Effect Of Interfacial Interactions On The Glass Transition Of Ultrathin PS Films Investigated By Dynamic Wetting Behaviour Of Droplets

With the advances in nanotechnology, much attention had been paid to the condensed physical properties of polymer thin film. When the thickness reduced to the scale of the polymer chain, the glass transition behavior of nanoconfined polymer thin film will deviate from bulk. The interfacial interaction between polymer film and substrate was one of the main factors that affect glass transition dynamic of the polymer thin film, and it becomes more significant with the reduction of thin film thickness. Exploring the physical parameters that evaluate interfacial interaction could indicate the nature of interface affected the glass transition temperature of polymer thin film.The dynamic wetting behavior of droplet is a new characterization method as it can directly define the glass transition temperature of polymer thin film according to its variation of mechanical properties. In this thesis, the interfacial interaction was controlled by the functional groups’ type and the molecular weight of PS, and the influence of interfacial interaction on the glass transition temperature were systematically investigated by dynamic contact angle and ellipsometry. Some conclusions were obtained as follows:(1) The Tjm of PS ultrathin film with different end functional groups and molecular weight decreased with the reduction of polymer film thickness. But the extent for Tjm reduction and the critical thickness that Tjm starts to deviate from bulk behavior changed with the types of end functional group and the molecular weight of PS. Under the same thickness, when the molecular weight is less than 120 kg/mol, PS with various group have the Tjm on the order as follows: HOPS-OH > HOOC-PS-COOH > H-PS-H, and the order of the critical thickness as: HO-PS-OH < HOOC-PS-COOH < H-PS-H. When the film thickness is identical, the Tjm of HO-PS-OH and HOOC-PS-COOH is inversely proportional to the molecular weight. However, the thicknessdependence of Tjm of H-PS-H thin film is irrelevant to the molecular weight. When the molecular weight is higher than 120 kg/mol, the thickness-dependence of Tjm of the PS ultrathin film is independant on the end functional groups. The discrepancy in the glass transition dynamic is own to the difference of interfacial interaction between functional groups and substrate. As well, the result obtained by ellipsometry was well consistent with the results obtained by the dynamic wetting behavior of droplet, which demonstrated that it is feasible and universal to measure the glass transition temperature by dynamic wetting behavior of droplet.(2) The ratio between the thickness of interfacial adsorption layer(Ra) and the polymer radius of gyration(Rg) can be used to evaluate the interfacial interaction which affects the glass transition temperature of polymer thin film. The extent of Tjm reduction(ΔTjm) of PS ultrathin film linearly increased with Ra/Rg, and the critical thickness linearly decreased with Ra/Rg. We discovered the relation Ra/Rg with glass transition temperature of polymer film, and the glass transition temperature can be estimated according to the Ra/Rg.(3) The “stick-slip” behavior of droplets on the surface of polymer film was also affected by interfacial interaction. The slope for the linear correlation between maximum jumping angle(Δθmax) of PS ultrathin film and the thickness(Kθ) as well as that for wetting rigde height(Kw) increased with the increasing of Ra/Rg ratio. It revealed that the interfacial interaction is in proportion to the growth rate between the PS film thickness and maximum jumping angle(Δθmax) as well as the height of wetting ridge.