Designing materials with tunable modulus and viscosity is key for applications such as three-dimensional (3D) printing, sound damping, and wearable devices. Vitrimers provide an ideal platform for viscoelastic design because their dynamic, conserved bond exchange allows for control of both cross-link density and exchange kinetics. Here, multiple boronic acid cross-linkers with different functionalities and kinetics were reacted with silicone diols to form poly­(dimethylsiloxane) (PDMS) vitrimers. Networks cross-linked with boric acid or two phenyl-substituted boric acids exhibited relaxation times and viscosities within one order of magnitude of each other. Conversely, a cross-linker with nitrogen neighboring groups led to a four order of magnitude acceleration in network relaxation time while still exhibiting a similar modulus to the slower systems. All of these samples demonstrate an increase in moduli with temperature due to entropic elasticity. To understand the effect of more than one dynamic bond on the viscoelastic response, multiple cross-linkers were then combined into a single network and the relaxation spectrum was characterized. The mixed vitrimers exhibit a single relaxation peak, which more closely follows the dynamics of their faster component. These rheological observations are essential for designing complex viscoelastic materials.

Imparting multiple, distinct dynamic processes at precise timescales in polymers is a grand challenge in soft materials design with implications for applications including electrolytes, adhesives, tissue engineering, and additive manufacturing. Many competing factors including the polymer architecture, molecular weight, backbone chemistry, and presence of solvent affect the local and global dynamics, and in many cases are interrelated. One approach to imparting distinct dynamic processes is through the incorporation of dynamic bonds with widely varying kinetics of bond exchange. Here, statistically crosslinked polymer networks are synthesized with mixed fast and slow dynamic bonds with four orders of magnitude different exchange kinetics. Oscillatory shear rheology shows that the single component networks (either fast or slow) exhibit a single relaxation peak, while mixing fast and slow crosslinkers in one network produces two peaks in the relaxation spectrum. This is in stark contrast to telechelic networks with the same mixture of dynamic bonds where only one mixed mode is observed, and here we develop the molecular design rules necessary to have each dynamic bond contribute a distinct relaxation mode. By controlling the polymer architecture and difference in the number of dynamic bonds per chain, we have elucidated the role of network architecture on imparting multimodal behavior in dynamic networks. A highly tunable and recyclable material has been developed with control of rubbery plateau modulus (through crosslink density), relaxation peak locations and ratio (through crosslinker selection and molar fractions), and tan δ (through the relationships of the rubbery plateau and relaxation peak locations).

Metrics & MoreArticle Recommendations I n the original manuscript "Relaxation of Vitrimers with Kinetically Distinct Mixed Dynamic Bonds", acknowledgments of key funding sources and instrument usage were regrettably not included. This section should read as given here.