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Fluorescence is the property of absorbing radiation and emitting instantaneously a radiation, which is shifted (compared to the absorption) to longer wavelengths. The figure illustrates the changes of a molecule's energy levels and the spectra of absorption (excitation) fluorescence.
By the use of fluorescent probes it is possible to measure amorphous orientation of PET and different polymers, which don't provide the special intrinsic fluorescence of PET. Probes with anisotropic absorption or emission are added to the polymer material (see picture left). The fluorescent probe has to provide polarised fluorescence
A suitable fluorescence probe is Eastobrite OB-1 (Eastman Kodak). It provides anisotropic absorption (see fig. right). It is often used as additive. By addition of 50 ppm or less it's possible to take measurements of PET, PBT, PA6 and PA 6.6 (see results). Also possible is the measurement of PP, but probably the overall orientation is detected in this case.
Structure (top and bottom) and spectra (right) of fluorescence probe Eastobrite OB-1 (Eastman Kodak)
The fibre is illuminated with UV radiation polarized either parallel and perpendicular to its axis. The fluorescence radiation is analysed either parallel and perpendicular to the fibre axis.
The measured four intensities are:
I33: intensity of the fluorescence
light polarizer and analyser are parallel to sample axis (3),
I31: polarizer parallel, analyser perpendicular to draw axis,
I13: polarizer perpendicular, analyser parallel to draw axis,
I11: polarizer and analyser perpendicular to draw axis.
the orientation values are calculated by:
and r is the emission anisotropy.
Biaxial material is characterised by the intensity ratios RTD and RMD:
Further information about the measurement principle and the application of this sensor is available in the literature.
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