Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: March 24, 2008
Publication Date: March 24, 2008
Citation: Jong, L. 2008. Fractal dimensions of soy protein nanoparticle aggregates determined by dynamic mechanical method [abstract]. Materials Research Society. Paper No. U8.18. Technical Abstract: Soy protein isolate (SPI) is obtained from soybeans by removing soybean oil and soy carbohydrates. Soy protein nanoparticles were prepared by alkaline hydrolysis of SPI and centrifugal separation process. Structurally, SPI is a globular protein and its aggregates in water consist of sphere-like protein particles. Light scattering measurements of hydrolyzed SPI (HSPI) indicate a narrow size distribution. In swollen state, the volume and number weighted mean particle size are 260 and 210 nm respectively. The fractal dimension of the protein aggregates can be estimated by dynamic mechanical methods when the particle aggregates are imbedded in a polymer matrix. Nanocomposites were formed by mixing HSPI nanoparticles with a rubbery styrene-butadiene (SB) latex, followed by freeze-drying and compression molding methods. The dynamic shear moduli of the elastomeric composites containing 20, 30, and 40% particle fractions were measured over a temperature range from -70 to 140 deg C at 0.16 Hz. A logarithmic plot of shear modulus vs. particle fraction in rubber plateau region at 140 deg C 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. In this study, strain sweep experiments were carried out on the composite samples repeatedly for 8 times at 140 deg C with a dynamic frequency of 1 Hz over a strain sweep range from 0.01% to 15%. The strain cycles generated a stress softening effect in the composites and eventually reached a reversible equilibrium state. 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. Even though these two types of experiments are quite different, the results show that the fractal dimensions extracted from both linear and non-linear viscoelastic data have a reasonably 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.