Time–Temperature Superposition of Polybutadiene Vitrimers

Abstract

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 glasstransition 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 h. 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, the application of these determined shift factors to stress relaxation curves fails 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 that 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.

Tuning the Mechanical and Dynamic Properties of Elastic Vitrimers by Tailoring the Substituents of Boronic Ester

“…There is a significant difference in the activation energies estimated from the 1/ e method and from rheological tTS. Apparently, the 1/ e method provided the relaxation time that significantly deviated from the actual value …”

“…Apparently, the 1/e method provided the relaxation time that significantly deviated from the actual value. 41 Although we have shown the correct method to estimate the relaxation time, there is one more piece of the puzzle that is still missing because the apparent E a seems to be still higher than the bond energy of the esterification process. 42 Our group has also shown that the segmental relaxation dynamic (τ α ) is non-negligible, and one should consider τ α as the prefactor of bond exchange…”

Unveiling the Multiscale Dynamics of Polymer Vitrimers Via Molecular Dynamics Simulations

“…For the macroscopic rheological properties of vitrimers, neither TTSP nor TESP is applicable, which arises from the complex multiscale dynamic behavior of the material. The experimental results conducted by Ricarte et al also indicate that, with increasing frequency, TTSP fails for the storage modulus of the polybutadiene vitrimers they prepared …”

Trialkylsulfonium-Based Reprocessable Polyurethane Thermosets

“…Frequency sweep measurements were performed on PUTS5–5% between 140 and 115 °C (Figure A) using a shear strain of 0.4%, which was well within the linear viscoelastic region (LVE-region) of the networks (Figure S19). The frequency sweeps were then combined in a master curve by shifting them toward a reference temperature (140 °C) . To ensure the validity of the time–temperature superposition principle for the materials, van Gurp–Palmen diagrams were first constructed by plotting the phase angle δ as a function of the absolute value of the complex modulus | G *| = ( G ′ 2 + G ′ ′ 2 ) 0.5 (Figure B).…”