Starch-based biocomposite films incorporating spherical cellulose nanocrystals from garlic stalks offer a novel application of an underused natural fiber.
Reinforcement is a critical feature of polymer technology. A neat polymer matrix often cannot meet the mechanical properties desired for a specific application. Consequently, the use of reinforcing filler is integral to the design of new polymer-based materials. Common reinforcing fillers used in polymer processing include carbon black, glass fiber, clay, talc, silica, and other metals.1 In a time of growing environmental concern, bio-based fillers have attracted substantial research interest.2 Among these fillers are cellulose nanocrystals (CNCs), that is, the crystalline portion of cellulose with at least one dimension equal to or smaller than 100nm.3 Here, we describe CNCs isolated from garlic stalks, agricultural waste that is frequently left untreated following the harvest season in the Philippines and is underused.4 We also employed the CNCs as a reinforcing filler for starch-based bioplastics.Figure 1 is a schematic of the procedure we used to isolate CNCs from garlic stalks. We delignified (i.e., pulped) the stalks in alkaline medium using sodium hydroxide and bleached them with hypochlorite solution. We then hydrolyzed the resulting cellulose fibers with 50% sulfuric acid at 35°C for 5h. Acid hydrolysis enabled the removal of amorphous cellulose by promoting hydrolytic cleavage of glycosidic bonds and, ultimately, releasing individual crystallites.5 The resulting suspension after hydrolysis was then washed, dialyzed, and sonicated, yielding a stable suspension of cellulose nanocrystals. The suspension had a density of 1.6g/ml and a concentration of 2.3% CNCs by mass.We monitored changes in crystallinity from the raw garlic stalks (RGSs) to the CNCs using x-ray diffraction analysis. Each diffractogram reveals two well-defined peaks at 2θ = 12.8° and 22.5° (where 2θ is the angle of diffraction), typical of cellulose (see Figure 2). As expected, the isolated CNCs gave the sharpest peak with the highest intensity, indicating the higher crystallinity relative to raw and delignified stalks. The removal of lignin, which is amorphous after delignification, increased the crystallinity index of RGSs from 40 to 55%. Further treatment with acid resulted in the isolation of more crystalline cellulose with a crystallinity index of 62%.Figure 1.Isolation of cellulose nanocrystals (CNCs) from garlic stalks. RGS: Raw garlic stalks. DGS: Delignified garlic stalks. The electron microscopy images in Figure 3(a–d) show the morphological changes brought about by different chemical treatments. The RGSs—see Figure 3(a)—exhibit no individualized cellulose fibers because of the presence of lignin and hemicelluloses (polysaccharides) that act as cementing components around the fiber bundles. These substances were removed during delignification, as is evident in the micrograph for the delignified garlic stalks—see Figure 3(b)—which shows smaller, more separated, and better defined cellulose microfibrils. The cellulose fibers have an average diameter of 8.5μm. Further treatment of the cellulose fibers by acid hydrolysis reduced them to nanometer size: see Figure 3(c) and (d). The isolated CNCs appear as black, spherical spots in the micrograph. We attribute the noticeable aggregation of cellulose nanocrystals to strong hydrogen bonding of the particles. Indeed, Lu and Hsieh6 have suggested that strong hydrogen bonding among cellulose nanocrystals overcomes the repulsion of surface negative charges when CNCs are in the dry phase. The isolated CNCs had diameters ranging from 30 to 50nm. The calculated value for the average diameter of the CNCs was 35nm.Table 1.Mechanical properties of the biocomposite films with varying starch:CNC ratios.TreatmentTensile strengthModulus(starch:CNCs)(MPa)(MPa)T0 (100:0)10.0327.3T1 (100:2.5)14.3416.2T2 (100:5)15.6439.6T3 (100:10)10.5392.5T4 (100:15)9.58349.98Figure 2.X-ray diffraction patterns of raw garlic stalks, delignified garlic stalks, and CNCs. Figure 3.Scanning electron microscopy images of (a) raw garlic stalks and (b) delignified garlic stalks, and transmission electron microscopy images of the isolated CNCs at different magnifications (c) and (d). We tested the reinforcing effect of the nanocrystals by using them as filler in starch-based bioplastics. We prepared films with different ratios of starch to CNC (100: 2.5, 100:5, 100:10, and 100:15) by solution casting using cornstarch as the matrix and glycerol as plasticizer. Film without CNCs served as control. We determined tensile strength and modulus in accordance with ASTM D88-02 using an Instron 5585H tensometer. We observed the highest improvement in tensile strength and modulus in sample T2 (see Table 1). Tensile strength increased by 56% relative to non-reinforced film. Similarly, the modulus of T2 increased by 34%. The reinforcing effect of CNCs in starch may be a function of good adhesion at the interface of CNC and starch, again, owing to hydrogen bonding. This interfacial interaction results in the formation of a rigid network of CNCs that serves to reinforce the matrix.7 However, we also observed that increasing the amount of CNCs led to deterioration of the mechanical properties of the biocomposites. For example, although samples T3 and T4 had higher CNC content, the reinforcing effect may have been offset due to agglomeration of the nanocrystals at higher loading.In summary, we were able to isolate spherical CNCs from garlic stalks by alkali pulping, acid hydrolysis, and sonication. We characterized the crystallinity and morphology of the crystals using x-ray diffraction and transmission electron microscopy, respectively. The reinforcing effect of CNCs as a filler in starch-based bioplastics was evident in the improvement of tensile strength and modulus at a starch:CNC ratio of 100:5. However, we did observe agglomeration at higher CNC loading. To overcome the strong hydrogen bonding that ultimately results in CNC agglomeration, as a next step we are planning to focus on surface modification of CNCs.AuthorsMelissa B. AgustinDepartment of Chemistry Central Luzon State UniversityMelissa Agustin is a lecturer. Her research focuses on the isolation, characterization, and application of nanocellulosic materials.Enna Richel De LeonDepartment of Chemistry Central Luzon State UniversityEnna Richel De Leon is an undergraduate student.Jerico BuenaobraDepartment of Chemistry Central Luzon State UniversityJerico Buenaobra is an undergraduate student.Joel R. SalazarDepartment of Chemistry Central Luzon State UniversityJoel Salazar is a lecturer. His research interests are the isolation, characterization, and application of nanocellulosic materials.Bashir AhmmadYamagata UniversityBashir Ahmmad is a professor in the Graduate School of Science and Engineering. His research interests focus on nanomaterials, surface science, and energy-related devices.Fumihiko HiroseYamagata UniversityFumihiko Hirose is a professor in the Graduate School of Science and Engineering. He is engaged in high-quality film deposition techniques, fast-switching devices, and highly efficient solar cells using interfacial nanotechnologies.ReferencesD. M. Bigg, Mechanical properties of particulate filled polymers, Polym. Compos. 8 (2), pp. 115-122, 1987. J. Shen, Z. Song, X. Qian and Y. Ni, Carbohydrate-based fillers and pigments for papermaking: a review, Carbohydr. Polym. 85, pp. 17-22, 2011. M. J. John and S. Thomas, Biofibres and biocomposites, Carbohydr. Polym. 71, pp. 343-364, 2008. M. B. Agustin, B. Ahmmad, E. R. P. De Leon, J. L. Buenaobra, J. R. Salazar and F. Hirose, Starch-based biocomposite films reinforced with cellulose nanocrystals from garlic
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