Broadening of the fluorescence spectra of hydrocarbons in ethylene-vinyl acetate copolymers and the dynamics of the glass transition

The steady-state fluorescence of two fluorescent condensed aromatic hydrocarbons (anthracene and pyrene) with different fluorescence decay rates (a few nanoseconds and hundreds of nanoseconds, respectively) were employed to investigate the relaxation processes of several random ethylene-co-vinyl acetate copolymers (EVA). Copolymers with various comonomer compositions (EVA-9, EVA-18, EVA-25 and EVA-33) were studied. Data for the EVAs were compared with the same aromatic molecules in low-density polyethylene (LDPE), high-density polyethylene (HDPE) and poly(vinyl acetate) (PVAc) homopolymer models. The fluorescence rate constants were measured by single photon counting at room temperature. The wavelength dependence of the emission spectra (the edge-excitation red-shift (EERS)) were studied at room temperature and at 77 K and the data were interpreted based on the correlation of the differences of decay rates for both molecules and the time correlation of the polymer relaxation processes. The spectral broadenings (full-width at half-maximum (FWHM)) of the fluorescence spectra were studied from 20 to 400 K, which included temperature ranges above and below the glass transition, and these broadenings are compared.