Preparation and properties of nanocellulose- reinforced environmentally adaptable PAM/PVA semi- interpenetrating highly conductive ion-conducting hydrogel

doi:10.21203/rs.3.rs-3113219/v1

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Preparation and properties of nanocellulose- reinforced environmentally adaptable PAM/PVA semi- interpenetrating highly conductive ion-conducting hydrogel

https://doi.org/10.21203/rs.3.rs-3113219/v1

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In this work, a semi-interpenetrating network (IPN) hydrogel reinforced with cellulose nanocrystals (CNC) and containing lithium chloride (LiCl) and dimethyl sulfoxide (DMSO) is composed of cross-linked polyacrylamide (PAM) and linear polyethylene alcohol (PVA) chains, which had excellent anti-drying, anti-freezing, and ionic conductivity performance. The experimental results show that the ion-conducting hydrogel has excellent cyclic stretching performance. Under the tensile strain of 200%, the relative resistance of the PAM/PVA/CNC/LiCl hydrogel remained unchanged after 10 stretching cycles and returned to its original length. The water retention rate of PAM/PVA/CNC/LiCl hydrogel is close to 80% at 60 °C. In addition, even at -20 °C, the hydrogel still has good electrical conductivity and exhibits excellent anti-freezing performance. The ion-conducting hydrogel has excellent conductivity and anti-freezing and anti-drying properties and has potential application prospects in flexible wearable devices and sensors.

cellulose nanocrystals

anti-freezing and anti-drying

ion-conducting hydrogels

sensors

Hydrogel ^[1] is a three-dimensional cross-linked polymeric network that can absorb and retain large amounts of water or other fluids without rupture. With its unique three-dimensional cross-linked polymeric network, hydrogels are widely used in adsorbents ^{[2, 3]}, agriculture ^[4], drug delivery ^[5–7], tissue engineering ^[8], and communication sensing ^{[9, 10]}, which has received extensive attention and applications. In recent years, with the development of artificial intelligence devices, the demand for sensors for the detection of pressure and strain has increased, and hydrogels have received extensive attention. The sensor converts the detected mechanical signals such as subtle stress and strain into electrical signals such as output current, resistance, and capacitance changes ^[11]. For better application in human detection, flexible sensors based on flexible and highly biocompatible materials need to be developed. Different from ordinary polymers (such as plastics and rubbers), hydrogels are a class of soft substances with polymer networks that swell with water and have the advantages of softness and high biocompatibility. Therefore, hydrogels can be used in tissue engineering, flexible electronics ^{[12, 13]}, and soft robotics ^[14]. In addition, electrical conductivity, mechanical strength, and anti-freezing performance are essential factors for flexible sensors. However, poor mechanical properties and environmental adaptability severely limit their applications in the above-mentioned fields.

A current effective strategy to improve the mechanical properties of hydrogels is to construct interpenetrating network (IPN) structures. Compared with hydrogels with a fully interpenetrating network, semi-interpenetrating network (semi-IPN) hydrogels do not restrict the interpenetrating elastic network and are co-crosslinked by physical and chemical, thus possessing high environmental adaptability and higher mechanical properties. Furthermore, adding functional nanoparticles with high specific surface area to hydrogels is another effective way to enhance the mechanical properties of hydrogels. At present, cellulose nanocrystals (CNC) ^[15], multi-walled carbon nanotubes ^[16], graphene oxide ^[17], nano bentonite ^[18], and even hybrid nanofillers have been widely mentioned to improve the mechanical properties of hydrogels.

Cellulose nanocrystals are highly available and low-cost biopolymers for the production of high-value products. Which has good biocompatibility and biodegradability and is combined with other materials for industrial applications. Cellulose nanocrystals are often combined with conductive fillers to make conductive nanocomposite composites ^[19] so that it has excellent mechanical properties. Dewi et al. ^[20] isolated nanocrystalline cellulose (CNC) and characterized its performance as a reinforcing agent for alginate-nanocrystalline cellulose hydrogel beads. The results show that CNC can be used as a reinforcing agent, which can improve the physical, mechanical, and thermal properties of the hydrogel. Polyvinyl alcohol (PVA) is a hydrophilic, semi-crystalline, biocompatible, and nontoxic polymer that contains many hydroxyl groups. It is a good choice for biocompatible hydrogels due to its good solubility and affinity for different polymers. PVA chains act as elastomeric fillers in hydrogels to improve mechanical properties. Liu et al. ^[21] used carboxylated polyvinyl alcohol and chitosan/Fe³⁺ as raw materials to prepare double cross-linked hydrogels with amide and coordination bonds. The hydrogel has a good swelling rate and good biodegradability, stronger mechanical properties, and adhesion.

In addition to insufficient mechanical properties, hydrogels lose water at higher temperatures and freeze at lower temperatures, causing them to lose flexibility and stretchability, reducing electrical conductivity, which limits the use of hydrogels in various environments in the application. At present, there are two main methods to improve the anti-freezing and anti-drying performance of hydrogels, that is coating elastomers on the surface of hydrogels ^{[22, 23]} and adding inorganic salts. The challenge for the coating approach is to enhance the interaction between the hydrogel and the elastomer. Since water molecules are likely to diffuse and evaporate through the elastomer, this method is not a long-lasting method for improving anti-freezing and anti-drying performance. Compared with the coating method, hydrogel adding inorganic salts can improve the anti-freezing and anti-drying performance more permanently.

Zhou et al. ^[17] prepared an IPN hydrogel composed of cross-linked polyacrylamide, polyvinyl alcohol, and graphene oxide, and used lithium chloride to improve anti-drying performance. They found that as the LiCl concentration increased, the rate of water loss decreased. By adjusting the content and type of salt, the anti-freezing and anti-drying performance of the hydrogel can also be greatly improved. Furthermore, the addition of salt also improved other properties of the hydrogel. Wu et al^[24] used polyvinyl alcohol/cellulose nanofibers (CNFs) as a composite substrate to prepare ion-conducting hydrogels for strain sensors, and discussed the effect of Fe³⁺ concentration on the mechanical properties and conductivity of ion-conducting hydrogels. For example, Wang et al. ^[25] reported that hydrogels would become conductive in the presence of sufficient water-soluble salts, which would expand their applications in areas where electrical conductivity is required.

In this work, a semi-IPN hydrogel network was constructed by cross-linking the PAM network and linear polyvinyl alcohol chains. Due to the hydrogen bonds generated by PVA ^[26], it is considered to be the material of choice for forming stable semi-IPN polymer networks. The addition of LiCl and DMSO can improve the anti-freezing and anti-drying performance of the hydrogel, enhance its environmental adaptability, and facilitate its application in various environmental conditions. And different ions were introduced to analyze the conductivity difference of the hydrogels. Furthermore, cellulose nanocrystals are used to reinforce hydrogels for mechanical stability. The prepared hydrogels exhibit excellent cyclic tensile recovery performance, excellent anti-freezing, and anti-drying properties, and high electrical conductivity, proving that the cellulose nanocrystal-reinforced semi-IPN structure is beneficial to improve the mechanical properties of hydrogels. In addition, the loading of high-concentration ions can significantly improve environmental adaptability, and the prepared ion-conducting hydrogels have potential applications in the fields of soft robots, sensors, and flexible wearable devices.

2.1. Materials

Polyvinyl alcohol-1750 (PVA) was purchased from Sinopharm Chemical Reagent Co., Ltd. Acrylamide (AM), N, N'-methylene bisacrylamide (MBA), and potassium persulfate (KPS) were supplied from Shanghai Macklin Biochemical Co., Ltd. Nanocellulose Suspension (CNC 4.1wt%) were acquired from Guilin Qihong Technology Co., Ltd. Lithium Chloride (LiCl) and Dimethyl Sulfoxide (DMSO) were purchased from Shanghai Titan Technology Co., Ltd. All reagents are used directly without further purification.

2.2. Preparation of PAM/PVA/CNC/LiCl Hydrogels

CNC solutions of different qualities were mixed with 1.2 g of PVA and DMSO of different qualities to prepare PVA/CNC solutions. Then, 4g AM and LiCl of different qualities were added to the above solution and stirred evenly. Subsequently, 0.02g MBA and 0.2g KPS were added into the beaker and stirred. Air bubbles were removed before pouring the mixture into Teflon molds. Finally, the PAM/PVA/CNC/LiCl hydrogels were obtained after polymerization in an oven at 70°C for 1 h.

The samples were named PAM/PVA/CNC_x/LiCl_y hydrogels, where: x represents the weight percentage (wt%) of CNC in the solution; y refers to the content of LiCl.

Table 1

Compositions of the hydrogels
Sample	CNC	DMSO	H₂O	LiCl
PAM/PVA/CNC₀/LiCl_2.4	0g	8g	8g	2.4g
PAM/PVA/CNC_0.25/LiCl_2.4	1.56g	8g	8g	2.4g
PAM/PVA/CNC_0.5/LiCl_2.4	3.34g	8g	8g	2.4g
PAM/PVA/CNC_0.75/LiCl_2.4	5.38g	8g	8g	2.4g
PAM/PVA/CNC₁/LiCl_2.4	7.75g	8g	8g	2.4g
PAM/PVA/CNC_0.5/LiCl₀	3.34g	8g	8g	0g
PAM/PVA/CNC_0.5/LiCl_1.2	3.34g	8g	8g	1.2g
PAM/PVA/CNC_0.5/LiCl_3.6	3.34g	8g	8g	3.6g
PAM/PVA/CNC_0.5/CaCl_{2 2.4}	3.34g	8g	8g	2.4gCaCl₂
PAM/PVA/CNC_0.5/AlCl_{3 2.4}	3.34g	8g	8g	2.4gAlCl₃
PAM/PVA/CNC_0.5/LiCl_2.4/H	3.34g	0g	16g	2.4g
PAM/PVA/CNC₀/LiCl_2.4/H	0g	0g	16g	2.4g

2.3. Scanning electron microscope

The surface morphology and cross-sectional morphology of PAM/PVA/CNC₀/LiCl_2.4 and PAM/PVA/CNC_0.5/LiCl_2.4 hydrogels were observed with a JEM-2100F scanning electron microscope. The PAM/PVA/CNC/LiCl hydrogel samples were processed by freeze-drying to obtain dehydrated hydrogels. And the hydrogel samples were dried and sprayed with gold to study the pore structure of the hydrogel.

2.4. Fourier transform infrared spectroscopy

The PAM/PVA/CNC/LiCl hydrogel samples of different control groups were weighed. And then use the CX-9600 Fourier transform infrared spectrometer (FTIR) to test the chemical structure of the dehydrated hydrogel samples, and perform spectral analysis on the samples according to the standard. The resolution was ± 2 cm^− 1, the wavenumber range was 400–4000 cm^− 1, and the step size was 4 cm^− 1.

2.5. Mechanical properties

The above-prepared PAM/PVA/CNC/LiCl hydrogel samples of different specifications were cut respectively to make sample strips of 50mm×10mm×2mm, and then the mechanical properties of the sample strips were tested by a universal material testing machine. The loading speed was 50mm/min.

2.6. Ionic conductive property

The voltage of the PAM/PVA/CNC/LiCl hydrogel self-powered strain sensor was recorded by the VICTOR VC9801A + digital multimeter, and the impedance value data of the hydrogel was obtained by the LCR TH2832 digital bridge tester.

The ion-conducting hydrogel samples (50×10×2mm) were quantitatively tensile tested by a universal material testing machine, and the relative resistance change and sensitivity factor of the hydrogel under tensile cyclic strain was tested by a digital bridge tester. Calculate the relative change of resistance (ΔR/R₀, where ΔR = R-R₀), the sensitivity factor calculation formula was as follows (1):

$$\text{G}\text{F}=\frac{\frac{{\Delta }\text{R}}{{\text{R}}_{0}}}{\frac{\text{l}-{\text{l}}_{\text{o}}}{{\text{l}}_{0}}}$$

(1) In the formula: l₀ and l represent the length (m) of the hydrogel before and after the strain is applied; R₀ and R represent the resistance (Ω) before and after straining.

2.7. Sensor characterization and test

The prepared PAM/PVA/CNC/LiCl hydrogel samples were cut to make a sample strip of 60mm×10mm×2mm, which was taped to the index finger, and the finger was bent at an angle (0° to 90°). The digital bridge tester was used to record the resistance of the hydrogel under different strains and calculate the relative change in resistance (ΔR/R₀, where ΔR = R-R₀).

2.8. Anti-freezing test

The PAM/PVA/CNC_0.5/LiCl_2.4/H and PAM/PVA/CNC_0.5/LiCl_2.4 hydrogel control groups were made, and cut into 40mm×10mm×2mm samples, and put in a refrigerator below − 20°C to be frozen, taken out every 24 hours to test conductivity, the conductivity calculation formula was as follows (2):

$${\sigma }=\frac{L}{R\text{*}S}$$

where σ (S/m) represents the ionic conductivity of the hydrogel, L (m)represents the length of two adjacent electrodes of the hydrogel, R (Ω)represents the resistance value of the hydrogel, and S(m²) was the contact area of the hydrogel.

2.9. Anti-drying test

All samples were cut into strips of 40 mm×10 mm×2 mm, and the PAM/PVA/CNC/LiCl hydrogel samples were placed in an oven at 60°C for 8 hours, and the weight W_t of the hydrogel was measured every 1 hour. The initial weight was W₀.

The formula for calculating the weight ratio was:

Weight ratio = W_t/W₀*100%.

2.10. Swelling performance test

The PAM/PVA/CNC/LiCl hydrogel samples were made into a cylindrical shape and placed in a beaker filled with water to stand still. The weight V_t of the hydrogel was recorded every 24 hours, and the initial weight was V₀.

The calculation formula for the swelling ratio was as follows:

Swelling ratio = V_t/V₀*100%.

3.1. Preparation of PAM/PVA/CNC/LiCl hydrogel

As shown in Fig. 1, the schematic diagram of the preparation process of PAM/PVA/CNC/LiCl hydrogel was introduced. The hydrogel constructed a semi-IPN hydrogel network by cross-linking the PAM network and linear polyvinyl alcohol chains with the introduction of dimethyl sulfoxide (DMSO) and cellulose nanocrystals (CNC) as a cryoprotectant reinforcement. The freezing point of water could be lowered due to the formation of strong hydrogen bonds between the sulfinyl groups in DMSO and water molecules. The co-solvent system could effectively lower the freezing point of water and inhibit the formation of ice crystals in the gel network. The addition of LiCl contributed to the high ionic conductivity. In addition, LiCl was attached to PVA molecular chains through ionic interaction with COO⁻, and its strong hydration strengthened the binding of PVA to surrounding water molecules while achieving excellent water association ability^[27].

3.2. Morphology observation

To explore the microstructure of the prepared PAM/PVA/CNC/LiCl hydrogel, the morphology of the freeze-dried hydrogel was observed by scanning electron microscopy (SEM). As shown in Fig. 2, compared with PAM/PVA/CNC₀/LiCl_2.4 hydrogel (Fig. 2a), the pore size of PAM/PVA/CNC_0.5/LiCl_2.4 hydrogel became smaller (Fig. 2b). The pores of the hydrogel would generate many new pores after the addition of CNC, which could be seen that the successful mixing of CNC proved that the addition of CNC had a strengthening effect on the skeleton of the hydrogel. The porous structure of the hydrogel would allow the free Li⁺ in the hydrogel to have more flow channels, thereby improving the sensing performance of the hydrogel and making it better applied to flexible sensors.

3.3. FTIR Analysis

As shown in Fig. 3, for the FTIR spectrum of the hydrogel, the characteristic absorption peaks of the amide group were obvious, including the N-H bending vibration peak at 1641 cm^− 1 and the C-N stretching vibration peak around 1326 cm^− 1. This indicated that all hydrogels had the same PAM amide groups. In addition, for PAM/PVA hydrogels, the absorption peaks of primary amino groups tended to merge into a broad band at 3339 cm^− 1, which was due to the effect of hydrogen bonds between PAM and PVA, when the hydrogel was added after CNC, the absorption peaks of the primary amino groups of the hydrogel began to shift to the low wave number band, which was caused by the intermolecular and intramolecular hydrogen bonds between CNC and PVA. The peak at 1012cm^− 1 was the absorption vibration peak of the sulfoxide S = O bond. The change of the absorption vibration peak of the sulfoxide S = O bond could be understood as the physical effect of DMSO. These indicated the formation of a robust semi-IPN network in the prepared PAM/PVA hydrogels.

3.4. Mechanical Properties

As shown in Fig. 4, the PAM/PVA/CNC/LiCl hydrogel exhibited excellent ductility and good mechanical properties. The PAM/PVA/CNC/LiCl hydrogel could be stretched more than twice its original length without breaking (Fig. 4a), and it could also be stretched to twice the length even when twisted (Fig. 4b), which shows that the good ductility of PAM/PVA/CNC/LiCl hydrogel. The PAM/PVA/CNC/LiCl hydrogel could be compressed and returned to its original height after being stressed (Fig. 4c). In addition to good ductility, PAM/PVA/CNC/LiCl hydrogel also had good mechanical properties, which was specifically shown by using a thin PAM/PVA/CNC/LiCl hydrogel through a rubber band and a 500g weight. It could be seen that it can easily lift a 500g weight (Fig. 4d). Therefore, the PAM/PVA/CNC/LiCl hydrogel had good mechanical properties and ductility, making it an ideal material for flexible sensors.

As shown in Fig. 5, the tensile stress-strain curves of all PAM/PVA/CNC/LiCl hydrogels showed high stress and strain. Wherein the elongation at break of the hydrogel without adding CNC was 202%, and the tensile breaking stress was 80kPa, while for the hydrogel of PAM/PVA/CNC_0.5/LiCl_2.4, the elongation at break of the hydrogel could be up to 354%, the tensile fracture stress could reach 170kPa. As shown in Fig. 5b, as the CNC content in the hydrogel increased, the mechanical properties of the hydrogel gradually increased and then decreased slowly. The introduced CNCs could be regarded as the cross-linking points of semi-IPNs because they could form a large number of hydrogen bonds with functional group polymer chains with high specific surface area. This explained why the mechanical properties of hydrogels increase with the increase of CNC content. When the content of CNC in the hydrogel was too much, it would lead to the aggregation of CNC in the hydrogel or the formation of more microcrystals, which would make the three-dimensional network structure unevenly distributed, resulting in a decrease in the mechanical properties of the hydrogel.

3.5. Self-Adhesiveness

Unlike traditional wearable sensor devices, flexible sensors assembled based on hydrogels could detect signals without sticking to the skin with materials such as tapes. To conveniently monitor human motion and physiological signals and avoid interface delamination under repeated deformation as a sensor, proper adhesion must be maintained between wearable devices and human skin, and flexible wearable devices based on hydrogels had high excellent adhesion performance, and would not cause discomfort or even damage to the human body due to long-term high-current stimulation. As shown in Fig. 6, the as-prepared PAM/PVA/CNC/LiCl hydrogel exhibited excellent adhesion to various surfaces including glass, iron, leather, plastic, rubber, and other materials. The weight of 200g could also be easily stuck by PAM/PVA/CNC/LiCl hydrogel. The results showed that the PAM/PVA/CNC/LiCl hydrogel could be widely used in the adhesion of various material surfaces.

3.6. Ionic conductivity

As shown in Fig. 7a, the PAM/PVA/CNC/LiCl hydrogel was connected to the circuit through a wire and a 3V power supply, and it could be found that the LED light was lit, even when the hydrogel was stretched, the LED light still lit but with a noticeable change in bulb brightness. The reason was that when the hydrogel was stretched, its conductive pathways were stretched, the distance between ions became longer, and the transmission of electrons in the pore structure of the hydrogel was blocked, increasing the resistance of the hydrogel and a decrease in the conductivity. As shown in Fig. 7b, the hydrogel was assembled with zinc and copper sheets to form a simple self-powered device, and electrons flowed from the negative electrode to the positive electrode through the wire, thereby forming a potential difference of about 0.838 V, which converts chemical energy into electrical energy well. This self-powered sensor device freed itself from the limitation of external power sources and expanded the potential of hydrogels for flexible and wearable sensors.

As shown in Fig. 8a, the electrical conductivity of PAM/PVA/CNC/LiCl hydrogels with different CNC contents was tested. The experimental results showed that when the CNC content increased, the electrical conductivity of the hydrogel would first increase. Then began to decline, this was because the strong hydrogen bonds formed by CNC in the PAM/PVA/CNC/LiCl hydrogel network would significantly change the conduction path of the hydrogel, so that its conductivity would continue to rise until it reaches a peak, when the CNC excessive content would lead to the aggregation of CNC in the hydrogel or the formation of more crystallites, resulting in the uneven distribution of the three-dimensional network structure, resulting in a decrease in the conductivity of the hydrogel. The resistance value of PAM/PVA/CNC/LiCl hydrogel with different Li⁺ ion concentrations was recorded with the digital bridge, and the change in its conductivity was also calculated. It can be seen from Fig. 8b that when the Li⁺ concentration gradually increased, ion channels were opened, and more and more conductive ions could move, which was manifested as an increase in conductivity. When the Li⁺ concentration increased to a certain level, due to the limited cross-sectional area of the channel, it was difficult for the ion channel to ensure that all ions pass through, and the collision of conductive ions reduced the conductivity, which was manifested as a decrease in conductivity. In addition, a large amount of Li⁺ was immobilized through ionic interactions, leading to a decrease in the cross-sectional area of ion channels and a decrease in conductivity^[27]. Hydrogel samples with different ions were prepared, and their conductivity tests were compared. The results showed (Fig. 8c) that the conductivity of PAM/PVA/CNC/LiCl hydrogel containing Li⁺ was better than that of hydrogels containing Ca²⁺ and Al³⁺. Since the ion mobility of Li⁺ was better than that of Ca²⁺ and Al^3+[28], it had more significant transport performance, which greatly enhanced the conductivity, demonstrating the excellent electrical conductivity of PAM/PVA/CNC/LiCl hydrogels.

3.7. Sensing Performance

As shown in Fig. 9a, the PAM/PVA/CNC/LiCl hydrogel was subjected to a stretch release cycle from 0% strain to 200% strain, and the change of its relative resistance (ΔR/R₀) was quantitatively studied, which could be seen that as the strain increases, the relative resistance of the PAM/PVA/CNC/LiCl hydrogel also increases, which indicates that the PAM/PVA/CNC/LiCl hydrogel strain sensor has a positive strain effect. The strain sensitivity factor (GF) was an important index to evaluate sensor performance. The GF value corresponding to the 0%-300% strain of the PAM/PVA/CNC/LiCl hydrogel was shown in Fig. 9b. The graph of the relative resistance change was divided into three parts. It could be seen that the GF value of the PAM/PVA/CNC/LiCl hydrogel is 1.44 when the strain is 0–50%, the GF value of the hydrogel is 2.38 when the strain is 50–150%, and when the strain is 150–300%, the GF value of the hydrogel reaches 4.31. This showed that the prepared hydrogel had good sensitivity to large strains and could effectively sense the generation of large strains. In addition, the PAM/PVA/CNC/LiCl hydrogel sensor produced stable output signals over 10 cycles at 200% strain (Fig. 9c), which demonstrated the good stability, reproducibility, and excellent sensing properties of the prepared hydrogels.

The prepared PAM/PVA/CNC/LiCl hydrogel could make sensitive feedback to the pressure. Based on the assembled hydrogel pressure sensor, the relative resistance change of the hydrogel under the pressure of 200g weight was − 22%, and it still tended to be stable after cycling. It could be seen from Fig. 10 that the hydrogel could give feedback at the moment when the weight was put on it, and it tended to be stable within 1s. When the weight was picked up, the relative resistance returned to the initial position due to the good shape recovery, which illustrated the excellent pressure response characteristics of the hydrogel, and also showed that the prepared PAM/PVA/CNC/LiCl hydrogel had the potential to act as a pressure sensor.

To test the potential of PAM/PVA/CNC/LiCl hydrogel-based wearable strain sensors, the hydrogel was taped to the human index finger and made various bending angles. As shown in Fig. 11a, when the index finger was bent to different angles, the relative resistance of the hydrogel changed differently, and when the finger returned to the original angle, the relative resistance of the hydrogel was the same as before. When the finger was flexed cyclically in the range of 0–90°, the relative resistance of the hydrogel exhibited a steady periodic change (Fig. 11b). The results showed that the prepared PAM/PVA/CNC/LiCl hydrogel had good sensitivity and electrical stability, and the relative resistance at different bending angles could be observed in real-time. A wearable strain sensor based on PAM/PVA/CNC/LiCl hydrogel could clearly distinguish the difference in the bending angle of the index finger, which had the potential to fabricate wearable electronic devices.

3.8. Anti-freezing Performance

The anti-freezing performance of hydrogel was very important for flexible sensors based on the hydrogel. For this reason, the PAM/PVA/CNC/LiCl hydrogel samples were placed in a -20°C refrigerator to test the anti-freezing performance. As shown in Fig. 12, the PAM/PVA/CNC_0.5/LiCl_2.4 hydrogel still had good conductivity after 7 days, and it could be seen that the change curve of its conductivity was not so drastic. However, after 7 days of PAM/PVA/CNC_0.5/LiCl_2.4/H hydrogel, the conductivity was extremely low and the change curve was sharp. When the frozen hydrogel samples were connected to small light bulbs, it could be observed that the light bulbs connected with PAM/PVA/CNC_0.5/LiCl_2.4 hydrogels were brighter. This showed that the addition of DMSO could improve the anti-freezing performance of the hydrogel, because the sulfinyl group in DMSO could form a strong hydrogen bond force with water molecules, thereby inhibiting the formation of ice crystals, making PAM/PVA/CNC/LiCl hydrogel was not easy to freeze and lose conductivity.

3.9. Anti-drying Performance

Since ion-conducting hydrogels generally had a high water content, in daily use, the water in the hydrogel would inevitably evaporate, resulting in a decrease in the water content of the hydrogel and affecting the biocompatibility of the hydrogel as well as the performance of various aspects. To improve the anti-drying performance of the hydrogel, solvents, such as glycerin and DMSO, were usually added when preparing the hydrogel. As shown in Fig. 13, after placing the PAM/PVA/CNC_0.5/LiCl_2.4 hydrogel and the PAM/PVA/CNC_0.5/LiCl_2.4/H hydrogel in a 60°C oven, the PAM/PVA/CNC_0.5/LiCl_2.4 hydrogel sample still maintained the basic shape, while the PAM/PVA/CNC_0.5/LiCl_2.4/H hydrogel became hard, had no flexibility and turned yellow, according to the hydrogel weight change curve, which could be found that the weight of the PAM/PVA/CNC_0.5/LiCl_2.4 hydrogel sample changed by 22%, while the weight of the PAM/PVA/CNC_0.5/LiCl_2.4/H hydrogel sample changed by 50%. It could be seen that after adding the DMSO solvent, the weight change of the hydrogel was relatively gentle, and the anti-drying performance is better. This was because DMSO, as an organic solvent, acted as a protective agent in the ion-conducting hydrogel, interacting with the water molecules in the hydrogel, thereby reducing the rate of water volatilization.

3.10. Swelling Performance

Since hydrogels would absorb a large amount of water and swell, the degree of swelling had a great impact on physical properties and applications. To study the swelling behavior of PAM/PVA/CNC/LiCl hydrogels, distilled water was used as the swelling medium to conduct swelling kinetics measurements. The swelling kinetics of PAM/PVA/CNC/LiCl hydrogel, as shown in Fig. 14, the swelling capacity of PAM/PVA/CNC/LiCl hydrogels increased greatly in the initial period after immersion in distilled water, and then the swelling capacity increased slowly until reaching the equilibrium swelling capacity. Since the hydrogel formed a three-dimensional cross-linked polymeric network, this significantly improved the hydrogel swelling capacity. The results showed that the PAM/PVA/CNC/LiCl hydrogel had a higher swelling ratio, which greatly broadened its application in other fields and prolonged the use time of the hydrogel.

A PAM/PVA/CNC/LiCl hydrogel with a semi-interpenetrating network was prepared, and its mechanical properties were analyzed and studied, which confirmed the reinforcement effect of nanocellulose on the mechanical properties of the hydrogel. In addition, its electrical conductivity, anti-drying, and anti-freezing were evaluated. The results showed that the PAM/PVA/CNC/LiCl hydrogel had excellent mechanical properties, which was manifested in its good stretch recovery, and its relative resistance was unchanged after 10 cycles of stretching under 200% tensile deformation. The introduction of DMSO significantly improved the anti-drying and anti-freezing properties of PAM/PVA/CNC/LiCl hydrogels even at 60°C and − 20°C, they still had good electrical conductivity. At the same time, the addition of LiCl greatly improved the conductivity of PAM/PVA/CNC/LiCl hydrogel, which was significantly better than that of other metal ion hydrogels (Ca²⁺, Al³⁺). The results showed that PAM/PVA/CNC/LiCl hydrogel had good application prospects in wearable devices, flexible sensors, and other fields.

Acknowledgments

The authors gratefully acknowledge the financial support provided by the Graduate student scientific research innovation projects in Jiangsu province (Project Number: SJCX22-XY054); The Open Fund of Key Laboratory for Advanced Technology in Environmental Protection of Jiangsu Province (JBGS026); the Industry-Academia-Research project of Jiangsu Province [grant number BY2022456].

Author Contributions

All authors contributed to the study conception and design. Material preparation, data collection and analysis were performed by [Yaning Wang], and [Xinmin Huang]. The first draft of the manuscript was written by [Yaning Wang] and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.

Funding

“This work was supported by [SJCX22-XY054] and [JBGS026].

Competing interests

The authors have no relevant financial or non-financial interests to disclose.

Conflict of interests

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Ethics approval and consent to participate

Not applicable.

Consent for publication

All the authors have agreed and given full consent for publication of this manuscript.

Availability of data and materials

The data supporting the findings of this study are openly available as a DOI to an electronic repository.

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No competing interests reported.

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Preparation and properties of nanocellulose- reinforced environmentally adaptable PAM/PVA semi- interpenetrating highly conductive ion-conducting hydrogel

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Abstract

Figures

1. Introduction

2. Experimental section

2.1. Materials

2.2. Preparation of PAM/PVA/CNC/LiCl Hydrogels

2.3. Scanning electron microscope

2.4. Fourier transform infrared spectroscopy

2.5. Mechanical properties

2.6. Ionic conductive property

2.7. Sensor characterization and test

2.8. Anti-freezing test

2.9. Anti-drying test

2.10. Swelling performance test

3. Results and discussion

3.1. Preparation of PAM/PVA/CNC/LiCl hydrogel

3.2. Morphology observation

3.3. FTIR Analysis

3.4. Mechanical Properties

3.5. Self-Adhesiveness

3.6. Ionic conductivity

3.7. Sensing Performance

3.8. Anti-freezing Performance

3.9. Anti-drying Performance

3.10. Swelling Performance

4. Conclusion

Declarations

References

Additional Declarations

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