Abstract
This paper demonstrates the preparation of transparent biocomposites from chitin nanofiber using a series of simple mechanical treatments after the removal of proteins and minerals. Field emission scanning electron microscopy (FE-SEM) images show that the prepared chitin nanofibers are highly uniform with a width of less than 50 nm and a high aspect ratio. Due to the nano-size, the fibers are small enough to retain the transparency of the neat polymethylmethacrylate resin. Light transmission of the obtained chitin/PMMA biocomposite was 90.2%, in comparison to the neat resin, which was 92.6%. Mechanical property tests showed that chitin nanofibers significantly improved the tensile strengths and Young’s modulus of the neat PMMA, which increased from 43.8 MPa to 102 MPa and 1.6 GPa to 3.43 GPa, respectively. PMMA resin was found to be well dispersed in the biocomposite and had little effect on the tensile properties of the material. The properties mentioned above qualify the chitin nanofiber as a green and high-performance candidate having potential to be applied in next-generation optical electronic and building systems as a commercially available material.
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Optically Transparent Biocomposites: Polymethylmethacrylate Reinforced with HIGH-PERFORMANCE Chitin Nanofibers
Chuchu Chen,a Dagang Li,a,* Qiaoyun Deng,a and Botao Zheng a
This paper demonstrates the preparation of transparent biocomposites from chitin nanofiber using a series of simple mechanical treatments after the removal of proteins and minerals. Field emission scanning electron microscopy (FE-SEM) images show that the prepared chitin nanofibers are highly uniform with a width of less than 50 nm and a high aspect ratio. Due to the nano-size, the fibers are small enough to retain the transparency of the neat polymethylmethacrylate resin. Light transmission of the obtained chitin/PMMA biocomposite was 90.2%, in comparison to the neat resin, which was 92.6%. Mechanical property tests showed that chitin nanofibers significantly improved the tensile strengths and Young’s modulus of the neat PMMA, which increased from 43.8 MPa to 102 MPa and 1.6 GPa to 3.43 GPa, respectively. PMMA resin was found to be well dispersed in the biocomposite and had little effect on the tensile properties of the material. The properties mentioned above qualify the chitin nanofiber as a green and high-performance candidate having potential to be applied in next-generation optical electronic and building systems as a commercially available material.
Keywords: Chitin nanofiber; Polymethylmethacrylate(PMMA); Optically transparent; Biocomposite; Mechanical properties; High-performance
Contact information: a: College of Wood Science & Technology, Nanjing Forestry University, 210037，Nanjing, P.R. China; * Corresponding author: njfuldg@163.com
INTRODUCTION
Nanofibers have received a great deal of attention in the research community because of the recent revolution in nanotechnology. Latest studies have demonstrated that nanofibers can be produced from a number of polymers. The unique characteristics plus the functionalities of the polymers themselves impart nanofibers with many desirable properties for advanced applications (Shams et al. 2011). For example, cellulose nanofibers have been shown to be great reinforcement nanocomposites (Siqueira et al. 2009; Nakagaito and Yano 2005).
Apart from cellulose, chitin, poly (ß-(1-4)-N-acetyl-D-glucosamine), is the most abundant source of polysaccharides found in nature, with an annual bioproduction of around 1010 to 1011tons (Gopalan Nair and Dufresne 2003). It is the main component of the exoskeleton of arthropods such as crabs, prawns, and insects. These exoskeletons have a fine hierarchical organization consisting of α-chitin nanofibers and various proteins and minerals (Raabe et al. 2005). Recently, Ifuku (2009) succeeded in isolating α-chitin nanofiber from crab shells with a uniform width of approximately 10 to 20 nm and a high aspect ratio by simple process. It shows excellent mechanical properties, including a high Young’s modulus and high tensile strength. In particular, Nakagaito and Yano (2005) reported a unique cellulose nanofiber composite film. Because of the nano-size effect, the
nanocomposite, which incorporated with an acrylic resin, was optically transparent. Lately, it has been shown that this finding is applicable for chitin nanofibers to obtain an optically transparent nanocomposite (Ifuku et al. 2010b). However, many reports have introduced the preparation of chitin nanofibers under acidic conditions (Ifuku et al. 2010a), which are severely damaging to the machines. So this has been a big problem that needs to be solved as soon as possible.
Additionally, chitin has different characteristics from cellulose, such as biocom-patibility, wound healing activity, high purity, and hydrophobicity, which strongly distinguishes it from cellulose nanofiber composites (Shams et al. 2011). More recently, Ifuku and Shams (2011) demonstrated that chitin nanofibers extracted from crab shells exhibit much higher transparency than cellulose nanofibers. Although native chitin is a semicrystalline biopolymer with microfibrillar morphology and excellent material properties, most of the biomass is thrown away as industrial waste without effective utilization, especially in the case of crab shells. Thus, it is important to make efficient use of this biomass resource as a natural and environmental friendly material.
Because nanocomposites are considered to have great potential, various nanofillers have been used for their preparation, including cellulose, carbon nanofibers, nanoclays, and so on (Johnsen et al. 2007). However, a detailed study of composites using chitin nanofiber derived from completely sustainable and renewable natural resources as reinforcement has still not been conducted particularly to the point of discussing its mechanical properties. Specific characterization of the effects of chitin nanofibers on various resins would be especially valuable for designing advanced nanocomposite materials. In this paper, we prepared chitin nanofibers using a series of different mechanical treatments from waste crab shell powder compared with traditional methods, such as just grinding. Then PMMA was selected as the matrix in this study because it presented excellent transparency which could reach as high as 92% compared with acrylic resin which was 91% (Ifuku et al. 2011a). At the same time, PMMA is light but with high strength, such that it could be used as a building material. Furthermore, PMMA is a kind of avirulent environmental protection material with good chemical stability. In this paper we compounded PMMA with the obtained chitin nanofiber sheet and characterized its transparency, Young’s modulus, tensile strength, and microstructure.
EXPERIMENTAL
Materials
Dried crab shell powder was purchased from Zhejiang Golden-shell Biochemical Co., LTD. at low cost. Polymethylmethacrylate (PMMA, 8N) resin was obtained from Evonik Degussa (China) Co., LTD. The other chemicals and distilled water were all purchased from Nanjing Chemical Reagent Company and used without further purification in this study.
Methods
Removal of matrix components
According to the general methods (Gopalan Nair and Dufresne 2003), crab shell powder was treated with 7% hydrochloric acid (HCl) solution for 24 hours at room temperature
firstly to remove mineral salts such as calcium carbonate. After the suspension filtered and rinsed with an abundance of distilled water, the obtained sample was dispersed in 5% potassium hydroxide (KOH) solution for 6 hours to remove proteins, and this process was repeated four times in order to wipe out all the residual proteins. The pigment compo-sition in the sample was then removed using 95% ethanol at room temperature for 6 hours followed by filtration and washing with distilled water. At last, chitin powder was obtained, as Fig. 1 shows.
Fabrication of chitin nanofibers and chitin sheet
Purified powder was dispersed in water at 1wt%. The suspension was then passed ten times through a grinder (MKCA6-2; Masuko Corp., Japan) at 1500 rpm with a clearance gauge of -1.5 and -3, respectively. The position was determined as the point of slight contact between the two grinding stones. In order to further fibrillate the chitin fibers, the sample was treated with high-pressure homogenization (EmulsiFlex-C3, AVESTIN, Inc, Canada) for 20 min at a concentration of 0.1wt%. Finally, the suspension was placed in a high-speed centrifuge (H-1650, Hunan Xiangyi Laboratory development Co., LTD) and blended for 10 minutes at a speed of 10000 rpm. In the end, the obtained chitin fiber was nano-scalar through the FE-SEM observation. Morphological change of the chitin during the preparation of nanofibers is shown in Fig. 2. Nanofiber suspension, 1000 mL, which was the supernatant removed by decantation after the process of centrifugation, was vacuum-filtered using a polytetrafluoroethylene (PTFE) membrane with a 0.2 μm pore size to produce a thin paper sheet of 11 mm in diameter. The wet sheet was dried at 55 oC under slight pressure overnight and the sample’s thickness was 35 μm. Then, another part of the nanofiber suspension was freeze-dried for the purpose of analyzing its diameter distribution later.
Preparation of chitin/PMMA nanocomposite membranes
PMMA was used as a matrix. Five grams of PMMA particles were immersed in 100 mL of dimethyl formamide. After mixing them for 1 hour at 55 oC, a liquid PMMA rein was obtained. The dried sheet, which was vacuum filtered using PTFE membrane were impregnated with neat PMMA resin under reduced pressure for 5 hours. Then, the impregnated sheets were taken out of the solution and oven-dried at 50 oC for 12 hours. Chitin nanofiber reinforced plastic membranes thus obtained were approximately 68 μm thick, and the fiber content was about 60 wt%, calculated based on the dry weights of the chitin nanofiber sheets and nanocomposites. The whole process is shown in Fig. 2.
FE-SEM observation
The morphology of chitin nanofiber and chitin/PMMA nanocomposite was observed using a field emission scanning electron microscope (HITACHI S-4860, HITACHI Japan) and a scanning electron microscope (SEM, FEI, qunta200). The fracture surface of the films was obtained by snap. The sample was freeze-dried in a Freeze Drying Machine (Zhongke, XIANOU-10) at -40 oC for 48 hours, using silica gel to maintain dryness. Then a small part of the specimen was taken to the sample stage and coated with approximately a 2 nm layer of gold using a vacuum sputter coater (SCD 005); the sputtering time was about 30 s, and the electric current was 10 mA.
Fig. 1. Process of preparation of chitin from waste crab shell powder
Fig. 2. Overall process of fabrication of chitin nanofibers and their nanocomposite
UV-visible spectra
Light transmittance of the membrane was observed with a UV-VIS near-infrared spectrometer (U-4100.HITACHI) with an integrating sphere 60 mm in diameter from 200 nm to 1000 nm in visible light wavelength range at 20 oC. Three replicates were conducted for each sample. The measurements were carried out three times to ensure the accuracy.
Tensile test
The samples were investigated with a universal material-testing machine equipped with a 100 N load cell (SANS, Shenzhen, China) at room temperature. The length of the specimen was 20 mm, 5 mm wide, and a crosshead speed of 1 mm/min was used for the tests. Ultimate tensile strength at break of the specimen was evaluated. The results were reported as the average value from measurements of at least five specimens.
RESULTS AND DISCUSSION
Characterization of Chitin Nanofibers
Crab shells are composed of around 20 to 25% chitin nanofibers that are associated with other constituents (Chen et al. 2008). Figures 4a and b show photographs of the Burma crab shell, also known as black crab, before and after the removal of matrix components, respectively. Interestingly, after using the same chemical treatments applied in the waste crab shell powder described above, the crab shell turned completely white and kept its original shape and substantial morphological detail, including the eyes. Compared with the raw material, the treated shell feels softer due to the loss of some matrix, and its microstructure is shown in Fig. 5. A number of relatively uniform ellipse-type macropores with dimensions of 1×0.5 µm were apparent; these were the spaces that had been occupied by the protein and mineral salts before the treatment. Additionally, the bundles of chitin fiber could also be clearly seen having an oriented arrangement. This image (Fig. 5a) was similar to the one obtained in the latest research performed by Iftekhar et al. (2012).
Fig. 3. Morphological change of the chitin during the preparation of nanofibers. (a) chitin powder after chemical treatments, (b) chitin powder turbid liquid after grinding treatment, (c) suspension after high pressure homogenization, and (d) suspension after centrifugation
Thus, in order to obtain nano-scalar chitin fibers with a high aspect ratio, mechanical treatments are necessary to effectively separate the bundles. It can be confirmed via Fig. 3. During the preparation of chitin nanofibers, the fiber suspension was gradually clear and homogeneous. Because of sequential mechanical treatments, fiber bundles are successfully dispersed step by step until reaching the expected nanometer level. Therefore, the final obtained sample, as Fig. 2d represents, is almost transparent through the macroscopic observation.
As many specialists have described (Raabe et al. 2005), exoskeletons of crustaceans have a strictly hierarchical organization, and the fracture structure is shown in Fig. 5b. Obvious layers were exhibited and each layer was connected with plenty of chitin fibers. The whole structure looks complex but uniform.
Fig. 4. (a) Raw Burma crab shell and (b) crab shell after removal of matrix components
Fig. 5. FE-SEM image of (a) surface of the crab shell and (b) fracture surface of the crab shell
Figure 6 (a and b) shows FE-SEM pictures of the surface of the chitin sheets prepared using the waste crab shell powder as the starting material. Under low magnification (×2000), it appears very smooth and dense. In the case of mechanical treatments, the chitin fibers were fibrillated successfully with an average width of less than 50 nm. Under 40,000 times magnification, the surface of the sheet exhibited a very fine nanofiber network, which was uniform and compact with a number of holes. This result explains that sufficient mechanical treatments are just enough to separate the bundles into nano-scalar chitin fibers though in neutral condition. This would represent an improvement relative to the conventional methods, which require the addition of acid during the preparation (Ifuku et al. 2010a), because acid tends to corrode the equipment.