Dana Ezzeddine

Vitrimers are polymer networks that undergo structural rearrangement through dynamic associative bond exchange without compromising 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 photo-initiated 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 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.

Vitrimers are polymer networks connected by associative cross-links – covalent linkages that maintain network connectivity but exchange through reversible chemical reactions. Associative cross-links significantly change the dynamics of the molten polymer. This study focuses on the linear viscoelasticity of polystyrene vitrimers (PS-v) bearing imine cross-links. PS-v samples were prepared by condensation between precursor copolymers with pendant aldehydes and 1,6-hexanediamine cross-linker. The number average molecular weights of the precursors were 6 and 8 kDa, and the amine-to-aldehyde molar ratio (r) ranged between 0.8 and 2.4. The glass transition temperature exhibited a non-monotonic relationship with r. The linear viscoelasticity of PS-v was evaluated using a combination of small amplitude oscillatory shear (SAOS), stress relaxation, and creep and recovery. Time-temperature superposition analyses indicated two distinct relaxation regimes: (I) fast high frequency dynamics with a Williams-Landel-Ferry temperature dependence and (II) slow low frequency dynamics with Arrhenius behavior. The fast regime represented the segmental relaxations of the vitrimer backbone. The slow regime was described as a Slow Arrhenius Process (SAP), in which the long time dynamics have a temperature-independent rheological activation energy. For all PS-v samples in this study, the observed SAP had a much weaker temperature dependence than expected from sticky Rouse model predictions. Increasing r altered the plateau modulus and SAOS cross-over frequency but did not affect the temperature dependences of the segmental motions or SAP. To describe the origin of the SAP, three hypotheses are proposed: cross-linker diffusion, polymer matrix effects, and local elasticity fluctuations.

A vitrimer has covalent cross-links that preserve network connectivity but permit topology fluctuations through dynamic exchange reactions. In this work, we investigate the linear rheology of polybutadiene (PB) vitrimers bearing cross-links that exchange via dioxaborolane metathesis. PB vitrimers are cross-linked in solution using photoinitiated thiol-ene click chemistry. As the targeted cross-link density is increased, both the insoluble fraction and glass transition temperature increase. Linear viscoelasticity is studied using a combination of small amplitude oscillatory shear (SAOS), creep, and stress relaxation measurements. In SAOS, the elastic modulus is approximately constant while the viscous modulus increases as angular frequency decreases. In creep, the compliance displays power law behavior that persists for at least 8 hrs. In stress relaxation, the modulus transitions from a rubbery plateau into a power law regime. Both SAOS and creep data are visually superposed into master curves using horizontal shift factors that exhibit Arrhenius behavior. However, application of these determined shift factors to stress relaxation curves fail to align the data into a single master curve. SAOS shift factors (a_SAOS) collapse the short time relaxation data and their activation energy (E_a^SAOS) matches the effective Williams-Landel-Ferry activation energy for PB homopolymer, indicating that the short time dynamics correspond to network strand segment relaxations. Creep shift factors (a_creep) collapse the long time relaxation data and their activation energy (E_a^creep) is significantly larger than the energies predicted by established theoretical models for transient networks. We speculate the discrepancy between experiment and theory is due to the temperature dependence of the cross-linker mobility within the vitrimer matrix, which is not fully captured by established theoretical models.