The practical applications of soft materials and gels are wide-ranging and diverse. Because of this, there is an interest in developing materials that can be easy manipulated and optimized to fit a specific application. Here, we report an easily tunable polyvinyl alcohol (PVA) organogel formed with mixtures of boric acid (BA) and various diboronic acid crosslinkers. With BA alone, PVA forms weak aggregations and yields viscous dispersions in DMSO. However, we observe that exchanging small amounts of BA for 1,4-benzenediboronic acid (1,4-BDBA) dramatically improves the material properties and gelation. Specifically, BA/1,4-BDBA organogels form at lower overall crosslinker concentration, have improved thermal and mechanical properties, and are stiffer and more stable than those with either crosslinker alone. We propose that the enhancement in these material properties is the result of cooperative PVA crosslinking between 1,4-BDBA and BA. That is, we theorize that 1,4-BDBA is not only introducing new crosslinks between PVA and itself but is also improving the ability of BA to function as a crosslinker. Interestingly, the structure of the diboronic acid influences this cooperativity and thus the morphology and material properties of the gel. We find that 1,3-benzenediboronic acid (1,3-BDBA) has little to no impact on the stiffness and stability of the PVA-BA material. As 1,4- and 1,3-BDBA vary only in substitution pattern and linearity, these findings suggest that cooperative crosslinking within the PVA-BA gel can be activated or deactivated with structural changes. Thus, BA/BDBA organogels can be readily tuned by simply varying the ratio of crosslinkers or by changing the identity of the diboronic acid.

Manipulating boric acid crosslinking in polyvinyl alcohol organogels

ACS Spring 2022

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