Conferring dichroic properties and optical responsiveness to polyolefins through organic chromophores and metal nanoparticles

Abstract

Properly designed organic chromophores based on functionalized terthiophenes by push–pull substituents and stilbene derivatives were efficiently incorporated into polyolefins (polyethylene (PE), polypropylene (PP) and their copolymers, even with polar functionalities) at various concentration (from 0.05 to 2 wt.%) either by casting from solvents or by processing in the melt. The uniaxial orientation of the derived films by mechanical drawing provided highly anisotropic films with optical dichroic behaviour in the visible both in absorption and in emission. A different optical response was obtained with bis-benzoxazolyl-stilbene (BBS) optical brighteners incorporated into PE and PP films (ranging from 0.05 to 0.5 wt.%) by melt-processing showing for the last polymer matrix a well-defined excimer green emission by increasing dye concentration flanked by a clear change of the luminescent properties of the films. The original blue colour of the film provided by the radiative transitions of the isolated BBS molecules, may be restored after polymer stretching that promotes the breakup of the aggregates and the alignment of the single chromophore molecules along the stretching direction thus providing a very sensitive tool for determining the polymer orientation. With the same approach nanocomposites with unusual and anisotropic optical properties were obtained through the dispersion of preformed gold nanoparticles and gold-binding chromophores in a stretched PE matrix (∼4 wt.%) or produced directly inside a polymer matrix by a photo-reduction process. This kind of nanostructured materials can be applied to different fields that ranges from sensors ( i.e., sensitive devices for UV-degradable substances, molecular strain sensor for polymer matrices) to photonics ( i.e., linear absorbing polarizer, displays, non-linear optical devices).