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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 1 H 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.
Ezzeddine et al. (Fri,) studied this question.