Infrared and fluorescence spectroscopy investigation of the orientation of two fluorophores in stretched polymer films

The simultaneous orientation of poly(1,4-butylene)succinate (PBS) and 4,4′-bis(2-benzoxazolyl)stilbene (BBS) or 2,5-bis(5-tert-butyl-benzoxazol-2-yl)thiophene (BBT) in PBS–BBS and PBS–BBT films was investigated during stretching at 80 °C, about 33 °C below the PBS melting temperature. The PBS orientation was first investigated using the 3430 cm−1 infrared band and its order parameter <P2> goes from 0.36 at a local strain of 359% for pure PBS to 0.66 in blends containing 6 wt % of BBT at a local draw ratio of ≈350% (or 0.56 with 5 wt % of BBS). At the same time, BBT shows in the same films an orientation parameter <P2> reaching a maximum of 0.31 at a local strain of ≈350%, using the 1580 cm−1 band. Since polarized FT-IR was unable to provide the orientation of BBS due to overlapping bands with PBS, polarized fluorescence spectroscopy was then used and reveals an apparent order parameter of 0.32 for BBS monomers (or 0.34 for BBT monomers), but to no orientation of BBT aggregates and BBS excimers. In other words, the two small molecules behave similarly in terms of orientation independently of their molecular shape and packing. BBS and PBS both exhibit a sharp increase of S and <P2> when reaching a BBS concentration of ≈0.08 wt %, whereas BBT and PBS show a smooth increase of S and <P2>, respectively, in films containing BBT. The sharp increase, or transition in orientation, is accompanied by a conversion of BBS from monomers to excimers, whereas BBT-containing films show a regular increase of the number/size of BBT aggregates. These results indicate that the dye (BBS or BBT) has a profound influence on the orientation of the semi-crystalline polymer (PBS) since its <P2> almost doubles at dye concentrations above 0.08 wt %. The results also suggest that the use of a fluorescent probe to follow the polymer chain orientation is not applicable for such systems.