Molecular motion and anisotropy in polymers investigated by fluorescence techniques

Fluorescence polarization has been used to investigate the correlation between the orientation distribution function odf^G of quasi rigid rod fluorescent guest molecules and the odf^H of anisotropic host polymer in which the guests are imbedded, e.g. fluorescent molecules incorporated in the non-crystalline regions of uniaxially stretched polyethylene (LDPE). The orientation correlation between odf^G and odf^H is discussed with respect to the thickness to length ratio t/1 and the length of the probe molecules. For a rigid rod length around 2 nm there is a linear dependence between the factorizable orientation parameter P₂^G of odf^G and the length of rigid rods at given orientation of LDPE. This behaviour is obtained after correcting the different ratios t/1 of the probes. The variation of probe geometry, i.e. reference length and ratio t/1, gives information not only about the mechanism of orientation but also about the change of rotational mobility depending on size and fluorescence lifetime of the probes. Selecting probes of various length in the lifetime region of 1–6 ns, it results that microregions with reference length larger then 2 nm are rotationally immobile in the non-crystalline regions of LDPE at ambient temperature.