Study on the microstructural evolution of silica gel during sol-gel-gel-glass conversions using the fluorescence polarization of rhodamine B

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Abstract
The fluorescence polarization of rhodamine B is used as an environment probe to study the microstructural evolution of silica gel during sol to gel to gel glass transitions. The wet gel is an amorphous porous solid of which organics and solvents (water and ethanol) exist in the pores. During gelation and heat treatment, the solvents evaporate gradually and the inorganic network of the matrix forms and strengthens as a result of the collapse of pores and the structural relaxation. The rhodamine B molecules lose mobility gradually and are finally steadily isolated and adsorbed into the tiny pores of the inorganic network, which makes the rotational diffusion of rhodamine B molecules difficult and increases the fluorescence anisotropy and polarization of rhodamine B.