Study On The Nanomechanical Properties Of Ultrathin Amorphous PS Films

With the rapid development of nanotechnology,ultrathin amorphous polymer films are not only used in the fields of flexible sensors,semiconductor materials and nano-coatings.The issue of the physical properties of ultrathin amorphous polymer films,such as structural stability,tensile strength,flexural strength and ductility,has received considerable critical attention.It is now well established that the glass transition temperature(Tg)of polymer ultrathin film changes with the decrease of film thickness.At the same time,due to the influence of confinement,the mechanical properties of amorphous polymer films,such as elastic modulus and viscoelastic behavior,deviate from the mechanical properties of the bulk.The issue of modulus for ultrathin amorphous polymer film has been a controversial subject within the influence of the characterization method,the structure of the polymer and other factors.In order to explore the relationship between the mechanical properties of amorphous polymer films with the bulk,and to provide a theoretical analysis for the application of the polymer films.In this paper,polystyrene(PS)films were studied by using a new method of micromechanics.The results are as follows:(1)The force-deformation experimental data of the PS film was measured by AFM.The Young's modulus of PS film was calculated with contact mechanics models,such as Hertzian model,Derjaguin-Muller-Toporov(DMT)model and Johnson-Kendall-Roberts(JKR)model.The relationship between the modulus of the PS film above the thickness threshold and the bulk was compared.The experimental results show that the modulus of the PS film with the JKR model is consistent with the bulk.The method of the AFM nanomechanical mapping(AFM-NMM)with JKR model is suitable for the micromechanical measurement of PS films.(2)The effects of probe radius,adhesion force,load and substrate with the JKR model was used to study on the modulus of PS film.The experimental results show that the probe radius was 15 nm,the sharp tip was not worn with the AFM-NMM experimental process,and the absolute value of the adhesion force between the tip and the sample was close to or less than the load.The Young's modulus of the PS film above the thickness threshold supported by the silicon substrate was consistent with the bulk modulus.(3)The tendency of modulus changing for PS confined films was studied by buckling experiments and AFM-NMM experiment.The buckling experiment show that the modulus of ultrathin PS film decreases with the decrease of film thickness(-50 um).The results of AFM-NMM experiment show that the modulus of PS film supported by rigid(Si)substrate increases with the decrease of film thickness(near 100 nm),and the heterogeneity of amorphous PS film increases with the decrease of film thickness.At the same time,the Young's modulus of ultrathin films increases due to rigid substrate and residual stresses in the films.