| The adhesion and tribological performance of soft materials are affected by interfacial interactions.Strong adhesion and hydration lubrication can be achieved through designing the network of soft materials,investigating related interfacial interaction mechanism and tuning surface force.Serving as a type of polymers,soft materials are widely used in the fields of adhesion and hydration lubrication.The adhesion performances of soft materials require improvement and the tribological performance should be further investigated.With the development of synthesis technology,new soft materials and testing technology,the adhesion and friction of soft materials as well as related interfacial interaction mechanism have attracted tremendous interest.Therefore,understanding the design principles of new soft materials as well as revealing the effects of surface force on adhesion behavior and tribological performance are key issues that require being solved in the fields of adhesion and lubrication.This dissertation focuses on the adhesion mechanisms and tribological performance of soft materials,and various functional soft materials are designed for reversible adhesion in air,gelid adhesion,underwater adhesion and hydration lubrication.The properties of those new soft materials are tested,and the interfacial interaction mechanisms during adhesion are analyzed.The surface forces at the interface as well as the influence factors of adhesion and tribology are investigated.In addition,the adhesion energy is employed to establish the adhesion model at the interface of soft materials.Based on the experimental results and theoretical analysis in this work,a hydrogel coating and a flexible dual-responsive sensor are designed and fabricated,serving as the engineering applications of surface interaction mechanism of the soft materials.The specific investigation in this work is as follows:First of all,a highly stretchable hydrogel and an anti-freezing hydrogel are developed,and the reversible adhesion mechanism in air and gelid adhesion mechanism are systematically investigated.By ultraviolet curing method,a highly-stretchable and self-healing polyacrylamde-polyethylenimine(PAM-PEI)hydrogel is fabricated,which shows reversible adhesion with various substrates.The reversible adhesion is mainly caused by the physical interactions at the interface including hydrogen bonds,metal complexation,hydrophobic interaction and van der Waals force.In addition,the hydrogel can be further updated by introducing CaCl2 and glycerol into PAM-PEI hydrogel network,and a new type of freezingtolerant hydrogel is fabricated.The stretchability of the hydrogel improved from 53 times to 300 times,and the self-healing time shortened from 30 seconds to 3 seconds.Besides,the updated hydrogel obtained new properties including freezing-tolerance,water-retaining and gelid adhesion.The freezing-tolerant hydrogel is able to adhere substrates at-40℃,showing gelid adhesion.And tensile adhesion tests suggested that the adhesion strength increased from 102 kPa to 1.1 MPa when the ambient temperature decreased from 25℃ to-40℃.Therefore,the gelid adhesion of soft materials is proposed,and the temperature-related gelid adhesion model at interface is established.The gelid adhesion mechanism of the freezing-tolerant hydrogel is systematically investigated,and the increased adhesion at gelid condition is attributed to the enhanced mechanical strength of the bulk hydrogel as well as the increased surface interaction at gel-substrate interface.After that,a strong underwater adhesive is fabricated by employing covalent crosslinks at the interface and the underwater adhesion mechanism is investigated.A hexamethylene diisocyanate-polydimethylsiloxane(HDI-PDMS)polymer is developed through the addition reaction between the isocyanate group and the amino group.The soft polymer is totally cured in water and exhibits strong underwater adhesion with various substrates with the highest adhesion strength of 2.8 MPa.The underwater adhesion behavior with curing time is further investigated,and the effects of salt concentration and temperature are revealed.In addition,the surface interactions of the adhesive on various substrates are analyzed,and the underwater adhesion mechanism is revealed.The adhesion model during curing process is established to clarify the effect of curing time on adhesion strength.Result shows that the adhesion strength increases with curing time and then remains constant as the curing time further increases.Because extending the curing time will increase the molecular weight of polymer chains and enhance the surface interactions,which results in the increase of dissipated energy and interfacial energy.As the curing time further increases,the adhesive is fully cured and the adhesion strength remains constant.In addition,the salt ions in solution can impair the surface interactions through screening the electrostatic charges of the adhesive polymeric chains and the substrate surface,which would weaken the adhesion.And increasing the ambient temperature can also decrease the surface interaction,leading to a decrease in adhesion.The underwater adhesion of the HDI-PDMS polymer is mainly attributed to the combination of the chemical crosslinks,hydrogen bonds and other physical interactions at the interface.Then,the time-dependent behavior of hydrogel friction and related influence factors are investigated.A rheometer is used as the testing equipment of hydrogel friction,and the effects of time,shear velocity,load force and pH of the solution on the friction coefficients for three typical polyacrylamide-based(PAM)hydrogels are investigated.The time-dependent friction behavior of hydrogel is discovered,and the friction coefficients for the hydrogels dramatically decrease at the beginning and then remain constant or gradually increase.Two distinct regimes including the friction-decrease regime and friction-constant/increase regime are observed.In the friction-decrease regime,the decreasing behavior in friction coefficient is determined by the waiting time before experiments.Increasing the contact time between the hydrogel and substrate leads to a longer timescale of the friction-decrease regime.In the frictionconstant/increase regime,the friction behavior is determined by the hydrogel network and the charge behavior at the interface.In addition,the friction behavior of hydrogel is affected by velocity.At low velocities,the friction coefficient increases with shear velocity,which is constant with hydrodynamic friction.When the velocity is larger than a critical value,the fluid begins to intrude into the interface as the velocity further increases,and the lubrication layer is formed through rehydration,which leads to a decrease in the friction coefficient.When the velocity further increases,the friction coefficient begins to increase again with velocity,which can be explained by elastohydrodynamic lubrication theory.Besides,the friction coefficient rapidly decreases at the beginning and then slowly increases as the load force increases.At the beginning,the decrease in friction coefficient is caused by the desorption of the polymer chains from the substrate surface.However,when the load force further increases,the friction coefficient gradually increases,which is caused by an increase in the viscosity and a decrease in the thickness of the solution at the interface.Finally,the adhesion model at the interface of soft materials is established,and engineering applications of soft materials are designed and fabricated based on the investigation of surface interaction mechanism.When the adhered soft interface is damaged,the total adhesion energyΓ that required to break the adhesion in a unit area can be divided into the dissipated energy ΓD and the interfacial energy ΓI.The interfacial energy between the soft material and the substrate is described by the cohesive zone model,and the dissipated energy during the deformation of the soft material is analyzed by Ogen-Roburgh model.According to the experimental results and theoretical analysis,a flexible anti-fog and anti-frost coating as well as a flexible temperature and strain dual-responsive sensor are designed and fabricated,serving as the engineering applications for surface interaction mechanism of the soft materials.The freezingtolerant hydrogel shows stable surface force with the substrate,which can be used to fabricate flexible anti-fog and anti-frost coatings.And the anti-fog and anti-frost performances are caused by the hydrophilic,water-absorbing and freezing-tolerant properties of the hydrogel network.The water molecules can be absorbed immediately into the gel network when the water vapor in air reaches the coating surface without water droplets or crystals condensing on the surface,leaving a transparent surface and exhibiting the anti-fog and anti-frost performances.In addition,a flexible temperature and strain dual-responsive sensor is fabricated by enclosuring the freezing-tolerant hydrogel with an elastomer layer.The surface between the hydrogel and elastomer can be enhanced by ultraviolet irradiation,and the thermo-and strain-sensitive mechanisms of the flexible sensor are explored. |
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