(283d) Eco-Friendly Fabrication and Characterization of Mechanically Strong, Thermally Stable, Largely Impermeable and Biodegradable Zein-Graphene Oxide Nanocomposites

Eco-friendly fabrication and characterization of mechanically strong, thermally stable, largely impermeable and biodegradable Zein-Graphene Oxide Nanocomposites

Tahrima B. Rouf, Jozef L. Kokini

Department of Food Science, Purdue University, West Lafayette, IN­47907

Email: trouf@purdue.edu, jkokini@purdue.edu

Synthetic polymers are polluting the environment at an alarming rate. It has been reported that the “Great pacific ocean garbage patch”, which was twice the size of Texas [1], is now thrice the size of France and consists of 79,000 tons of plastic waste [2]. As an eco-friendly alternative, natural polymers have received tremendous attention, due to their nontoxicity, biodegradability, biocompatibility, low cost, non-dependence on petroleum sources, and availability from renewable resources. Zein, the most abundant protein of corn, is a byproduct of ethanol production and is one of the most commonly used plant-based proteins. Due to its unique hydrophobic film forming properties, low cost, abundance and lack of essential amino acids for human consumption, zein is often used in non-food applications [3]. Zein has found application in drug delivery [4–6], coating material for tablets, nanoparticles and nanofibers [7, 8], high strength adhesives [9] and packaging materials [10, 11]. Typically standalone zein proteins often require some chemical or physical modifications, like addition of plasticizers, crosslinker or reinforcing nanofillers to have comparable properties to those of synthetic polymers [10–12]. The objectives of this study were to successfully fabricate zein-graphene oxide (Z-GO) nanocomposite films and characterize the obtained product, for offering an in-depth insight regarding the effect of graphene oxide (GO) nanofiller in the mechanical, thermal, barrier and surface properties of zein. Graphene oxide (GO), is a reinforcing nanomaterial made of single or few layers of carbon atoms, that has reportedly increased mechanical, thermal, barrier, electrical properties of different plant based biopolymers [13]. GO nanosheets were prepared using Tour’s method. Z-GO nanocomposites were fabricated using solvent casting technique, by applying GO in concentrations of 0%, 1%, 3%, and 5%, where 70% ethanol was used as a solvent, oleic acid used as a plasticizer and monoglyceride as an emulsifier. The prepared GO nanosheets and the Z-GO nanocomposite films were characterized using TEM, AFM, FT-Raman, FTIR, TGA, as well as mechanical property and water vapor permeability measurements. The TEM of the prepared GO nanosheets showed wrinkled morphology, which resembles crumpled silk veil waves [14] and the transparency of the GO sheets indicated exfoliation to few layer structured graphene material. This effect resulted in obtaining higher surface area and further increased the possibility of achieving good biopolymer/filler interface. The FT-Raman spectra for the multi-layered GO sheets exhibited two characteristic peaks located at 1309 and 1590 cm^–1, which can be attributed to D and G bands. The G band is caused by in plane bond stretching vibrations of sp² bonded carbon atom and the D band is due to vibrations of the carbon atom of disordered and defected graphite [15]. The prominent D peak in the FT-Raman spectra is an indication of the presence of disorder, which is likely related to both the structural disorder induced by the tears and folding over of the GO sheets as well as the presence of residual oxygen and point defects. Chemistry of bond formation in GO nanosheets, as well as, Z-GO nanocomposite was investigated using FTIR. FTIR spectra of pristine GO shows characteristic peaks at 1373 cm^-1 (C–O–C vibrations), 1623 cm^-1 (sp²-hybridized C=C, in-plane vibrations, as well as vibrations from ketonic species (C=O) ), 1070 cm^−1 (C–OH vibrations), 1737 cm^-1 (C-OH vibrations from –COOH), and 3207 cm^-1 (C–OH vibrations from COOH and H₂O) [16]. FTIR of Z-GO nanocomposites showed, the characteristic protein peaks from zein, as well as, formation of a new peak at 1048 cm^-1 which can be attributed to C-O stretching vibrations from GO. As the nanofiller concentration was gradually increased, the intensity of the new peak also increased, which indicated that GO has been distributed uniformly throughout zein matrix. Surface roughness analysis using AFM showed that Z-GO nanocomposites had reduced roughness compared to control zein films, which is another indication of the uniform dispersion of GO throughout zein matrix and water contact angle measurement showed significant changes in hydrophobic/hydrophilic balance, on addition of GO. TGA results showed that the 50% thermal decomposition temperature increased by 16 ^oC, and the 90% thermal decomposition temperature increased by 34^oC at only 1.5% GO loading. Our studies also showed significant improvement of mechanical properties as well as a decrease in water vapor permeability for the nanocomposites with only a small loading of GO. The impact of GO on properties of zein films strongly depended on the percentage as well as the preparation technique. This research provides an in-depth analysis of the impact of GO nanofillers on the physicochemical properties of zein biopolymer and aims at providing a comprehensive understanding of the properties of nanocomposites, with an ultimate goal to find novel applications for bio-based food grade materials in the fields of packaging, sensors as well as other biodegradable functional films.

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