Peculiarities in the solvent absorption characteristics of epoxy-siloxane hybrids

Epoxy-siloxane hybrids (bi-continuous nanocomposites) were produced from mixtures of a silane-functionalized resin and tetra-ethoxysilane by the sol-gel method. Absorption of tetrahydrofuran (THF) in thin samples was found to take place by a two-stage process, consisting of an induction period and a subsequent step-increase to the equilibrium level. The induction time increased with increasing siloxane content, while the equilibrium amount absorbed decreased accordingly. By assuming that solvent uptake of the system follows a classical Case 11 absorption, a graphical relationship between induction time, diffusion coefficient, and velocity of the advanced plasticized front was proposed as a basis for the control of the solvent absorption characteristics of this type of material. Absorption of methanol (CH3OH) took place rapidly and without an apparent induction stage, but the amount absorbed at equilibrium was much lower than for THF absorption. Increasing the siloxane content did not significantly decrease the amount of CH3OH absorbed at equilibrium, but considerably reduced the rate of solvent uptake, while the sorption process assumed the features of a two-stage process. The cyclic absorption of both solvents was found to cause structural changes in the epoxy network, in the form of a combination of physical aging and increased cross-linking density.