A Three-Dimensional Porous Conducting Polymer Composite with Ultralow Density and Highly Sensitive Pressure Sensing Properties

1School of Material Science & Engineering, University of Science & Technology Beijing, Beijing 100083, China 2Collaborative Innovation Center for Nanomaterials & Devices, College of Physics, Qingdao University, Qingdao 266071, China 3College of Chemical Science & Engineering, Qingdao University, Qingdao 266071, China 4Industrial Research Institute of Nonwovens & Technical Textiles, Qingdao University, Qingdao 266071, China


Introduction
Various sensors were widely used in all walks of life, and the fabrication of high-performance sensor has drawn extensive attention.Using nanomaterials (nanoparticles [1,2], nanofibers [3,4], nanotubes [5,6], and aerogels [7,8]) especially nanofibers or nanofibrous membrane as the element of sensors is effective means to perform high-performance, pint-sized, and lightweight sensors because the low density and high specific surface area can reduce the weight and improve performance of sensor.
Since aerogel was first discovered by Kistler [9] in the 1930s, many efforts have been paid to the development of aerogels because of many kinds of excellent properties.Aerogel, as the lowest density solid material at present, has an extreme porosity of approximately 90%-99%.The particular three-dimensional (3D) porous structure made it have good performance in thermal insulation [10,11], absorption of oil [12,13], catalyst support [14,15], drug carrier [16], and lithium-ion batteries [17,18].Aerogels as gas sensor element have good performance and because of that the large mesopores can provide favorable diffusion for free molecules to move reversibly from the external environment to the pore interior [19].Nowadays, aerogels have been used in humidity sensing [19,20], oxygen sensing [21], nitrogen dioxide sensing [22] and other sensors.However, most aerogels need to composite conductive materials to enhance conductivity.Polyaniline (PANI), as a conductive polymeric material, has low density, good electrical conductivity, and plasticity.Combining PANI with other materials can obtain good conducting composite materials.There are many reports about in situ polymerization of PANI on the surface of organic poly(methyl methacrylate) (PMMA) nanofibers [23], poly(vinylidene fluoride) (PVDF) nanofibers [24], and so on to obtain composite materials which have good sensing property.However there are little reports about in situ polymerization of PANI on the surface of nanofibers in fibrous aerogels.
Here, we used the SiC/polyacrylonitrile (PAN) composite nanofiber aerogels as support to fabricate a porous conducting material with ultralow density by the in situ polymerization of PANI on the surface of fibers in aerogels.The mass increase is 400% after in situ polymerization.The PANI deposited on the fibers in aerogel and became the majority of PANI/SiC/PAN composite, so the composite has high electrical conductivity.The conductivity of PANI/SiC/PAN composite is about 0.013 S m −1 .The 3D porous PANI/SiC/PAN composite is of low density and high porosity.Its density is 0.211 g cm −3 and its porosity is 76.44%.The sample has high performance in pressure sensing.The response of pressure is reversible and the response time is short.Under the constant voltage, the current can increase 500% after pressing for 1 s and increase 750% after pressing for 30 s.

Experimental
2.1.Fabrication of Aerogels.The polyacrylonitrile (PAN) nanofibers were prepared by electrospinning.The electrospinning precursor solution contained 10 wt% PAN (  ∼ 5,000,000) and 90 wt% DMF (AR).0.75 wt% PAN nanofiber membrane, 0.25 wt% SiC nanowhiskers (diameter of 200-500 nm), 20 wt% tert-butyl alcohol, and 79 wt% deionized water were put in beakers, and then they were dispersed by high-speed dispersion homogenizer (A-25, China) with 15,000 rpm for 15 min.Then the dispersed mixture was poured into weighing bottles in a plastic box.Liquid nitrogen was poured into plastic box to freeze the solution quickly.Then the weighing bottles with freeze casting PAN/SiC were transferred to freeze-dryer (FD-1-50, China) under 50 Pa and −50 ∘ C for two days.So the SiC/PAN aerogels were prepared.The samples have no elasticity, so the samples were put in air dry oven for 300 ∘ C heating modified to obtain good elasticity and morphological stability.

In Situ Polymerization of PANI on Fibers in SiC/PAN
Aerogel.The PANI/SiC/PAN aerogels were fabricated by the in situ polymerization of PANI on the surface of PAN or SiC fibers.In the progress of in situ polymerization, ammonium persulfate (APS) was used as oxidant and 5-sulfosalicylic acid dihydrate (SSA) was used as catalytic agent, and the aniline was the monomer to form polymer by in situ polymerization.The specific procedures were as follows.Firstly, two solutions were prepared: 4.66 g APS was dissolved in 100 mL deionized water marked as solution A and 7.98 g SSA and 2.26 g aniline were dissolved in 100 mL deionized water marked as solution B. Secondly, SiC/PAN aerogels were put into solution B, and then solution A was instilled into solution B with ultrasonic concussion by ultrasonic vibrating machine (KQ2200DE).Thirdly, the mixed solution was put in freezer at 3 ∘ C for 24 h.Fourthly, the aerogels were taken out and washed by deionized water.At last, the PANI/SiC/PAN porous composites were dried by natural drying.The fabrication process is shown in Figure 1.

Pressure Sensing Test.
As shown in Figure 2, pressure sensing property was tested by simple homemade device.Two copper wires were fixed on the sides of the sample as electrodes by silver colloids.Then two glass slides were affixed on the sides of the sample to stable the electrodes and suffer the pressure.Two electrodes were connected to Keithley 6487 high resistance meter system to test the change of electrical conductivity.In the process of testing, a constant voltage of 5 V was supplied, so the changes of instantaneous current can reflect the changes of electrical conductivity of sample under pressures.

Characterization.
The microstructure of aerogel before and after in situ polymerization was measured by a scanning electron microscope (SEM, Nova NanoSEM 450).The sample before polymerization was sputtered with gold and tension of 10 kV was applied.Tension of 20 kV was applied to the sample after polymerization.The thermal stability was detected by thermogravimetric analysis (TGA, Mettler-Toledo TGA/DSC).The analysis was performed at 10 ∘ C min −1 from 25 ∘ C to 500 ∘ C under air atmosphere.The thermogravimetric analysis testing under air atmosphere is because we want to know the temperature which can be practically used.And the resistances under temperature from 25 ∘ C to 250 ∘ C were tested by the High Precision Heating Stage (SET3625, ShenZhen, China) and multimeter; the resistance increased acutely under 225 ∘ C, so the testing was only up to 250 ∘ C not 500 ∘ C. The phases were identified by X-ray diffraction (XRD, RIGADU D/MAX-2500) in the angular 10−90 ∘ with a scan speed of 4 ∘ min −1 .The chemical compositions were analyzed by Fourier transform infrared spectroscopy (FT-IR) using a Thermo Scientific Nicolet iN10 spectrometer and transmittance data were processed for the wave number range 700-4000 cm −1 .The conductivities of the sample before and

Results and Discussion
The conducting aerogel was prepared by the in situ polymerization of PANI on the surface of fibers in the aerogels.
The pores (diameter of about 1 m) in as-prepared PAN/SiC aerogel are bigger than pores in common aerogels, so the PANI is more easily polymerized in the inside of aerogel and the coating layer of PANI can be thicker.We can see that the color of the sample was translated to bottle green as shown in Figure 3, and it is due to the PANI being bottle green.In fact, in the process of polymerization of PANI, the solution and the sample changed color gradually.And the color of sample would not change back after cleaning by deionized water.It indicated that the PANI was in situ polymerization on the surface of fibers in aerogel firmly and stably.It can be seen from Figure 3(b) that there is shrinkage after polymerization compared with original aerogel.It should be noted that the shrinkage of the sample happened in the process of drying, and there are almost no changes of shape and volume in the process of polymerization and washing.The shrinkage may be due to the surface tension of water in the process of drying.The weight of the sample was increased from 0.0323 g to 0.1623 g; the increase of relative mass is 400%.So the density after polymerization was increased (about 0.211 g cm −3 ).The porosity of aerogel was calculated by the formula  = (1 −   /  ) × 100%.Here,   is the bulk density,   is the skeletal density, and  is the porosity.The porosity of PANI/SiC/PAN composite was 76.44%.The I-V curves of the sample before and after in situ polymerization were shown in Figure 4.The resistance of sample after polymerization was increased 2 × 10 7 times compared with the original sample.The specific morphology change of aerogel after polymerization was shown in Table 1.The conductivity changes were not equal to resistance change because of the shrinkage of sample.The specific conductivity was changed from 2.9 × 10 −8 S m −1 to 1.3 × 10 −2 S m −1 calculated by the formula  = / = /; here  is the conductivity,  is the specific resistance,  is the area of surface,  is the resistance, and  is the length.
The FI-IR spectra of the sample before and after in situ polymerization are shown in Figure 5(a).The measurement range was 700-4000 cm −1 .The SiC/PAN aerogel spectra (curve (A)) exhibited the absorption peaks of a stretching vibration at peaks at 2933 cm −1 (CH stretching in CH and CH 2 groups), 2242 cm −1 (C≡N), 1451 cm −1 (CH blending), and 1119 cm −1 (ether band) [25,26], and those four peaks that come from PAN nanofibers cannot be observed in the spectrum of PANI/SiC/PAN composites which may due to the PAN nanofibers being clad by PANI.Absorption peaks at 802 cm −1 and 917 cm −1 are ascribed to the transversal optic (TO) mode and longitudinal optical (LO) vibration of the Si-C stretching vibration [27].Those two peaks were weakened after polymerization.The characteristic peaks at 1657 cm −1 and 1567 cm −1 are attributed to the stretching vibration of quinoid and benzenoid rings on PANI molecular chain, respectively [28].The peak observed at 1080 cm −1 is designated to vibration mode of N=Q=N which is an electronic band (Q refers to the quinonic type rings) [29].The C-H vibration of aromatic ring at 3059 cm −1 and C-N of from aromatic amines at 1297 cm −1 [30] were also observed in curve (B).All those peaks proved the in situ polymerization of PANI on the surface of fibers in aerogels.
The thermogravimetric (TG) curve of PANI/SiC/PAN is shown in Figure 6(a).The weight loss from room temperature to 78 ∘ C is due to the presence of moisture.The PANI/SiC/PAN composite can absorb a large number of water molecules because of the high porosity.The obvious weight loss from 234 ∘ C to 500 ∘ C is attributed to the degradation polymer backbone of PANI [34] and the pyrolysis of PAN [35].
Figure 6(b) shows the resistance variation with increasing temperature from 25 ∘ C to 250 ∘ C of PANI/SiC/PAN composite.The increase of resistance from 50 ∘ C to 100 ∘ C may be due to the water evaporation because water can increase the electrical conductivity of polyaniline through an increase in the interchain electron transfer and/or by increasing the mobility of dopant ions [36][37][38].The increase of resistance beyond 100 ∘ C is due to the pyrolysis of PANI.The pyrolysis of PANI is drastic after 234 ∘ C as shown in Figure 6(a) and the increase of resistance is also drastic from 225 ∘ C to 250 ∘ C.
To test the pressure sensing property, the resistances of the sample under different pressures at 10 −3 bar levels were tested.As shown in Figure 7, the imposed pressures were from 0.001 to 0.010 bar.The electrical resistivity declined gradually with the increase of pressure.And the decrease of resistance was about 18% and even the pressure was only 0.002 bar.So the sample can detect the light contact pressure at 10 −3 bar levels.The change of electrical resistance was nonlinear may be due to the fact that the closing of pore was not completely in proportion to the pressure.
The reversible pressure sensing property of sample was also tested.A constant voltage of 5 V was applied on the sample, so the electric current reflects the change of resistance.The test period was 60 s (30 s for pressure and 30 s for relax).As shown in Figure 8(a), when the sample was pressed by the pressure of 0.02 bar, the current increased clearly.The current rise quickly at first; the increase of current was up to about 500% when the sample was pressed for 1 s.Then the current rise rate was slowed down.The increase was up to 750% when the sample was pressed for 30 s.When the sample was relaxed, the resistance was regained.So the response to pressure was fast and reversible.The instantaneous response of pressure was also tested.The means is similar to the reversible response testing; a constant voltage of 5 V was supplied on the sides of sample.But in order to obtain quick pressure and recovery, we use index finger to press sample in a very short time.Figure 8(b) shows the response curve of instantaneous pressure.The response and recovery time is quite short, and the response of current is clear (increased by about 50%).There is a little increase (about 10%) of current after pressing and recover compared with the original current, and it may be due to the unrecoverable deformation in the process of testing.
Figure 9 shows the diagrammatic drawing of pores changing in the process of pressing and relaxing.After in situ polymerization the fibers were packaged by PANI.The fiber became thicker and the pores became tiny.The electric current was conducted from one fiber to another in the sample.Under the effect of pressure, the sample deformation occurred.A portion of pores was closed when the sample was pressed while they were open when the pressure was canceled.The closure of pores can connect conducting fibers so the electrical conductivity of sample increased with the pressure.The shape changes become bigger and the closed pores become more with the increase of pressure, so the electrical conductivity becomes higher with the increase of pressure.

Conclusion
Here, a 3D porous conducting material was prepared by a simple and efficient method-in situ polymerization of PANI on the surface of fibers in the aerogels.After polymerization of PANI, the weight was increased by 400%.And its density was still low (about 0.211 g cm −3 ).The porosity of PANI/SiC/PAN composite is 76.44%, and the conductivity of it is 0.013 S m −1 .The sample has good pressure sensing property because of its porousness and good conductivity.The pressure sensing property was tested by homemade simple device.The resistance of sample was reduced gradually after the increase of pressure.The response is reversible and the response time is short, so the composite can detect even instantaneous or tiny pressure.The 3D porous conducting PANI/SiC/PAN composite material may be used in pressure sensors.

Figure 2 :
Figure 2: Diagrammatic drawing of pressure testing equipment.

Figure 9 :
Figure 9: Diagrammatic drawing of changing process of pores when pressed and relaxed.

Table 1 :
Morphology, conductivity, and porosity changes of the aerogel after in situ polymerization.