Temperature and Pressure Effects on the Dynamics of Elastomeric Dioxaborolane Vitrimers

Abstract We report the synthesis, and the thermal, viscoelastic and dynamic properties of a series of dioxaborolane-based poly(n-butyl methacrylate) (PBMA) vitrimers, as a function of molar mass and cross-linking density. By combining calorimetry, dielectric spectroscopy and rheology, we explored the different characteristic temperatures that influence the dynamics of vitrimers. Three main processes could be identified by dielectric spectroscopy, the local β-process, the segmental α-process and the merged αβ-process at higher temperatures. Pressure was employed to examine their sensitivity through an apparent activation volume. We found that the local β-process was only weakly affected by pressure, and that the rubber-to-glass temperature continuously increased with vitrimer cross-linking density. Our results revealed that the dynamic bonds in vitrimers contribute to an effective unit of relaxation (e.g. an effective segment) that is larger than that in the corresponding homopolymer. In addition, the ratio of activation energies at constant volume and at constant pressure revealed that the segmental dynamics in the vitrimers is controlled by intra-molecular barriers. The viscoelastic properties are influenced by the topology freezing temperature, TV, separating segmental-controlled dynamics at lower temperatures from the bond exchange mechanism at higher temperatures initiating the flow. The derivative approach applied to the viscoelastic shift factors was found to be a practical method to locate TV. In the vitrimers, the bond exchange mechanism had a high apparent activation energy (E ∼ 90 kJ/mol) similar to that of the segmental dynamics. We present the first high-pressure investigation of the relaxation dynamics in vitrimers, elucidating the origin of the segmental process and re-defining the fundamental effective unit of relaxation.