Structural Design And Functional Study Of Multi-stimulus Responsive Polyvinyl Alcohol Hydrogels

Stimulus-responsive hydrogels are among the most promising smart materials,and there have been countless studies on basic exploration and industrial applications surrounding this class of materials,including biomedical,soft actuators,and sensors.The hydroxy polymer polyvinyl alcohol(PVA)has received a lot of attention in the hydrogel direction due to its biodegradability,biocompatibility,and ease of molding,etc.The single structure and lack of significant functions of PVA limit its practical applications.Polysaccharide derivatives include carboxymethyl cellulose(CMC),carboxymethyl chitosan(CMCS),chitosan(CS),etc.These materials can give PVA hydrogels the advantages of high strength,high modulus,biocompatibility,biodegradability,stimulation response,etc.,and are successfully used as PVA hydrogel reinforcing components and functional components.Physical cross-linked hydrogels are not stable enough to meet the demand,and other networks are usually introduced to enhance their stability and multi-stimulus response.The internal network structure of homogeneous hydrogels is more homogeneous,which is different from heterogeneous hydrogels,to achieve complex deformation without basic structural design,for which the anisotropic structure of hydrogels is regulated to prepare stimulus-responsive bilayer hydrogels to achieve actuation function.Based on the design of poly(vinyl alcohol)hydrogel network structure,the bilayer hydrogel was firstly designed and prepared using carboxymethyl cellulose with different molecular weights and substitution degrees to modulate the mechanical properties of poly(vinyl alcohol)hydrogel using a simple method such as cyclic freeze-thaw(F-T)and from the network construction of physical cross-linking as a substrate.The dynamic B-O bonding was introduced to construct multiple networks to enhance the mechanical properties and functional response of the gels,and the contribution of the bilayer structure to each network component under external stimuli was investigated.Finally,a multifunctional bilayer hydrogel humidity sensor was prepared by regressing the property differences of the pendant groups of the polysaccharide sugar ring itself.The study provides a more comprehensive understanding of the modulation of heterogeneity of polyvinyl alcohol hydrogels in response to multiple stimuli to prepare bilayer hydrogel smart devices with driving and sensing capabilities.The study mainly consists of the following work:(1)To investigate the effect of structure and function of polyvinyl alcohol/carboxymethyl cellulose hydrogels:hydrogels were prepared using carboxymethyl cellulose with different molecular weights Mw(9×104 g/mol and 2.5×105 g/mol)and degrees of substitution DS(0.7 and 1.2)with polyvinyl alcohol,and enh anced by hydrogen bonding of-OH,-COO-on the CMC chain and-OH on the PVA chain.mechanical properties.The effects of Mw and DS of CMC on the mechanical properties and pH response of PVA hydrogels were investigated when they acted together.The most significant effect on the properties of PVA hydrogels was obtained for CMC with DS=0.7 and Mw=2.5× 105 g/mol.The presence of free Na+ in the solution after dissolution of CMC and the presence of-COO-on the sugar ring structure after ionization led to the preparation of pH-responsive CMC-H ionic hydrogels,which provided the feasibility for subsequent stimulus response and sensor development.(2)To investigate the effect of boron ester bonding on the properties and functions of polyvinyl alcohol/carboxymethyl cellulose bilayer hydrogels:from the regulation of heterogeneity of hydrogel structure.The cross-linked network formed by physical crystallization of PVA was used as the primary network,the hydrogen bonding interaction between CMC and PVA was used as the secondary network,and the B-O bond was introduced inside the gel to form chemical cross-link as the tertiary network.The CMC/BCMC-H bilayer hydrogel was prepared by anisotropic modulation.The bilayer interface is tightly bound by hydrogen bonding and B-O bonding,and the introduction of B-O bonding enhances the mechanical properties and strengthens the bilayer hydrogel to be sensitive to pH and temperature.The differences in osmotic pressure,cross-link density and cross-linking mode are used to generate macroscopic bidirectional driven deformation in response to environmental stimuli.By relying on the ion channel deformation,a flexible device with integrated pH response,temperature actuator and temperature sensor was developed to detect the environmental pH and temperature changes in real time.(3)Exploring the heterogeneity regulation and actuator of polyvinyl alcohol/polysaccharide bilayer hydrogels:based on the presence of anionic carboxymethyl cellulose(CMC DS=0.7,Mw=2.5×105 g/mol),polysaccharides such as cationic chitosan(CS)and amphoteric carboxymethyl chitosan(CMCS)were introduced as two other structural building blocks and driving control units to prepare physically cross-linked polyvinyl alcohol/polysaccharide bilayer hydrogels.The type and concentration of different layers of polysaccharides were changed to adjust the spatial heterogeneity for the purpose of bilayer hydrogel pH and humidity driving.Based on the distortion of ion channels during humiditydriven deformation of the bilayer hydrogel,a two-in-one flexible device integrating humidity actuator and humidity response sensor was developed,which can detect the change of environmental humidity in real time.In addition,the bilayer hydrogel has good tensile toughness,interfacial bonding,and resistance to strain effects,and has the ability to recognize human motion as a strain sensor.