Chemical modification of the hydrophilic surface of poly(ethylene terephthalate) (PET) was examined by the selective alkylation of the acid salt on the surface using acyl bromides as an electrophile with catalytic potassium fluoride. The hydrophobicity of the PET surface increased as the alkylation reaction of the hydrolyzed surface proceeded. The chemical incorporation of the alkylation reagents was confirmed by the presence of fluorine peaks in X‐ray photoelectron spectroscopy (XPS) from the fluorinated reagents. Exponential increases were observed in the emission and excitation intensities of fluorescence spectra during the reaction of the surface with a fluorescent reagent. This indicated that the esterification of carboxylic acids on the surface proceeded without deterioration of the reaction even at the later stages. The relation between changes in contact angles and the fluorescence spectra revealed that the hydrophobicity of a surface was quickly restored at the beginning of a reaction.
