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.
Polymer membranes are commonly pursued for passive separations, which require less energy than distillation. Typically, glassy materials are chosen for gas separations due to extreme sensitivity to size differences in penetrants, whereas rubbers are used for liquid separations due to solubility differences. Vitrimers, dynamic polymer networks with associative bond exchange, are an emerging class of polymers that have gained much attention as self-healing and recyclable materials. Here, in a new direction for vitrimers, we demonstrate the utility of the dynamic bond for eliciting large differences in molecular transport through dense polymer networks. Specifically, permanent and dynamic polymer networks with boronic ester crosslinks are synthesized across a broad range of dynamic bond densities, and the diffusion of a large aromatic dye is measured using fluorescence recovery after photobleaching. When dynamic bond exchange is accelerated by the presence of neighboring nitrogen groups, penetrant transport is enhanced relative to permanent networks by a factor that increases with increasing dynamic bond density and can exceed 1 order of magnitude. Dynamic bonds without the neighboring group effect produce no enhancement of diffusion. These results are interpreted in terms of the ratio of the bond exchange time (inferred from small-molecule experiments) to the hopping time of the penetrant (extracted from the diffusion coefficients). Our results point to a general route for imparting selectivity into polymer membranes through dense crosslinking and dynamic covalent chemistry.
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.
There is growing interest in polymers with high ionic conductivity for applications including batteries, fuel cells, and separation membranes. However, measuring ion diffusion in polymers can be challenging, requiring complex procedures and instrumentation. Here, a simple strategy to study ion diffusion in polymers is presented that utilizes ion-chromic spiropyan as an indicator to measure the diffusion of LiTFSI, KTFSI, and NaTFSI within poly(ethylene oxide)-based polymer networks. These systems are selected, as these are common ions and polymers used in energy storage applications, however, the approach described is not specific to materials for energy storage. Specifically, to enabling the study of ion diffusion, these salts cause the spiropyran to undergo an isomerization reaction, which results in a significant color change. This colorimetric response enables the determination of the diffusion coefficients of these ions within films of these polymers simply by optically tracking the spatial-temporal evolution of the isomerization product within the film and fitting the data to the relevant diffusion equations. The simplicity of the method makes it amenable to the study of ion diffusion in polymers under a range of conditions, including various temperatures and under macroscopic deformation.
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).
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