BULK VS SURFACE CONTROL OF POLYMER ADSORPTION FROM SOLUTION - AN X-RAY EVANESCENT-WAVE INDUCED FLUORESCENCE STUDY

We have measured the adsorption of poly(dimethylsiloxane) from solutio ns of toluene (good solvent) and 2-butanone (Theta solvent) to the sol ution/vapor interface at 20.0 degrees C. The adsorption was studied us ing surface tensiometry and X-ray evanescent wave induced fluorescence (XEWIF). In the XEWIF method, the atomic fluorescence of Silicon, con tained in the polymer, was measured as a function of an X-ray excitati on depth beneath the liquid/vapor surface. Cu K alpha(1) X-rays from a conventional tube source were used to excite the fluorescence in a sp ectrometer of our design. The data were interpreted by fitting model s tep adsorption profiles and then calculating the surface adsorption ex cesses as a function of molecular weight and concentration. Two polyme r samples were studied, with weight average molecular weights of 38.9 x 10(3) and 631 x 10(3) g/mol and M(w)/M(n) equal to 2.84 and 1.73, re spectively. No strong molecular weight or concentration dependencies w ere observed. Although the addition of polymer reduced the surface ten sion more in toluene solution than in 2-butanone, a greater surface ex cess was observed in the 2-butanone solutions. The comparison of the p olymer adsorption in the two solutions allows us to discuss the relati ve influence of the surface and the bulk solution quality in determini ng the surface excess.