Phase Behavior and Structural Changes in Tetraethylorthosilicate-Derived Gels in the Presence of Polyethylene Glycol,...

A study has been done on how different concentrations and molecular weights of polyethylene glycol (PEG) affect the phase behavior of the sol-gel reaction. The sol-gel reaction of tetraethylorthosilicate (TEOS) in acidic media in the presence of PEG has been studied by means of visual observations and rheological investigations. The observations revealed that a macrophase separation occurred at intermediate concentrations for the two highest molecular weights of PEG. The proposed mechanism was bridging flocculation between organic and inorganic polymers. At high concentrations of PEG, steric stabilization took place and thus the phase transition evolved without a macrophase separation. Gelation times have been determined by visual observations and two different rheological methods, and there were no significant differences in the gelation times determined by the different methods. Flow curves revealed Newtonian, shear thinning, rheopectic, and thixotropic flow behavior at different stages of the sol-gel reaction, giving information about the structure formed. At the gelation point a viscoelastic scaling law G'(omega) approximately G"(omega) approximately omegan has been observed with values for the critical exponent n in the range of 0.71-0.83. Values of the critical exponent have been used for gaining information about the degree of branching of the gel when comparing different phase behavior and growth mechanisms. It was shown that the phase-separated gels were less branched and more open than the others. Copyright 1998 Academic Press.