The effects of soy protein on viscoelastic behaviors of polymer materials

Abstract

The interest in “green” materials, or materials made with biocompatible and biodegradable components is growing, including research into natural polymers, namely proteins and polysaccharides, due to their structures and properties. Soy protein isolate (SPI) has shown promise as a modifier for polymer composites with broad applications. However, little is known about how these protein structures following the denaturation and re-aggregation processes during material processing affect their interactions with one another as well as with the polymer matrix. This study intended to achieve better understandings on this research problem by exploring relaxation behaviors of polyethylene oxide (PEO)/SPI composites via rheological and dielectric analyses. Composites of different SPI concentrations were fabricated in three ways: ball milling (BM), solution processing with deionized water (H$_2$0) and dimethyl sulfoxide (DMSO) as the solvent. The study revealed the distinctive interaction mechanisms between PEO and SPI, subjected to the materials processing, showing varying networking ability of SPI in PEO matrix and dielectric polarizability. Ball-milled SPI had the weakest ability to form SPI networks in PEO, even at 50wt% SPI-BM loading, with the melt still showing typical liquid-like behavior. Composites made in DMSO showed the best ability to form SPI networks, leading to not only solid-like behaviors, the highest melt modulus and viscosity, but also the thermally stable dielectric polarization. Composites made in both solvents with 1wt% SPI concentration produced the largest dielectric constant, with PEO/SPI- H$_2$0 composites achieving higher dielectric constant than PEO/SPI-DMSO composites. The α-relaxation of PEO was substantially affected by both PEO-SPI interactions and the presence of SPI networks. While the PEO-SPI interactions favored α-relaxation, the SPI network structures had the opposite effects, to different levels subjected to materials processing.