The Crucial Role in Controlling the Dynamic Properties of Polyester-Based Epoxy Vitrimers: The Density of Exchangeable Ester Bonds (υ)

Abstract

Vitrimer materials that obscure the line between ″thermoset″ and ″thermoplastic″ have already attracted a great deal of interest since they exhibit dynamic properties of stress relaxation or reprocessability without losing their permanently cross-linked networks. While much research has been aimed at developing new exchangeable dynamic bonds to broaden the scope, there is a dearth of effort in exploring the key factors that influence the dynamic properties of vitrimer materials. These explorations are not only useful in the exact design and synthesis of vitrimer materials but also quite important in their theory establishment and computer simulation. However, only a few ways including catalyst control have been confirmed to be the key factors in controlling the dynamic properties of classical vitrimer materials like the polyester-based epoxy vitrimer. Here, we proposed that the density of exchangeable ester bonds (υ) in networks also has a crucial role in adjusting the dynamic properties of epoxy vitrimers. Four polyester-based epoxy vitrimers were synthesized from the tricarboxylic acid curing agent and different epoxy monomers, resulting in various υ’s. These epoxy vitrimers were named ″E202-vitrimer″, ″E380-vitrimer″, ″E500-vitrimer″, and ″E640-vitrimer″ according to the different molecular weights of the epoxy monomer. It was revealed that the dynamic properties of these vitrimer materials varied with the υ value. From the E640-vitrimer to E202-vitrimer, the stress relaxation behavior sped up with the rise in υ and the characterized relaxation times (τ’s) obviously decreased. Meanwhile, the activation energy (E a) and the Arrhenius prefactor (τ0), which are two important intrinsic parameters in evaluating the dynamic properties of vitrimers, exhibited heavy dependence on υ, while linear models were applied to describe their relationships. Moreover, the τ at various temperatures could be also predicted from a linear model that relied on the υ in polyester vitrimers. These results indicated that υ has a crucial role in controlling the dynamic properties of polyester-based epoxy vitrimers and the value of υ could be applied to calculate the τ, E a, and τ0. Besides, the influence of υ on the dynamic properties of polyester-based epoxy vitrimers and the resulted models might be workable in other vitrimer materials, which can be utilized to design desired vitrimer materials for various applications.

Boronic Acid Esters and Anhydrates as Dynamic Cross-Links in Vitrimers

“…One must remember, however, that the E a for the exchange reactions of the analogous small molecules is not the same E a that describes the sensitivity of the viscosity or stress relaxation behavior to temperature changes. The latter depends not only on the activation barrier of the reaction itself but also on the structure of the network [ 62 , 63 , 64 ].…”

“…If T g < T v then the vitrimer shows the rubbery state in between these temperatures, while above T v it flows with following Arrhenius dependence with temperature, since it is mainly controlled by the cross-link exchange kinetics. In this case, high E a values for vitrimers indicate the rapid decrease in viscosity upon the temperature rising, while vitrimers with low E a exhibited less pronounced viscosity [ 62 ]. In the opposite case, when T g > T v , the vitrimer is an elastic solid below T g .…”

Microscopic Dynamics and Viscoelasticity of Vitrimers

“…Molecular dynamics simulations in conjunction with mode-coupling theory have been used to explain this strong temperature dependence . Besides temperature, relaxation in vitrimers has been shown to depend on many parameters: the presence and the nature of a catalyst, , the bond exchange kinetics, − the structure − and the dynamics of the network, , and the characteristics of the molecular linkers used in linker-mediated vitrimers. , Sciortino and coworkers introduced models to account for the dependence of the viscosity on the bond exchange time scale and showed that topological defects accelerate relaxation. , Time–temperature superposition has been used to empirically extend the frequency window of observation in so-called thermo-rheologically simple vitrimers, ,, but it fails for vitrimers with many relaxation processes involving several temperature dependences. , Clearly a predictive understanding of the molecular mechanisms that control the macroscopic rheology of vitrimers is highly desirable. In this context, it appears crucial to go a step forward beyond macroscopic rheology by relating the microscopic molecular dynamics of vitrimers to their linear viscoelastic properties.…”

Regulating the Dynamic Behaviors of Transcarbamoylation-Based Vitrimers via Mono-Variation in Density of Exchangeable Hydroxyl

“…In addition, values of τ 0 (Arrhenius prefactor) calculated from eq 3 were explained as the characteristic relaxation time at an infinite temperature. 37 For convenience, values of τ 0 were treated directly using the logarithm and noted as lg(τ 0 ). As shown in Table 2 and Figure 8C, the values of lg(τ 0 ) in the CL-PTMC-OH z series exhibited a linear correlation with [M] ex-OH .…”

“…The fitting parameters, slope and intercept (a 3 and b 3 ), were dependent solely on the construction of vitrimers. 37,40 Once a 3 and b 3 were kept fixed, T v could be predicted from calculated values of [M] ex-OH .…”