Modulating Thermal Stability and Flexibility in Chitosan Films with Neutral Polyol-Boric Acid Complexes

The incorporation of boron into bioplastics offers the potential for diverse applications, with the structure-property relationship between polymer chains and boron species being the key for design. Here, we report the ability to modulate the flexibility and thermal stability of chitosan materials by varying the concentrations of erythritol and the molar equivalents of boric acid. Erythritol and boric acid form neutral complexes that alter the hydrogen-bonding face of erythritol while maintaining free diol units. 1D and 2D NMR experiments indicate preferential formation of the 1,3-isomer (85%) with minor amounts of 1,2- and 2,3-isomers. Structural, thermal, mechanical, and morphological characterization was performed using ATR-FTIR, TGA and DSC, DMA, and SEM, respectively. Molecular-level interactions of the complexes and d-glucosamine, the repeat unit of chitosan, showed increased aggregation and hydrogen-bonding interactions of the free diol units with the NH of d-glucosamine, supporting the trends in flexibility observed in the polymer system.