides the amide linkages, and (c) the absorbed moisture molecules penetrating the composite film and lodging between the junction of two graphene sheets to produce a resistance to electron movement.


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Figure 8. Reversibility test curves. (a) Three-dimensional plots of resistivity, temperature, and conductivity of graphene-nylon-6 polymer composite film, (b) change in resistivity and conductivity according to response time, and (c) change in resistivity with increasing temperature, and its corresponding response time.



Figure 9. Reaction mechanism of moisture molecules absorption. (a) Water molecules absorption reaction mechanism-1 in which water molecules absorption, desorption, finally again absorption at the junction point of graphene through the steps-2, 3, 4, and 5, and (b) water molecules absorption reaction mechanism-2 in which water molecules absorption, desorption, and finally again absorption at the junction point of graphene through the steps- 6, 7, 8, and 9, respectively.


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“TOC Graphic”


Synergistic Effect in Moisture Sensing of Nylon-6 Polymer Films

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