Reversibly Cross-Linking Amino-Polysiloxanes by Simple Triatomic Molecules. Facile Methods for Tuning Thermal, Rheological, and Adhesive Properties

The physical properties of five siloxane polymers with different types and frequencies of amino functional groups along the polymer side chains have been manipulated from flowing liquids to gels and to rubber-like materials by the simple addition or subtraction of a neutral triatomic molecule, CO2 or CS2, at room temperature. The chemical changes, formation of ammonium carbamates and ammonium dithiocarbamates, create materials whose properties are totally different from those of the parent polymers as a result of the introduction of ionic cross-links. These materials can be returned to their original forms by heating (in the case of the CO2 adducts) or to their protonated original forms by treatment with an acid (in the case of the CO2 and CS2 adducts). Heating the ammonium dithiocarbamates leads to loss of H2S and permanent (covalent) thiourea cross-links between the polymer chains. The new materials adhere strongly to other surfaces and can be swelled to several times their original volumes by different liquids. The rheological, swelling, and physical properties of the new materials have been correlated with the structures of the original polymers to provide a comprehensive picture of how changes at the nanometric length scale are translated to macroscopic changes. At least for the polysiloxanes examined here, the properties of the adducts do not correlate with the molecular weights of the original polymers, but do with the frequency of amino groups. The results demonstrate a simple, new method to cross-link polysiloxanes (and, in principle, a wide range of other polymers), transforming them into materials with totally different and potentially commercially useful properties.