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1、Film Thickness Dependence of the Surface Structure of Immiscible Polystyrene/Poly(methyl methacrylate) Blends Keiji Tanaka, Atsushi Takahara, and Tisato Kajiyama* Department of Chemical Science Revised Manuscript Received December 20, 1995X ABSTRACT: The film thickness dependence of surface structur
2、e for immiscible polystyrene/poly(methyl methacrylate) (PS/PMMA) films was investigated on the basis of atomic force microscopic observation and X-ray photoelectron spectroscopic measurement. In the case of the PS/PMMA film of 25 m thickness, the air-polymer interfacial region was covered with a PS
3、rich overlayer due to its lower surface free energy compared with that of PMMA and a well-defined macroscopic phase-separated structure was formed in the bulk phase. Also, in the case of the PS/PMMA thin film of 100 nm thickness, the phase-separated structure, in which the PMMA rich domains separate
4、d out of the PS rich matrix, formed at the film surface. The formation of the surface structure for the PS/PMMA thin film can be attributed to either the chain conformation or chain aggregation structure being frozen at the air-polymer interfacial region before the formation of a PS rich overlayer d
5、ue to the fairly fast evaporation of solvent molecules. On the other hand, the two-dimensional PS/PMMA ultrathin film of 10.2 nm thickness did not show distinct phase-separated structure. When the film thickness became thinner than 10.2 nm, the two-dimensional PS/PMMA ultrathin film of 6.7 nm thickn
6、ess showed fine and distinct phase-separated structure with the domain size of a few hundred nanometers. This structure can be designated as “mesoscopic phase- separated structure”. The surface phase state for the two-dimensional PS/PMMA ultrathin films can be explained by the film thickness depende
7、nce of both the interaction parameter and the degree of entanglement among polymer chains. Introduction Investigations of surface structure for multicompo- nent polymer systems have been extensively done for the last decade, both experimentally1-8and theoreti- cally,9with respect to associated funct
8、ional properties such as blood compatibility, lubricant, wetting, perme- ability, and so on. It has been revealed that the surface structure of multicomponent polymer systems is fairly different from that in the bulk, that is, a lower surface free energy component is generally enriched in the surfac
9、e region in order to minimize the air-polymer interfacial free energy. Also, for the past few years, surface molecular motions of polymeric solids have been paid great attention by several groups10-16due to their importance in practical applications.Both vigorous thermal molecular motion and thermal
10、 instability of polymeric chains at the surface can be explained by the larger free volume fraction at the surface region com- pared with that in the bulk due to the preferential segregation of chain end groups at the surface re- gion11,13,14and/or the unsymmetrical environment at the air-polymer in
11、terface.13,14 Polymeric films of whose thicknesses are less than about twice the radius of gyration of an unperturbed chain, 2Rg, can be defined as two-dimensional ultra- thin films.17-19Then, the polymeric blend films with thicknesses less than 2Rgof the higher molecular weight component can be def
12、ined as the two-dimen- sional ultrathin blend films.20A flexible polymer chain in an ultrathin film is in a nonequilibrium state, since the conformational entropy of an individual chain in a constrained thin region is reduced in comparison with that in a three-dimensional solid state.19Since polymer
13、ic chains at the interface, in general, are thermally unstable, the molecular aggregation struc- ture in the two-dimensional ultrathin film of binary polymer blend must be greatly different from that in the thick film. The authors have investigated the film thickness dependence of surface structure
14、for the mis- cible polystyrene/poly(vinyl methyl ether) (PS/PVME) blend films.20Even though the blend system was miscible in the bulk region, it was revealed that sur- face phase-separated structure was formed even at the temperature below the bulk phase separation temperature in the case of the two
15、-dimensional ul- trathin state. The mechanism of surface phase sep- aration for the PS/PVME ultrathin film could be ex- plained by two factors: the negative spreading coef- ficient of PVME on the PS matrix and the remarkably reduced conformational entropy of a PVME chain due to stretching. Internal
16、phase-separated morphology for immiscible polymer blend films has been studied by several groups.21-23Shiraga et al. investigated the bulk phase- separated structure of the immiscible PS/poly(methyl methacrylate) (PS/PMMA) blend film on the basis of field-emission scanning electron microscopic observa- tion.21They revealed that macroscopic phase-separated domains with a few micrometers in diameter were formed in the bulk region of the thick film. Winnik and co-workers investigated the depth pro
