Radiative and nonradiative excited state processes for studying the sol to gel evolution

M. CLAUDIA MARCHI; ALDABE BILMES, S; RM NEGRI

LANGMUIR

AMER CHEMICAL SOC

Lugar: Washington; Año: 2002 vol. 18 p. 6730 - 6735

0743-7463

The sol to gel evolution of systems based on the hydrolysis of titanium n-butoxide, Ti(OBun)(4), in 1-butanol was investigated by monitoring the changes of the radiative and nonradiative electronic excited state processes of embedded dyes (cresyl violet and 4-(dicyanomethylene)-2-methyl-6(p-dimethylaminostyryl)-4H-pyran). Fluorescence anisotropy experiments (FA) allow determination of changes in the microviscosity of the medium surrounding the fluorophore through the sol-gel evolution. The increase of the anisotropy parameter ,, is explained in terms of solvent confinement in cavities enclosed within cross-linked polymeric chains. The acoustic signal recorded in laser-induced optoacoustics experiments (LIOAS) is attenuated as the system loses fluidity, with a minimum at t(g), thus providing an alternative method for determining the gelation point. In addition, within the theoretical approach of percolation theory, the exponent of the viscosity power law is obtained from the attenuation of the sound wave. Although both FA and LIOAS provide information on the degree of cross-linking between polymeric chains, there is a clear difference between the behavior of the macroscopic shear viscosity determined by LIOAS and the local friction or microviscosity obtained from FA.