Preparation Of Polyacrylamide-based Composite Hydrogel And Its Solar-driven Water Evaporation Performance

Solar driven water evaporation technology combines solar energy collection and water purification,and directly transfers heat through photothermal conversion materials to promote water evaporation.It is an effective method to solve the current shortage of water resources.Hydrogel has a cross-linked network with highly adjustable physicochemical properties,which can meet the stringent requirements of high-efficiency solar driven water evaporation.Using hydrogel to convert solar energy to heat,the technology of sewage treatment and seawater desalination has been developed.However,the hydration mechanism and the effect on the evaporation process of hydrogel polymer network is not clear.The stability of hydrogel during long-term evaporation cycle has not been fully studied.To solve the above problems,hydrogels with high efficiency and stable cycling performance were prepared by adding different oxygen-containing functional groups,using polyacrylamide as the matrix and dopamine as additive.The influence of oxygen-containing functional groups on water activation and water evaporation enthalpy was explored.Polyacrylamide/polydopamine/polyethylene glycol composite hydrogels were prepared by freeze-drying method using polyethylene glycol as dispersant.The photothermal conversion property of pure polyacrylamide hydrogel is weak,while the modified composite hydrogel has good light absorption and hydrophilicity.The addition of hygroscopic polyethylene glycol results in the formation of intermediate water in the composite hydrogel.The results of Raman spectroscopy show that the ratio of intermediate water to free water is1.37,and the equivalent evaporation enthalpy in the composite hydrogel is reduced from2460 J g^-1 of free water to 1508 J g^-1.The evaporation rate of the composite hydrogel reaches2.40 kg m^-2 h^-1 under light intensity of 1 k W m^-2,and has stable evaporation rates under different light intensities and salt concentrations.Low-cost and environment-friendlyβ-cyclodextrin was chosen as modifier to replace polyethylene glycol to prepare polyacrylamide/polydopamine/β-cyclodextrin composite hydrogels.The addition amount of multi-walled carbon nanotubes was analyzed,and the results showed that the absorptance reached 97.2%when the addition amount of multi-walled carbon nanotubes was 1 m L.The increase of oxygen-containing functional groups inβ-cyclodextrin leads to an increase to 1.48 in the ratio of intermediate water to free water in composite hydrogels.The water evaporation enthalpy in the composite hydrogel calculated according to differential scanning calorimeter fitting was 1428 J g^-1,which was close to the equivalent evaporation enthalpy of 1410 J g^-1.The evaporation rate of composite hydrogel under one sun intensity is 2.65 kg m^-2 h^-1,and has good performance stability.During 40 cycles of testing,the evaporation rate remains unchanged.Polyacrylamide/polydopamine/polyvinyl alcohol composite hydrogels were prepared by pre-crosslinking polyvinyl alcohol,glutaraldehyde and polydopamine to form a hydrophilic skeleton.The composite hydrogels polymerized at low temperatures exhibit excellent swelling properties,capable of storing 6 times water with their own weight to meet the continuous evaporation needs in one day.The results of Raman spectroscopy and nuclear magnetic resonance show that the ratio of intermediate water to free water in composite hydrogel is 1.73.The water evaporation enthalpy measured by differential scanning calorimetry and the equivalent evaporation enthalpy are 1135 J g^-1 and 1094 J g^-1,respectively.Benefiting from the expansion of the evaporation area and the net energy gain from the environment,the cylindrical composite hydrogel can achieve a high stable evaporation rate of 4.85 kg m^-2 h^-1 under one sun intensity.The unique cross-linked porous structure of hydrogel not only makes the photothermal conversion material evenly dispersed to obtain high absorptivity,but also ensures the rapid transportation of water to the evaporation surface.More importantly,the polymer network of hydrogels has been optimized through hydrophilic polymer design.The hydration and interaction of oxygen-containing functional groups adjust the content and distribution of intermediate water,and reduce the energy demand of water evaporation in hydrogels,which promotes the evaporation of water.