Structure-Thermomechanical Property Correlations of Highly Branched Siloxane-Urethane Networks

A series of B-3 core-terminated highly branched siloxane urethane polymers was synthesized through the A(2) + B-3 route. Isophorone diisoc-yanate (IPDI)-terminated polydimethylsiloxane (PDMS) was used as the A(2) unit and triethanol amine as the B-3 core. Size exclusion chromatography (SEC) studies revealed decreasing number-average molecular weights for the branched polymers and increasing tendency toward lower molecular weight species formation with increased proportion of B-3 core in the branched polymers. The degree of branching and fraction of dendritic units, evaluated from H-1 NMR, increased monotonically with increasing B-3 core in the branched polymers. Cross-linked networks of the highly branched polymers were prepared by reaction of the terminal hydroxyl groups with tetraethoxysilane (TEOS) at room temperature. The sol fractions obtained for the networks from solvent extraction studies were consistent with the non-network-forming low molecular weight fractions obtained from the deconvoluted SEC traces. The solubility parameter, Flory-Huggins interaction parameter, and cross-link density of the networks were evaluated from swelling studies. FTIR spectroscopy was used to evaluate the degree of hydrogen bonding of the branched networks. The thermomechanical properties of the networks were evaluated by stress strain measurements and dynamic mechanical analysis, and the results were correlated with the structural parameters, such as degree of branching, extent of hydrogen bonding, and cross-link density.