Polymer gels are used for a wide variety of applications ranging from diaper linings to drug delivery systems. To induce gelation, polymer chains must be crosslinked in order to form a fibrous gel network. Since crosslinking is critical to the gelation process, developing methods that manipulate polymer crosslinking would allow us to influence and optimize the properties of the material. Recently, we reported the enhancement of boric acid (BA) crosslinks within polyvinyl alcohol (PVA) organogels in the presence of 1,4-benzendiboronic acid (1,4-BDBA). With mixtures of BA and 1,4-BDBA, we observed improvements in the rheological properties, gelation times, and critical gelation concentrations (CGC). We attribute these findings to cooperative crosslinking between 1,4-BDBA and BA. In contrast, PVA-BA gels formed with the addition of 1,3-benzenediboronic acid (1,3-BDBA) do not follow the same trend. Namely, at concentrations lower than its CGC, the addition of 1,3-BDBA has little impact on the storage modulus (G′) of the PVA-BA gel and no new crosslinks formed. At higher ratios of 1,3-BDBA to BA, we indeed observed an increase in G′. However, the G′ values of these BA/1,3-BDBA gels are lower than the expected combination of the individual crosslinkers alone. This suggests that while 1,4-BDBA stabilizes BA crosslinks, 1,3-BDBA interferes and competes with BA for PVA binding sites. Structurally, 1,4- and 1,3-BDBA vary only in substitution pattern and linearity. Thus, these findings indicate that cooperative crosslinking within the PVA-BA gel can be activated or deactivated with structural changes. Interestingly, while both 1,4- and 1,3-BDBA have significant impacts on BA’s ability to crosslink PVA, these crosslinkers do not cooperate or compete with one another. Instead, when used in combination, the G′ of the resulting PVA/1,4-BDBA/1,3-BDBA gel is equal to the sum of gels formed with either 1,3-BDBA or 1,4-BDBA alone. We attribute this non-competitive relationship to a similarity in crosslinking efficiency and mechanism. In summary, these findings suggest that the structure of the crosslinker dictates the cooperativity in PVA-BA gels. Small structural changes in diboronic acids can dramatically impact the properties of the gels and the activation of BA crosslinks. Thus, this mechanism is useful for manipulating and tuning the properties of PVA-BA material.

Don’t get bent out of shape: Cooperative crosslinking within PVA-BA gels

ACS Spring Meeting 2021

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