Ion-induced effective control of morphologies of soy protein biocomposites

Soy protein isolate (SPI) has received immense research attention recently for its potential use in biocomposites. As a protein material, SPI is highly complex; it possesses four levels of structure arising from many types of intermolecular interactions. However, practical knowledge on how to alter these interactions to create protein biocomposites with adjustable morphologies is sparse. This study presents one of very few studies on tuning the morphology of protein in biocomposites by modulating these interactions. In this study, SPI/poly(ethylene oxide) (PEO) biocomposites with various morphologies were successfully fabricated by introducing salts (LiClO4, Li-2 CO3, NaClO4, and NaCl) with different solubilities in PEO and in water into the biocomposite. The results showed that all four salts increased the size of the SPI phase; the magnitude of this effect increased when salts with higher solubility in water were used, and when the salt concentration was increased, in accordance with a greater number of ions available to interact with SPI. Similarly, the salts (LiClO4 and NaClO4) soluble in PEO dramatically reduced the crystallinity of the PEO matrix, resulting in highly stretchable biocomposites; while the salts with poor solubility in PEO (Li2CO3 and NaCl) preferentially distributed in SPI-rich phase and did not show obvious effects on PEO crystalline structures, leading to rigid biocomposites with higher fracture toughness. Importantly, these findings enable the fabrication of protein-based biocomposites with a variety of morphologies for both the protein phase and the polymer phase with great potential to achieve various properties and applications.