HYDROXYL-TERMINATED OLIGOMERS CROSS-LINKED BY ALKOXYSILANE SOL-GEL ORPOLYURETHANE CHEMISTRIES - A COMPARISON

The structures of in situ generated clusters and the level of physical interactions in two types of networks differing from the chemistry of crosslinking were studied by means of small-angle X-ray analysis and dynamic mechanical spectroscopy. For the first type of networks, the c rosslinks result from the hydrolysis and condensation of ethoxysilane endgroups, thus generating silicon-rich dispersed phase. In the second case, the crosslinks result from the formation of urethane units by i ntroducing a triol. In the two cases, different types of soft segment precursors having different polarities are considered. alpha,omega-hyd roxyl-terminated oligomers of hydrogenated polybutadiene or polycaprol actone or a polyester from oleic acid are used. The miscibility of the soft-segment chains with the relatively polar crosslinks is the most important parameter for understanding the morphology and the mechanica l behavior of such materials. The main difference obtained from the SA XS analysis and DMS experiments is that the silicon-rich clusters appe ar to be stiffer and well, separated in comparison with the trimethylo lpropane-urethane crosslinks. In addition, in the case of silica clust ers generated in situ, the phase separation plays an important role. I n the polycaprolactone-based systems, the formation of clusters is mai nly governed by the nature and the reactivities of the functional grou ps. As a consequence, the clusters are more fractallike. The mechanica l behavior, i.e., the mechanical losses and the high-temperature behav ior, is discussed as a function of the existing interactions and the c oncentration of elastically active network chains in the different typ es of networks considered. (C) 1997 John Wiley & Sons, Inc.