Effect of Cross-link Density on Polybutadiene Vitrimer Networks

Vitrimers are polymer networks with dynamic cross-links that can rearrange themselves through associative bond exchange. When placed in a good solvent they remain insoluble because the cross-links maintain network connectivity. When heated, however, the network undergoes a topological rearrangement. In this work, we evaluate how varying the cross-link density (N_XL) affects the network structure and properties. Polybutadiene (PB) vitrimers with dioxaborolane cross-links were synthesized by cross-linking commercial PB via a photo-initiated thiol-ene click reaction. The targeted N_XL was varied from 1.1 to 7.6 cross-links per chain. To compute the number of thiol-ene linkages per PB chain, the networks were cleaved with excess diols. Analysis of nuclear magnetic resonance spectra of the cleaved networks yielded a direct measurement of the number of thiol-enes per chain. As N_XL was increased, both the glass transition temperature and plateau modulus increased. The measured plateau modulus from variable temperature small-amplitude oscillatory shear was compared to predictions from the phantom network model. Vapor swelling experiments in chloroform showed that the swelling degree increases before reaching a plateau above N_XL of 4.4. This study demonstrates the influence of dynamic associative cross-links on polymer network structure.