Relaxation of Vitrimers with Kinetically Distinct Mixed Dynamic Bonds

Abstract

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.

Characterising different molecular landscapes in dynamic covalent networks

“…1). 53–55 Apart from the dynamic cross-link itself, also its topological location and surrounding backbone chemistry will impact its dynamic and mechanical behaviour. When applied to stress-relaxation experiments, the generalised Maxwell model can be used to consider each of these contributions separately as distinct Maxwell elements (adhering to a specific single exponential decay).…”

Transcarbamoylation in Polyurethanes: Underestimated Exchange Reactions?

“…In a recent publication, Evans and co-workers investigated the relaxation profile of two kinetically distinct boronic ester dynamic bonds within the same network. 113 The mixed vitrimers exhibited also a single relaxation peak, which more closely follows the dynamics of their faster component. These two investigations implied that the kinetically predominant exchange reaction will govern both stress−relaxation time and profile, thus masking the occurrence of more complex mechanisms.…”

“…This result suggested a narrow distribution of relaxation times and was attributed to the occurrence/involvement of one main mechanism governing the exchange dynamic, i.e., the transcarbamoylation. In a recent publication, Evans and co-workers investigated the relaxation profile of two kinetically distinct boronic ester dynamic bonds within the same network . The mixed vitrimers exhibited also a single relaxation peak, which more closely follows the dynamics of their faster component.…”

Tuning the Dynamics of Enamine-One-Based Vitrimers through Substituent Modulation of Secondary Amine Substrates

“…We also envision that a strategy of mixing secondary amines with different substituents or combining primary and secondary amine substrates can be applied to prepare materials with high creep resistance in an ambient environment and simultaneously fast dynamics under reprocessing conditions, as has been demonstrated by a recent study. 41,66 ■ ASSOCIATED CONTENT * sı Supporting Information…”

“…As such, the molecular tuning of dynamics with a wide window and in a catalyst-free manner would be desirable. Several methods have been reported to realize this goal, for instance, tuning the abundance of dynamic motifs, ,, adjusting the stereoelectronic effects, ,, incorporating multiple associative dynamic chemistry, , and so on. Recently, Kalow and co-workers reported a series of Meldrum’s acid crosslinkers that allows for the tuning of the stress–relaxation rate in the range of 4 orders of magnitude .…”