Molecular Design of Multimodal Viscoelastic Spectra Using Vitrimers

Imparting multiple, distinct dynamic processes at precise time scales 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 a 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 cross-linked polymer networks are synthesized with mixed fast and slow dynamic bonds with 3 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 cross-linkers 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 provide molecular design guidelines for having each dynamic bond contribute a distinct relaxation mode. By comparing the polymer architecture and the difference in the number of dynamic bonds per chain, we have elucidated the role of network architecture in imparting multimodal behavior in dynamic networks. A highly tunable and recyclable material has been developed with control of rubbery plateau modulus (through cross-link density), relaxation peak locations and ratio (through cross-linker selection and molar fractions), and tan delta (through the relationships of the rubbery plateau and relaxation peak locations).