| Polystyrene-polyisoprene (polybutadiene)-polystyrene triblock copolymers are thermoplastic elastomeric materials and widely used as adhesives and sealants. Surface properties, such as adhesion, friction and wettability, are affected by the surface structures of polymers. Hence, to achieve perfect surface properties of polymer materials, it is meaningful to clearly understand the surface structures at the molecular or nanoscale level in situ.The surface chemical structures of these block copolymers remain largely unstudied and poorly understood because only few surface techniques are effective to probe such copolymer surface at the molecular level. In the nearest decades, sum-frequency generation vibrational spectroscopy (SFG) has been developed into a surface-specific spectroscopic tool with monolayer sensitive and has been successfully applied to various kinds of surfaces and interfaces. SFG is a powerful and versatile in situ surface probe, which not only permits identification of surface molecular species but also provides information about orientation of functional groups at the surface.In this thesis, the surface structures and properties of polystyrene- polydiene-polystyrene triblock copolymers were investigated by contact angle measurement, atomic force microscopy (AFM) and SFG. The influence of casting solvent and chemical architecture of polydiene on film surface structure formation was explored.The surface structures and solvent-adaptive of polystyrene-block-poly isoprene-block-polystyrene triblock copolymer (SIS) films cast with various solution were systematically studied. The results show that the surface of SIS film cast with cyclohexanone solution was composed of PS and PI. Such surface structure show reversible switching properties if it is alternately exposed to PI-selective solvent cyclohexane and PS-selective solvent butanone vapor. While for films prepared with toluene and cyclohexane solution, PI segment tends to segregate and covers the surface, and the outmost surface was irreversible after exposure to different selective solvent vapor treatment. These results indicated that the existence of PS on the surface of initial SIS films would remarkably affect the reversible responsive to solvent.Surface morphology and chemical composition of polystyrene-block- polydiene-block- polystyrene triblock copolymer films casted with toluene solution depend on the chemical structure of mid-block (PB or PI). When the mid-block was PI segment, SIS film easily formed a structure of PS cyclinders parallel to the surface. The outmost surface of the film was occupied by pure PI which has lower surface energy. While for PB as mid-block, the PS cyclinders near the surface was perpendicular to the surface, PB coexist with PS at the outmost surface of SBS cast film.It has found that the chemical structure of PI block would affect the surface structure of SIS films. The surface of SIS containing 1,4-PI was dominated by 1,4-PI segment which has lower surface energy. While both PS and PI coexist on the outmost surface of SIS film, when 3.4-PI was selected as a mid-block, even though the surface energy of 3,4-PI is lower than the PS segment.When SIS-1 with 1,4-PI is blended with SIS-4 containing 3,4-PI, SIS-4 tends to segregate and remain in the air-polymer interface since it has a lower surface energy compared to SIS-1 in the blend regardless of the content of SIS-4. The enrichment of SIS-1 near the surface would induce the orientation of allyl group of 3,4-PI tilted more toward the surface. The enrichment of plasticizer will reduce the content of C=CH2 of allyl group on the SIS-4 surface.The relationship between the surface structure and adhesion force of SBS film was investigated by contact angle measurement, SFG, AFM and attenuated total reflection spectroscopy (ATR-FTIR). It is found that the adhesion force at the nanoscale of SBS films was mostly affected by the content of PS near the surface, but was insensitive to the composition at the outmost surface. |
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