Preparation Of Polyacrylamide Composite Hydrogels And Their Application Study Solar Evaporation And Flexible Sensors

Being a soft material with a three-dimensional mesh structure,hydrogels present good water absorption and excellent biocompatibility.However,the drawbacks of low mechanical strength,poor self-healing ability and single functionality greatly restrict the application of hydrogels.To solve these problems,various strategies have been developed,among which composite hydrogels based on functional fillers have become a promising topic that benefit from the simple preparation process and excellent performance.Thus this paper firstly constructed a multilayer structured hydrogel solar evaporator（ML-SVG）with PAM as the base network and investigates mechanism and performance;then we constructed a double network structure by introducing polyacrylic acid（PAA）into the PAM network.Dopamine（DA）and metal ions(Ni²⁺and Al³⁺)were added as functional fillers to prepare flexible wearable PAM/PAA/PDA/（Ni+Al）hydrogels and the application in flexible sensors has been investigated.The detail as shown below:（1）In this work,a multilayer structured PAM composite hydrogel solar evaporator（ML-SVG）was designed based on in situ free radical polymerisation reaction.The hydrogel consists of PAM/cellulose nanofiber（PAM/CNF）water transport layer at the bottom,photothermal conversion layer of PAM/carbon nanotubes/titanium nitride（PAM/CNTs/Ti N）in the middle,and rough surface layer composed of"island"PAM/hollow MXene microspheres（PAM/HSMX）at the top.The microstructure of the hydrogels was observed by scanning electron microscopy（SEM）.Compared with pure PAM hydrogel,the bottom PAM/CNF hydrogel showed more uniform three-dimensional pore structure.This high porosity three-dimensional structure of PAM/CNF provides a good capillary driving force for water transport in the hydrogel,thus accelerating the transport of water to the bottom.Hydrogel simulation evaporation experiment showed that compared with PAM/CNF-PAM/CNTs/Ti N[DL-SVG（1）]and PAM/CNF-PAM/HSMX[DL-SVG（2）],ML-SVG reached a stable evaporation rate first.The evaporation rate increased from1.8 kg·m^-2·h^-1for DL-SVG（1）and 1.6 kg·m^-2·h^-1for DL-SVG（2）to 2.2 kg·m^-2·h^-1,and the corresponding evaporation efficiency also increased from 57%and 50%to70%.This is because the multi-layer hydrogel solar evaporator（ML-SVG）has different wettability and photothermal conversion efficiency between each layer.The lower layer is water conveying heat insulation,the middle layer is heat collecting material,and the upper layer is constructed with uneven surface,can further improve the utilization of sunlight,thus giving ML-SVG excellent evaporation performance.In addition,ML-SVG has excellent recycling performance,efficient sewage purification and brine desalination performance.（2）In this work,highly conductive self-healing PAM/PAA/PDA/（Ni+Al）hydrogel based on PAM and PAA as substrates was prepared based on in situ free radical reaction.The mechanical properties and adhesion properties of hydrogel were analyzed by TPA test.The test showed that the hydrogel could be stretched 11.5 times of itself,the breaking strength was 180.8 k Pa,and the mechanical properties remained stable after 5 times of cyclic stretching.This is because the addition of metal ions and PDA can increase the cross-linking sites of hydrogels,leading to the increase of the cross-linking density between macromolecular chains,which is conducive to improving the mechanical properties of hydrogels.The hydrogel was cut for self-healing test.After 12 h of self-healing,the hydrogel could be stretched to 10.8times its own size,with a breaking strength of 160 k Pa and a self-healing efficiency of94%.This is because the highly active phenol hydroxyl group in PDA can form a variety of non-covalent synergies with polymer networks,which makes the hydrogels have excellent self-healing properties.In addition,the whole-texture test showed that the adhesion strength of the hydrogel to plastic,metal and glass was 3.8 N·Sec,8.9N·Sec and 11.1 N·Sec,respectively.SEM tests showed that PAM/PAA hydrogels had a more complete three-dimensional network structure after adding metal ions and PDA,because carboxyl groups in PAM and PAA can provide a large number of binding sites for metal coordination with metal cations.The hydrogel had a sensitive electrical response,and its measuring factor（GF）is up to 8.4 in 0%-1000%tensile strain test.So that the PAM/PAA/PDA/（Ni+Al）hydrogel has great potential for use as a flexible strain sensor for monitoring the response to changes in water flow,sound,and human movement.