Stretching Structuring Nanofibrillar Polyvinyl Alcohol Based Ionic Organohydrogels And Application

Hydrogel is defined as a three-dimensional polymer network that containing a large amount of water.Classical hydrogels can be synthesized by cross-linking hydrophilic polymer molecular chains or water-soluble monomers with cross-linkers.However,compared with natural hydrogels such as biological soft tissues,synthetic hydrogels are soft and weak due to its simple and loose in structure,which greatly limit their broad application.Therefore,by designing hydrogel with superstructure network inspired from natrual hydrogels is of great significance to develop new synthetic hydrogels with excellent functions and properties.In recent years,many efforts have been made toward developing various kinds of bioinspired hydrogels with energy dissipation structures from the molecular to macroscopic scale.Some of them have excellent mechanical properties comparable to muscle and cartilage.These structured hydrogels have shown great application prospects in various fields such as soft machines,artificial organs,and regenerative medicine.Nowadays,many special strategies have been developed to fabricate structured hydrogels with different structures,activities and functions,such as 3D printing technology,electrospinning technology,gradient ice template technology,tissue engineering tecnology,etc.However,these methods usually require long processing time and high cost,which largely limit the large-scale preparation of structured hydrogel materials.Therefore,the development of methods for new hydrogel network fabrication is very important to explore high-performance hydrogels and their composite materials with highly organized structure.In this paper,crystalline,biodegradable PVA was selected as the hydrogel matrix.First,glycerol was used as a co-solvent to prepare a PVA organohydrogel with nanofibrillar network.Then,the uniaxial and biaxial stretching strategy was used to prepare PVA organohydroel with excellent omini-directional mechanical performance.Moreover,a small amount of cellulose nanofiber(CNF)was added as toughening agents to PVA organohydrogel to explore the strengthening and toughening effects of long nanofibers in the organohydrogel.Furthermore,this paper discussed the functional application of structured PVA based gels,and shown the potential application of structured hydrogels.The research content and main conclusions are as follows:(1)Introducing glycerol as a co-solvent and regulating the content of glycerol,a series of PVA gels with different topological network were prepared.The introduction of glycerol not only made PVA oranohydrogel a high anti-drying performance,but also improved the crystallinity of the organohydrogel by inhibiting the intermolecular interaction between the PVA molecular chains and cross-linking density.When the ratio of glycerol/water reaches 1:1,the size of the internal framework of the gel is reduced from submicron scale to tens of nanometers(10 times than that of PVA hydrogel).Such nanofibrillar gel has high transparency(>90%)and good mechanical properties.The toughness of the nanofibrillar gel reached to 3.2MJ/m~3,which is 10 times tha that of the PVA hydrogel,and the strength and elongation at break of the gel have been greatly improved.Based on the aforementioned excellent optical and mechanical properties of nanofibrillar gel,a high-performance PVA ionic organohydrogel was further prepared by soaking in a Na Cl solution.PVA ionic organohydrogel has high sensitivity(GF=1.56),high linearity(R~2>0.998)and good cycle stability(>1000cycles).Furthermore,we demonstrated the potential application of high-performance PVA gel in smart contact lense,facial expression recognition and motion monitoring.The promising ionic organohydrogel,which is also applicable to other systems,enables opportunities of the next-generation electronics and shows a great potential in human-interactive sensing and artificial intelligence.(2)In order to prepare a gel that mimics the highly ordered fabrillar network in natural tissues,a PVA organohydrogel with anisotropic nanofibrillar structure is prepared by pre-stretching and fixing at 60°C through the special interaction of glycerol and PVA molecular chains.When the prestretch is 4.2,the orientation factor of gel reaches at0.79.After fixing,the gel exhibits obvious stress-induced crystallization,and a smaller size and a larger amounts of PVA nanocrystalline domains was formed.These nanocrystalline regions serve as cross-linking points to help improve its mechanical properties.As the prestretch ratio increases,the mechanical properties of the oriented gel are significantly improved.When the prestretch is 4.2,strength(5.6±0.4 MPa)and modulus(2.7±0.1 MPa)of the gel are 3 times than that of unstretched gel.Although the elongation at break is slightly decreased,but still remain at a high level(240%).The toughness is 11.5±1.3 MJ/m~3,which is 2.7 times that of unstretched.Perpendicular to the prestretching direction,the fracture energy of the gel(3.6±0.6 J/m~2)is significantly lower than the another direction(19.3±3.3J/m~2),which is almost eqavolent to the unstretched gel(4.2±0.7 k J/m~2).The fatigue threshold of the anisotropic gel reaches 1130 J/m~2,which is 5 times than that of the AG-0 gel,as well as relatively high level in literature reports.Due to the presence of glycerin as a co-solvent,the anti-drying property of the gel is significantly improved,and the water loss rate after 40 days is only about 20%.Moreover,this physically cross-linked gel also has good recycling processing performance,which reflects its excellent eco-friendly function.Moreover,the antibacterial properties are further investigated based on the nanofiburillar structure of the gel.The AG-4.2 gels exhibite a high antibacterial property(99.3%against E.coli,95.6%against S.aureus).Excellent anti-drying,recycling and antibacterial properties provide basic functions for its wide use on human-machine interface.The ionic gels were prepared by soaking in Na Cl solution.The ionic gel not only has good sensitivity and linearity in strain sensing(GF=0.79,R~2>0.99),but also exhibits excellent durability performance.It still maintain a stable durability after 20,000cycles even with pre-cut crack.Excellent fatigue-resistant cycling sensing performance makes it suitable for long-term strain sensing.Therefore,a wearable respiratory monitoring system based on the anti-fatigue ionic gel sensor was designed.Furthermore,by obtaining signals from the gel sensor,the system can conduct long-term monitor the respiratory status of clinical patients and help rate the respiratory risk.The system provide a feasible path for clinical patient respiratory risk assessment,and has a good potential application.(3)To further fabricate high-performance gel with hierarchical network,an anisotropic PVA/CNF gel with the introduction of CNF into the nanofibrillar PVA organohydrogel was prepared by uniaxial stretching/fixing method.When the prestretch is 5.8,the birefringence of gel reaches at 0.88 and the orientation factor of gel is 0.82,indicating that an anisotropic structure is formed inside the gel.When the prestretch is 4.2,the orientation factor of is 0.79,which is similar to the anisotropic PVA gel.As the prestretch increases,the mechanical properties of the oriented gel are significantly improved.For the anisotropic PVA/CNF gel,its strength(16.1±0.6MPa)and modulus(20.9±4.9 MPa)are 7 times and 34.8 times than that of the unstretched gel,respectively.The elongation at break remains about 170%.The AC-4.2 gel exhibits high toughness(35.6±5.3 MJ/m~3)due to its high elongation at break.The fracture energy of the anisotropic PVA/CNF gel is 41.4±0.3 J/m~2,which is 7.7times than that of the unstretched gel(5.4±1.3 J/m~2)and 2.1 times than that of anisotropic PVA gel(19.3±3.3 J/m~2).It is indicated that the anisotropic PVA/CNF gel are capable of excellent crack resistance.The fatigue threshold of the AC-5.8 gel is 1360 J/m~2,which is 16%higher than the uniaxially stretched PVA gel(1130 J/m~2).The small amount of CNF can consolidate the PVA network so as to inhibit the propagation of crack.The ionic PVA/CNF gel was further prepared by soaking in Na Cl solution.Its high strength and high modulus performance can avoid large deformation caused by external stress and damage the stability of the temperature-resistance signal,so it is applied to strain sensors and temperature sensors under small strain conditions.The ionic PVA/CNF gel shows good cycling stability,sensibility in the strain range of 1-50%,and frequency independence.Meanwhile,the ionic PVA/CNF gel can be used at the range of-20-60°C,and has a high conductive activation energy(27.2 k J/mol).In addition,the resistance change of the AIC-5.8 gel is 2.8%/°C at 20°C,indicating a good temperature sensitivity at room temperature.(4)The uniaxially structured gel has anisotropy and large differences in mechanical properties in different directions.Therefore,the biaxially structured PVA gel was further prepared by the biaxially stretching and fixing method.As the prestretch increases,the surface size of the biaxially structured PVA gels increases and the thickness size decreases.As the prestretch increases,the strength of the gels increases,and the elongation at break decreases slightly.Along the both directions,the strength of the gel are 8.3±1.2 MPa and 8.0±0.8 MPa,which are in between the both aforementioned anisotropic gels.The modulus of the gel(>4.3 MPa)and the elongation at break(>340%)are at a relatively high level.The toughness of the gel in both directions is excess to 17 MJ/m~3,and the fracture energy is excess to 23 k J/m~2,both of which are in between the both aforementioned anisotropic gels.on the one direction,the fatigue threshold of the gel is 600 J/m~2,which is 2.7 times than that of the unstretched gel.Meanwhile,the fatigue percolation on the another direction of the gel is 770 J/m~2.The gel exhibit excellent omni-directional fatigue resistance,which is related to the dense polymer network and the more perfect crystal region as the cross-linking point to increase the dissapation energy when crack propagated.Due to the excellent mechanical properties of the biaxially stuctured gel in all directions,it can meet the mechanical strain and fatigue resistance requirements of the flexible energy storage device.Furethermore,by immersing in a Li Cl solution,the ionic gel electrolyte was prepared and assembled into a supercapacitor for electrochemical performance characterization.The ionic gel electrolyte shows a low electrochemical impedance(<50Ω)and a good cycle stability(>1000 cycles).The ionic gel electrolyte exhibits high mechanical properties for the electrochemical application and shows potential application in flexible supercapacitor.(5)In view of the good mechanical properties of the above-mentioned biaxially structured gel in various directions,we further prepared a gel with richer molecular chains entanglement.With introduction of a small amount of CNF,the biaxial structured PVA/CNF gel was prepared by the biaxial stretching and fixing method.The surface pore size of the gel increases,and a denser network was formed in the thickness.The gel improves the strength of the gel by 15%and the modulus by 30%compared with the biaxial structured PVA gel.However,due to the good interaction between CNFs and PVA,during the setting process,the crystallization of PVA is instead inhibited,resulting in a decrease of toughness and fracture energy.The fatigue threshold of the gel in the two directions are 700 J/m~2 and 610 J/m~2,respectively,which are equivalent to the the biaxial structured PVA gel,indicating a good fatigue resistance in both directions.The ionic gel was further immersed in Li Cl solution to prepare the biaxial structured PVA/CNF gel electrolyte and assembled into a supercapacitor for electrochemical performance characterization.The biaxial structured PVA/CNF gel electrolyte also shows lower electrochemical impedance(<50Ω)and good cycle stability(>1000 cycles).It is indicated that the ionic gel electrolyte with high strength,modulus and fatigue resistance is suitable for supercapacitor application in the electrolyte of supercapacitors.It provides a feasible idea for the preparation of new high-strength composite hydrogel electrolyte materials.