水凝胶材料的力学性能在很大程度上决定其应用范围，因此开发和完善制备具有优异力学性能水凝胶的方法对水凝胶的理论研究和实际应用具有重要意义。基于Hofmeister效应的盐析浸泡处理能够向水凝胶网络中引入特定盐离子，改变聚合物分子的聚集状态，从而影响水凝胶的机械性能，同时还能赋予水凝胶良好的导电性、抗冻性等功能。我们以基于氢键交联的聚乙烯醇（PVA）水凝胶为研究对象，开发了新的盐析浸泡处理方法，设计制备了一种各向同性和两种各向异性高强度PVA基水凝胶，研究了制备条件对凝胶力学结构和性能的影响，并通过一系列测试和表征手段深入探讨了盐析处理对水凝胶机械性能和交联结构的影响机理。主要的研究结果如下：
（1）开发了一种在盐析过程中同步引入易形成氢键的多官能团分子来增强体系内氢键作用以提高水凝胶机械强度的方法。将PVA水溶液短暂冷冻后浸泡在柠檬酸钠-聚丙烯酸（Na3Cit-PAA）混合水溶液中制备了具有高力学强度的PVA/PAA/Na3Cit水凝胶。与在Na3Cit中浸泡的水凝胶相比，在Na3Cit-PAA混合水溶液中浸泡的PVA水凝胶的拉伸强度和弹性模量有了更显著的提高，分别达到了21.83 MPa和4.63 MPa。结构表征证实浸泡过程中PAA进入了水凝胶网络并与PVA分子链有效形成了氢键。Na3Cit的盐析作用诱导PVA分子链的聚集和结晶以及PAA与PVA链间氢键作用的形成，共同提高了水凝胶的交联密度和机械强度。通过改变Na3Cit和PAA浓度，可以对水凝胶的力学性能进行优化和调控。将传统盐析方法中的单一无机盐溶液替换为无机盐和易形成氢键的多官能团分子混合溶液的方法，为通过简单盐析浸泡法提高水凝胶的机械性能扩展了新的思路。
（2）利用水凝胶保持拉伸状态下在盐溶液中浸泡的方法制备高强度各向异性PVA-海藻酸钠（SA）水凝胶。先通过冷冻-解冻法或浇筑法制备出力学性能较弱的PVA-SA水凝胶，再将水凝胶按一定拉伸率预拉伸并固定在模具中，随后加入(NH4)2SO4水溶液使水凝胶在拉伸状态下盐析，制备了具有取向结构的高强度PVA-SA水凝胶。在拉伸-盐析过程中，预拉伸使水凝胶内的柔性PVA分子链和半刚性SA分子链沿着拉伸方向进行取向，取向排布的高分子链更利于采取适当构象来形成更多氢键。同时，(NH4)2SO4的盐析作用促进了聚合物分子链的聚集和水分子的排出，导致水凝胶网络的各向异性收缩。结构表征和流变测试证实了拉伸-盐析处理后的水凝胶中形成了取向结构和更强的氢键作用。拉伸-盐析的处理方法赋予了PVA-SA水凝胶各向异性的微观结构、优异的力学性能以及良好的导电性。通过调节SA含量、拉伸率可以有效调控PVA-SA水凝胶的力学性能。对通过浇筑法制备的PVA-SA水凝胶进行拉伸-盐析处理后，水凝胶的拉伸强度可达到35.29 MPa，弹性模量可达到10.15 MPa。
（3）对PVA-DMSO有机凝胶同步进行溶剂交换和拉伸-盐析处理制备具有取向结构的高强度PVA水凝胶。将PVA-DMSO溶液进行冷冻-解冻处理制备具有较为均匀网络结构的PVA-DMSO有机凝胶，再将其保持一定的拉伸率并浸泡在(NH4)2SO4水溶液中，溶剂交换和盐析效应协同诱导了PVA分子链的聚集和结晶，而拉伸处理使得PVA分子链在应力作用下取向排列，促进了PVA分子链间或链内更多氢键的形成。结构表征和流变测试证明拉伸盐析处理促进了水凝胶中取向结构的形成和PVA链之间的氢键交联。在适当的(NH4)2SO4浓度、浸泡时间和拉伸率下可以制备出力学性能显著增强的PVA水凝胶，其拉伸强度和弹性模量分别可达26.10 MPa和4.89 MPa。拉伸-盐析-溶剂交换结合法有效地提升了水凝胶的力学性能，这进一步证明了拉伸状态下盐析处理方法的有效性和普适性。本工作拓展和完善了高强度各向异性PVA水凝胶的制备方法。

The mechanical properties of hydrogel materials largely determine their applications. Therefore, it is of great significance to develop and improve the methods for preparing hydrogels with excellent mechanical properties for both basic research and practical applications of hydrogels. The salting out treatment based on Hofmeister effect can introduce specific salt ions into the hydrogel network, change the aggregation state of polymer chains, thus affecting the mechanical properties of hydrogels, and also endow the hydrogels with good conductivity, anti-freezing and other functions. In this work, focusing on polyvinyl alcohol (PVA) hydrogels cross-linked by hydrogen bonding, we developed new salting out soaking strategies based on Hofmeister effect and prepared an isotropic and two anisotropic high-strength PVA based hydrogels, studied the influence of preparation conditions on the structure and mechanical properties of the hydrogels, and explored the mechanism of salting out treatment on the mechanical properties and cross-linked structure of the hydrogels through a series of tests and characterizations. The main results are as follows:
(1) Preparing high-strength hydrogels by simultaneously introducing molecules with multiple functional groups capable of forming hydrogen bonds into hydrogels during the process of salting out. Hydrogels with high mechanical strength are produced by soaking the briefly frozen PVA aqueous solution in a mixed aqueous solution of sodium citrate (Na3Cit) and polyacrylic acid (PAA). Compared with the PVA hydrogel soaked in a Na3Cit solution, the tensile strength and elastic modulus of the PVA/Na3Cit/PAA hydrogel are significantly enhanced, reaching 21.83 MPa and 4.63 MPa, respectively. Structural characterizations confirm that PAA molecules are introduced into the hydrogel and form hydrogen bonds with PVA chains during the soaking process. The aggregation and crystallization of PVA chains induced by Hofmeister effect and the formation of hydrogen bonds between PAA and PVA chains lead to the increased crosslinking density and enhanced mechanical strength of the hydrogel. The mechanical properties of hydrogels can be adjusted by controlling the concentrations of Na3Cit and PAA. The use of a mixed solution of inorganic salt and molecules with multiple functional groups capable of forming hydrogen bonds rather than only inorganic salt in salting out method provides a new idea for improving the mechanical properties of hydrogels. 
(2) Preparing high-strength anisotropic PVA-sodium alginate (SA) hydrogel by salting out under drawing. PVA-SA hydrogels with weak mechanical properties are firstly prepared by freezing-thawing method or casting-reswelling method, then they are fixed in a mold with a pre-set drawing ratio and then soaked in a (NH4)2SO4 solution. During the drawing-salting out process, the stretching leads to orientation of the flexible PVA chains and semi-rigid SA chains in the hydrogel along the stretching direction. More hydrogen bonds are easier to form between these oriented polymer chains with an appropriate conformation and distance. At the same time, Hofmeister effect of (NH4)2SO4 promotes the aggregation of polymer chains and the expulsion of water, leading to the anisotropic shrinkage of the hydrogel network. Structural characterization and rheological tests confirm the formation of oriented structure and stronger hydrogen bonds in the PVA-SA hydrogels after salting out under drawing. The mechanical properties of PVA-SA hydrogel can be effectively controlled by adjusting the SA content and drawing ratio. Tensile strength and elastic modulus of this hydrogel reach 35.29 MPa and 10.15 MPa, respectively. Salting out under drawing treatment endows PVA-SA hydrogel with anisotropic microstructure, excellent mechanical properties and good conductivity. 
(3) Preparing high-strength anisotropic PVA hydrogels by soaking the pre-stretched PVA-dimethyl sulfoxide (DMSO) organogels in a (NH4)2SO4 aqueous solution. PVA-DMSO organogels with a more homogenous network are prepared by freezing-thawing treatment of PVA dissolved in DMSO solution, and then they are pre-stretched to different drawing ratios and immersed in a (NH4)2SO4 aqueous solution. The solvent exchange and the Hofmeister effect synergistically induce the aggregation and crystallization of PVA chains, and meanwhile, PVA chains and crystallites are oriented along the stretching direction, promoting the formation of more hydrogen bonds between or within the oriented PVA chains. Structural characterization and rheological tests show that oriented structure and more cross-links are formed in PVA hydrogels during the solvent exchange and salting out under drawing. PVA hydrogels with significantly enhanced mechanical properties can be prepared under proper concentration of (NH4)2SO4, soaking time and drawing ratio. Tensile strength and elastic modulus of the hydrogel can reach 26.10 MPa and 4.89 MPa, respectively. The combination of solvent exchange and salting-out under drawing can effectively enhance the mechanical properties of hydrogels, proving the effectiveness and universality of the salting-out under drawing treatment. This work expands and improves the methods for preparing high-strength anisotropic hydrogels.