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-vinylpyri dine) [P(St-b-4VP)] symmetric diblock copolymers were synthesized by a sequ ential anionic polymerization. The surface chemical composition, surface mo rphology and surface mechanical properties of the diblock copolymer thin fi lms were investigated on the basis of the angular-dependent X-ray photoelec tron spectroscopic (ADXPS) measurement and transmission electron microscopi c (TEM) observations. The XPS results revealed that in the case of the prot on-terminated P(St-b-4VP) film, the surface weight fraction of poly(4-vinyl pyridine)(P4VP) decreased dramatically after annealing above the glass tra nsition temperature, Tg of P(St-b-4VP), resulting in the formation of polys tyrene (PS) surface layer in order to minimize the interfacial free energy at air-polymer interface. Whereas, in the cases of the fluoroalkyl-terminat ed P(St-b-4VP) firms IP(St-b-4VP)-C2CFxF where -C2CxF shows fluoroalkyl end group], the surface weight fraction of P4VP decreased slightly and still w as more than 30% even after the annealing treatment. TEM observation reveal ed that the lamellar structure was formed in all P(St-b-4VP). The surface o rder 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 surf ace and the outermost layer was composed of PS. On the other hand, in the c ase of P(St-b-4VP)-C-2 C-x(F), the well-developed lamellae was tilted ca. 4 5 deg to the film surface and the alternating PS and P4VP layers were expos ed to the air interface. These results indicate that the surface order and orientation of microphase-separated lamellar structure strongly depend on t he chemical structure of the chain end group.