Structure And Properties Of Eco-friendly Soy Protein Functional Materials

The environmental crisis caused by the extensive use of petroleum-based nondegradable materials and the impending petroleum finite resources have directly threatened human being's sustainable development. Therefore, environment-friendly polymers based on natural renewable resources are attracting more and more attentions now. Soy protein, the coproduct of the soybean oil industry, is readily available from renewable resources and agricultural processing industry, and is regarded as a viable alternative for petroleum based polymers. However, there are some drawbacks for soy protein materials, such as brittleness and water sensitivity, which limit their extensive use. In this work, we used soy protein isolate (SPI) as the starting material and prepared a series of soy protein blends and nanocomposites through a green method. The structure and properties of the resulting materials were characterized by using analytic methods like FTIR spectroscopy (FTIR), UV-vis spectroscopy (UV-vis), scanning electron microscopy (SEM), transmission electron microscopy (TEM), differential scanning calorimetry (DSC), thermal gravimetric analysis (TGA), contact angle (CA), X-ray photoelectron spectroscopy (XPS), dynamic mechanical analysis (DMA), tensile tests, conductivity tests, magnetization tests, microwave absorbing tests and so on. The structure and properties as well as the correlation between them for these novel materials are discussed.The innovation of this work is described as follow:(1) for the first time, we prepared the alkaline soy protein solution using ammonia, and fabricated the waterborne polyurethane (WPU)/SPI blend films by solution casting methods; (2) Hydrophobic materials were successfully prepared from SPI and natural rubber (NR) by blending via freezing and lyophilizing method. (3) A novel kind of conductive nanocomposites was prepared from SPI and carbon black (CB). (4) A series of SPI／γ-Fe2O3 nanocomposites with good magnetic and microwave absorbing properties were successfully prepared.The general contents and conclusions of this work are generalized as follow:Soy protein solution was prepared under high temperature (90℃) and alkline conditions (ammonia). SPI/WPU blend films were fabricated by mixing protein solution and WPU dispersion through casting method. It revealed that there were fine interface contact and strong hydrogen bonding between soy protein and WPU. The blend films exhibited good compatibility and optical clarity. The surface and bulk hydrophobicity was improved with incorporation of WPU. The blend films exhibited high flexibility under both dry and wet conditions. The blend films exhibited low cytotoxicity and good biocompatibility compared to WPU, and hence can find potential application as biomaterials.The mixing between SPI solution and NR latex exhibited obvious separation tendency resulted by the thermodynamical immiscibility. Thus the SPI/NR blend foams were prepared by the freezing and lyophilization technique. The protein moiety and rubber moiety were dispersed in the blend sheets homogeneously. The freezing played an important role in the improvement of the compatibility of the SPI/NR materials, leading to high transparent blend sheets. The water resistant properties of the blend sheets were significantly improved compared with pure SPI. The calculated values of the total surface free energy according to Van Oss-Good theory indicated a low surface energy of NR in the blends, therefore the surface segregation of NR occurred in SPI/NR materials, leading to hydrophobic surfaces of the blend sheets. A slight increase of N and P elements was observed for the inner surface compared with the free surface from the XPS experiments, which proved the surface enrichment of NR. The flexibility of the blend was significantly improved and phase inversion phenomenon was observed. The blend sheets were found to possess good biodegradability, and microorganism in the soil could directly attack the surface of the sheets to speed up metabolism. Furthermore, the blend sheets exhibited good biocompatibility and were capable of supporting cell adhesion and proliferation, thus indicating a potential bioapplication as novel tissue engineering materials.Hydroxyamines with amino, imine and hydroxyl functional groups possess good compatibility with protein. Soy protein thermoplastics plasticized with triethanolamine (TEA) and diethanolamine (DEA) were successfully prepared. TEA possessed a long molecular chain, which could disrupt the hydrophilic shell and then penetrate into the hydrophobic region of soy protein, resulting in a good compatibility. The plastics with hydroxyamines possessed good optical transparency. The highest optical transmission at a wavelength of 800 nm was 64～72% for DEA plasticized SPI and 82～85% for TEA plasticized SPI. The glass transition temperature was significantly reduced by adding plasticizer. Plasticizer with higher molecular would lead to a higher glass transition temperature of the plastics.With an increase of plasticizer in the films, the water uptake and water diffusion coefficient of soy protein plastics were increased. Of the two, the plastics with TEA possessed a lower water uptake. The flexibility of soy protein was highly improved by adding plasticizers. TEA plasticized soy protein exhibited higher tensile strength, Young's modulus and elongation at break compared with DEA plasticized soy protein. In addition, plastics with TEA also possessed better thermal stability due to the strong interactions between TEA and soy protein.The SPI/CB conductive nanocomposites were fabricated by dispersing CB into soy protein matrix through water media. It revealed that hydrogen bonding interactions occurred between soy protein and CB. The distribution of CB in soy protein was not homogeneous and there were CB-rich and CB-poor domains in the matrix. The mechanical and thermal properties were significantly enhanced with incorporation of CB. CB could both interact with the plasticizer-rich and protein-rich domains of SPI/glycerol system, resulting in the restriction of molecular motions of soy protein and the enhancement of glass transition temperature as well as the modulus of the composites. CB particles could reduce the distribution width of both glycerol-rich and protein-rich domains, which resulted in the homogeneity of the systems. TGA revealed that CB enhanced the stability at the protein decomposition stage, therefore improved the thermal stability of the nanocomposites. The conductivity of the SPI/CB nanocomposites was increased with CB and the percolation threshold was about 0.76 vol%. The multiple-percolation and nonrandom distribution of CB are likely to make the conducting composites deviate from the classical percolation theory. The SPI/CB nanocomposites possessed a higher permittivity compared to pure soy protein, exhibiting stronger capability to absorb electromagnetic waves.Maghemite (γ-Fe2O3) nanoparticles were created by precipitation followed by oxidation, and using them, SPI/Fe2O3 magnetic nanocomposites were successfully prepared. The nanoparticles were found to be dispersed in the matrix homogeneously and strong hydrogen bonding occurred between the filler and matrix. The incorporation of Fe2O3 enhanced the mechanical properties of the resulting soy protein materials. The synergistic interactions between Fe2O3 and soy protein as well as between the nanoparticles make the reinforcement deviate from the Guth sphere-model theory. Fe2O3 particles mainly improved the stability at the protein decomposition stage, therefore improved the thermal stability of the nanocomposites. The resulting nanocomposites exhibited good superparamagnetic behavior and good absorbing property for electromagnetic waves with a frequency of 2-18 GHz. This work provided a novel way of fabricating magnetic soy protein materials through a low cost and simple method.As a result, the novel materials were prepared in the present work using green and simple method. The correlation between the structure and properties of these materials was estabilished. In this work, SPI, the environment friendly and renewable source material, was used as the base (matrix) material. These biocompatible and biodegradable materials were found to be potential bioactive materials. Therefore, this thesis possesses academic value and application perspective, and well accords with the target of our country and the strategy of sustainable development.