X-RAY EVANESCENT WAVE-INDUCED FLUORESCENCE STUDY OF POLYMER ADSORPTION BELOW THE THETA-POINT

X-ray evanescent wave-induced fluorescence (XEWIF) has been used to measure the monomer segment profile of poly(dimethylsiloxane) adsorbed from a dilute solution in bromocyclohexane to the solution/vapour interface. Rather than employment of a fluorescent label on the polymer chain, the bromine in the solvent provided the fluorescent label and sufficient intensity was observed to perform the experiments using a conventional X-ray source. The theta-point for this system has been reported as 29-degrees-C and our measurements were performed at (23 divided-by 24)-degrees-C with no indication of bulk phase separation. Our results cannot be interpreted by scaling or mean-field models for polymer adsorption from a theta solvent. However, a step profile, in which a polymer-rich phase with phi(polymer) = 0.32 +/- 0.02 extends (14 +/- 3) nm into the bulk phase fits the data well. The step profile is consistent with the interpretation of prewetting near the solution critical point.