Poly(Styrene- b -Butadiene- b -styrene) Self-organized Ultrathin Films
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(MNO is 139,000 and its weight fraction of polystyrene(PS) blocks is 40%. The polystyrene (PS) was prepared through polymerization of styrene initiated with radical 2.2'-azobis(2-methylpropionitrile). Its Mw is 10,000. Preparation of Self-organized Ultrathin Films The SBS dissolves in toluene, xylene and THF, respectively, to form solutions with a concentration of 1% (weight/volume). The three solvents are good solvents. Using a syringe, a small drop (about 0.005ml) of the SBS solution is injected on the surface of a clean hydrophobic silicon wafer (1 cm x 1 cm) (film-forming condition 1). The thickness of the solution layer is about 50nm. After some minutes, the color interference fringes appear at the film surface. If the silicon wafer is placed in a vessel sealed with a rubber septum (film-
forming condition 2), the times of appearance of the interference fringes were delayed in these systems. The measurements of AFM show that the structures of the films will do not change further after appearance of interference fringes. So the time of appearance of the interference fringes provides a qualitative estimate of the evaporation rate of the film-forming solvent. They are indicated as ta and listed in table I.
Table I The t, under a variety of the film-forming conditions . _€o......d..t.0 .n...................................................................................! .................................................................................................. 2........................................................... ..................................................... f. P -f !! .a.i xylene toluene THF xylene toluene film-forming solvent 60 6 60 10 10 ta (mlin) AFM Observations The AFM measurements were obtained on a commercialized instrument, Nanoscope III AFM (Digital Instruments, Santa Barbara, U.S.A). All the images were obtained in ambient air at room temperature with standard silicon nitride tips in a tapping model. Determination of intrinsic viscosity [TI]. The intrinsic viscosity of the solutions of SBS and PS in toluene and xylene was measured with an Ubbelohde viscometer at 25 VCand the [il] values are listed in table II. Table 11 [il] of the solutions of SBS and PS in the toluene and xylene []] .toluene PS (ml/g) SBS (ml/g)
40.38 98.05
xylene 40.02 98.41
RESULTS AND DISCUSSION Figure 1 shows AFM images of the self-organized ultrathin films cast from 1% xylene and toluene solution of used SBS under the film-forming condition 1. There are spherical domains with 25nm diameter in the xylene solution cast film. These spherical domains assemble in a perfect hexagonal closed-packed array (Fig. la). There are also the spherical domains with 25 nm diameter in the toluene solution cast film. They have assembled in a line array rather than in a hexagonal array as in Fig. la.and have some deformation (Fig. Ib).
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Figure 1 The AFM images of self-organized ultrathin films cast from 1%solution of SBS in xylene (a. b) and toluene (c) under film-forming condition I These spherical structures are
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