Microscopic Evidence for the Topological Transition in Model Vitrimers

Abstract

In addition to the glass transition, vitrimers undergo a topological transition from viscoelastic liquid to viscoelastic solid behavior when the network rearrangements facilitated by dynamic bond exchange reactions freeze. The microscopic observation of this transition is elusive. Model polyisoprene vitrimers based on imine dynamic covalent bonds were synthesized by reaction of α,ω-dialdehyde-functionalized polyisoprenes and a tris(2aminoethyl)amine. In these dynamic networks nanophase separation of polymer and reactive groups leads to the emergence of a relevant length scale characteristic for the network structure. We exploited the scattering sensitivity to structural features at different length scales to determine how dynamical and topological arrests affect correlations at segmental and network levels. Chains expand obeying the same expansion coefficient throughout the entire viscoelastic region, i.e., both in the elastomeric regime and in the liquid regime. The onset of liquid-like behavior is only apparent at the mesoscale, where the scattering reveals the reorganization of the network triggered by bond exchange events. The such determined "microscopic" topological transition temperature is compared with the outcome of "conventional" methods, namely viscosimetry and differential scanning calorimetry. We show that using proper thermal (aging-like) protocols, this transition is also nicely revealed by the latter.

Model Polyisoprene Vitrimers

“…Therefore, the topology restricts the motion of the PI chains among cross-linking points and slows down the imine dynamic exchange process in PI 11K -Vit , resulting in an increase of the activation energy (see Figure S16). These values are in good agreement with the recent values of activation energies for PI 2K -Vit and PI 6K -Vit based on segmental dynamics, which were reported to be 66 ± 20 and 79 ± 7 kJ/mol, respectively . The relative increase with the arm molecular weight is also consistent.…”

“…These values are in good agreement with the recent values of activation energies for PI 2K -Vit and PI 6K -Vit based on segmental dynamics, which were reported to be 66 ± 20 and 79 ± 7 kJ/mol, respectively. 82 The relative increase with the arm molecular weight is also consistent. A thermomechanical analysis was performed in DMA from −80 to 120 °C with a heating rate of 3 °C/min, as depicted in Figure S10.…”

“…Conventionally, T v is defined as the temperature where the viscosity of the melt is 10 12 Pa s. − It has been suggested that T v can be estimated by extrapolating the Arrhenius fitted curve to a relaxation time of 3 × 10 5 s . Recently, by performing careful DSC measurements at different low heating rates, we have obtained the unambiguous experimental evidence for T v , which is in the range of ∼272 ± 14 to 274 ± 5 K . Therefore, during heating, the topology-changing exchange reactions essentially governed the decrease in relaxation times (viscosity).…”

“…Another critical characteristic of a vitrimer is the topology freezing transition temperature (T v ), which refers to the transition temperature from the solid to the liquid state via dynamic exchange reactions in a polymeric network. Conventionally, T v is defined as the temperature where the viscosity of the melt is 10 12 Pa s. 83−85 It has been suggested that T v can be estimated by extrapolating the Arrhenius fitted curve to a relaxation time of 3 × 10 5 s. 24 Recently, by performing careful DSC measurements at different low heating rates, we have obtained the unambiguous experimental evidence for T v , which is in the range of ∼272 ± 14 to 274 ± 5 K. 82 Therefore, during heating, the topology-changing exchange reactions essentially governed the decrease in relaxation times (viscosity). All the PI vitrimers exhibited very short molecular relaxation times, practically due to the existence of low T v (∼0 °C) along with low T g (T g < T v ) of the PI backbone.…”

Vitrimer Transition Phenomena from the Perspective of Thermal Volume Expansion and Shape (In)stability

“…In this respect, the results in the literature are contradictory: either no anomaly at T v is observed at all, 17 or peculiarities in the data sets are so small that they are considered marginal on their own. 22 Retaining the perspective of a glass transition at T v for now, another question, with regard to the relaxation behavior, is what type of glass transition can actually occur at T v if it is already frustrated (with respect to an ordered state) in the high-temperature phase and is accordingly not a real liquid? Here, it should be mentioned that other types of glasses and glass transitions, besides the canonical one, exist.…”