The Relationships Between Viscoelasticity Of Polymer Films And The Stick-slip Behavior Of Liquid Droplet On Their Surfaces

During the past decade, ultrathin polymer films have been the focus of numerous studies. The glass transition temperature(Tg) is one important parameter characterizing the physical properties of polymers. The behavior of the glass transition at nanoscale deviates substantially from that of the bulk. Thus, studies of the Tg-confinement in polymer thin film are of significance both in term of optimizing the properties of nanomaterials and in advancing our understanding polymer chain dynamics under confinement. Recently we developed a novel method to measure the glass transition of polymer nanometer films by detecting the dynamic wetting behavior of liquid droplet on their surface. This thesis makes a further study aiming to correlate the relationship between the viscoelasticity of the polymer films and the dynamic wetting behavior of liquid droplets on their surface. In the meanwhile, dynamic contact angle measurement was adopted to study the effect of film thickness and polymer molecular weight on the glass transition temperature. The conclusions were shown as follows:The dynamic wetting behavior of liquid droplets on polymer film surface was shown to be related to the viscoelasticity of the polymer films. Jumping angle(△θ) was put forward to sale the stick-slip behavior and the relationship between the △θ and temperature is parabolic curve. When the molecular weight of PS is below 425.0 Kg/mol, the maximum jumping angle(△θmax) decreases with increasement of molecular weight. And when molecular weight is above 425.0 Kg/mol, it keeps constant. The height of weight ridge at Tjm has similar molecular weight dependence with that of △θmax. It was found that the relationship between △θmax and modulus is a linear fashion, i.e. △θmax decreases linearly with increasing polymer elastic modulus.The relationship between △θ and temperature is dependent on the polymer molecular weight. The onset temperature of stick-slip behavior(Tonset) is independent on polymer molecular weight. However, the temperature where the stick-slip behavior disappear(Tend) decrease with increase of molecular weight, then keeps constant when molecular weight is above 425.0 Kg/ mol. The relationship between the height of weight ridge and temperature( h~T) is similar to the deformation-temperature curve of polymer, which can reflect the mechanical properties of polymer film. By comparing the ―h~T‖ and ―△θ ~ T‖ curves, it was found that the Tonset is the temperature where the elastic modulus of PS begin to decline and the Tend is related to the viscous flow temperature(Tf) of polymer film. The difference in mechanical properties of polymer with various molecular weight is responsible for the different Tend value.The effect of thickness and molecular weight on the glass transition of temperature of polymer thin films was studied. For films supported on silica, Tg decrease with decreasing thickness for PS but increased for PMMA. Polymer films with larger molecular weight had a higher T g and a thicker adsorbed layer. Based on layer model, the interface layer would suppress the mobility of polymer thin film, thus Tg of PS and PMMA with larger molecular weight is higher than that with low molecular weight.