The preparation of PVA/PBS/LiCl hydrogels and their performance as conductive gels

Abstract

The article describes the preparation of polyvinyl alcohol (PVA)/polybutylene succinate (PBS)/lithium chloride (LiCl) hydrogels, their structural features, as well as their electrical and mechanical properties. Firstly, by using a twin screw extruder, the PBS was blended with plasticized PVA and then PBS fibres were formed during the process. Secondly, the PVA/PBS composite materials were dissolved in the binary mixed solvent of glycerol and water containing LiCl to prepare PVA/PBS/LiCl hydrogels by the sol‐gel method. The morphology, structure, mechanical property, and conductivity of the hydrogels were investigated. The mechanical property of the hydrogels is improved with PBS fibres significantly and LiCl can improve the electrical conductivity of the hydrogels. The strength and conductivity of the PVA/PBS/LiCl (3 wt%) hydrogels are 4.67 MPa and 8.3 s/m, respectively. The PVA/PBS/LiCl hydrogels show good mechanical strength and conductivity and can be used in the wearable sensor field.

INTRODUCTION

With the rapid development of electronic technology, wearable electronic devices have entered people's daily lives, and flexible conductive materials have attracted great attention. Hydrogel, which is one of the most important candidate materials for the preparation of soft electronic equipment [1–3], has been widely used in medical detection and health monitoring [4, 5], intelligent robots [6], flexible sensors [5, 7], flexible batteries [7], motion monitoring [5], flexible supercapacitors [8], and other aspects. Polyvinyl alcohol (PVA) is an ideal material for preparing hydrogels. Recently, some research works have paid more attention to the enhancement of their mechanical and electrical properties. Bian et al. [9] reported that lignocellulose nanoparticles (LCNF) were used as reinforcement materials to prepare PVA hydrogels. The rheology and mechanical properties of the hydrogel system could be adjusted by changing the content of LCNF. De et al. [10] studied the influence of microfibres and coarse celluloses on the preparation of PVA hydrogel by freezing and thawing. It was pointed out that as a reinforcement material, cellulose microfibres can lead to the structure reconstruction of polymer and hydrogel. This can enhance the mechanical properties of PVA hydrogel. Qian et al. [11] introduced the conductive polymer polyaniline (PANI) into the double network poly (N-isopropyl acrylamide-co-acrylamide)/polyvinyl alcohol (PNA/PVA) hydrogel and prepared a kind of multi-functional conductive hydrogel. By adjusting their composition, the prepared PNA/PVA/PANI hydrogel has excellent conductivity and an ultra-high photothermal conversion efficiency. Closed-loop monitoring and sensing feedback are realized through the change of real-time resistance. Gong et al. [12] reported the preparation of PAANA/PEDOT:PSS/PVA hydrogel with high strength and high conductivity by using the semi-interpenetrating network (SIPN) strategy and cyclic freeze–thaw treatment. In the hydrogel, PVA, polystyrene sulfonate (PEDOT: PSS), and polyvinyl pyrrolidone (PVP) are played as the matrix network, the conductive medium, and the reinforcement, respectively. Yi et al. [13] prepared conductive PVA hydrogels by freezing and thawing and they can be used as humidity sensors. Wei et al. [14] prepared PVA/graphite oxide (GO) conductive hydrogels for strain sensors by freezing the mixture of PVA and GO and immersing themselves in a high concentration of sodium chloride (NaCl) solution. Their good conductivity can be contributed to the synergistic effect of GO and NaCl.

Until now, the mechanical property of hydrogels is still a restriction for the application of them in sensors. The trade-off between conductivity and mechanical property is also facing challenge in constructing hydrogels with excellent comprehensive properties. Here, PBS nanofibres were used as reinforcement materials and LiCl was used as conductivity improvers to prepare PVA/PBS/LiCl hydrogels with good electrical conductivity and mechanical strength. Firstly, PVA/PBS composites containing PBS in-situ nanofibres were prepared by a twin-screw melt mechanism. Secondly, prepared PVA/PBS composites were mixed with LiCl to disperse LiCl into the composites and form PVA/PBS/LiCl hydrogels by the sol-gel method. Simultaneously, the effect of LiCl content on the morphology, structure, and conductivity of PVA/PBS/LiCl hydrogels was investigated.

EXPERIMENTAL

Materials

PVA was purchased from Shanghai Yingjia Industrial Co., Ltd., lithium chloride (LiCl) was purchased from Tianjin Fengchuan Chemical Reagent Technology Co., Ltd., polybutylene succinate (PBS) was purchased from Tianjin Hengxing Chemical Reagent Co., Ltd., and glycerol was purchased from Tianjin Fuyu Fine Chemical Co., Ltd.

Preparation

Preparation of PVA/PBS composite materials

PVA/PBS composite fibres were prepared by using in-situ fibre forming technology with a twin screw extruder. The weight ratio of PVA and glycerol is 55:45, and the mixture of them was kept at 75°C for 6 h. Then, PBS (5 wt%) was added into plasticized PVA and they were stirred well and blended for extrusion. The obtained fibres were cut into composite particles.

Preparation of PVA/PBS/LiCl hydrogel

The PVA/PBS/LiCl hydrogels were prepared by introducing LiCl into the PVA/PBS. Firstly, 17.2 g PVA/PBS composites and a certain amount of LiCl (0, 0.5, 1.0, 1.5, 2.0 g) were added into distilled water (20.5 g) and then keep the solution at 25°C. Secondly, the temperature was raised gradually and kept at 40, 60, and 90°C for 1.5, 1, and 1 h, respectively. Finally, it was kept at 95°C and stood for 5.5 h to obtain PVA hydrogel solution. The hydrogel solution was poured onto the glass mould that was kept at 95°C in the oven. The PVA/PBS/LiCl hydrogels were formed by natural cooling. The hydrogel samples with LiCl content of 0, 1, 2, 3, and 4 wt% were recorded as PVA/PBS/LiCl (y), respectively, and y represented the mass fraction of LiCl.

Scheme 1 shows the process of preparation of PVA/PBS/LiCl hydrogels. PVA/PBS composite fibres were prepared by using in-situ fibre forming technology with a twin screw extruder and then they were cut into composite particles. Subsequently, LiCl was added to the solution containing PVA/PBS particles, and keep them at 95°C for a definite time. In the end, the PVA/PBS/LiCl hydrogels were formed by natural cooling. For the introduction of PBS and LiCl, the mechanical property and conductivity of hydrogels will be improved and the designed hydrogels can be used as sensors.

[IMAGE OMITTED. SEE PDF]

Characterization

Scanning electron microscope (SEM)

PVA in PVA/PBS composite material was dissolved in water, and the morphology of residues was examined by scanning electron microscope (SEM) (Phenom Pro X, Shanghai Funa Scientific Instrument Co., Ltd). The cross-section of PVA/PBS/LiCl hydrogels was also observed.

Fourier transform infrared spectroscopy (FTIR)

The chemical structure surface groups of hydrogels were studied by Fourier transform infrared spectroscopy (FTIR) (Nicolet210-IR, Shanghai Jinghong Experimental Equipment Co., Ltd). Before the test, the samples and spectral pure potassium bromide were dried fully. The wavenumber range of the tested sample was 4000–400 cm⁻¹.

X-ray powder diffractometer (XRD)

The crystal structure of the hydrogel was characterized by X-ray powder diffractometer (XRD) (D8 Advance, Bruker Instrument Co., Ltd.). The XRD spectra of the hydrogel materials in the 2ɵ range of 10 to 70^o were obtained with a scanning rate of 10^o/min.

Mechanical properties

The mechanical properties of the hydrogel were tested with a universal tensile tester (AI-7000S1, High-Speed Railway Technology Co., Ltd.). The testing speed was 500 mm/min. The size of the hydrogel for the tensile test was 100 × 10 × 1 mm (long × wide × thick). During the test, each hydrogel sample with the same content was measured 10 times to ensure the accuracy of the experiment.

Conductivity

The conductivity of the hydrogel was measured by a four-probe tester (SB118, Shanghai Qianfeng Instrument Co., Ltd.). The hydrogel with uniform size was selected for testing, with a length of 50 mm, a width of 10 mm, and a thickness of 1 mm. Each sample was measured 20 times to reduce experimental errors.

Sensor performance

LCR meter (E4980A, Agilent Co., Ltd.) was used to investigate the sensor performance of the hydrogel. The hydrogel was placed at different joints to monitor the moving of these joints. The resistance changes of the prepared hydrogel were recorded to analyze the sensor performance of the hydrogel.

RESULTS AND DISCUSSION

SEM imaging of PBS nanofibres and PVA/PBS/LiCl hydrogels

To investigate the PBS fibres in the PVA/PBS composites, PVA/PBS composites were washed and the residues of them were studied with SEM. Figure 1 shows the SEM image of the residues of PVA/PBS composite materials after the dissolution of PVA. It indicates the PBS nanofibres in PVA/PBS have remained, and the multilevel interspace reticulate structure of them is a benefit to the mechanical property. The diameter of PBS nanofibres is about 40 to 200 nm. This proves that the in-situ PBS nanofibres were formed during extrusion and stretching processes.

[IMAGE OMITTED. SEE PDF]

The microstructure of PVA/PBS/LiCl hydrogels was also researched and shown in Figure 2. As the SEM images show, it is clearly observed that the pore size of PVA/PBS/LiCl hydrogels increased with the increase of LiCl content. This is the main reason for the decrease in the strength of hydrogel. The large pore size makes the hydrogel provide a wider channel for the free movement of electrons, which is the most important reason for the excellent conductivity of hydrogels.

[IMAGE OMITTED. SEE PDF]

FTIR analysis of PVA/PBS/LiCl hydrogels

Figure 3 shows the FTIR spectrums of PVA, PVA/PBS, and PVA/PBS/LiCl (3 wt%) hydrogels. The characteristic peaks of PVA hydrogel are displayed in the FTIR spectrum. PVA, as the main substrate for forming the hydrogel skeleton, plays a very important role in the hydrogel structure and the concentration of it is far higher than PBS and LiCl. Comparatively, there are not any obvious shifts of peaks or the new characteristic peaks observed in the spectrums of PVA/PBS and PVA/PBS/LiCl (3 wt%). It can be seen that there is no strong interaction between LiCl and hydrogels in PVA/PBS/LiCl hydrogels. Clearly, all the spectrums of prepared hydrogels show a wide and strong peak at 3300 to 3650 cm⁻¹. This can be attributed to the stretching vibration absorption of – OH and demonstrates the presence of hydroxyl groups in PVA and PBS molecules. The corresponding peak in PVA/PBS hydrogel and PVA/PBS/LiCl (3 wt%) is at 3438 cm⁻¹ to 3426 cm⁻¹ respectively. There is a little shift observed, which indicates that the original hydrogen bonds in PVA were destructed and some new hydrogen bonds between the PBS and PVA molecular chains were formed. The peak at 1718 cm⁻¹ is corresponding to the stretching vibration absorption of C = O in the carbonyl groups of PBS. This means that PBS exists in PVA/PBS hydrogels and PVA/PBS/LiCl hydrogels. During the preparation of hydrogels, part of PVA forms intermolecular hydrogen bonds to construct crystalline regions. Hydrogen bonds are also formed between PVA and PBS, resulting in the formation of physical intersections. The shift of the resonance absorption peak indicates that the intermolecular interaction in the hydrogels is enhanced due to the formation of hydrogen bonds in the blend system. And this can improve the mechanism property of PVA/PBS/LiCl hydrogels.

[IMAGE OMITTED. SEE PDF]

The analysis of XRD

XRD is also used to analyze the crystalline property of PVA, PVA/PBS, and PVA/PBS/LiCl (3 wt%) hydrogels. Figure 4 shows the XRD patterns of them. There are four obvious peaks at 11.2, 19.6, 23.4, and 40.6°, representing the (001), (101), (200), and (112) crystal planes of PVA respectively [15, 16]. This result consists with the literatures and belongs to the strong interaction between PVA chains through intermolecular hydrogen bonding [17, 18]. It can be seen that there are not any obvious changes of the peaks after introducing PBS into the composites. Compared with PVA and PVA/PBS, the diffraction peak of PVA/PBS/LiCl (3 wt%) at about 23.4° is weaker and with a little shift, which indicates that LiCl weakens the (200) crystal planes of the PVA. This can be attributed to the destruction of the intermolecular hydrogen bonds between molecular chains.

[IMAGE OMITTED. SEE PDF]

Mechanical properties of PVA/PBS/LiCl hydrogels

Figure 5 shows the strength change of PVA/PBS/LiCl hydrogels. Figure 5a is the stress–strain curves of PVA/PBS/LiCl hydrogels after standing in the air for 6 h. The mechanical properties of PVA/PBS/LiCl hydrogels with different content of LiCl were discussed. It could be seen that with the increasing content of LiCl, the breaking strength and elongation at the break of the PVA/PBS/LiCl hydrogels decreases. While the content of LiCl changes from 0 to 4 wt%, the breaking strength of hydrogel decreased from 3.88 to 1.34 MPa and the breaking elongation gradually decreased from 427.94% to 297.63%. Relatively, the strength decreased by 65.46% and the breaking elongation decreased by 30.45%. Figure 5b shows the stress–strain curves of PVA/PBS/LiCl hydrogels after standing in the air for 12 h. With the increase of LiCl, the breaking strength and elongation of PVA/PBS/LiCl hydrogels are also decreased clearly. The breaking strength of PVA/PBS/LiCl hydrogels gradually decreases from 5.52 to 4.20 MPa and the breaking elongation of them decreases from 653.60% to 402.27%. In contrast, the strength and the breaking elongation of PVA/PBS/LiCl hydrogels decrease by about 24.32 and 38.45%, respectively. This means that there is a huge effect of LiCl on the strength and the breaking elongation of the hydrogels. With the addition of LiCl, the intermolecular interaction between polymer chains is destroyed and the pore size of PVA/PBS/LiCl hydrogels is larger. In the meantime, the three-dimensional network structure of the hydrogels becomes larger. This is in accord with the result of SEM images which are shown in Figure 2.

[IMAGE OMITTED. SEE PDF]

Conductivity of PVA/PBS/LiCl hydrogels

The conductivity of PVA/PBS/LiCl hydrogels with different content of LiCl was tested with a four-probe tester and is shown in Figure 6. As Figure 6a shows, with the initial enhancement of LiCl in PVA/PBS/LiCl hydrogels, the conductivity increases clearly. The conductivity of PVA/PBS/LiCl (4 wt%) is close to the value of PVA/PBS/LiCl (3 wt%). At the initial stage, the positive and negative electric charges in hydrogels increase with the content of Li⁺ and Cl⁻ ions. However, due to the limited saturation solubility, there is not a higher conductivity when the content of LiCl is higher than 3 wt%. PVA/PBS/LiCl (3 wt%) hydrogels were chosen as conductors to investigate the sensor performance of PVA/PBS/LiCl hydrogels.

[IMAGE OMITTED. SEE PDF]

Sensor performance of PVA/PBS/LiCl hydrogels

Figure 7 shows the relative resistance changes (ΔR/R₀) with the gradual bending of the elbow, knee, and index finger from 0° to 90° and the flexing of the wrist, thumb, and neck were bent from 0° to 45°. The relative resistance changes increase significantly with the bending and flexing of different joints, and their recovered resistances are close to the initial values. Figure 7d indicates that the minimum resistance change rate was less than 5% in the process of detecting movement. Meanwhile, when the hydrogel was stretched from 0% to 33.3%, the value of ΔR/R₀ was 89.5% (Figure 7g). They exhibit a high stability and strain-sensitive conductivity, indicating its great potential as an intelligent wearable device to detect human movement. Based on the relative resistance change and the relative strain change, the value of Gauge factor (GF), which is the key parameters for flexible sensors, is about 2.69.

[IMAGE OMITTED. SEE PDF]

CONCLUSION

By using PVA as hydrogel matrix materials, in situ forming PBS nanofibres as reinforcement materials and LiCl as conductivity improvers, PVA/PBS/LiCl hydrogels with good mechanical property and well conductivity were prepared by the sol-gel method. After introducing PBS nanofibres into hydrogels, the strength and the breaking elongation of PVA/PBS/LiCl hydrogels enhances significantly. With increasing the content of LiCl, the strength and the breaking elongation of PVA/PBS/LiCl hydrogels decrease obviously and their conductivity increases at the initial stage. Overall, when the content of LiCl is 3 wt%, the conductivity of the hydrogel is up to the maximum value (8.3 s/m) and the strength of the hydrogel could keep at 4.68 MPa. The PVA/PBS/LiCl hydrogels exhibit good mechanical and electrical properties and can be used in the wearable sensor field.

AUTHOR CONTRIBUTIONS

Zheng Guo: Conceptualization; funding acquisition; investigation; methodology; resources; writing—original draft; writing—review & editing. Zebo Wang: Data curation; formal analysis; methodology; writing—review & editing. Wei Pan: Data curation; formal analysis; methodology; writing—review & editing.

ACKNOWLEDGEMENTS

We are grateful to the Science and Technology Guiding Project of China Textile Industry Federation (2022007).

CONFLICT OF INTEREST STATEMENT

The authors declare no conflict of interest.

DATA AVAILABILITY STATEMENT

The data that support the findings of this study are available from the corresponding author upon reasonable request.

References

Wang, C., Wang, C., Huang, Z., Xu, S.: Materials and structures toward soft electronics. Adv. Mater. 30, [eLocator: 1801368] (2018). [DOI: https://dx.doi.org/10.1002/adma.201801368]