Studies On Structure And Properties Of Nanocomposite Based On Soy Protein And Starch Nanoparticle

It is a good way to reinforce material that nano-sized structure uniformly dispersed into polymer matrix by strongly interacting with matrix and restricting the motion of polymer segments. Soy protein isolate (SPI) is one of few natural polymers that can be thermoplastic processed assisted with small molecules. However, the water-sensitivity, low rheology and high rigidity of soy protein, as well as the competition of petroleum-based products with economic advantages, limit the development and application of soy protein plastics. Recently, it becomes a hot topic to develop soy protein plastics. At the same time, starch nanocrystal (StN), originated from natural starch granule, can be also used to produce bionanocomposites. For the effects of inorganic nanoparticles on human's health, the research of StN-based nanocomposite has become a new focus of material sciences. This work aims to produce potential nanocomposite material with high mechanical properties mainly based two natural polymer, SPI and StN. The content of this thesis is divided into two aspects: the one is the SPI-based nanocomposites filled by inorganic and organic nanoparticles, and the other is the nanocomposites modified by StN grafted poly(e-carprolactone) (StN-g-PCL).The detailed work in this thesis is described as follows. The nano-SiO₂ and multi-walled nanotube (MWNT) were compounded into soy protein by simple solution mixing, and then compression molded as the nanocomposite sheets. Meanwhile, the StN was filled into soy protein matrix to produce full-degradable nanocomposite. In addition, the StN-g-PCL was prepared according to the opinion of "grafting from" via microwave-assisted ring-open polymerization, and then added into poly(lactic acid) (PLA) and waterborne polyurethane (WPU), resulting in the enhancement of strength and elongation. In order to disclose the structure-mechanical properties relationship, such nanocomposite materials were characterized by X-ray diffraction (XRD), differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), transmittance electron microscope (TEM), scanning electron microscope (SEM), and tensile test etc. The spherical nano-SiO₂ and tubular MWNTs resulted in a simultaneous increase in strength and elongation for soy protein plastics. Meanwhile, the water uptake of the MWNT/SPI nanocomposite was decreased due to the strong interactions between MWNTs and SPI component. The StNs, derived from natural starch granule, were also used to prepare soy protein nanocomposite, and produce a reinforced full-degradable nanocomposites. It was attributed to a uniform dispersion of small-scale StN in amorphous region of SPI matrix as well as enduring stress of rigid StN and stress transferring mediated with interfacial interactions between active StN surface and SPI matrix. In addition, the StN-g-PCL was used to reinforce and toughen PLA and WPU. Herein, the grafted PCL in StN-g-PCL can strongly interact with polymer matrix and even form co-continuous phase structure, which enhances the strength and elongation at one time.As a result, the creative points in this thesis can be concluded as follows. (1) Spherical nano-SiO₂, tubular MWNTs and platelet-like StN were successfully compounded into soy protein to produce reinforced nanocomposites, which overcame the problem of decreasing strength caused by the plasticization of soy protein plastics. The nano-SiO₂ particles self-aggregated as ca. 100 nm clusters, and showed superior reinforcing function in contrast to isolated nanoparticles, namely an enhancement in strength and elongation at one time. The isolated MWNTs and network-like structure dispersed into SPI matrix to form the strong interfacial adhesion by the wrapping and penetrating of SPI chains to MWNTs. When the size and content of MWNT were moderate, the strength and elongation were simultaneously enhanced. In addition, the hydroxyl groups onto the StN surface can strongly adhere SPI matrix and then reinforce the materials. (2) The effects of various shapes and characters of nanoparticles on the structure and properties of nanocomposites were disclosed. The simultaneous reinforcing and toughening of nano-SiO₂ / SPI nanocomposites was attributed to the moderate nano-clusters aggregated by nano-SiO₂ as well as all kinds of strong interfacial interactions. The higher performances of MWNT/SPI nanocomposite resulted from high effective specific surface area for adhering SPI matrix and easy-to-wrapping character of small-size objects as well as the interactions between penetrating SPI chains and internal wall of MWNTs and the associations with SPI matrix mediated by stretching SPI segments inside MWNTs. The reinforcing of StN mainly depended upon the polar hydroxyl groups onto StN surface and induced the interfacial interaction. (3) This work is the first report on grafting PCL onto StN by microwave-assistant ring-open-polymerization. The resultant StN-g-PCL was compounded with various polymer matrix, such as WPU and PLA. The grafted PCL chains induced a co-continuous phase structure, which not only reinforced materials, but also enhanced elongation.Based on the study above mentioned, the strength of soy protein plastics was enhanced by introducing nanoparticles, and the water-resistance was also improved to a certain extent. Thus, the problem that the plasticization caused the decrease of strength was solved. At the same time, the StN-g-PCL simultaneously reinforced and toughened the WPU and PLA. Moreover, it is disclosed that the mechanical properties of nanocomposites strongly depend on the shape, size, surface character of nanoparticles and their distribution in polymer matrix. There are always three kinds of interaction which contribute to the strength of nanocomposite and even elongation, such as original interaction in polymer matrix, the interfacial interactions between matrix and nanoparticles, and the self-aggregated affinity among nanoparticles. In conclusion, the structure-properties relationship for all the systems are established while a strategy to design and modify nanocomposite has been submitted.