Estudo do comportamento térmico de polifluorenos in situ por espectroscopia Raman

Abstract

This work aimed to study the thermal behavior of conductive polymers belonging to the polyfluorene family: poly(9,9-dioctylfluorenyl-2,7-diyl) - PF8; poly[(9,9-di(3,3'-N,N'- trimethyl-ammonium)propylfluorenyl-2,7-diyl)-alt-(9,9-dioctylfluorenyl-2,7-diyl)] bromide - PFN-Br and poly(9,9-dioctylfluorene-alt-benzothiadiazole) - F8BT through Raman spectroscopy. In situ Raman spectra using excitation at 785 nm were recorded in the range of - 196 ºC (77.15 K) to 325 ºC (593.15 K) for polymers PF8 and PFN-Br, while for F8BT the range from -196 °C (77.15 K) to 300 °C (573.15 K) was employed. For PF8, a shift to a lower wavenumber was observed with increasing temperature of the bands at 1415 cm-1 (νC‒C inter meros) and 1345 cm-1 (ρC‒H + νC‒C fluorene ring) which indicates a decrease in chain conjugation length. An increase in the relative intensity of the band at 1345 cm-1 was observed for temperatures above the glass transition temperature (Tg) and of the bands at 1280 cm-1 and 1305 cm-1 at a temperature close to the phase transition, indicating that these bands are related to the change in the torsional angle of the polymeric chain (ϕ). The Raman spectrum of PFNBr presents a profile similar to that of PF8 since the spectrum is dominated by bands related to the fluorene group. The variation in the intensities of the bands associated with the change in ϕ as a function of temperature is smaller in PFN-Br than in PF8, indicating that alkylammonium substituents induce greater rigidity in the polymeric chain. The Raman spectrum of the F8BT presents the band at 1360 cm-1 , attributed to the νC‒C of the benzothiazole unit; the behavior of this band (position and intensity) as a function of temperature can be related to the variation in the conjugation length and thermal events such as the Tg and crystallization.