Cooperative crosslinking in polyvinyl alcohol organogels.

Manipulating and optimizing the properties of gels is important for practical applications but can be both synthetically difficult and expensive. In this work, we report an easily tunable polyvinyl alcohol (PVA) organogel formed with boric acid (BA) and 1,4-benzenediboronic acid (1,4-BDBA) as crosslinkers. While PVA and BA alone form weak aggregations in DMSO, adding small amounts of 1,4-BDBA dramatically improves the material properties and gelation. PVA organogels made with mixtures of BA and 1,4-BDBA have improved thermal properties, lower CGCs, and higher G' than those with either crosslinker alone. We propose that these enhanced material properties are the result of cooperative PVA crosslinking between 1,4-BDBA and BA. As the properties of this system can be improved by simply varying the ratio of crosslinkers, these organogels are highly adjustable and are a practical alternative to PVA hydrogels.

Tuning glycerol plasticization of chitosan with boric acid.

Chitosan-based bioplastics are attractive biodegradable alternatives to petroleum-derived plastics. However, optimizing the properties of chitosan materials to fit a particular application or obtain a desired property is not a trivial feat. Here, we report the tunability of glycerol-plasticized chitosan films with the addition of boric acid. In combination, glycerol and boric acid form neutral complexes that alter the hydrogen-bonding face of the plasticizer and ultimately limit glycerol's ability to plasticize chitosan. Thus, we found that chitosan films containing glycerol-boric acid complexes were less flexible, had increased thermal transition temperatures, and showed more uniform morphologies. Structural, thermal, mechanical and morphological characterization was performed using ATR-FTIR, TGA and DSC, DMA, and SEM respectively. Molecular-level interactions of the neutral boron complexes and D-glucosamine, the repeat unit of chitosan, were also investigated used NMR and ATR-FTIR. The results of this work demonstrate the necessity of specific hydrogen-bonding interactions between the plasticizer and the polymer for effective plasticization, an important insight into the plasticization mechanism of chitosan films. Furthermore, the formation of complexes with glycerol is a novel and convenient method for tuning the physical properties of chitosan films.