2.1. Synthesis
2.1.1. Chemicals and Materials
Poly-phenylene terephthamide (PPTA) fibers were bought from Du Pont Holding Co. Ltd., USA. Potassium hydroxide (KOH, AR) and dimethyl sulfoxide (DMSO, 99%) were used to prepare ANF, which were purchased from Kermel company and Alfa Aesar Chemical Co. Ltd., China, respectively. Pyrrole (Py, AR) and iron (Ⅲ) chloride hexahydrate (FeCl3.6H2O) were provided by Shanghai Aladdin Biochemical Technology Co. Ltd., China. All chemicals were analytical grade and used without purification. Ethanol absolute was obtained from Tianjin Carton Company, China. Hydrochloric acid was purchased from JDTZ Company. Nylon membranes with a pore size of 0.2 µm were used to fabricate composite film via a vacuum filtration technique and purchased from JINTENG Company, China.
2.1.2. Preparation of ANF solution
ANF was prepared by deprotonation of PPTA fibers according to the previously reported method in literature.[28, 29] Firstly, PPTA fibers of 1.0 g and KOH of 1.5 g were slowly added to 480 mL DMSO under magnetic stirring. Subsequently, 20 mL deionized (DI) water was added into the above mixed solution and continuously magnetically stirred for 4 h at a room temperature. An evenly, crimson and viscous solution was prepared due to the deprotonation of PPTA fibers. Afterwards, ANF was obtained after alternately cleaning with ethanol absolute and DI water by vacuum filtration technique until the pH of the solution was 7. Finally, pre-prepared ANF was dispersed again in DI water to get ANF suspension of 2.0 mg mL-1 for further fabricating a ANF based composite film.
2.1.3. Preparation of ANF@PPy composite films
ANF was successfully coated by PPy via In-situ polymerization technique and then was used to synthesize ANF@PPy composite films via the vacuum filtration technique following a pressing method. The schematic illustration of preparation process for ANF@PPy composite films was presented in Fig. 1. The brief description of preparation process for ANF@PPy films was as follows: Firstly, a certain amount of Py was added to ethanol absolute and then stirred for achieving a uniform dispersibility of Py. Py solution was added to the pre-prepared ANF solution (2.0 mg mL-1) of 10 mL and then continuously magnetically stirred for 5 min to achieve a uniform dispersibility of Py in ANF suspension, marked as solution A. 0.06 mol L-1 FeCl3.6 H2O was dispersed in 2 mol L-1 hydrochloric acid solution, respectively, named as solution B. B was slowly dripped into solution A under the ice water bath for initiating the polymerization of Py, during which the magnetically stirred for 6 h. Thus, PPy was successfully polymerized and covered on surface of ANF for forming ANF@PPy suspension. After that, ANF@PPy was alternately washed by a vacuum filtration technique with ethanol absolute and DI water for removing acid and other impurities. Finally, ANF@PPy was dispersed in DI water and then sonicated for 15 min using an ultrasonic cell crusher instrument in order to obtain the smooth composite film. ANF@PPy composite films were prepared from ANF@PPy suspension via using a vacuum filtration technique and then dried by a freeze-dried for 1 day. ANF@PPy films were pressed by two plates at 20 MPa for 5 min in order to enhance the connection. In this manuscript, the resultant ANF@PPy composite films were prepared according to the different amounts (0.01 g, 0.02 g, 0.03 g, 0.04 g) of PPy polymerization in ANF, which were named as ANF@PPy-1, ANF@PPy-2, ANF@PPy-3, ANF@PPy-4, respectively.
2.1.4. Characteristic
The morphology of ANF@PPy composite films were characterized by transmission electron microscopy (TEM, Talos F200×, Japan)) and a filed-emission scanning electron microscope (SEM, S-4800, Hitachi, Japan). The micro-confocal Raman spectrometer (LabRAM HR Evolution, Horiba, French) and Fourier transform infrared (FTIR) spectroscopy (EQUINOD55X, Bruker, USA) are used to evaluate the chemical structure of materials. Thermogravimetric analysis (TGA) was performed under an argon atmosphere from 25 to 1000°C at a heating rate of 10°C min− 1, which is collected from (TGA2050, America). The composite films are cut into a rectangular stripe of 10 × 30 mm2 and their mechanical strength is measured under an extension rate of 5 mm min− 1 via a tensile testing machine (ZQ-990 L-9, ZHIQU, China). The electrical conductivity of film was measured with four-point probes method (HP504, China). EMI shielding performance of ANF@PPy composite films were evaluated by using a vector network analyzer (VNA, AV3672D, Ceyear, China) with waveguide of C band (6–8 GHz), X band (8–12 GHz), Ku band (12–18 GHz), K band (18-26.5 GHz). Electromagnetic scattering parameters (S11, S21, S12, S22) were collected by VNA and then used to calculate SER (reflection SE), SEA (absorption SE), SE (the total EMI SE) for evaluating the EMI shielding performance of ANF@PPy film. The dynamic resistance signal of ANF@PPy film sensor was recorded during the human motions by LCR digital bridge (TH2830, TongHui, China). The relative resistance change was used to evaluate the wearable sensing performance of ANF@PPy film. The experiments involving human subjects had been conducted with the full, informed consent of the volunteers. For Joule heating performance, resultant ANF@PPy composite films were cut into rectangular strips of 10×30 mm2. In addition, the temperature of film was recorded under different voltages by an infrared thermal image camera (PI 400, Optris, Germany). For photothermal conversion performance, the applied area of films was π × 25×25 mm2 and sunlight simulator (PLS-SXE 300, Perfect, China) was used for providing the light irradiation.
2.2 structure and characterizations
ANF is firstly coated by PPy via an in-situ polymerization technique and then ANF@PPy solution as precursor is used to obtain film via a vacuum filtration followed a mechanical pressing. The schematic diagram of preparation process for ANF@PPy films is displayed in Fig. 1. ANF@PPy prepared via an in-situ polymerization is more stable and uniform than the infiltration or mixed ANF/PPy films. [30], which is beneficial to improve physical properties of composite films. Morphologies of ANF and ANF@PPy composite are characterized via using TEM and SEM, as shown in Fig. 2. As shown in TEM image of Fig. 2a, ANF was successfully fabricated by a deprotonation of PPTA and intertwined with each other, its diameter is about 20 nm. The introduction of DI water as a proton donor can effectively decrease the deprotonation time of PPTA to 4 h, which can be served to reduce an inherent viscosity of PPTA, add the electrostatic repulsion for chain expansion and promote the recovery of hydrogen bonds between ANF.[17] After the in-situ polymerization of PPy on surface of ANF, TME images shown in Fig. 2b and 2c indicate that ANF had been coated PPy and core-shell structure forms due to the introduction of PPy. In addition, it is obvious that PPy is polymerized on surface of ANF and the diameter is enlarged, as shown in Fig. 2c. The thickness of PPy shell layer in ANF@PPy-4 is in range of 15–20 nm. As shown in Fig. S1 and Fig. 2c, the diameter of ANF@PPy increase with increasing the amount of Py monomer due to more PPy polymerization. Surface morphologies of ANF and ANF@PPy composite films are observed by SEM and SEM images are presented in Fig. S2 and Fig. 2d. The surface of ANF@PPy-4 composite film becomes denser and smoother than that of ANF film because large amount of PPy is coated on the surface of ANF. Meanwhile, SEM image with high-magnification for ANF@PPy-4 composite film is presented in Fig. 2e, indicating that ANF is intertwined with each other which is consistent with TEM images shown in Fig. 2b. Moreover, the mechanical pressing can improve the densification and interconnect between ANF. So, the cross section of ANF@PPy-4 composite film is shown in Fig. 2f. After mechanical pressing under 10 MPa via two plates, there is no obvious delamination found in cross section SEM image of ANF@PPy-4 composite film which is beneficial for connect between ANF@PPy. Thus, mechanical pressing not only reduces the overall thickness of composite films but also improve the electrical conductivity of film due to the intertwine connection. Those are beneficial for improving EMI shielding performance, Joule heating, photothermal conversion and wearable sensor for human movements due to the construction of the intertwine transmission network for electron and heating and high flexibility.
Raman spectra and FTIR spectra are collected to characterize the structure of ANF and ANF@PPy composite films, as shown in Fig. 3a and 3b. ANF is prepared via the deprotonation of PPTA fibers via using the DMSO and potassium hydroxide. Compared with Raman of PPTA fibers, there is no change in Raman of ANF shown in Fig. 3a, indicating that the structure of PPTA was not destroyed during the deprotonation process. Characteristic peaks located at 1354 cm− 1 and 1565 cm− 1 are obvious in Raman of PPy and ANF@PPy-4 composite film (as shown in Fig. 3a), which are corresponding to the C-C backbone stretching vibration and ring stretching vibration of PPy, respectively [31]. In addition, the presence of PPy shell layer on surface of ANF is further confirmed by FTIR spectra and TGA. As shown in Fig. 3b, the typical characteristic peaks of PPy are presented in FTIR spectrum of ANF@PPy-4 composite film, which are located at 1038, 1451 and 1541 cm− 1 corresponding to the C-H in-plane vibration, C-N scaling and ring peak of PPy, respectively.[30, 31] In addition, peak located at 1643 cm− 1 comes from the C = O stretching vibration of ANF, which has a blue shift in comparison with the FTIR spectrum of pure ANF due to the formation of H-bond between ANF and PPy.[30, 31] The combination of strong H-bond can offer outstanding mechanical properties of ANF@PPy composite film. Moreover, TGA is carried out for further confirming the existence of PPy in ANF@PPy film and studying the thermal stability of ANF@PPy. In comparison with TGA curve of ANF, mass of ANF@PPy composite film stars to lose at a lower temperature owing to the weaker thermal stability of PPy as shown in Fig. 3c. Raman, FTIR spectra, TGA and TEM results indicate that ANF coated by PPy had been synthesized.
Optical image of ANF@PPy-4 composite film is shown in Fig. 3d and obtained via filtering ANF@PPy suspension, which represents a uniform film with a large transverse dimension and turned black after introduction of PPy. As shown in inset of Fig. 3d, ANF@PPy film can be folded into an airplane and show any structure failure due to an excellent mechanical flexibility of composite film. In addition, the tensile stress-strain curves of ANF@PPy composite films are collected by a tensile testing machine for evaluating the mechanical properties of composite films, as shown in Fig. 3e. The tensile strengths of ANF@PPy composite films increase from 88 MPa to 132 MPa with increasing the additive amount of Py. The increasing trend of tensile strength might be attributed to the in-situ growth PPy on surface of ANF making composite film denser, which enhances the mechanical properties of ANF@PPy composite films. It is obvious that ANF@PPy-4 composite film exhibits shorter fracture strain at 5.2% and tensile strength of 132 MPa. It is easy to clearly and intuitively observe the changing trend of tensile strength and fracture strain of ANF@PPy composite films summarized in Fig. 3f, indicating that the increasing content of PPy comprehensively enhance the mechanical performance.
2.3. Electrical conductivity and EMI shielding performance
Due to the mechanical flexibility of ANF and high electrical conductivity of PPy, flexible ANF@PPy composite films can be used as EMI shielding materials. EMI SE is relationship with electrical conductivity according to Simon’s formalism as following:[32, 33]
$$SE=50+10\text{log}(\sigma /f)1.7t\sqrt{\sigma f}$$
1
where σ (S m− 1), t (cm) and f (MHz) are the electrical conductivity and thickness of EMI shielding materials, and electromagnetic wave frequency, respectively. EMI SE is used to characterize the attenuation of electromagnetic wave for EMI shielding materials, which is larger than 20 dB indicating that 99% electromagnetic waves are attenuated and EMI shielding materials can reach the commercial standard.[33] EMI SE of ANF@PPy composite films can be researched via using VNA equipped a waveguide of C-band (6.0–8.0 GHz), X-band (8.0–12.0 GHz), Ku-band (12.0–18 GHz) and K-band (18.0-26.5 GHz). EMI SE of EMI shielding materials can be expressed by power coefficients of reflectance coefficient (R), absorbance coefficient (A), and transmittance coefficient (T) are satisfied the equations as following:[32]
According to Eq. (1)~(3), the reflectance coefficient (R), absorbance coefficient (A) and transmittance coefficient (T) can be calculated from scattering electromagnetic parameters collected by VNA. Thus, coefficients R, A and T of ANF@PPy composite films have been calculated as function of electromagnetic frequency and shown in Fig. 4a-4c, respectively. It is obvious that reflectance coefficient (R) is larger than transmittance coefficient (T), indicating that EMI shielding performance of ANF@PPy composite films mainly come from the reflection of electromagnetic wave. In addition, a small portion of electromagnetic wave is absorbed by ANF@PPy films. Thus, a tiny portion of electromagnetic wave is transmitted because T is approximately 0.
Moreover, the total EMI SE comes from three parts: reflection SE (SER), absorption SE (SEA) and multiple reflection SE (SEM), which can be calculated according to the following equations:
When the total EMI SE is greater than 15 dB, SEM can be ignored. Total EMI SE, SER, and SEA of ANF@PPy composite films and pure ANF film as function of electromagnetic wave frequency (6.0-26.5 GHz) are displayed in Fig. 4d and Fig. S3. It is very clear that EMI shielding performance of ANF based shielding materials is effectively improved after the introduction of PPy and ANF@PPy composite films exhibit excellent EMI shielding properties. EMI SE increases from about 10 dB for ANF@PPy-1 to about 35 dB for ANF@PPy-4 in wide frequency range, as shown in Fig. 4d. ANF@PPy-4 composite film exhibits the EMISE of 35 dB in wide frequency range of 6.0-26.5 GHz covering C-band, X-band, Ku-band and K-band, indicating that ANF@PPy-4 composite film can be applied in wide bandwidth application as EMI shielding materials. Electrical conductivity of ANF@PPy composite films are collected and shown in Fig. 4f. ANF@PPy-4 composite film has the largest electrical conductivity of 3546.0 S m− 1 in all composite films. The total EMI SE mainly comes from SER and SEA. EMI SER values of ANF@PPy composite films increase with the increase of PPy content due to the improvement of electrical conductivity of films (as shown in Fig. 4e and Fig. S3), indicating that more electromagnetic wave is reflected by the conductive surface of composite films. Thus, high electrical conductivity of PPy polymer materials plays a significant role in improving EMI shielding performance according to the Simon’s formation (Eq. (1)).
The thickness of EMI shielding materials is one of the most important parameters, which has a significant influence on EMI shielding performance and practical application of EMI shielding materials in many electric devices. Figure 4g and Table S1 show total EMI SE and thickness of ANF@PPy composite film and polymer based materials.[34–43] It is clear that ANF@PPy-4 with thin thickness exhibits excellent EMI shielding performance, which is beneficial for its practical application. The EMI SE divided by thickness of shielding materials (SE/t, dB cm− 1) can be used to evaluate the EMI shielding performance of materials. As shown in Table S1, the SE/t of ANF@PPy-4 composite film can reach to 826.0 dB mm− 1, which is higher than those of many materials, such as GNPs/PEDOT:PSS of 87.5 dB mm− 1,[34] CF/Fe3O4/PPy of 360.0 dB mm− 1,[39] Fe3O4/C/PPy of 35.0 dB mm− 1,[42] PPy/MXene,[41] S-doped rGo[38] and so on. ANF@PPy-4 composite film has a potential application in wide frequency range (C-, X-, Ku- and K-band) due to its high EMI SE, thin thickness and mechanical flexibility.
2.4. Wearable sensor of human motions
Wearable sensors for strain and pressure have attracted people's widespread attention due to the demand of disease diagnosis, human body monitor and other issues in recent years,[24, 44, 45] Sensing nanomaterials with excellent mechanical properties and conductivity have been efficient conductive components of flexible electronic devices for wearable devices and sensors.[21] ANF@PPy composite film has good mechanical flexibility and high conductivity, resulting in a potential application in wearable sensor for monitoring the human motions. ANF@PPy-4 film was chosen to characterize the sensing ability owing to mechanical robustness (ultimate stress of 132 MPa) and conductivity of 3546 S m-1 (as shown in Fig. 3e and 4f). ANF@PPy-4 composite film as sensing material is assembled into the wearable sensor with PDMS as flexible substrate and copper foil as electrodes, which is applied to monitor the human physiological motions, such as finger, wrist, elbow, knee, ankle, forearm and vocal cord, as shown in Fig. 5 The resistance of ANF@PPy-4 composite film can be changed via bending or stretching under force, as shown in Fig. 5a. The relative resistance variation is defined by, where and are the initial resistance and the real-time resistance during deformation, respectively.[46] The relative resistance variation of ANF@PPy-4 composite film under stretching is measured with LCR digital bridge combined the manual tensile testing machine. Figure 5b presents the relative resistance variation of ANF@PPy-4 composite film under stretching-releasing at various applied forces (2, 4, 6 and 8 N). It is observed that the relative resistance variation linearly increases with the increase in applied force, which is consistent with the tensile stress-train curves of ANF@PPy-4 composite film in Fig. 3e. The resistance of ANF@PPy-4 composite film increases during stretching after applied force due to the destruction of the electron transfer network. The human motion recognition and monitoring can be achieved via the flexible sensor, which has broad application prospects in the fields of prosthetic technology, sign language translation, robot technology, and other fields.[22, 47] The sensitive stretching sensing capability of ANF@PPy-4 composite film provides a flexible sensor possibility in monitoring human motions, such as finger, wrist, elbow, knee, ankle, forearm and throat, as shown in Fig. 5c-5i The ANF@PPy-4 composite film is fixed at the finger displayed in the inset of Fig. 5c. The resistance of ANF@PPy-4 composite film increases and remains stable after the bending of finger. When the finger returns to the straightened state, the resistance will return to its initial value due to the release of stretching on ANF@PPy based sensor. The relative resistance variation increases with the increase of the bending angles of finger (as shown in Fig. 5b) because of the strain level variation owing to the bending of finger, which can be used to the gesture recognition. In addition, the resistance variation can almost reach the same level in multiple cycles of the bending at fixed angle and release, suggesting that the sensor has good stability. Similar response characters can be found in other human body parts motion, such as wrist (Fig. 5d), elbow (Fig. 5e), knee (Fig. 5f), ankle (Fig. 5g). Additionally, a slight movement can be detected for the slight forearm twists in Fig. 5h. Moreover, the sensor based on ANF@PPy can detect the vocal cord vibration coming from pronounce. The ANF@PPy-4 composite film is fixed in the throat for detecting the pronunciation. It is clearly seen in Fig. 5h that the relative resistance variation signals had been recorded toward the pronunciation of word “nihao” via throat motion caused by the vocal cord vibration. All the above results suggest that ANF@PPy composite film is one of potential candidates for monitoring various human motion.
2.5. Joule heating properties
Due to the excellent mechanical flexibility of ANF and high electric conductivity of PPy, ANF@ANF composite film with flexibility and good conductance has a potential application in thermal management according to Joule heating effect. Joule heating comes from the heat energy converted from electric energy according to the Joule’s law Q=(U2/R)t (where Q, U, R and t represent the heat generated by current, the input voltage, resistance of materials and time, respectively.). ANF@PPy composite films can be used as a positive electrical heater due to their Joule effect owing to high electrical conductivity. Direct voltage source table is used to load voltage between both ends of ANF@PPy composite films, current flows composite film and then heat is generated due to Joule heating effect. The surface temperature of ANF@PPy composite film is captured by an infrared thermal image camera. The Joule heating performance of ANF@PPy composite films are collected and shown in Fig. 6 and S4. Surface temperature of all ANF@PPy composite films as function of time is shown in Fig. 6a under same input working voltage of 5.0 V. It is obvious that the saturated surface temperature of ANF@PPy composite films increase from 23 ºC to 77 ºC under input voltage of 5.0 V with the increase of PPy content inducting the improvement of electric conductivity. Figure 6b displays the surface temperature-time curve of ANF@PPy-4 composite film under different input voltages (1.0, 2.0, 3.0, 4.0 and 5.0 V). When input voltage is 1V, surface temperature of ANF@PPy-4 composite film reaches only 25 ºC and there is no obvious increase. Saturated surface temperature can reach approximately 32 ºC, 42 ºC, 59 ºC and 77 ºC under input voltage of 2.0 V, 3.0 V, 4.0 V and 5.0 V, respectively. In addition, surface temperature of ANF@PPy-4 composite film is uniform in all input working voltages as seen in insets of Fig. 6b due to uniformity of composite film. From the perspective of safety and miniaturization, low voltage driving Joule hearting performance is an important factor for an application in wearable devices.
Moreover, the long-term stability of saturated surface temperature under input voltage is significant to materials practical application. Figure 6c shows the long-term heating stability measured under voltage of 5.0 V within the long duration of 1200 s, suggesting the long-term temperature stability of ANF@PPy-4 composite film as Joule heating materials. Additionally, the gradually increasing input voltage from 1.0 to 5.0 V and then gradually decreasing to 0 V for ANF@PPy-4 and ANF@PPy-1 composite film induces Joule heating as shown in Fig. 6d. There is a terraced field shape curves for ANF@PPy composite films in Fig. 6d, indicating the stability of frequency-switching voltage. In addition, the temperature stability and repeatability of ANF@PPy-4 composite film is evaluated via a cyclic ON-OFF curve of input voltage of 5.0 V for 5 cycles and 1000 s as shown in Fig. 6e, indicating the superior heating stability as well as repeat ability. ANF@PPy-4 composite film with mechanical flexibility exhibits excellent Joule heating performance, such as low voltage drive, long-term temperature stability and repeat ability. Those results make ANF@PPy composite film to extend more application fields of electric thermal.
2.6. Photothermal conversion performance
Additionally, PPy polymer has strong solar light absorption, thus resulting in high photothermal conversion efficiency.[48, 49] So PPy based on composite material had been investigated in photothermal conversion, solar membrane distillation, cancer theranostic, and so on.[50] Fig. 7a displays the schematic diagram of photothermal performance measurement via using a sunlight simulator as light source. The sunlight of 100 mW cm− 2 power density (one sun intensity) radiates on materials, which is provided by the sunlight simulator at 50 cm above materials. Photothermal conversion performances of ANF film and ANF@PPy composite films are investigated and shown in Fig. 7 and S5. The temperature of ANF and ANF@PPy-4 composite film as function of irradiation time is shown in Fig. 7b under the one sun irradiation intensity (power density of 100 mW cm− 2) for 120 s. The saturation temperature of ANF@PPy-4 composite film can reach to 53 ºC, which is higher than that of pure ANF film due to the introduction of PPy with high light absorption. For further investigating the stability of photothermal for ANF@PPy-4 composite film, the temperature is collected under a long time (1200 s) irradiation of 100 mW cm− 2 power density as shown in Fig. 7c. The temperature of ANF@PPy-4 composite film can rapidly reach to 53 ºC and then slowly increase to 56.2 ºC during the duration irradiation of 100 mW cm− 2 power density, indicating the photothermal stability of composite film. Moreover, Figs. 7d and 7e show the photothermal properties of ANF@PPy-4 composite film with increasing the irradiation light power density (from 100 to 250 mW cm− 2). The saturation surface temperature of ANF@PPy-4 composite film linearly increases from 53.5 to 87.2 ºC with the increase of irradiation light power density, indicating the well-controllable photothermal property of ANF@PPy-4 composite film. In addition, the repeat ability and photothermal response of ANF@PPy-4 composite film is investigated via the temperature-time curve under the cycling ON/OFF of sunlight for 10 cycles and 1000 s. It is clear that the response temperature and repeat ability are steady during the cycling ON/OFF process, which manifests excellent reliable heat stability and repeatability.