Supporting Information

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1 Supporting Information Stable, Strain-Sensitivity Conductive Hydrogel with Anti-freezing Capable, Remoldability and Reusability Chengxin Hu, Yulin Zhang, Xiangdong Wang, Lu Xing, Lingying Shi and Rong Ran* (College of Polymer Science and Engineering, Sichuan University, Chengdu , China) * address: S-1

2 Contents 1. Materials Methods Preparation of samples Characterization of samples Mechanical performance test Antifreeze performance test Conductive test Results The optimal ratio of the prepared sample Characterization of the gel Reference...24 S-2

3 1. Materials Aniline (ACS, 99%), Phytic acid solution (50% in H 2 O, M w =660.04) and Poly (vinyl alcohol) 1799 (Degree of alcoholysis=98-99% mol/mol) were purchased from Aladdin Industrial Corporation. Ammonium persulfate (AR, 99.5%) was provided by Chengdu Kelong Chemical Reagent Factory. Glycerol (AR, 99.5%) was obtained from Fu Chen (Tianjin) Chemical Reagent Co., Ltd. Deionized water was provided by Chengdu Changzheng Chemical Reagent Co., Ltd. All chemicals used here were analytical grade and used as received without further purification. 2. Methods 2.1Preparation of samples PVA/Gly/PANI gels (PGA gels): The polyvinyl alcohol (PVA) network is formed by a method of circulating freeze-thaw. The polymerization of aniline (ANI) is obtained by doping phytic acid (PA) and initiating polymerization by Ammonium persulfate (APS) at a low temperature. Because glycerol (Gly) and water are miscible with each other in any ratio, it can be added directly to the reaction solution. The resulting PVA/Gly/PANI gels were named as P x G y A z (x represents the ratio of the mass of PVA to the volume of the water (w/v %), y represents the volume ratio of glycerol and water (v/v %) and z represents the ratio of aniline/phytic acid solution to glycerin/water solution). Take P 25 G 50 A 25 gel as an example. First, 10ml, 25w/v% PVA aqueous solution was prepared at 90, then 10ml glycerin was added and stirred evenly until the S-1

4 solution was colorless and transparent. After that put the above solution in the refrigerator and cool to 3~4. Then 2.366g 50wt% phytic acid solution and 0.916g aniline were added to form a pale yellow solution A. In addition, 0.57g ammonium persulfate was dissolved in 1ml deionized water to obtain solution B. The solution A and B were placed in the refrigerator and cooled to 3~4. Then the A and B solution were mixed together quickly, and injected into the mold. Subsequently, a green gel was obtained after 5 hours of reaction at an ambient temperature of 3-4 C. Put the green gel in freezer (-20 C) about 12 h to make PVA freezing, and then placed the gel on glass plate at room temperature about 6 h for thawing. The above steps are noted as one freeze-thaw cycle. Repeat the freeze-thaw cycle three times to get the final gel. PANI samples: 0.57g APS was added to 1ml deionized water, stirring evenly and forming solution A. Then 0.916g aniline was added to the 2.366g 50wt% phytic acid solution, stirring to form a light yellow solution B. After that, place A, B solution in the fridge and cool it to 3~4 C. Then, solutions A and B were rapidly mixed and reacted at 3 to 4 C for 7 hours to obtain a dark-green powder dispersion. Deionized water was added to dilute the dispersion. The dispersion was filtered and then washed three times with alcohol and deionized water respectively to give a black powder. The powder was dried in an oven at 50 C for 12 hours to obtain PANi. S-2

5 2.2Characterization of samples Preparation of dry gels: Due to the good moisturizing effect of glycerol, all tests requiring dry samples (SEM, XPS, XRD, and FTIR) herein are obtained by removing glycerol. The gels were first soaked in excessive deionized water for 12 hours to exchange glycerol, and then freeze-dried to obtain gels without solvent. Fourier transform infrared spectroscopy (FTIR): spectra of PVA gel, P 25 G 50 A 25 gels and PANi powders were recorded on a Nicolet 6700 (Thermal Scientific, USA) spectrometer. The PANi powders were firstly obtained by the above method and used directly. The dry PVA gel and PGA gel were ground into a powder with a flat file and dried. FT-IR is measured by KBr tablet method and the test range of wavenumbers is 4000cm -1 to 400 cm -1. X-Ray Diffraction (XRD): XRD measurements were performed on the D/MAX 2400 X-ray diffractometer operating in the 2Theta/Theta mode with continuous scanning the step of 0.02 (2 θ). Scanning Electron Microscope (SEM): The micro-topography structure of the PGA gels after freeze-dried were characterized by a Quanta 250 scanning electron microscope (SEM) (FEI, USA) at an acceleration voltage of 20kV. X-Ray Photoelectron Spectroscopy (XPS): X-ray photoelectron spectroscopy (XPS, XSAM 800, Kratos Analytical) was employed to examine the chemical state of C, N and O in PVA and PGA gels. The XPS survey scan from 0 to 1200 ev of PANI and PVA gels. The anode of X-ray was Al Kα ( ev). The pressure in analysis chamber is less than Torr. S-3

6 Density Functional Theory (DFT) Study: Method: To further discuss the internal forces of the gel, we use density functional theory (DFT) that Lu Xiong et al. have reported to calculate the interaction energy 1. The simulation was performed using the density functional theory program DMol3 in Material Studio (Accelrys, San Diego, CA). The physical wave functions were expanded in terms of numerical basis sets, Dmol3/GGA-PBE/DNP (3.5) basis set 2. The core electrons were treated with DFT semi-core pseudo potentials. The exchange-correlation energy was calculated with Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation (GGA). A Fermi smearing of Ha (1 Ha = ev) and a global orbital cutoff of 5.2 Å were employed. The convergence criteria for the geometric optimization and energy calculation were set as follows: (a) a self-consistent field tolerance of Ha/atom; (b) an energy tolerance of Ha/atom; (c) a maximum force tolerance of Ha/Å; (d) a maximum displacement tolerance of Å. Model: Before the calculation of binding energy, we established three interrelated and progressive models. (1) When we discuss the interaction energy between water molecules and glycerol molecules, we use two water molecules, two glycerol molecules, one water molecule and one glycerol molecule for comparison and discussion. (2) Two water molecules, one glycerol molecule and one PVA molecule are used (This PVA molecule has 6 repeating structural units and a random configuration), S-4

7 respectively, to discuss the interaction energy between glycerol molecules, water molecules and PVA molecules. (3) When discussing PGA gels (including PVA, polyaniline, phytic acid, water, and glycerin), we use a PVA molecule, two water molecules, a glycerol molecule, a phytic acid molecule, and a polyaniline molecule as a model. The polyaniline molecule contains two benzene rings and one fluorene ring, which are connected by -NH- and =N=, respectively. The calculation method of interaction energy: The strength of the interaction between the components in the model is represented by the interaction energy (E int ) and can be derived from the following equation: E E E int total component (Equation S1) Where E total and E component represent the total energy of the system, and the energy of each component in the system, respectively. E int is negative, and the greater the absolute value, the stronger the interaction. 2.3Mechanical performance test Uniaxial tensile test: Uniaxial tensile test of the hydrogels was performed using an Instron universal test instrument (Model 5576, Instron Instruments, U.S.A) with a 1kN load cell at 25 C with a tensile speed of 100mm/min. In addition, the PGA gels were stretched uniaxially at - 20 C with other conditions remained. The Young s S-5

8 module is calculated from the slope at 3-10% of the stress-strain curves. Rheology measurements: Constant temperature test: Rheology measurements of the hydrogels were conducted with HAAKE MARS 40 Rheometer (Thermo Fisher scientific) using a parallel plate of diameter 20 mm. The frequency sweep was performed over the range of rad s -1 at a fixed strain of 1%. All measurements were performed at 25±0.1 controlled by a constant temperature table. Variable temperature test: The temperature sweep was performed over the range of 25~95 C at a fixed strain of 1% and a fixed frequency of 7.96Hz (angular velocity =50rad/s). The rate of temperature rise and decrease was 3 C /min. Dynamic thermo mechanical analysis (DMA): The freezing effect of gels below 0 C are tested by Q800 Dynamic thermomechanical analyzer (TA Instruments). The uniaxial tensile model was carried out. The fixed deformation frequency was 7.59Hz, and the tensile strain was 1%. The temperature was changed from 25 to -60 C or -120 C at the speed of 3 C /min. 2.4Antifreeze performance test Glycerin/Water solution: Glycerol/water solutions with different glycerol volume ratios were prepared (0%-100%) and placed at 20 C for 48 h. Removed from the cold room and combined with an infrared imager to see if the solution freezes through S-6

9 the bottle inversion method at room temperature. PGA gels: A series of PGA gels were prepared according to the different glycerol concentrations described above (0%-70%), and the resulting gels were frozen in a freezer at -20 C for 24 hours. Then the gels were removed from the freezer to discuss the degree of icing of the gels in combination with an infrared imager at room temperature. 2.5Conductive test This article discusses the conductivity of gels by testing the voltage-current relationship. The conductivity of bulk PGA mixed gels were characterized by the conventional volt-ampere curve method. The samples were prepared and cut into rectangular shape (with a dimension of 20 mm 5 mm 3mm). The volt-ampere curves of the sample in the static state was measured from the two-probe machine (Keithley 2601B system source meter). S-7

10 3. Results 3.1The optimal ratio of the prepared sample Solution A Solution B Variabl e code PVA (g) Wate r (ml) Glycero l (ml) Aniline (g) Phytic acid (g) APS (g) Wate r (ml) P 10 G 50 A P 15 G 50 A PVA P 20 G 50 A P 25 G 50 A P 30 G 50 A P 25 G 30 A Glycerol P 25 G 50 A P 25 G 70 A P 25 G 50 A Aniline P 25 G 50 A P 25 G 50 A P 25 G 50 A P 25 G 0 A Special P 25 G 50 A PANi Table 1. The amount of reagents in the sample. code Solvent content (wt%) E (MPa) ε f (%) σ f (MPa) P 10 G 50 A P 15 G 50 A P 20 G 50 A P 25 G 50 A P 30 G 50 A Table 2. The change of solvent content, Young's modulus E, fracture elongation ε f and fracture stress σ f with PVA concentration. ( indicates that it cannot be measured) S-8

11 variable code Solvent content (wt%) E (MPa) εf (%) σf (MPa) Fracture Energy (MJ/m 3 ) Conductivity (S/m) P 25 G 50 A P 25 G 50 A Aniline P 25 G 50 A P 25 G 50 A P 25 G 50 A Table 3. The change of solvent content, Young's modulus E, fracture elongation ε f, fracture stress σ f and conductivity with the ratio of aniline and phytic acid solution to glycerin. S-9

12 Figure 1. Specific experimental procedures. Antifreeze effect of glycerol/water solution G vol (%) 0% 10% 30% 50% 70% 90% 100% Glycerol (ml) Water (ml) Table 4. The volume ratio (G vol ) of glycerin/water Figure 2. a) Photographs of glycerol and aqueous solutions before freezing. b) Photographs of glycerol and water solutions after freezing for 48 h. c) Infrared imaging of Figure b, where CS points represent the lowest temperature in the block. S-10

13 A solution of glycerol and water was first prepared in a series of volume ratios (Table 4), and the solution was then frozen at -20 C for 48 hours. As shown in Figure 3 we can see that the solutions with glycerol concentration of 0%-30% are frozen. Glycerol concentrations of 50% and 70% are clear liquids, they are easy to flow and no ice. The glycerol solution with a volume of 90%-100% freezes in the refrigerator and quickly melts to a viscous state after being taken out of the refrigerator. In combination with the near-infrared imaging image, the lowest temperature point is found in the middle of the bottle, indicating that the solution flows down slowly along the bottle wall. Therefore, we can think that when the glycerol concentration is too low or too high (0%-30% or 90%-100%), the antifreeze effect is not good. Figure 3. a) Change in the freezing point for different glycerol/water mass ratios. b) Change in freezing point for different glycerol/water volume ratios (Converted from figure a). Combining with the changes in the freezing point of the glycerin/water solutions with different glycerol and water ratios in the previous study 3, it can be seen that the visual images of the above glycerol solution frozen are the same as those of previous studies. When the volume ratio of glycerol/water is between 50% and 70%, the S-11

14 antifreeze effect is best. The corresponding antifreeze temperature is -29.3~-42.9 C. Calculation of effective cross-linking points of PGA gel Use Equation 1 4 to further calculate the effective cross-linking density (ν eff ) of the gel, G 0 represents the platform value achieved when the storage modulus changes with frequency. R is the gas constant, T the absolute temperature and φ 0 the polymer volume fraction, which can be calculated by the feeding ratio at sample preparation. G A RT 0 eff 0 2 A 1- f (1) (2) ƒ represents functionality of the cross-links. Because the crosslinking point of the PVA gel in this paper is the crystal area formed by the cycle freeze-thaw, f >>2 and hence A <<1. G0 eff Obtained RT 0 (3) T (K) G 0 (Pa) φ 0 (%) ν eff (mol/m 3 ) 30% % % Table 5. The change of density of effective crosslinking points of PGA gel with glycerol concentration S-12

15 From the above table, it can be seen that as the concentration of glycerol increases, the effective crosslinking point density of the gel gradually decreases. This is due to the small molecule of glycerol introduced, which acts as a plasticizer and increases the mobility and chain of the molecular chain. The space between them makes the density of effective cross-linking points decrease, and the mechanical strength of gels decreases 5. Effect of ANi concentration on gel P 25 G 50 A 25, P 25 G 50 A 50, P 25 G 50 A 75, and P 25 G 50 A 100 gels were prepared by changing the concentration of aniline. Uniaxial tensile tests and conductivity tests were performed on the gels. The relationship between the stress-strain curve, conductivity, Young's modulus and fracture energy of the gel with the amount of aniline was obtained. Figure 4 a) Electrical conductivity changes with the amount of aniline. b) Stress and Strain S-13

16 Curves of P 25 G 50 A 25, P 25 G 50 A 50, P 25 G 50 A 75, P 25 G 50 A 100 Gels Figure 5. a) Fracture energy and conductivity change with the amount of aniline. b) Young s module changes with the amount of aniline. It can be found that the conductivity increases significantly with the addition of aniline, while the fracture stress, fracture strain, and fracture energy decrease. The main reason is that the polyaniline molecular chain contains a large number of benzene rings, showing a rigid structure. It will hinder the movement of molecular chains and reduce the mechanical properties. In addition, although the mechanical strength decreased, the Young's modulus of the gel slightly increased with the amount of aniline, which was also due to the rigid ring structure of polyaniline. S-14

17 3.2Characterization of the gel. FTIR Figure 6. FTIR spectrum of PVA gel, P25G50A25 gel and pure PANi power. In the spectrum of PVA, the absorption peaks at cm -1 and 1446 cm -1 correspond to the stretching vibration absorption peak and bending vibration absorption peak of -CH 2, respectively. The absorption peak at corresponds to the stretching vibration peak of C-OH. In the FTIR spectrum of PANi, 1560 cm -1 and 1486 cm -1 correspond to the absorption peaks of the quinoid rings and benzene rings 6-7, respectively. The absorption peak 1301 cm -1 corresponds to the stretching vibration peak of C-N. And the absorption peak 1146 cm -1 corresponds to the stretching vibration peak of N=Q=N (Q stands for quinoid ring) 8 It indicates that polyaniline can be successfully synthesized by solution low temperature polymerization. In the FTIR S-15

18 spectrum of the P 25 G 50 A 25 gel, all of the above absorption peaks can be found, indicating that the P 25 G 50 A 25 is successfully compounded by PVA and PANi. SEM Figure 7. Scanning electron microscope pictures of PANi Power a), P 25 G 50 A 5 b), P 25 G 50 A 25 c) and P 25 G 50 A 100 d). From left to right are original and enlarged images, respectively. S-16

19 XPS Figure 8. XPS spectra of PVA gel and P 25 G 50 A 25 gel. S-17

20 Density Functional Theory (DFT) Study 1) Water molecules and glycerol molecules Figure 9. The interaction of (a) G-G, (b) G-W. Form Interaction energy (Ha) W-W G-G W-G Table 6. The calculated interaction energy of bonding pairs in W-W, G-G and W-G. The calculated W-G interaction energy is lower than that between W-W and G-G, indicating that the stability of water molecules and glycerol molecules coexisting is stronger than that of water molecules alone or glycerol molecules alone. S-18

21 2) PVA molecules coexist with water molecules and glycerol molecules Figure 10. The interaction of (a) PVA-W, (b) PVA-G, (c) PVA-G-W. S-19

22 Form Interaction energy (Ha) PVA-W PVA-G PVA-G-W Table 7. The calculated interaction energy of bonding pairs in PVA-W, PVA-G and PVA-G-W The interaction energy of PVA-G-W is higher than that of PVA-G or PVA-W, indicating that the stability of PVA molecules is better in the case where water molecules and glycerol molecules coexist. This indicates that glycerol and water molecules form more hydrogen bonds with the PVA molecular chain. S-20

23 3) PVA molecules and Polyaniline molecules coexist with water molecules, glycerol molecules and phytic acid molecules. Figure 11. The interaction of (a) PVA-W, (b) PVA-G, (c) PVA-G-W. S-21

24 Form Interaction energy (Ha) PVA-PANi PVA-PANi-W PVA-PANi-G PVA-PANi-PA PVA-PANi-All Table 8. The calculated interaction energy in PVA-PANi, PVA-PANi-W, PVA-PANi-G, PVA-PANi-PA and PVA-PANi-All. Figure 12. The uniaxial tensile curves of PGA gel at 25 C and 20 C. The illustration is an intuitive picture of stretching PGA gel by hand at - 20 C. As shown in Fig.12, comparing the stress-strain curves at 25 C and -20 C, it S-22

25 can be found that the fracture stress and fracture strain of PGA gel at -20 C is 90.4 % and 92.9 % of those at 25 C, respectively. We can clearly know that PGA gel can still maintain good mechanical properties at -20 C. Figure 13. Deformation sensitivity of PGA gel at - 20 C We first connected the strip-shaped PGA gel with two copper wires and then placed it in an environment of -20 C for 12h. After that, we connected the test system and recorded the change of current with deformation during the stretching process of the PGA gel. We recorded the deformation sensitive process of PGA gel (Movie S2) with a camera and calculated the change of resistance with time to get Figure 2. We can find that PGA gel still has good deformation responsiveness even at -20 C, which also reflects that PGA gel still has good performance as a sensor at low temperature. S-23

26 4. Reference (1) Han, L.; Liu, K.; Wang, M.; Wang, K.; Fang, L.; Chen, H.; Zhou, J.; Lu, X. Mussel Inspired Adhesive and Conductive Hydrogel with Long Lasting Moisture and Extreme Temperature Tolerance. Adv. Funct. Mater. 2018, 28 (3), , DOI: /adfm (2) Delley, B. From molecules to solids with the DMol3 approach. The Journal of Chemical Physics 2000, 113 (18), , DOI: / (3) Lane, L. B. Freezing Points of Glycerol and Its Aqueous Solutions. Industrial & Engineering Chemistry 1925, 17 (9), (4) Nie, J.; Du, B.; Oppermann, W. Swelling, Elasticity, and Spatial Inhomogeneity of Poly(N-isopropylacrylamide)/Clay Nanocomposite Hydrogels. Macromolecules 2005, 38 (13), , DOI: /ma050589s. (5) Sanyang, M.; Sapuan, S.; Jawaid, M.; Ishak, M.; Sahari, J. Effect of Plasticizer Type and Concentration on Tensile, Thermal and Barrier Properties of Biodegradable Films Based on Sugar Palm (Arenga pinnata) Starch. Polymers 2015, 7 (6), (6) Wanwan, L.; Fengxian, G.; Xiaoqian, W.; Ning, Z.; Mingming, M. Strong and Robust Polyaniline-Based Supramolecular Hydrogels for Flexible Supercapacitors. Angew. Chem. Int. Ed. 2016, 55 (32), , DOI: doi: /anie (7) Zeng, X.-R.; Ko, T.-M. Structures and properties of chemically reduced polyanilines. Polymer 1998, 39 (5), , DOI: /S (97) (8) Pan, L.; Yu, G.; Zhai, D.; Lee, H. R.; Zhao, W.; Liu, N.; Wang, H.; Tee, B. C.-K.; Shi, Y.; Cui, Y. Hierarchical nanostructured conducting polymer hydrogel with high electrochemical activity. Proceedings of the National Academy of Sciences 2012, 109 (24), , DOI: /pnas S-24