Reversible Reactions, Mesh Size, and Segmental Dynamics Control Penetrant Diffusion in Ethylene Vitrimers

Abstract

The diffusion of two aromatic dyes with nearly identical sizes was measured in ethylene vitrimers with precise linker lengths and borate ester cross-links using fluorescence recovery after photobleaching (FRAP). One dye possessed a reactive hydroxyl group, while the second was inert. The reaction of the hydroxyl group with the network is slow relative to the hopping times of the dye, resulting in a large slowdown by a factor of 50 for a reactive probe molecule. A kinetic model was fit to the fluorescence intensity data to determine rate constants for the reversible reaction of the dye from the network, which confirms the role of slow reaction kinetics. A second network cross-linker was also investigated with a substituted boronic ester showing ∼10,000 times faster exchange kinetics. In this system, the two dyes show the same diffusion coefficient, as the reaction is no longer the rate-limiting step. The role of dense meshes on small and large dyes is also discussed in the context of the existing theories. These results highlight the potential of dynamic networks to control penetrant transport through synergistic effects of the mesh size, dynamic bond kinetics, and penetrant–network interactions.

Self-Consistent Theory for Structural Relaxation, Dynamic Bond Exchange Times, and the Glass Transition in Polymeric Vitrimers

“…Significant progress in this direction was recently made in the ECNLE theory framework for homopolymers melts . Vitrimers are also potentially new separation membrane materials with novel functionality, as suggested by the recent experimental study of one of us. , This motivates our ongoing efforts to extend the presented approach to create a predictive theory for penetrant transport in dynamic bonding materials. Of critical importance for understanding penetrant mobility is how bond exchange affects the matrix segmental relaxation time, and hence the penetrant hopping rate and diffusivity.…”

“…Vitrimers also have other varied proposed applications such as solid polymer electrolytes, superhydrophobic coatings, shape-memory materials, , and adhesives . Although a large body of experimental work has been directed toward investigating the self-healing, recyclability, and stress relaxation behavior of vitrimers, ,, quantitative studies of the effects of bond exchange in dense melts and on phenomena such as crystallization, phase separation, self-assembly, , and ion and molecular transport , are relatively rare. From a broad perspective, a more comprehensive understanding of dynamics across length and time scales is required for materials design and optimization, in addition to creating a mechanistic understanding of dynamic cross-linking on monomer scale relaxation in vitrimers.…”

Molecular Simulation of Covalent Adaptable Networks and Vitrimers: A Review

“…The slow dynamic bonds do not seem to influence the diffusion of the dye through the material because the rate at which bond exchange occurs is not high enough to differentiate the material from a permanent network. However, in the case of fast dynamic bonds, the bond exchange reactions (BERs) occur on timescales where penetrant hopping is possible, enhancing the diffusion of the penetrant as a consequence [39,40].…”