The viscoelastic properties of thin films



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There is a considerable interest in studying material properties at micro-, nanometer size scale. Thin films exhibit significant different behaviors from that of bulk systems. Both liquid dewetting technique and particle embedment method have been used to investigate viscoelastic properties of thin films. The liquid dewetting technique which uses liquid as substrate is a direct mechanical method to study the viscoelastic properties of polymer (polystyrene (PS) and polycarbonate (PC)) thin films. Results show that the glass transition temperature (Tg) dependence for PS and PC thin films on liquid surface is more like the supported film rather than the freely standing film. Both Tg depression and film stiffening were observed when PS film thickness is less than 20 nm. No molecular weight effect was found for the dewetting behaviors of the PS films prior to the terminal flow regime, where the majority of the current work was carried out. For PC thin films, similar Tg was observed when film thickness is less than 35 nm. As the film thickness increases during the dewetting experiment, the present results show that the film dewetting process for the polymer/glycerol couple has to be described as a “non-isothermal” experiment. A numerical method is proposed to correct the experimental data to the “isothermal” and constant thickness condition. A novel temperature-step method has been described to direct obtain the Tg of thin films from the liquid dewetting measurement based on the liquid dewetting experiment is a “non-isothermal” experiment. Different from all of the previous methods in literature used to obtain the glass transition temperature of thin films, we can get polymer thin film glass transition temperature-thickness dependence only from one sample within a single experiment by change operating temperature steps. Two liquids (glycerol and 1-Butyl-3-methylimidazolium trifluoromethanesulfonate) have been chosen to study the liquid substrate effects on polymer thin film dewetting behaviors by this temperature-step mehod. It was found that PS thin film dewet faster on glycerol than that on ionic liquid and its Tg has a stronger film thickness dependence compared to PS thin film on ionic liquid. A sub-micron particle embedment technique has been used to determine the shear modulus of 1, 2-dipalmitoyl-Sn-glycero-3-phosphotidylcholine (DPPC) lipid multibilayers by two different particles (PS particle and silica particle) from atomic force microscope (AFM). Methods used to determine particle size has been discussed and results show that AFM might be the best way to obtain particle size in the present work. The standard JKR model was used to relate the shear modulus of the lipid multibilayer films with the particle embedment depths. The so-determined modulus of the DPPC is consistent with the reported literature values for DPPC, while silica particles give more reliable results compare to polystyrene particles which has a small amount of surfactant in the sample.