Submitted to: Journal of Polymer Science Part B: Polymer Physics
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/4/2005
Publication Date: 12/1/2005
Citation: Jong, L. 2005. Viscoelastic properties of ionic polymer composites reinforced by soy protein isolate. Journal of Polymer Science Part B: Polymer Physics. 43(24):3503-3518. Interpretive Summary: In the rubber composite applications, most rubber products are filled with reinforcement materials. From the viewpoint of renewal materials and environmental reasons, it is desirable to use natural materials that have the same or better functions. In the continuous effort to find new industrial applications for soy protein, the structure and function of soy protein in a rubber composite are characterized to give a better understanding of its utility. Reinforcement mechanism of soy protein in rubber using carboxylated poly(styrene-butadiene) as composite matrix is studied by a comparison between experiment and theory. The comparisons were also made with rubber composites and demonstrate soy protein composites in dry state have higher elastic modulus at the same volume fraction of filler, implying a higher elastic modulus in dry soy protein aggregates. The dry soy protein is therefore an effective reinforcement additive when used in appropriately formulated products and selective applications. This basic study will be of general interest to technologists developing new rubber products and will be beneficial to soybean farmers by creating new markets for soy protein products.
Technical Abstract: Both linear and nonlinear viscoelastic properties of ionic polymer composites reinforced by soy protein isolate (SPI) were studied. Viscoelastic properties were related to the aggregate structure of fillers. The aggregate structure of SPI consists of submicron size of globule protein particles that form an open aggregate structure. SPI and carbon black (CB) aggregates characterized by scanning electron microscope and particle size analyzer indicate that CB aggregates have a smaller primary particle and aggregate size than SPI aggregates, but the SPI composites have a slightly greater elastic modulus in the linear viscoelastic region than the CB composites. The composite containing 3 to 40 wt % of SPI has a transition in the shear elastic modulus between 6 and 8 vol % filler, indicating a percolation threshold. CB composites also showed a modulus transition at less than 6 vol %. The change of fractional free volume with filler concentration as estimated from WLF fit of frequency shift factor also supports the existence of a percolation threshold. Nonlinear viscoelastic properties of filler, matrix, and composites suggested that the filler-immobilized rubber network generated a G’ maximum in the modulus-strain curves and the SPI formed a stronger filler network than the CB in these composites.