Chain end group-induced surface ordering in poly(styrene-b-4-vinylpyridine) symmetric diblock copolymer films

The proton-terminated and fluoroalkyl-terminated poly(styrene-b-4-vinylpyridine) [P(St-b-4VP)] symmetric diblock copolymers were synthesized by a sequential anionic polymerization. The surface chemical composition, surface morphology and surface mechanical properties of the diblock copolymer thin films were investigated on the basis of the angular-dependent X-ray photoelectron spectroscopic (ADXPS) measurement and transmission electron microscopic (TEM) observations. The XPS results revealed that in the case of the proton-terminated P(St-b-4VP) film, the surface weight fraction of poly(4-vinyl pyridine)(P4VP) decreased dramatically after annealing above the glass transition temperature, Tg of P(St-b-4VP), resulting in the formation of polystyrene (PS) surface layer in order to minimize the interfacial free energy at air-polymer interface. Whereas, in the cases of the fluoroalkyl-terminated P(St-b-4VP) films [P(St-b-4VP)-C₂C^F_X where -C₂C^F_X shows fluoroalkyl end group], the surface weight fraction of P4VP decreased slightly and still was more than 30% even after the annealing treatment. TEM observation revealed that the lamellar structure was formed in all P(St-b-4VP). The surface order and orientation of lamellar structure were strongly influenced by the chain end group species. That is, in the case of the proton-terminated P(St-b-4VP) film, the lamellar structure was oriented parallel to the film surface and the outermost layer was composed of PS. On the other hand, in the case of P(St-b-4VP)-C₂C^F_X, the well-developed lamellae was tilted ca. 45 deg to the film surface and the alternating PS and P4VP layers were exposed to the air interface. These results indicate that the surface order and orientation of microphase-separated lamellar structure strongly depend on the chemical structure of the chain end group.