Model polydimethylsiloxane telechelic vitrimers with dynamic boronic ester bonds were synthesized to investigate the viscoelastic properties of dynamic networks with extremely low T g via multiple rheological approaches. Frequency sweeps and stress relaxation tests, conducted at more than 120 °C above T g, show the anticipated Arrhenius behavior of relaxation time with inverse temperature and give the same activation energy for a fixed molecular weight, obtained using a variety of analysis methods. Time–temperature superposition demonstrates that the flow regime is thermorheologically simple, while the modulus of the plateau regime increases with increasing temperature, consistent with a conserved network topology and associative bond exchange. As relaxation times decrease, the rubbery plateau modulus increases, indicating a decoupling of terminal dynamics from mechanics. Below 40 °C, a second Arrhenius regime with lower activation energy emerges, which is attributed to a transition from relaxation dominated by reaction exchange kinetics to relaxation dictated by local polymer dynamics. Our work points to the importance of assessing a broad temperature window and using multiple approaches in probing vitrimers and dynamic networks.

Durable hydrophobic materials have attracted considerable interest in the last century. Currently, the most popular strategy to achieve hydrophobic coating durability is through the combination of a perfluoro-compound with a mechanically robust matrix to form a composite for coating protection. The matrix structure is typically large (thicker than 10 μm), difficult to scale to arbitrary materials, and incompatible with applications requiring nanoscale thickness such as heat transfer, water harvesting, and desalination. Here, we demonstrate durable hydrophobicity and superhydrophobicity with nanoscale-thick, perfluorinated compound-free polydimethylsiloxane vitrimers that are self-healing due to the exchange of network strands. The polydimethylsiloxane vitrimer thin film maintains excellent hydrophobicity and optical transparency after scratching, cutting, and indenting. We show that the polydimethylsiloxane vitrimer thin film can be deposited through scalable dip-coating on a variety of substrates. In contrast to previous work achieving thick durable hydrophobic coatings by passively stacking protective structures, this work presents a pathway to achieving ultra-thin (thinner than 100 nm) durable hydrophobic films.

Vitrimers have been investigated in the past decade for their promise as recyclable, reprocessable, and self-healing materials. In this Viewpoint, we focus on some of the key open questions that remain regarding how the molecular-scale chemistry impacts macroscopic physical chemistry. The ability to design temperature-dependent complex viscoelastic spectra with independent control of viscosity and modulus based on knowledge of the dynamic bond and polymer chemistry is first discussed. Next, the role of dynamic covalent chemistry on self-assembly is highlighted in the context of crystallization and nanophase separation. Finally, the ability of dynamic bond exchange to manipulate molecular transport and viscoelasticity is discussed in the context of various applications. Future directions leveraging dynamic covalent chemistry to provide insights regarding fundamental polymer physics as well as imparting functionality into polymers are discussed in all three of these highlighted areas.

Materials that absorb shock wave energy from blasts and high-speed impacts are critical for protection of structures, vehicles, and people. Incorporating dynamic bonds into polymers has enabled precise control over the time-dependent response and energydissipating modes, but this work has focused on much slower time scales and lower forces than those associated with shock waves. Here, we design polymers networks with dynamic covalent bonds, called vitrimers, where reversible exchange reactions provide a potential mechanism for shock wave energy dissipation. Increasing the density of dynamic bonds leads to a systematic increase in energy dissipation, measured by the drop in peak pressure of a laser-induced shock wave. An analogous permanent polymer network shows no dependence of dissipation on cross-link density. The vitrimers can absorb shock multiple times while maintaining performance, attributed to bond exchange and the intrinsic self-healing ability of the polymer. Our results are the first to demonstrate that vitrimers are an effective route to the design of energy-dissipating materials, particularly at the high frequencies and pressures associated with shock waves.