Preparation And Performance Of High-strength And High-toughness Polyvinyl Alcohol Hydrogels Based On Solvent Exchange Strategy

Hydrogels are a typical class of soft materials with a hydrophilic three-dimensional polymer network as the main support structure and are swollen by large amounts of water(typically greater than 50%)without dissolving,and are widely used in underwater wearable smart sensors,implantable artificial tissues,and soft robots.However,conventional hydrogels cannot be used as reliable load-bearing materials due to their weak mechanical properties,especially for applications under extreme mechanical conditions of high stress,large deformation,and cyclic loading.Although classical crosslinking strategies based on freezethaw cycles,dry annealing,biomineralization,and salt precipitation have been proposed to enhance the mechanical properties of hydrogel networks,their strength gains are often achieved at the expense of stretchability and even toughness.Therefore,it remains a great challenge to develop hydrogels with simultaneously high strength,high fracture strain,good fracture toughness,and excellent fatigue fracture resistance to meet the reliability and service life for different applications used under extreme mechanical conditions.In this paper,polyvinyl alcohol(PVA)with simple molecular structure,flexible conformation,abundant hydrogen bonding sites and tunable crystallization domains is chosen as a model for the study.The relationship between the strength,toughness and fatigue resistance of the hydrogel and these structural effects is investigated in detail.The research of this thesis is divided into three main parts as follows:1.Preparation and performance of PVA hydrogels based on the pre-freezing-assisted solvent exchange strategyThe pre-freezing-assisted solvent exchange strategy is based on two processes:prefreezing and solvent exchange,where the solvent exchange includes both the exchange of good and bad solvents of the polymer and the salt precipitation process.Firstly,PVA in its good solvent DMSO obtains a homogeneous polymer solution with a spreading conformation by breaking the inter-and intra-molecular interactions of the original polymer and by strong hydrogen bonding with the solvent molecules.The polymeric chains in this solution are extruded by the growing DMSO ice crystals during the pre-freezing process,which leads to microphase separation in the good solvent and the formation of tiny crystalline domains that act as cross-linking points for the pre-gel network.As the solvent exchange proceeds,the good solvent DMSO is completely replaced by the bad solvent H2O so that the hydrogen bonding between PVA molecules is changed from weak to strong.At the same time,the salting out of PVA by NaCl further increases the entanglement of polymer chains and crystallization domains.These restored strong non-covalent bonding interactions together build the tough gel network.The formation of the preconfigured network limits the movement of PVA chain segments and provides a template for subsequent solvent exchange to form more new physical cross-linking sites(e.g.,crystalline domains),which further increases the denseness of the gel network and thus improves the mechanical properties of the gels.The strength of hydrogels obtained based on this strategy reached 5.66 ± 0.54 MPa,toughness 35.62 ± 5.84 MJ/m3,strain at break 1161±40%,fracture energy 24.17 ± 0.93 kJ/m2,and fatigue threshold 735 J/m2.2.Preparation and performance of PVA hydrogels based on glycerol mixed solvent exchange strategyBased on the design concept of introducing NaCl into the poor solvent H2O to form a less soluble solvent for PVA to regulate the polymer chain conformation in the previous chapter,in this chapter,we introduce glycerol(Gly)into the poor solvent H2O to obtain a binary solvent mixture that is less soluble for PVA,and its abundant hydroxyl interaction sites will build up a tight polymer-solvent-polymer compared to NaCl.The solubility of PVA in DMSO is the same as in Chapter 1.During the exchange of the good solvent DMSO with the bad solvent(mixture of water and glycerol),the stretched PVA chain conformation shrinks and physical cross-linking domains such as entanglement,crystalline domains,and hydrogen bonding are formed between the polymer chains to build a tough gel network.The strength of the gels obtained based on this strategy reached 8.39 ± 0.51 MPa,toughness reached 52.63 ± 1.98 MJ/m3,strain at break reached 1127 ± 123%,and energy at break reached 16.34 ± 5.60 kJ/m2.This result exceeds Chapter 1 and proves that the polymer-solvent-polymer hydrogen bonding interaction constructed in the hydrogels by this strategy is more effective for its mechanical properties improvement is more effective.Moreover,the hybrid solvent exchange strategy can also impart excellent freeze resistance(up to-65℃),high temperature resistance(up to 110℃),and drying resistance(up to 14 d of environmental stability)to the gels by varying the ratio of glycerol to water compared to the single solvent exchange.3.Preparation and performance of PVA hydrogels based on solvent-exchange-assisted wet annealing strategyBased on the materials prepared in Chapter 2,glycerol gels were wet annealed and then solvent exchanged in water to obtain hydrogels.the dissolution and gelation processes of PVA were based on the design ideas in Chapters 1 and 2.The polymer chains can easily optimize their conformation by high temperature in the solvent-swollen state thanks to the difficult volatility of glycerol.A suitable wet annealing temperature and time(e.g.,30 min at 120℃)will contribute to the aggregation of molecular chains in the hydrogel,the improvement of crystallinity and the reduction of the distance between adjacent crystalline domains.With the increase of the number and size of physical cross-linking sites and the decrease of the distance in the hydrogel,a more denser network structure was formed.Moreover,the results of numerous comparative experiments show that this strategy is more effective in improving the mechanical properties of the hydrogels than the conventional one-step solvent exchange,cyclic freeze-thaw and dry annealing strategies in terms of modulating the molecular chain conformation,crystallinity and densification of the network structure.The obtained hydrogels achieved strength of 11.19 ± 0.27 MPa,toughness of 82.28 ± 2.89 MJ/m3,strain at break of 1879 ± 10%,fracture energy of 25.39 ± 6.64 kJ/m2,and fatigue threshold of 1233 J/m2.