Polymers can dampen vibration or sound waves if the frequency of peaks in their relaxation spectrum match that of the incoming wave. Controlling the placement, breadth, and height of these peaks is a key goal when designing damping materials. Vitrimers, or dynamic polymer networks with associative covalent bonds, provide an ideal platform for chemical and mechanical tunability. In this work, we used four different boronic acid crosslinkers each reacted with silicone diols to form PDMS vitrimers of dynamic boronic esters. Networks with boric acid, phenyl diboronic acid, or biphenyl diboronic acid experienced relaxation times within approximately one order of magnitude at a given temperature, with aromatic groups leading to faster times due to a destabilization of the boronic ester. In contrast, an aromatic diboronic acid with nitrogen neighboring groups led to a four order of magnitude drop in the network relaxation time. For all samples, the modulus remains nearly constant regardless of exchange kinetics. All vitrimers show modulus increase with temperature, in line with expectations for networks of conserved topology. When a single crosslinker is used, only one peak is observed in the relaxation spectra. When two crosslinkers are mixed in the same sample, systems without the nitrogen-bearing boronic acid still show a single relaxation mode dominated by the faster-exchanging crosslinker. When the fastest crosslinker with nitrogen neighboring groups is mixed with another crosslinker, an intermediate relaxation time is observed. Thus, when relaxation times are within an order of magnitude, the faster of two crosslinkers controls the relaxation time, and when modes are further apart, a blending of dynamics occurs. These rules are critical to understand for damping applications.

Using dynamic covalent chemistry to control viscoelasticity in vitrimers for vibration damping applications

ACS Spring 2022

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