Evaluation of the polarity of polyamide surfaces using the fluorescence emission of pyrene

This paper reports the static and dynamic fluorescence emission of pyrene sorbed onto the surface of 6 different polyamides classified in two categories: AB (polyamide-6 and -11) or AABB (polyamide-6,6, -6,9, -6,10 and -6,12). Pyrene is a well known fluorescent probe for the polarity evaluation of different media for which the ratio of intensities of the vibronic bands (I1/I3) decreases in less polar medium and the fluorescence decay from the singlet electronic excited state increases with decreasing polarity. The steady-state vibronic ratio I1/I3 of the pyrene fluorescence emission decreases with decreasing polarity for polyamides AB but for polyamides AABB, this ratio undergoes a significant decrease for polyamides-6,9 compared with -6,6 but remains almost constant for polyamides-6,10 and -6,12. This shows that the steady-state fluorescence of pyrene is only sensitive to the polarity of sorption sites whose dimensions are similar to the van der Waals radius of the molecule. In contrast, fluorescence decays decrease with decreasing polarity of the polyamide from τF = 250 ns for polyamide-6,6 to τF = 310 ns for polyamide-6,12 and thus this parameter is always sensitive to the polarity of the medium. We also discuss the advantages of these spectroscopic methodologies that include their applicability to samples without any further sample manipulation and which is independent of their form, powder, pellet or film, and of their thicknesses and sizes.