Effect of dangling chains on the structure and physical properties of a tightly crosslinked poly(ethylene glycol) network

Abstract

Major imperfections in crosslinked polymers include loose or dangling chain ends that lower the crosslink density, thereby reducing elastic recovery and increasing the solvent swelling. These imperfections are hard to detect, quantify and control when the network is initiated by free radical reactions. As an alternative approach, the sol–gel synthesis of a model poly(ethylene glycol) (PEG-2000) network is described using controlled amounts of bis- and mono-triethoxy silyl propyl urethane PEG precursors to give silsesquioxane (SSQ, R–SiO_1.5) structures as crosslink junctions with a controlled number of dangling chains. The effect of the number of dangling chains on the structure and connectivity of the dried SSQ networks has been determined by step-crystallization differential scanning calorimetry. The role that micelle formation plays in controlling the sol–gel PEG network connectivity has been studied by dynamic light scattering of the bis- and mono-triethoxy silyl precursors and the networks have been characterized by ²⁹Si solid state NMR, sol fraction and swelling measurements. These show that the dangling chains will increase the mesh size and water uptake. Compared to other end-linked PEG hydrogels, the SSQ-crosslinked networks show a low sol fraction and high connectivity, which reduces solvent swelling, degree of crystallinity and the crystal transition temperature. The increased degree of freedom in segment movement on the addition of dangling chains in the SSQ-crosslinked network facilitates the packing process in crystallization of the dry network and, in the hydrogel, helps to accommodate more water molecules before reaching equilibrium.