Temperature and pressure effects on the dynamics of vitrimers

The molecular dynamics are studied in a series of dynamic covalent polymer networks (vitrimers based on dioxaborolane crosslinks) of poly(n-butyl methacrylate) (PnBMA) as a function of temperature and pressure. Neat homopolymes (H), homopolymer precursors (P) and vitrimers (V) of PnBMA are investigated for different molar masses and vitrimer content by temperature-modulated DSC (TM-DSC), rheology and dielectric spectroscopy (DS). TM-DSC revealed the following order in the liquid-to-glass temperatures: . In rheology, time-Temperature superposition (tTs) works relatively well for the homopolymers and the precursors and only approximately for the vitrimers. The analysis of the shift factors, , for the vitrimer samples, revealed two regions with distinctly different temperature dependences: a WLF at low temperatures and an Arrhenius dependence at high temperatures. The temperature at which this dependence changes is indicated here as T_V. Contrary to rheology, tTs in DS fails due to the merging of the dielectrically active alpha- and beta-process. In agreement with rheology, crosslinks increase the relaxation time for the segmental process, leading to an increased T_g. Pressure strongly affects the segmental dynamics in all samples with the effect being stronger in the vitrimers. The pressure sensitivity is quantified through the apparent activation volume being higher for the latter systems at all temperatures. Overall, the findings underscore the significant impact of T and P on the dynamics of vitrimers.