Fractal Dimensions of Soy Protein Nanoparticle Aggregates determined by Dynamic Mechanical Method
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1086-U08-18
Fractal Dimensions of Soy Protein Nanoparticle Aggregates determined by Dynamic Mechanical Method Lei Jong National Center for Agricultural Utilization Research, United States Department of Agriculture, 1815 North University Street, Peoria, IL, 61604 ABSTRACT The fractal dimension of soy protein aggregates can be estimated by dynamic mechanical methods when the particle aggregates are imbedded in a polymer matrix. Composites were formed by mixing hydrolyzed soy protein isolate (HSPI) nanoparticle aggregates with styrenebutadiene (SB) latex, followed by freeze-drying and compression molding methods. The dynamic shear moduli of the elastomeric composites containing different particle fractions were measured. A logarithmic plot of shear modulus vs. particle fraction in rubber plateau region at 140 oC can be fitted with a linear line. From the slope of the fitted line, the fractal dimension of the particle aggregates was estimated using the Cluster-Cluster Aggregation (CCA) model developed by Kluppel and Heinrich. The CCA model can also be used to extract fractal dimension from dynamic strain sweep experiments. The reversible strain sweep data was then fitted with a CCA model expression developed by Huber and Vilgis to yield the fractal dimension of the particle aggregates. The results show that the fractal dimensions extracted from both linear and non-linear viscoelastic data have a good agreement with each other. The model fitting indicates HSPI has a greater fractal dimension and therefore a more compact structure than the un-hydrolyzed soy protein aggregates. INTRODUCTION Replacement of petroleum-based fillers (e.g., carbon black) in polymer composites with renewable agricultural based fillers is desirable for sustainability. Fractal dimension is a way to describe structure of particle aggregates and to understand their reinforcement effects in polymer composites. Dynamic mechanical method is a convenient alternative to light scattering method for measuring the fractal dimension of particle aggregates. Soy protein isolate (SPI) is obtained from soybean by removing soybean oil and soy carbohydrates [1]. Structurally, SPI is a globular protein and its aggregates in water consist of sphere-like protein particles, which further consist of nano-sized aggregates of subunits (polyamino acids). In scanning electron micrographs, soy protein aggregates appeared to consist of particles with a diameter of ~0.3 μm [2]. The average aggregate size of soy protein in alkaline water is ~ 4 μm after the protein is cooked at 55 oC for one hour. The ~0.3 μm particles further consist of nano-sized aggregates of subunits [3-5]. For example, 11S soy protein globulins (glycinin) have a size of ~10 nm [3], which is further an aggregation of acidic and basic subunits. Soy protein aggregates are therefore considered nanoparticle aggregates. The size reduction of SPI nanoparticle aggregates were accomplished by alkaline hydrolysis of SPI and centrifugal separation process.
EXPERIMENTAL DETAILS The soy protein isolate used in this
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