Preparation and characterization of poly (lactic acid)/ethylene vinyl acetate/graphene oxide polymer composites for water purification
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Date
2020-10
Authors
Mokoena, Lesia Sydney
Journal Title
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Publisher
University of the Free State
Abstract
Water contamination by heavy metals, due to most industries is a global problem. The released heavy metals are in the form of ions, which do not naturally degrade, but rather form part of water that is consumed. This has numerous negative effects on humans, animals, plants as well as the environment. The purpose of this study was to synthesize graphene oxide (GO) from expandable graphite powder (EG) and prepare environmentally friendly poly(lactic acid) (PLA)/ethylene vinyl acetate (EVA)/graphene oxide (GO) composites through melt mixing to be used to remove lead ions from solution. The GO was synthesized following the modified Hummer’s method and verified with FTIR, XRD and SEM-EDS. FTIR spectra showed numerous peaks denoting the successful attachment of oxygen containing functional groups to EG. XRD analysis for GO resulted in a peak at 7.5˚, with an increased interlayer spacing (0.347 nm for EG and 1.18 nm for GO), which validated the presence of oxygen containing functional groups. SEM images showed GO layers as well as their exfoliated surfaces and in EDS the oxygen content was proven to be higher in GO (39.4%) as compared to EG (12.3%). The PLA/EVA/GO blend (70/30, 50/50, and 30/70) composites, with different amounts of GO (1, 3, and 5 wt.%), were prepared by melt mixing. The morphology, surface, flow, and thermal properties as well as water studies (Water absorption and Pb (II) adsorption), were performed using Scanning electron microscopy (SEM), Surface energy evaluation system (SEES), Melt flow indexer (MFI), Differential scanning calorimetry (DSC), Thermogravimetric analyser (TGA), water intake experiments and Atomic absorption spectroscopy (AAS), respectively. The morphology of the blends showed that PLA and EVA were immiscible. In the blend composites, it illustrated that GO was mostly situated on the interface between PLA and EVA, although it could also settle in either of the polymer phases. The highest PLA containing blend composites showed cracks on the polymers and gaps on the interface, while in the other composites there were no visible cracks and gaps. GO sheets were present in the blend composites, and it brought about partial miscibility to the polymer matrices. The interfacial tensions, through SEES analysis, calculated for PLA/GO and EVA/GO pairs were similar, indicating an equal chance of GO to disperse in either one of the polymers, or even the interface. The wetting coefficient value of 0.523 suggested GO to be on the interface of the two polymers. Thermal analysis (melting and crystallization by DSC runs) showed that PLA and EVA are immiscible, as two melting peaks were observed for these polymers, 149 ℃ and 97.9 ℃, respectively. EVA was seen to have a hindering effect on the re-crystallization and melting process of PLA, this was because the cold crystallization temperature peak of PLA disappeared leading to no melting of PLA. While PLA had a semi-catalytic effect on the crystallization process of EVA, as the crystallization temperature of EVA was observed earlier, for all blends, though there was a decrease in degree of crystallinity. With the introduction of GO to the polymer matrices, the glass transition temperature of PLA moved to lower values, and in the 50/50 w/w PLA/EVA composites (with 1, 3 and 5 wt.% GO loadings) the individual melting peaks approached one another. This meant that GO brought partial miscibility to the polymer matrices. GO seemed to have a hindering effect on the crystallization of EVA, and a nucleating effect on the chains of PLA. This was observed through the drastic reduction in the degree of crystallinity of EVA, and the reappearance of the cold crystallization temperature peak of PLA, resulting in the re-definition of melting, for all the blend composites. In terms of thermal degradation studies through TGA analyses, the polymers degraded separately in the blends, proving their immiscibility. The introduction of GO to the blends showed the autocatalytic effect on the thermal degradation of the polymers (early degradation at lower temperatures, and delayed degradation with polymer char at higher temperatures). There was a presence of char in all the blend composites, which showed the presence of polymer (either PLA or EVA) as well as GO. This suggested that the polymers masked GO and in turn, the GO improved their thermal stability to some extent. Water absorption studies showed that the 66.5/28.5/5 PLA/EVA/GO composite had the highest water absorption degree, which was because of the cracks and spaces observed on it. The 50/50 w/w PLA/EVA composites also showed a high-water intake, and the exposed GO layers observed on their morphology was the attribution made to these observations. Then in Pb(II) intake studies through AAS, all the analysed samples had very high metal adsorption capacities, and the 66.5/28.5/5 PLA/EVA/GO composite adsorbed more metal ions under a basic medium and 5 hours contact time. Adsorption kinetic modelling showed a favourable heterogeneous and homogeneous adsorption as both the Langmuir and Freundlich isotherms gave linear fits. However, the Langmuir fit was more precise and therefore the conclusion was that adsorption took place mostly on a homogenous surface.
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Keywords
Dissertation (M.Sc. (Chemistry))--University of the Free State (Qwaqwa Campus), 2020, Water contamination, Graphene oxide (GO), Poly lactic acid