Mechanical Measurement Of Micro/Nano Polymer Interface In Liquid Phase

Underwater polymer interfaces play a significant role in various applications such as underwater coating protection,seawater desalination,and drug delivery.The mechanical properties of these interfaces determine their application scenarios and lifespan.To address the challenge of measuring the mechanical properties of underwater polymer nanofilms and droplets,developing advanced experimental mechanical measurement methods will provide novel insight into materials science,surface science,and related fields.Therefore,this dissertation systematically investigates the mechanical behaviors of underwater polymer films and droplets.The study includes the following aspects:（1）The temporal evolution of dynamics of underwater blister that was spontaneously produced at the interface of polystyrene（PS）nanofilms adhering to solid substrates was studied using an interference microscope system.The research results indicate that when a PS nanofilm sample supported by a glass substrate was covered with ultrapure water,dynamic phenomena such as blister nucleation,growth,and coalescence occur at the interface.The dynamics of interface blisters can be divided into three stages.In the first stage,the number of blisters increases rapidly,dominated by blister nucleation.In the second stage,the number of blisters decreases rapidly with time,mainly due to blister coalescence and growth.In the third stage,the number of blisters slowly decreases with time,and the dynamics of blisters tend to stabilize.Nanofilm thickness,the annealing process in fabricating the nanofilm and the wetting properties of substrate influence the dynamics of blister growth.For samples that are not annealed,as the thickness of the film increases,the area coverage and average radius of interface blisters also increase.After undergoing an annealing process,the growth dynamics of interface blisters slow down.The wettability of the glass surface is less than or equal to 20°,and nucleation occurs at the interface.By studying the circular isolated blisters,it was found that the radial radius（a）of a single blister on a 135 nm-thick film takes a power-law relationship with respect to time（t）((6∝^0.28).Finally,a physical model describing the morphological characteristics of circular blisters was established based on the principle of energy conservation,and a novel method for measuring the interfacial tension between polymer nanofilms and hydrophilic substrates was proposed.（2）The mechanical behavior of blister underneath PS nanofilms was investigated using the atomic force microscopy（AFM）nanoindentation technique.It was found that the force-distance curve present linear elastic behavior,indicating a stiffness for characterizing the blister deformation.The results show that within a radial radius less than half of the distance from the blister center,the blister stiffness remains unaffected by the anchoring effect.During the elastic deformation,the stiffness near the blister apex remains constant（characteristic stiffness k_b）,which can be used to characterizes the blister’s ability to resist elastic deformation under load.The characteristic stiffness exhibits significant size effects,i.e.k_b increases with decreasing the blister size or increasing PS film thickness.A physical model based on classical thin shell theory was establishedto explain the size effect of blister stiffness during the indentation experiment.Finally,this model was applied to measure the elastic modulus of thin films,suggesting a new method for measuring the elastic modulus of polymer nanofilms.（3）The morphological and static mechanical properties of underwater polymer nanodroplets were studied using AFM high-resolution imaging and nanoindentation techniques.The morphological parameters such as height,radial radius and contact angle of polydimethylsiloxane（PDMS）nanodroplets were obtained through high-resolution imaging with small scanning forces and three-dimensional fitting.It was found that the contact angle of nanodroplets is smaller than the macroscopic droplet contact angle（θ_∞≈84°）and decreases with decreasing radial radius.The apparent line tension of underwater PDMS droplets is negative and independent of droplet size.The indentation results of the nanodroplets indicate that the stiffness is not affected by the anchoring effect within a radial radius smaller than 0.75 times of the distance from the droplet center.Based on an adhesive force interaction model when the probe separates from the droplet,the interfacial tension at the liquid-liquid interface was calculated,suggesting a new method for measuring the liquid-liquid interfacial tension.Finally,the effect of biofilm materials on mechanical response was studied by using the method of nanodroplet morphology measurement.The end of spindle shaped mouse fibroblasts would produce stress response to different scanning force,and the number of pseudopods increased when the scanning force was greater than 100 p N.