EXCIMER FLUORESCENCE IN POLYPHOSPHAZENES .2. MORPHOLOGY AND DYNAMICS IN POLYMER-FILMS

The effects of substituent on morphology and fluorescence behavior in poly(organophosphazenes) are studied. Side-group and main-chain motions in two representative poly[(aryloxy)phosphazenes], poly(diphenoxyphosphazene) (PBPP) and poly(di-p-cresoxyphosphazene) (PBCP), are investigated along with a copolymer, poly[(trifluoroethoxy)phenoxyphosphazene] (PFPP) in both amorphous and crystalline phases in terms of the excimer-to-monomer intensity ratio, I(D)/I(M), excimer bandwidth, and excimer band position. The glass transition temperature, T(g), and mesomorphic temperature, T(1), obtained by differential scanning calorimetry, correlate well with transitions observed in I(D)/I(M) and excimer bandwidth measurements. Multiple excimer-forming sites are found in all polymers, especially in PFPP where isomorphic forms of monoclinic and orthorhombic structures are found. A comparison of fluorescene spectra from solutions and the bulk phase yields qualitative assignments of the excimer bands, whereas the fluorescence spectra for annealed PBPP and PBCP films illustrate the effect of crystal structure. In PBPP, the transformation of the alpha-form to the beta-form of the monoclinic crystal structure upon annealing does not increase the overall excimer emission but causes a blue shift in the composite excimer band. By contrast, the transformation of the alpha-form to the gamma-form of the orthorhombic structure in PBCP results in an increase in excimer emission as well as a red shift of the excimer band. The increase in excimer emission in PBCP upon annealing below T(1) suggests that the orthorhombic structure is more conducive to excimer formation.