| Abstract
| - The effect of colloidal silica fillers on the cross-linking behavior of a model UV-curablepolymer system (thiol−ene) is studied using in situ rheology and real-time FTIR spectroscopy. The validityof the Winter−Chambon criterion (convergence of the loss tangents at the gel point) is examined for thecross-linking of these filled polymers, some of which are strongly flocculated dispersions (physical gels)prior to chemical cross-linking. Two different types of colloidal silica particles are studied: one with octylchains tethered to the surface and the other with methyl surface groups. The Winter−Chambon criterionis satisfied for all samples containing the methyl-terminated silica. However, the criterion breaks downfor samples containing the octyl-modified silica, with the loss tangents not converging at any single point.This suggests the absence of a self-similar critical gel at the gel point in the latter case. Neither type ofsilica particles alters the mechanism of the cross-linking reaction, as revealed by FTIR spectroscopy, butthey do retard the cross-linking kinetics. An alternate method is suggested for determining the chemicalgel point of filled systems that fail to obey the Winter−Chambon criterion. This method involves monitoringthe critical strain (limit of the linear viscoelastic region) at various UV exposure times. A dramatic increaseis observed in the critical strain at the gel point, indicating a transition from weak, physical bonds tostrong, covalent cross-links.
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