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Materials with tunable modulus, viscosity, and complex viscoelastic spectra are crucial in applications such as self-healing, additive manufacturing, and energy damping. It is still challenging to predictively design polymer networks with hierarchical relaxation processes, as many competing factors affect dynamics. Here, networks with both pendant and telechelic architecture are synthesized with mixed orthogonal dynamic bonds to understand how the network connectivity and bond exchange mechanisms govern the overall relaxation spectrum. A hydrogen-bonding group and a vitrimeric dynamic crosslinker are combined into the same network, and multimodal relaxation is observed in both pendant and telechelic networks. This is in stark contrast to similar networks where two dynamic bonds share the same exchange mechanism. With the incorporation of orthogonal dynamic bonds, the mixed network also demonstrates excellent damping and improved mechanical properties. In addition, two relaxation processes arise when only hydrogen-bond exchange is present, and both modes are retained in the mixed dynamic networks. This work provides molecular insights for the predictive design of hierarchical dynamics in soft materials.

 

The kinetics of crystallization and crystal-crystal transformations in ethylene vitrimers are studied by time-domain NMR. These vitrimers previously exhibited polymorphic transition of crystal structures, which are shown here to be distinguishable by NMR via their dipolar line widths based upon different proton densities and fast internal motions. The conditions under which the polymorphs are formed and interconvert are identified via time-resolved NMR experiments, with a focus on recrystallization after full and partial melting. DSC experiments are used to clarify an unexpected superheating effect, which challenges the determination of actual melting points. We further identify a strong memory effect in isothermal (re)crystallization. Implications of the dynamic nature of the vitrimers in relation to the kinetics of crystallization are discussed. We find that internal perfecting of crystals, enabled by the vitrimeric exchange process, can have a large effect on the DSC-detected melting enthalpy without change in overall crystallinity.

 

Vitrimers, dynamic polymer networks with topology conserving exchange reactions, have emerged as a promising platform for sustainable and reprocessable materials. While prior work has documented how dynamic bonds impact stress relaxation and viscosity, their role on crystallization has not been systematically explored. Precise ethylene vitrimers with 8, 10, or 12 methylene units between boronic ester junctions were investigated to understand the impact of bond exchange on crystallization kinetics and morphology. Compared to linear polyethylene which has been heavily investigated for decades, a long induction period for crystallization is seen in the vitrimers ultimately taking weeks in the densest networks. An increase in melting temperatures ( T m ) of 25–30 K is observed with isothermal crystallization over 30 days. Both C 10 and C 12 networks initially form hexagonal crystals, while the C 10 network transforms to orthorhombic over the 30 day window as observed with wide angle X-ray scattering (WAXS) and optical microscopy (OM). After 150 days of isothermal crystallization, the three linker lengths led to double diamond (C 8 ), orthorhombic (C 10 ), and hexagonal (C 12 ) crystals indicating the importance of precision on final morphology. Control experiments on a precise, permanent network implicate dynamic bonds as the cause of long-time rearrangements of the crystals, which is critical to understand for applications of semi-crystalline vitrimers. The dynamic bonds also allow the networks to dissolve in water and alcohol-based solvents to monomers, followed by repolymerization while preserving the mechanical properties and melting temperatures. 

Abstract 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.