| Cellulose is the most abundant and ancient natural polymer on earth, Luffa sponge is a natural renewable resources, mainly composing cellulose, lignin, and hemicellulose. Luffa cellulose fibers were purified and isolated from waste luffa sponges via subsequent chemical treatment with KOH-NaClO2system. Luffa sponge nanocellulose crystals (LNCC) were prepared by an ultrasound-assisted sulfuric acid hydrolysis method. The effects of sulfuric acid concentration, temperature, and ultrasonication time on the yield of LNCC were optimized with responge surface methodology based on Box-Behnken Design(BBD). The results showed that quadratic model is a suitable model to describe the relationship between the yield of Luffa sponge nanocellulose crystals and the observed factors. The determination coefficient and adjusted determination coefficient were99.95%and99.88%, respectively, which confirmed the excellent accuracy of the model. The ideal conditions were analyzed as follows:sulfuric acid concentration62%, temperature51℃, and ultrasonic treatment time46min, which gave rise to an optimal yield of93.64%for Luffa sponges nanocellulose crystals. This was in good agreement with the theoretical predicted value of93.20%. The results offered a significantly favorable guidance for high value-added utilization of waste Luffa sponges.The main chemical components of Luffa sponge were analyzed. The a-cellulose content in Luffa sponge was61.3%, which is higher than those of wood plants while similar to hemp plants. The morphology of Luffa sponge, purified Luffa sponge cellulose and Luffa sponge nanocellulose were characterized by Morfi Fibers Analyzer, Optical Microscopy, Transmission Electron Microscopy (TEM), and Field Emission Scanning Electron Microscopy (FESEM). The results showed that the purified Luffa cellulose fibers were potentially suitable for pulping, with characteristics of a fiber curl degree of6.8%, coarseness of0.5891mg/m, together with an average width of26.4μm and weight length of893μm. LNCC was determined to have diameter around10nm and lengths in the range of200-400nm. The crystallinity index and zeta potential of L-NCC displayed values of68.74%and-15.1mV, respectively. The FTIR spectra of purified Luffa cellulose and LNCC were similar, indicating the native chemical structure of cellulose was remained intact during the process of nanocellulose crystals fabrication.The as-prepared LNCC was further integrated with nano-sized manganese dioxide and polyvinyl pyrrolidone. respectively, to manufacture composite materials. The morphology, crystallinity, thermal, and mechanical properties of the composites were analyzed by FETEM, XRD, Thermogravimetric analysis(TG). The nanoparticals of MnO2with diameters around10～40nm were uniformly dispersed in the LNCC matrix, giving rise to a smooth brown appearance of MnO2-LNCC films. The UV-vis specra of MnO2-LNCC films show a maximum absorption peak at about370nm, being originated from the characteristic adsorption of nano-MnO2. The crystallinity degree derived from the XRD profiles of MnO2-LNCC films significantly increased as compared with that of pure MnO2-LNCC film. Carbon, oxygen, and manganese elemental signals were all found in the MnO2-LNCC film samples measured by XPS analysis. The increased percentage of oxygen content resulted in a declined C/O ratio in MnO2-LNCC composite by comparison with that of LNCC. The thermal stability of MnO2-LNCC film was remarkably improved verified by thermal gravimetric (TG) analysis. The optimal MnO2-LNCC film sample presents an initial dagration temperature (Ti) and the temperature at maximum decomposing (Tmax) of302.7C and345.8C, respectively, which were both much higher than the Ti (214.6C) and Tmax (225.3C) of pure LNCC film. Moreover, the maximum tensile strength of MnO2-LNCC film reaches29.92MPa, which was1.77times as high as the control sample of LNCC film.Transparent LNCC-PVP films were successfully prepared via a facile in situ solution casting method. XRD analysis results showed that the crystallinity of LNCC-PVP composites gradually improved with the increasing content of LNCC nanofiller. All the LNCC-PVP composites displayed good thermal stabilities as characterized by TG-DTG analysis, although no positive correlations between the initial dagration temperatures (Ti) of the LNCC-PVP samples and the content of LNCC added were observed. The reason could be mainly due to the much higher Ti (>390C) of the native PVP polymer in comparison with that of as-prepared LNCC. The maximum tensile strength of the observed LNCC-PVP films (14.25MPa) was increased by a factor of two as compared to the pure PVP film. The stress-strain curves of the LNCC-PVP samples were similar, all of which could be classified as thermoplastic materials. The LNCC-PVP composites had good rheological stability and exhibited pseudoplastic fluid behavior, since the shear stress was improved with the increase of shear rate, while the viscosity became smaller, presenting a typical non-Newtonian fluid properties. |
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