X-Ray Reflectivity and Fluorescence Measurements From Polystyrene-Co-Bromostyrene/Polystyrene Interfaces
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X-RAY REFLECTIVITY AND FLUORESCENCE MEASUREMENTS FROM POLYSTYRENE-CO-BROMOSTYRENE/POLYSTYRENE
INTERFACES 2
J. SOKOLOV', M. RAFAILOVICH', X. ZHAO', W.B. YUN , R.A.L. JONES3 , E.J. KRAMER 3 , R.J. COMPOSTO4, R.S. STEIN4, A.BOMMANNAVAR 5 ,M. ENGBRETSON6 1) QUEENS COLLEGE, FLUSHING, NY 2) ARGONNE NATIONAL LABORATORY, ARGONNE,IL
3) CORNELL UNIVERSITY, ITHACA, NY 4) UNIVERSITY OF MASSACHUSETTS AT AMHERST, AMHERST, MA 5) BROOKLYN COLLEGE, BROOKLYN, NY 6) OAK RIDGE NATIONAL LABORATORY, OAK RIDGE, TENN ABSTRACT X-ray fluorescence using synchrotron radiation at glancing angles of incidence was used to measure interface widths for the highly immiscible polymer blend system polystyrene/polybromostyrene (PS/PBrxS, with bromination level x - 0.8 bromine atoms per monomer). The interfacial widths are for bilayers annealed for 4 hours and 24 hours at 141±1C are found to be 1O0±20A and 110±20A respectively. Introduction
The characterization of interface in polymer blend systems gives valuable information on mechanical and thermodynamic properties of blends and can be used to discriminate between the various proposed theories of 11
polymer phase behavior. 1 When dealing with highly immiscible blends, such as PS/PBrS (for high levels of bromination), the interfaces between phases may be of the order of 1OOA or less and measurements of interfacial profiles require high spatial resolution. We report here the results of a study using the techniques of x-ray fluorescence under conditions of near 12 1 total external reflection (NTEF)" In the NTEF technique, the angular dependence of the x-ray-excited fluorescence of a labeled species is measured for angles of incidence near the critical angle 9c for total reflection - for polymers B. l-2mrad (see Table 1). Below B6, only evanescent waves penetrate the medium and the fluorescence is sensitive to the surface region. In the case of 15keV x-rays incident on polymers the sampling depth is typically 50-100A. As the incident angle is increased to 0 ) 8, deeper layers are probed, the total depth sensed being a function of 0 and the extinction length (Table 1) for x-rays in the scattering medium. The total fluorescence signal from the labeled species, in this case bromine, is given by,[2t
Ifr(6) - Kf'sr(z)
Ix(z,
0) exp(-IssBz)dz
(I)
where B is the x-ray incidence angle, z is the depth into the sample, Ob,(z) is the concentration of Br atoms of z, I,(z, 0) is the x-ray intensity at depth z, AB, is the linear absorption coefficient for the characteristic Pr x-rays in the polymer [as (l/Psr) ) 100pm and the bilayers studied were of thickness ( 0.5pm, Pur was taken as zero] and K is a constant taking account. of the atomic fluorescence cross-section and geometric factors such as detector acceptance angle, efficiency, etc. Calculations of the electric
Mat. Res. Soc. Symp. Proc. Vol. 171. ý1990 Materials Research Society
338
0) were done using the standard matrix propagation field intensity I(z, 31 methodf , in which the scattering medium is divided up into layers parallel to the surface o
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