Impact of Cross-Link Density on Polybutadiene Permanent and Vitrimer Networks

Vitrimers are polymer networks that undergo structural rearrangement through dynamic associative bond exchange without compromising the overall topological connectivity. In this study, we investigate the impact of cross-link density on the structural, viscoelastic, and glass transition properties of both vitrimer and permanent networks. We synthesized polybutadiene (PB) networks using a photoinitiated thiol-ene click reaction to incorporate either dynamic dioxaborolane or permanent 1,4-benzenedimethanethiol cross-links, with the number of cross-links per chain ranging from 2 to 15. To quantify the actual cross-link density, we developed a network disassembly procedure in which an excess of 1,2-octanediol de-cross-links the PB vitrimer, after which the resulting fragments are analyzed using ¹H nuclear magnetic resonance spectroscopy. Although both networks exhibit identical gel fractions, small-amplitude oscillatory shear measurements combined with phantom network theory analysis reveal that vitrimers have a higher effective cross-link density than do their permanent counterparts. Vapor swelling experiments further indicated that PB vitrimers have fewer defects than permanently cross-linked networks. Differential scanning calorimetry demonstrated that vitrimers and permanent networks exhibit distinct relationships between the glass transition temperature and cross-link density. Overall, our findings underscore the significant influence of dynamic associative cross-links on the behavior and performance of elastomeric materials.