Telechelic siloxanes with hydrogen-bonded polymerizable end groups. I. liquid rubbers and elastomers

摘要

Functional siloxanes are widely used in the synthesis of model networks used in rubber elasticity studies, and various functional siloxanes are also used in the synthesis of silicone rubbers. We present here the results of studies of two classes of telechelic, free-radically polymeriz-able siloxanes carrying either amide or urea groups attached at both ends of siloxane chain. The presence of polar, hydrogen-bonding end-groups favors the formation of aggregates which is reflected in the drastically increased viscosity of the resulting "liquid rubbers." In addition, such aggregation accounts for high local concentrations of reactive groups thus making it possible to have high molecular weight precursors undergo facile room temperature polymerization. In this article, we study the effect of the nature of these functional groups on the viscosity of polymerizable "liquid rubbers," finding that the different endgroups lead to dramatically different viscosities. We also study the silicone rubber films obtained by a UV-initiated free-radical polymerization of the liquid rubbers, using dynamic mechanical analysis and large-strain uniaxial deformation. We find here that important properties such as plateau modulus and Young's modulus are highly dependent on both endgroup type and precursor molecular weight. The cross-linking of telechelic siloxanes in dilution was also studied to further explore the effect of crosslink density of the mechanical properties of the model networks. Finally, we evaluated the role of dangling ends within the networks through the incorporation of monofunctional macromers.