Tunability and Mixing Rules in PDMS Vitrimers

Vitrimers, or dynamic polymer networks with associative covalent bonds, provide an ideal platform for sustainability due to their high mechanical strength and facile reprocessability. To utilize vitrimers as reusable materials in many applications, control of their properties will be essential. Tuning viscoelastic properties can be accomplished via chemical versatility. In this work, four different boronic acid crosslinkers were 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, which would be highly useful in applications such as additive manufacturing. All vitrimers show modulus increase with temperature, in line with expectations for networks of conserved topology. Mixing of multiple crosslinker moieties in one network was also investigated. When a single crosslinker is used, only one peak is observed in the rheological 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 viscoelastic patterns and mixing rules are essential for controlling vitrimers to be used as sustainable materials in an array of applications.