Creep Mechanics of Epoxy Vitrimer Materials

Abstract

Epoxy materials are an indispensable class of polymers due to their excellent mechanical properties and high thermal resistance. However, traditional epoxy materials are typically non-recyclable and face issues with fatigue and creep. To combat the issue of recyclability, research interest is increasingly centered on epoxy vitrimer materials, which exhibit a dynamic covalent bond exchange reaction at high temperatures allowing for self-healing, recycling, and shape reprogramming. Herein, we discuss the creep mechanics of epoxy vitrimers; we postulate that gaining a fundamental understanding of what causes creep in vitrimers will garner advances in vitrimer creep resistance for future applications. Isothermal creep results demonstrate the presence of three distinct creep regimes: primary creep associated with polymer chain mobility, secondary creep associated with network rearrangement due to the dynamic covalent bond exchange reaction, and tertiary creep associated with errors in this reaction. We note at low temperatures (T ≪ T v ) and catalyst concentrations, vitrimers simply behave as a traditional epoxy material. In contrast, at high temperatures and catalyst concentrations, vitrimers exhibit significantly more creep with strong correlations to catalyst concentration, heating rate, and temperature, which are quantified in this work.

Critical Effects of Branch Numbers at the Cross-Link Point on the Relaxation Behaviors of Transesterification Vitrimers

“…This may be reasonable from the much lower E P of CL-3 and relatively close E P of CL-4 and CL-6 because the creep behavior for vitrimers above T soft is correlated to the relative balance between the applied stress and potential modulus of materials . The difference of softening behavior was also explained in terms of the bond exchange efficiency: a recent study from Nepal et al demonstrated that changing rate of strain (i.e., Δ L exp‑linear in the present case) is related to the bond exchange efficiency (i.e., bond exchange rate), , and the following result of stress–relaxation will demonstrate the highest bond exchange rate for CL-3.…”

“…The result is in fact consistent with the simulation study by Ciarella et al, where the loosely cross-linked vitrimers with a larger fraction of network defects exhibited a faster relaxation than the more tightly cross-linked ones with no network defects. As another reason, some recent studies reported that the relaxation time of vitrimers is contributed from both the time for bond exchange reaction and the time for molecular diffusion. , In this study, we assume the molecular diffusion was different depending on f and faster for the sample with lower f , which should be another reason for the observed difference of relaxation time.…”

Vitrimer Composites: Understanding the Role of Filler in Vitrimer Applicability

“…Due to this shortcoming, it is often beneficial to perform isothermal creep experiments to investigate a material's part lifetime, study creep suppression, and confirm a vitrimer T v , as previously reported. 43 Isothermal creep experiments performed with temperatures ranging from 175 to 225 °C, as seen in Figure 2d−f, demonstrate clear creep suppression of the composite materials compared with the neat vitrimer, in accordance with the literature. 11,12,47 In fact, increasing the filler concentration allows for greater creep suppression, where the reinforcement effect of graphene and clay filler for creep suppression was found to be comparable.…”

“…42 These results will then be corroborated via isothermal creep, determining the amount of creep suppression present as a function of filler concentration. 43 In the case of vitrimer composites, we primarily explore the addition of graphene and clay (cf. Figure S1) in a range of concentrations (0.1−1.0 wt %) to an epoxy vitrimer matrix.…”

Rheology of vitrimers

“…Also, polymer materials are often utilized in conditions of large deformations, which is especially important when there is an increasing plastic creep. Here one may wish to consider the ‘effective viscosity’ of a vitrimer in the plastic-flow regime, a notion that has attracted increasing attention in the analysis of recent experiments 21 , 24 , 25 , although the underlying mechanism of dissipation (and thus viscosity) is quite different from the classical fluids. It is important to develop a full rheological understanding of vitrimer response that spans between small-deformation elastic (or viscoelastic limit) and large-deformation (plastic flow) regime.…”