Wang Xinping

Detection of free surface composition and molecular-level structural development of styrene(S)/butadiene(B) block copolymer films during a solution-to-film process

The surface chemical structure development in solution-cast styrene(S)/butadiene(B) block copolymer films as a function of solvent evaporation time was investigated using sum frequency generation vibrational spectroscopy (SFG). The surface structure formation of the styrene(S)/butadien(B) block copolymer (30 wt% PS) films during the solution-to-film process was found to be controlled mainly by dynamic factors, such as the mobility of the PB block in solution. For SB diblock copolymers, a pure PB surface layer was formed only when the film was cast by dilute toluene solution. With increasing concentration of casting solution, PB and PS components were found to coexist on the film surface, and the morphology of the PB component on the film surface changed from cylindrical rods to spheres. For SBS triblock copolymers, a small amount of PS component existed on the surface even if the film was cast by 1.0 wt% toluene solution. In addition, PS components at the outermost layer of the film increased and the length of PB cylindrical rods on the surface decreased with increasing concentration of casting solution.

Environmental adaptivity of the surface of styrene-isoprene-styrene tri-block copolymers films controlled by the film-preparation kinetics

The responses of the surface structures to the solvent vapor treatment for the films of styrene(S)/ isoprene(I)/styrene(S) tri-block copolymers (SIS) prepared by various casting solvents and film-formation methods (solvent-casting and spin-coating) were investigated by contact angle measurement, atomic force microscopy (AFM) and sum frequency generation vibrational spectroscopy (SFG). It was shown that the surface wettability of the SIS solvent-cast films (cyclohexanone as solvent) and spin-coated films (toluene as solvent) exhibited the reversibly switching properties, if alternately exposed them to the vapor of PI-selective (cyclohexane) and PS-selective (butanone) solvents. However, for the solvent-cast films prepared by cyclohexane and toluene solution, the wettability of the films surface did not change, regardless of which solvent was applied in vapor treatment. It was claimed that the difference in the phase-separation structure of the SIS films prepared using different casting solvents and film-formation methods is the main reason for the discrepancy in the surface adaptive behavior. If the films have the equilibrium phase structures, the energy barrier for the conformation adjustment is apparently larger, and therefore it is hard to change the aggregation structure to accommodate the new solvent-vapor environment. While for the films having the non-equilibrium phase-separation structures, the conformational adjustment under the different solvent vapor will become relatively easier. Owing to this effect, the prepared SIS films with various film-formations kinetics can form different aggregation structures after a specific solvent vapor treatment, and finally leads to dissimilar surface structure. As a result, the surface wettability of these films exhibit different responsiveness to solvent vapor.

Effects of substrate/polymer interfacial interactions on dynamics of thin polystyrene films

The glass transition temperature ( T g) and viscoelasticity of thin polystyrene (PS) films supported on Si/Si-H and Si/SiO2 substrates were investigated using an ellipsometry and force-distance (F-D) measurements of atom force microscopy. It was found that T g of thin PS films deposited on either Si/Si-H or Si/SiO2 decrease with decreasing of film thickness. However, T g of PS film on Si/SiO2 substrate depressed more dramatically than that on Si/Si-H. The threshold thickness for T g reduction of thin PS film on Si/SiO2 (110 nm) is higher than that of film on Si/Si-H (40 nm). Meanwhile, thin PS film supported by Si/SiO2 exhibits the higher thermal expansivity compared with that on Si/Si-H substrate. Surface F-D measurements reveal the film on Si/Si-H substrate is stiffened, showing a higher elastic modulus. In order to understand the mechanism of the substrate effect, the interfacial energy and long-range van der Waals potential ( Φ vdw ) were investigated and a strong attractive interaction between PS and Si/Si-H substrate was found. Thus, the packing density was enhanced and the molecular mobility of PS chains at Si/Si-H/PS interface was suppressed. This strong interaction resulted in higher T g and elastic modulus of thin PS film compared with that on Si/SiO2 substrate.