Study On The Mechanical Properties Of PS Ultrathin Films And Their Influence Factors

With the rapid development of emerging technologies such as new generation lithography,bionics,flexible organic sensors and transparent micro-devices,amorphous polymer ultra-thin films with a thickness of less than 100 nm have been widely used,of which the applications are closely related to physical properties.Ultrathin amorphous polymer films are nano-confined polymers,and the stability and reliability are critical to the practical application of these technologies.Due to the nanoconfinement,the molecular chain conformation and the degree of molecular chain entanglement in the ultra-thin amorphous polymer film are greatly changed from the bulk polymer,as the result,the mechanical properties of the ultra-thin amorphous polymer film are essentially different from the bulk.At present,there have been a large number of studies on the mechanical properties of ultra-thin amorphous polymer systems.However,the studies use different characterization methods to explore the mechanical properties of ultra-thin polymer films whereas often leads to conflicting experimental results.It may be attributed to the complex influencing factors of nanoconfinement on the mechanical properties of polymer materials.In order to systematically explore the influence of the microstructure of the amorphous polymer film on its mechanical properties,this thesis uses the water drop experiment,the buckling mechanics experiment and the atomic force nanomechanics(AFM-NMM)experiment to explore the mechanical properties of polystyrene(PS)thin films.The results are as follows:(1)The formulas of Young's modulus in the water drop experiment are rearranged and analyzed,and different boundary conditions are substituted into the calculation.The results show that the far threshold theory is more suitable for this study than the near threshold theory for amorphous PS ultra-thin film systems.Taking the approximate condition of ???/2 to simplify the original formula and compare the calculated results of the methods.The calculation results show that the difference between the calculated values of Young's modulus of the two is within the error range,which proves the reliability of the simplified formula.(2)The radius and wrinkle length of the water droplets are measured by the water drop experiment using the ultra-depth-of-field microscope,and are calculated combined with the film thickness,contact angle and other data to obtain Young's modulus of PS films of different thicknesses.The results show that the Young's modulus of the PS film increases as the thickness decreases,and there is a thickness transition point that increases with the increase of the PS molecular weight.When the film thickness is greater than the transition point,the Young's modulus no clear thickness dependence;when the thickness is less than the transition point,the Young's modulus of the PS film increases with the decrease of the film thickness.(3)Using a horizontal mechanical testing machine and an ultra-depth-of-field microscope,the Young's modulus of the substrate and the wavelength of wrinkles can be obtained through the buckling mechanics experiment.And then the Young's modulus of PS films of different thicknesses can be calculated.The results show that the Young's modulus of 100 nm PS film is about 4 GPa,and it decreases with the decrease of film thickness.Besides,there is no obvious effect of the molecular weight and its distribution on the film Young's modulus.(4)Using an atomic force microscope,the force-deformation curve of the film can be obtained through the AFM nanomechanics experiment.JKR linear fitting method was used to analyze the curve,and the Young's moduli of PS films with different thicknesses are calculated.The experimental results show that the Young's modulus of the PS film has no obvious thickness dependence and molecular weight dependence in this experiment.(5)The Young's modulus of the PS film under different loads,different positions,different speeds and different substrates was measured by AFM nanomechanical experiments.Results show that there is no obvious molecular weight dependence for the Young's moduli;Young's moduli of the PS films increase with the increase of the load,and at the load of 41 n N The thickness dependence appears when the thickness increases,and it decreases with the increase of the thickness;the Young's moduli distribution of the PS film at the edge of the film is wider than in the center,and the measured value of the Young's modulus of the PS film is lower when the test rate is higher.The Young's modulus of the PDMS-based PS film The modulus decreases as the thickness decreases,and the Young's modulus of the PLA-based PS film has no obvious thickness dependence.(6)This thesis use different characterization methods to measure the same PS films and obtain different thickness dependence of Young's modulus of the PS films.It shows that different characterization methods measure the different responses of ultrathin films due to their different force loading modes.In addition,the molecular weight and distribution of PS,substrate,etc.have effects on the mechanical properties of the obtained ultra-thin film.Therefore,these influencing factors should be considered when designing and preparing nanodevices using nano-confined polymers,and when evaluating the mechanical properties of ultra-thin films,suitable characterization methods should be selected according to their uses.