Studies On The Modification Of Biodegradable Soy Protein Films

Nowadays, the development and utilization of biodegradable polymer, based on renewable resources, has been of growing concern and interest in polymer science because of the petroleum crisis and ever-increasing pollution caused by the non-degradable synthetic polymers. Soy protein, as one of the most abundant plant proteins and potential renewable source for biodegradable films, has attracted much attention. It is noted that films made from soy protein alone possess good biodegradability but have poor mechanical properties and water resistance. Therefore, the dissertation have dealt with the modification of biodegradable soy protein films by chemical crosslinking, chemical grafting and blending in order to improve the mechanical properties and water resistance. At the same time, the relationships between the structure and properties of modified soy protein films were investigated by means of fourier transform infrared spectroscopy (FT-IR), dynamic mechanical analysis (DMA), thermogravimetric analysis (DTG), energy dispersive X-ray measurement (EDX), X-ray diffraction measurement (XRD), Scanning electron microscopy observation (SEM), transmitting electron microscopy observation (TEM), tensile testing, water uptake testing and et al.The innovative points in this work are as follows. (1) A series of soy protein isolate/dialdehyde starch/Ag nano-films (SPI/DAS/Ag) with good mechanical and antibacterial properties were successfully prepared using dialdehyde starch as a crosslinker. At the same time, the Ag nanoparticles were formed by reducing Ag+ in situ. (2)γ-glycidoxypropyltrimethoxysilane, a silane crosslinker, was used to crosslink the SPI films, which improved the mechanical properties and water resistance of the SPI films significantly. (3) Soy protein isolate/polycaprolactone graft copolymers (SPI-g-PCL) with various compositions and molecular weights were synthesized by coupling hydroxy-terminated polycaprolactone(PCL-OH) with the Soy protein isolate (SPI) molecules, and then were compression-molded into the films by using glycerol as the plasticizer. The resultant SPI-g-PCL films have good water resistance and mechanical properties at 98 % relative humidity when compared with pure SPI film. (4) The carbon nanotubes coated with cationic guar gum was used to reinforce the SPI films, and improved significantly the mechanical properties of the films. (5) A series of soy protein isolate/hydroxyapatite (SPI/HA) nanocomposites were successfully prepared by co-precipitation in aqueous media. The SPI/HA composites hold great potentials as tissue scaffolds for biomedical applications.The main research contents and conclusions are as follows:(1) A new kind of macromolecular crosslinker, dialdehyde starch (DAS), was used to cross-link SPI. The films based on SPI and DAS were prepared by casting the SPI/DAS blend solutions in Teflon mold and by drying at 35 oC for 24 h. The structure and properties of the films were characterized by FT-IR, SEM, DMA, tensile testing and water vapor permeability testing. The results indicated that the cross-linked films exhibited much higher tensile strength and less water vapor permeability than the pure SPI film. In addition, Ag + can be reduced to silver nanoparticle in situ by the aldehyde groups of DAS. Transmitting electron observation indicated that the silver nanoparticles with a size distribution of 30~60 nm could be distributed uniformly in the SPI/DAS/Ag films. The antibacterial efficacies of the SPI/DAS/Ag films against E. coli were about 93%~99%.(2) A series of soy protein films were prepared by coagulating process usingγ-glycidoxypropyltrimethoxysilane (KH560) as a cross-linker. In the 20 % ethanol solution, KH560 reacted onto SPI chains through the alkali-catalyzed amino-oxirane addition reaction. Meanwhile, the condensation reaction of the silanol groups would be performed mostly during the solvent evaporation period of the following film formation process. The Si–O–Si linkages formed from the condensation reaction could provide the inter-chain covalent bonds, resulting in a crosslinked structure. DMA, TGA, tensile testing and water uptake testing were used to characterize the films. The results revealed that the tensile strength, water-resistivity and thermal stability of the films were obviously enhanced after crosslinking by KH560. The tensile strength of the films with 4 wt% KH560 were 7.39 MPa, which were much higher than those of the films without KH560 modification (4.13 MPa).(3) A series of soy protein isolate/polycaprolactone graft copolymers (SPI-g-PCL) were synthesized respectively by ring-openning polymerization ofε-caprolactone in the presence of SPI, and by coupling the hydroxy terminated polycaprolactone (PCL-OH) with SPI using isophorone diisocyanate (IPDI) as the coupling agent. After that, the SPI and SPI-g-PCL were compression-molded into the films by using glycerol as the plasticizer. Tensile testing and water uptake testing revealed that the SPI-g-PCL films exhibited higher tensile strength and lower water uptake at 98 % relative humidity when compared with the SPI films. Furthermore, all of the films degraded well in soil, and the half-life of degradation was less than 30 days.(4) A series of soy protein isolate/hydroxyapatite (SPI/HA) composites were successfully prepared by co-precipitating Na2HPO4 and CaCl2 in the aqueous solution of SPI, and then were compression-molded into the films by using glycerol as the plasticizer. The SPI/HA composites and films were characterized by FT-IR, SEM, TEM, DMA, tensile testing and water uptake testing. It was shown that the needle-like hydroxyapatite nanocrystals (HA), with 100~150 nm in length and 10~20 nm in diameter, could be dispersed uniformly in the SPI matrix. The increase of the glass transition andα-relaxation temperature evidenced the effective confinement of the protein molecules by the strong hydrogen bonding interactions between the SPI and HA. Meanwhile, this kind of confinement significantly lowered the water uptake, and enhanced the tensile strength and modulus of the composite films even at high relative humidity environments. Furthermore, the SPI/HA composites hold great potentials as tissue scaffolds for biomedical applications.(5) A series of SPI/GUM/MCNT blend films based on soy protein isolate (SPI), cationic guar gum (GUM) and multiwall carbon nanotubes (MCNT) were prepared by solution casting and drying method. The results from FT-IR, XRD and SEM revealed that SPI and GUM were miscible because of the strong hydrogen bonding and electrostatic interactions among–OH, cationic groups in GUM and–NH2,–CONH–,–COOH groups in SPI. In addition, GUM was found to be capable of stably dispersing the MCNTs in aqueous solution, which contributed to obtain a homogeneous distribution of MCNTs in the SPI/GUM/MCNT blend films. With the MCNT amounts increased from 0 wt% to 3 wt%, the tensile strength increased from 4.75 to 6.47 MPa, and the Young's modulus increased from 74.9 to 121.5 MPa.(6) A series of microcrystal cellulose (MCC) reinforced SPI films (SPI/MCC) were manufactured by compression-molding. The effect of MCC and glycerol as plasticizer on thermal properties, mechanical properties and morphology of the films were evaluated with DMA, SEM, tensile testing and water uptake testing. The results indicated that the strong hydrogen bonding interactions between MCC filler and SPI matrix play an important role in reinforcing the films without interfering with their biodegradability. With the MCC amounts increased from 0 wt% to 30 wt%, the tensile strength increased from 4.8 to 7.9 MPa, and the Young's modulus increased from 57.4 to 117.8 MPa. Furthermore, the incorporation of glycerol would unfold the molecular chain of SPI, with the glycerol amounts increased from 10 wt% to 50 wt%, the physical state of the SPI/MCC films changed from hard and brittle to soft and weak.