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

Abstract

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.