Accepted Manuscript Title: PREPARATION AND CHARACTERIZATION OF TRANSPARENT PMMA-CELLULOSE-BASED NANOCOMPOSITES Author: Esra Erbas Kiziltas Alper Kiziltas Shannon C. Bollin Douglas J. Gardner PII: DOI: Reference:
S0144-8617(15)00232-5 http://dx.doi.org/doi:10.1016/j.carbpol.2015.03.029 CARP 9771
To appear in: Received date: Revised date: Accepted date:
9-11-2014 5-3-2015 7-3-2015
Please cite this article as: Kiziltas, E. E., Kiziltas, A., Bollin, S. C., and Gardner, D. J.,PREPARATION AND CHARACTERIZATION OF TRANSPARENT PMMACELLULOSE-BASED NANOCOMPOSITES, Carbohydrate Polymers (2015), http://dx.doi.org/10.1016/j.carbpol.2015.03.029 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Transparent PMMA/cellulose nanocomposites were successfully prepared by a solution casting method which included a combination of ultrasonication and mechanical stirring processes using acetone as the solvent. Thermogravimetric analysis (TGA) indicated retained thermal stability of the transparent nanocomposites. The transmittance of PMMA/cellulose nanocomposites was reduced with increased loading of cellulose nanofibrils. No significant differences were observed between the FTIR spectra of pure PMMA and cellulose nanocomposites. Optical micrograph of PMMA/cellulose nanocomposite that some cellulose nanomaterials might have reaggregated among themselves. CNF can serve as promising reinforced nanomaterials for the production of PMMA nanocomposites in diverse applications such as: packaging, flexible screens, optically transparent films and light-weight transparent materials.
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PREPARATION AND CHARACTERIZATION OF TRANSPARENT PMMA-
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CELLULOSE-BASED NANOCOMPOSITES
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Esra Erbas Kiziltas1, 2†*, Alper Kiziltas1, 3 , Shannon C. Bollin4 and Douglas J. Gardner 1
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Advanced Structures and Composites Center, University of Maine, Maine, 04469, USA
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The Scientific and Technological Research Council of Turkey (TUBİTAK), Tunus Cad, Kavaklıdere, Ankara, 06100, TURKEY
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Research and Advanced Engineering, Ford Motor Co. Dearborn, Michigan 48124, USA
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University of Bartin, Bartin, 74100 TURKEY
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Department of Forest Industry Engineering, Faculty of Forestry,
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†: Corresponding author.
E-mail: [email protected]
Tel: +1-207- 249-9346 Fax: +1-207-581-2074
*: First author.
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E-mail: [email protected]
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Tel: +1-207- 249-9346 Fax: +1-207-581-2074
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Abstract
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Nanocomposites of polymethylmethacrylate (PMMA) and cellulose were made by a solution casting method using acetone as the solvent. The nanofiber networks were prepared using three different types of cellulose nanofibers: (i) nanofibrillated cellulose (NFC), (ii) cellulose nanocrystals (CNC) and (iii) bacterial cellulose from nata de coca (NDC). The loading of cellulose nanofibrils in the PMMA varied between 0.25 and 0.5 wt. %. The mechanical properties of the composites were evaluated using a dynamic mechanical thermal analyzer (DMTA). The flexural modulus of the nanocomposites reinforced with NDC at the 0.5 wt. % loading level increased 23% compared to that of pure PMMA. The NFC composite also exhibited a slightly increased flexural strength around 60 MPa while PMMA had a flexural strength of 57 MPa. The addition of NDC increased the storage modulus (11%) compared to neat PMMA at room temperature while the storage modulus of PPMA/CNC nanocomposite containing 0.25 and 0.5 wt.% cellulose increased about 46% and 260% to that of the pure PMMA at the glass transition temperature, respectively. Thermogravimetric analysis (TGA) indicated that there was no significant change in thermal stability of the composites. The UV-vis transmittance of the CNF nanocomposites decreased by 9% and 27% with the addition of 0.25 wt. % CNC and NDC, respectively. This work is intended to spur research and development activity for application of CNF reinforced PMMA nanocomposites in applications such as: packaging, flexible screens, optically transparent films and light-weight transparent materials for ballistic protection.
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Keywords: Nanocellulose; Nanocomposites; Thermal analysis; Transparency; PMMA
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1. Introduction It is well known that cellulose, a linear homopolymer composed of β-1, 4-linked glucose molecules, is the most abundant biopolymer on earth (Pecoraro et al. 2008). Characteristic behavior of cellulose mostly depends on its unique structural hierarchy and its biological origin, which provides excellent properties including; high mechanical properties, such as elastic modulus of the crystalline region of cellulose I is ~138 GPa, biodegradability, high strength, high aspect ratio, high specific surface area, low thermal expansion, and low density (Tanpichai et al. 2012; Nishino et al., 1995). The current demand in materials research is to develop multifunctional materials which comprise excellent features such as enhanced mechanical properties and thermal stability, biodegradability, being eco-friendly, and low-cost. Thus, cellulose nanomaterials from different sources have been widely used as a reinforcing phase in nanocomposites to produce innovative products for science, and technology especially in medicine, electronics or energy production (Vitta et al. 2012).
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Nanoscale cellulose fibers can be obtained from four different methods: (1) microfibrillated/nanofibrillated cellulose plant cell fibers, (2) cellulose whiskers or cellulose nanocrystals, (3) bacterial cellulose nanofibers (BC) and (4) cellulose nanofibers by electrospinning (Gardner et al. 2008). Nanofibrillated cellulose (NFC) is typically a fibrous component of cellulose fibers that have nanoscale (less than 100 nm) diameter and lengths up to several micrometers (Stelte et al. 2009). Using NFC as a reinforcement agent in polymer composite materials has gained increasing attention because of its excellent properties such as large surface area, water retention value, transparency, sustainability, and its unique features such as high strength and stiffness, low weight and biodegradability (Turbak et al. 1983; Nair et al. 2013). Cellulose nanocrystals (CNCs), rod-like or whisker shaped crystalline particles isolated from cellulose, have gained wide attention over a range of research areas (Habibi et al. 2010 and Klemn et al. 2011) attributed to its good mechanical properties, unique optical and self assembly properties (Wang et al., 2013). These excellent material properties play a significant role in developing functionalized optically transparent materials and lightweight nanocomposites for many applications. On the other hand, bacterial cellulose, a microbial polysaccharide, has gained a great deal of attention owing to its impressive physico-mechanical properties being obtained through a bottom up process which is biosynthesis of a class of acetic acid producing bacteria that includes that includes Acetobacter xylinus (Ha et al., 2011; Hestrin & Schramm, 1954; Shaha et al., 2013). Although bacterial cellulose (BC) has the same molecular formula as plant cellulose, it exhibits a unique three-dimensional micro and nano-porous network structure that provides high purity, a high degree of polymerization, high crystallinity (of 70–80%), high water content to 99%, and high mechanical stability (Barud et al., 2011). In recent years there has been considerable interest in using bacterial cellulose as a reinforcement nanomaterial in the preparation of optically transparent materials (Yano et. al.2005 and Nogi et. al.2005). Its nanosized fiber structure, typically a width of 50-80 nm, and a thickness of 3-8 nm (Tabuchi, 2007), enables the reduction of light scattering. BC also has a low coefficient of thermal expansion, which is more important for reinforcement fillers in optoelectronic devices (Nogi et. al., 2008). The dimensions and mechanical properties of nanoscale cellulose fibers compared to microcrystalline cellulose are summarized in Table 1. (Edge et al., 2000; Chauhan and Chakrabart 2012; Vitta and Thiruvengadam 2012; Yano 2010; Eichorn and Young 2001; Lee et al., 2009). Although the diameters of cellulose nanomaterials are less than 100 nm in Table 1, purity, surface energy (NFC: 41mN/m, BC: 57mN/m and CNC: 69mN/m) and crystal structure
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of the cellulose nanomaterials are different. In this regard, it is important to compare the reinforcing efficiency of cellulose nanomaterials in polymer composites (Lee et al., 2012; Yuwawech et al, 2015).
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In the past years, many transparent polymers including poly (methyl methacrylate), polystyrene, and polycarbonate have gained great interest because of their excellent optical clarity and low density. Poly (methyl methacrylate) (PMMA), a glassy polymer with excellent transparency and good processing ability is also used as a model polymer for making nanofillerreinforced transparent nanocomposites. Despite its huge potential, there are some certain drawbacks including mechanical-dynamical properties (low strength, impact resistance and storage modulus etc.), which limit its efficient use in engineering applications (Liu et. al. 2010; Li et. al., 2013). PMMA are also often used in place of glass in certain applications in which both high mechanical-dynamical properties and optical transparency are required. However, mechanical strength of PMMA still does not have sufficient for many current applications (Day et al., 1997). These drawbacks can be overcome via reinforcements with nano- and microfillers (Nussbaumer et. al., 2003; Tang et.al. 2006; Chen et.al. 2009). In the Liu et al. study, transparent polymethylmethacrylate (PMMA) composites were fabricated using freeze dried cellulose nanocrystals (CNCs) through a solution casting method. Their research showed that transparent composite sheets had better mechanical properties and their thermal stability seemed retained with respect to the matrix polymer (Liu et al. 2010). A general need also exists for increasing the mechanical strength and stiffness of PMMA composites while still retaining their good optical transparency (Day et al., 1997).
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In cellulose-based transparent composites, it is important to maintain the optical properties. Because of this cellulose nanofibrils need to have a certain cross sectional size such as less than half of the shortest wavelength of visible light (380–570 nm) to have reduced interfacial light scattering (Althues et al. 2007). However, dispersion is an important factor in terms of optical properties. Poor dispersion of nanofillers in polymeric matrices, lead to composites will have inadequate transparency even at very low loadings (Xu et. al., 2013). With the homogenous dispersion of nanofillers in polmer matrices, nanocomposites continue to maintain optical properties (Ben Mabrouk et. al., 2014). Processing methods also play an important role on the fabrication of composite films because the ultimate properties depend on morphological and cellulose-polymer interactions. Thus, the solvent casting technique is a favorable processing method to synthesize cellulosic nanocomposites films because of its slow solvent evaporation that provides better nanofiller arrangement and also gives enough time for the cellulose nanofibrils to form a percolation network (Dufresne et. al., 2013). In the Hajji et al. study, the influence of three processing methods was compared and the solvent evaporation method had pronounced success in enhancing the mechanical properties of cellulose nanowhisker-based nanocomposites. Nanofiber-network reinforced optically transparent composites with improved properties have also been reported (Nogi et al. 2005 and Okahisa et al., 2011). Hence, PMMA and cellulose nanofiber reinforced PMMA-based nanocomposites were fabricated by the solvent casting technique in the present study.
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It has been recently reported in the literarture (Jonoobi et al. 2013 and Wang et al.2013) that mechanical, thermal and optical properties were improved with cellulose nanomaterials. However, it has been also reported that cellulose nanomaterials loading may often deteriorate 5 Page 5 of 22
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these properties. In this respect, the effects of cellulose nanomaterials in polymers are not yet fully understood. Therefore, the objective of this work is to elucidate the differences or similarities among cellulose nanomaterials (NFC, CNC and NDC from BC) in terms of mechanical, thermal and optical properties in PMMA matrix. This study also compared their origin (bacteria versus trees), morphologies and dispersion properties in PMMA matrix, interactions with matrix, and the resulting reinforcing effects on the matrix polymer. Thermal, mechanical and optical properties of nanocomposites were evaluated by differential scanning calorimetry (DSC), dynamic mechanical thermal analysis (DMTA), thermogravimetric analysis (TGA), and UV–visible spectroscopy, respectively. These results should allow us to further investigate the industrial application areas for certain types of cellulose nanofibers.
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2. Material and Methods 2.1. Materials Nanofibrillated Cellulose (NFC) suspension at 3.5 wt. % was kindly obtained from the University of Maine Chemical and Biological Engineering Department. Cellulose Nanocrystals (CNC) were provided by the United States Department of Agriculture (USDA) Forest Service, Forest Products Laboratory. Solids content of CNC was 6.5 wt. %. Bacterial Cellulose (NDC) was extracted from Nata de coco which is a food product from the fermentation of coconut milk using bacteria Acetobacter xylinum. PMMA (OPTIX ® CA – 75 CLEAR) was supplied from Plaskolite and acetone, (Purity ≥ 98%) was purchased from Sigma Aldrich, respectively.
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2.2. Bacterial Cellulose Production from Nata De Coco BC was extracted from 10 jars (450gr) of Nata-de-coco. First, Nata-de-coco was rinsed with de-ionized water (3×10 dm³) to wash away the sugar syrup. The washed Nata-de coco then was blended for 1 min using a laboratory blender. This BC blend was then homogenized for 5 min and centrifuged at 14,000 g to remove the excess water. To further purify the BC, the centrifuged material was redispersed in deionized water (10 dm³). Sodium hydroxide (40 g, 1 mol) was added into this mixture and heated to 80˚ C for 60 min, while stirring to remove any soluble polysaccharides. The purified BC was then successively centrifuged and homogenized back to neutral pH using deionized water (Juntaro 2009).
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2.3. Production of Transparent Composites PMMA (20g) was dissolved in 100 ml of acetone at 50°C. The maximum temperature for the curing process was selected as 50 °C because the boiling point of acetone is 56 °C (Kim et al. 2005). PMMA-cellulose nanomaterial blends were prepared by dissolving different amounts of cellulose nanofibers (0.25% and 0.5% by weight) in the PMMA-acetone solution under continuous stirring for 1 hour followed by ultra-sonication for several hours at 50°C. The content of PMMA was kept constant for all samples. Composites were prepared by casting this solution in a glass Petri Dish and allowed to evaporate the solution at 50°C in oven overnight and then in vacuum oven for two days. After this, the films were peeled from the glass plate using a doctor blade to obtain free-standing films (Tomar et al. 2011). 2.4. Characterization of the Composites The optical transmittance of the PMMA and transparent nanocomposites were measured at wavelengths from 400 to 1000 nm using an ultraviolet (UV)–visible spectrophotometer (HP8453). Transmission spectra were measured using air as a reference. Thermogravimetric 6 Page 6 of 22
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Analysis (TGA) measurements were carried out using a Mettler Toledo analyzer on samples of about 8 mg. Each sample was scanned over a temperature range from ambient temperature to 600 °C at a heating rate of 10°C/min under nitrogen with a flow rate 20 ml/min to avoid sample oxidation. The samples used for the TGA measurement were randomly picked 5 individual samples from ground samples (Kiziltas et al., 2014; Ozen et al., 2013). Differential scanning calorimetry (DSC) experiments were used to determine glass transition temperature (Tg) of the neat PMMA and nanocomposite materials using a TA Q2000 differential scanning calorimeter (TA Instrument Inc., New Castle, DE, USA). The samples were heated from room temperature to 165°C to remove any possible solvent residues, cooled to 0°C, and then heated to 200° C at the same heating ramp of 10° C /min under a nitrogen atmosphere. The data were collected on the second heating ramp. At least three specimens were tested for each composition, and the results are presented as an average for tested samples. The viscoelastic properties of the composites were determined with a Rheometric Scientific DMTA IV. The experiments were conducted in tensile mode under isochronal conditions at a frequency of 10 Hz. The strain amplitude was fixed at 0.01% to be in the domain of the linear viscoelasticity of the composites. The temperature range was from 0 to 100°C at a scanning rate of 5°C /min. The storage modulus (E’), loss modulus (E’’) and loss factor (tan δ) of the samples were measured as a function of temperature. At least three specimens were tested for each composition, and the results are presented as an average for tested samples. DMTA was also used to investigate flexural modulus of elasticity and flexural strength of neat PMMA and nanocomposites. All specimens were subjected to a three point bending at a test temperature of 21.7±0.3°C with a constant strain rate of 1x10-4 mm/s. At least three specimens were tested for each composition, and the results are presented as an average for tested samples. Attenuated Total Reflectance Fourier-Transform InfraRed (ATR-FTIR) spectroscopy analysis was carried out on the neat PMMA and cellulose nanocomposite samples on a Perkin Elmer Spectrum One FTIR spectrometer (Wellesley, MA, USA), equipped with a Universal ATR accessory, using 200 scans and a resolution of 4 cm−1, over the range 4000–400 cm−1. Before the analysis, samples were dried at 90 °C under vacuum for 24 h (Erbas Kiziltas et al., 2015). Optical microscopy samples were prepared with a Leica Ultracut Microtome, using a diamond blade at room temperature. Sections were cut to a 5 micron thickness and floated in oil between a glass slide and coverslip. The samples were viewed in transmission on a Nikon FX upright microscope.
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3. Results and Discussion Figure 1 shows images of the neat PMMA and nanocomposite sheets with 0.25 wt% NFC, CNC, NDC and 0.5 wt. % NFC. The patterns and letters in the background indicate that the nanocomposite sheets are transparent. It can be seen from Figure 1 that the CNC reinforced nanocomposites exhibited better transparency compared to the NFC and NDC reinforced nanocomposites at the 0.25 wt. % loading level. This result indicated the light transmittance was less affected by the cellulose nanocrystals at low loading levels because of their relatively small size and homogenous dispersion (Liu et al. 2010). Consequently, an increased cellulose nanomaterials loading level (0.5wt. %) became increasingly opaque because of CNF agglomeration in the polymer matrix (Liu et al. 2010). Similar results were also observed for PMMA/NFC composites with a loading level of 0.5 wt. % (Littunen et al. 2013). Figure 2 shows quantitatively measured profiles of light transmittance versus the wavelength of visible light for the pure PMMA matrix and cellulose nanocomposite sheets 7 Page 7 of 22
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examined by a UV–visible spectrometer at a visible wavelength range of 400– 800 nm. The percent transmittances at 400, 600 and 800 nm are also tabulated in Table 2. The pure PMMA sample transmitted about 90% of the incident light. Figure 2 also showed that the UV-vis spectra of PMMA nanocomposites reinforced with 0.25 wt. % CNC, NDC and 0.5 wt. % CNC reinforced transparent composites offered 72%, 58% and 55% light transmittance respectively at a wavelength of 600 nm. However, because of the heterogeneous nature of the composites, the transmittance of nanocomposite was reduced with the increase of the loading content of cellulose nanofibers. It has been shown that the optical transmittance of the CNF composites is largely dependent on the dispersion of CNFs in the polymer matrix (Liu et al. 2010). The decrease in light transmittance by adding NDC (81%) and NFC (94%) at a percentage of 0.5% was found to be relatively greater compared to NDC (26.5%) and NFC (73%) with a percentage of 0.25% compared to pure PMMA at 600 nm. These results suggest that inadequate dispersion and agglomeration of cellulose nanofibers in the nanocomposites lead to reduced transparency. On the other hand, further addition of NFC generated a significant decrease of composite transparency although NFC exhibited better dispersion in the composites. This could probably be attributed to NFCs relatively large lengths. However, high transparency and uniform dispersion can be achieved by filtering cellulose suspensions prior to solvent casting (Nogi et al. 2009).
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Figure 3 shows the thermal degradation behavior of PMMA, cellulose nanomaterials, and PMMA/cellulose nanocomposites. The initial weight loss below 200 °C observed for CNFs attributed to removal of moisture from the cellulose. The thermal degradation that occurred between temperatures of 200−380 °C is attributed to the depolymerization of hemicellulose (for NFC) and cleavage of glycosidic linkages of cellulose for all cellulose nanofibers (Lee et al., 2012). Figure 3 also shows that thermal degradation behavior of CNC, NFC and NDC were very similar, whereas the onset degradation temperature of CNC was higher than those of NFC and NDC. The degradation onset temperatures corresponding to CNC, NFC and NDC were 287°C, 273°C and 260°C, respectively. The PMMA and cellulose nanocomposites underwent thermal degradation at a higher temperature than that of cellulose nanofibers. This could be explained via hydrogen bonding interactions between CNC hydroxyl groups and carbonyl groups on the PMMA matrix to facilitate miscibility of polymer blends (Kuo et al., 2008; Dong et al., 2012) where hydrogen bonding had a significant effect on the thermal properties of polymer blends. However, there was no significant difference found in the degradation temperatures of PMMA and cellulose nanocomposites with different loadings probably because of the low contents of cellulose nanofibrils.
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The PMMA began thermal degradation at 305 °C with a mass loss of 10% which indicated its onset decomposition temperature whereas the cellulose nanocomposites degradation peaks varied in the range of 292–325 °C as shown in Figure 4.a. The maximum decomposition temperature of the cellulose nanocomposites was increased 5 to 20ºC with the variation of cellulose nanofibers content whereas CNC (0.25 wt.) and NDC (0.5 wt. %) showed lower decomposition temperature compared to other cellulose nanocomposites. Sample preparations (size, morphology and homogeneity) may influence heat transfer within the samples and thus influence the course of reaction and the thermogravimetric measurements (Bottom 2008). In NDC production, sodium hydroxide was added into this mixture and heated to 80˚ C for 60 min, while stirring to remove any soluble polysaccharides. Residues of the polysaccharides remaining after processing of NDC could be the reason for the lower thermal stability. The thermal stability 8 Page 8 of 22
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The Tg of the cellulose composites was determined as the inflection point of the specific heat increment at the glass-rubber transition in DSC experiments. Figure 5 shows the effect of cellulose nanofibers on the Tg of the composites measured by DSC. It can be seen that incorporation of low contents of cellulose nanomaterials (0.25% and 0.5%) slightly increased the glass transition temperature of PMMA. (~ 80°C and nanocomposites ~ 84°C). The trend of the glass transition shifting to a slightly higher temperature suggests a restriction of the mobility of polymer chains (Kuo, 2008). This result was expected since cellulose nanomaterials’ hydroxyl groups could interact with the ester functional group (COOCH3) of the PPMA polymeric matrix in its side chain and therefore hinder the rotation of polymeric chains (Kuo, 2008; Jonoobi et al., 2010). A Similar result was reported for cellulose nanoparticle-reinforced PMMA composites in the literature (Han et al. 2014).
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The mechanical properties of the PMMA and the nanocomposites were also studied to examine the influence of cellulose nanofibers on the deformation and viscoelastic behavior of the nanocomposites. Figure 6 shows representative stress–strain curves of neat PMMA and cellulose nanocomposites with varying cellulose nanomaterial contents from DMTA. The nanocomposites with different loading content of CNC and NDC possessed slightly decreased values of flexural strength attributed to higher surface tension for CNC (69 mN/m), NDC (57 mN/m) compared to NFC (41 mN/m). However, NFC had a slight increase in the flexural strength of around 60 MPa while the neat PMMA had a flexural strength of 57 MPa as presented in Figure 6. This could be attributed to PMMA (41 mN/m) has better compatibility with the reinforcing material with lower surface energy (Oporto et al., 2011). The flexural strength values decreased with respect to the strength of the PMMA matrix between approximately 7-13%. Figure 6 indicates that the flexural modulus of the nanocomposites reinforced with NDC at the 0.5 wt. % loading level was 2.56 GPa, 23% greater than to that of the pure PMMA (2.07 GPa). The values of the flexural modulus of elasticity for PMMA, PMMA/NDC (0.25%), PMMA/CNC (0.25%), PMMA/NFC (0.25%), PMMA/NDC (0.5%), PMMA/CNC (0.5%) and PMMA/NFC (0.5%) were 2.07, 2.15, 2.05, 2.13, 2.56, 2.19 and 2.35 GPa, respectively. It can be seen that the NDC reinforced nanocomposites showed better flexural modulus of elasticity compared to NFC and CNC reinforced composites. This increase in flexural modulus of elasticity indicates that the NDC acted as reinforcement in the polymer matrix by transferring load from the polymer matrix to the NDC. It was also observed that from testing, PMMA/NFC composites did not have greater strain to failure than PMMA/CNC and PMMA/NDC nanocomposites.
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of cellulose nanocrystals which were prepared by sulfuric acid hydrolysis was lower than that of the original cellulose because of the presence of acid sulfate groups which decreased the thermal stability by a dehydration reaction (Roman and Winter 2004; Wang et al. 2007). The maximum decomposition temperature increased 20ºC with the NFC (0.25 wt. %) in the nanocomposites. Figure 4.b. shows mass loss at 600°C. The maximum residual mass at 600 °C was obtained from the NDC (about 35 %). There were no residual mass obtained for neat PMMA and the cellulose nanocomposites at 600 °C attributed to the degradation of PMMA starting slowly at 220°C and completely degraded at a temperature higher than 305°C (Pielichowski et al. 2005).
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The storage moduli (E′) at room temperature and glass transition temperatures of the PMMA and nanocomposites with different cellulose nanofiber loadings are shown in Table 3. The glass transition of PMMA was detected at around 75°C, the glass transition temperature 9 Page 9 of 22
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values were not significantly changed as the cellulose nanomaterials content was increased since the viscoelastic properties of the composite are strongly influenced by the matrix polymer. The PMMA composites had glass transition temperatures between 73 and 76° C. The storage moduli of the nanocomposites slightly increased with cellulose nanofibers at loading contents of 0.25 wt. % and 0.5 wt. % at room temperature. At low temperatures, the PMMA matrix was very rigid, and therefore no vigorous reinforcing effect from cellulose nanomaterials was detected at the magnitude of the storage moduli of nanocomposites (Han et al. 2014). The storage modulus of neat PMMA at room temperature was 2.73 GPa. However, the addition of 0.25 wt. % of NDC had a positive effect and the storage modulus of the composite (3.04 GPa) increased 12% compared to neat PMMA. This result could be attributed to the higher crystallinity of 71% of the NDC compared to NFC (41%) (Lee at al. 2012). The storage moduli (E′) at the glass transition temperature and the tan δ maximum peak values of the neat PMMA and nanocomposites with different cellulose nanomaterial loadings are shown in Table 3. The storage modulus of the nanocomposites was greatly increased with cellulose nanofibers at loading contents of 0.25 wt. % and 0.5 wt. % at the glass transition temperature. The reinforcement effect of the cellulose nanomaterials was clear at the glass transition temperature because the PMMA matrix became softened and the CNFs restricted the motion of the PMMA chains which promoted the rigidity of the PMMA (Han et al. 2014). The magnitude of the tan δ peak values of the CNC reinforced nanocomposites were significantly decreased around the glass transition temperature. This can probably be attributed to restriction in chain segment mobility in the amorphous region of the polymer (Kiziltas et al. 2011). The highest moduli of the PMMA/CNC nanocomposites containing 0.25 and 0.5 wt. % cellulose were about 46% and 260% of that of pure PMMA at the glass transition temperature. This result confirmed the creation of enhanced reinforcement with the addition of cellulose nanocrystals into the PMMA matrix.
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Figure 7 depicts the FT–IR spectra of neat PMMA and cellulose nanocomposites. It is evident that all the four spectra are similar except for a few changes in the spectra of the nanocomposites. No significant differences were observed between the FTIR spectra of pure PMMA and cellulose nanocomposites. The fingerprint characteristic vibration bands of PMMA appear at 1728 cm-1 C=O stretching mode. The bands at 3100 and 2800 cm–1 correspond to the methylene C–H stretching while the bands at 1350 and 1450 cm–1 are associated with C–H symmetric and asymmetric stretching modes, respectively. The 1240 cm–1 band is assigned to antisymmetric C-C-O stretch and the 1150 cm–1 band corresponds to skeletal vibrations coupled to C-H deformations, while C–C stretching bands are at 1000 and 800 cm–1. Absence of any additional bands by the presence of cellulose nanomaterials other than those of PMMA in the spectrum of PMMA and further they remaining unperturbed in all the four spectra indicate (1) the purity of the polymer obtained, (2) formation of the nanocomposites and (3) no chemical interaction or chemical bond formation between PMMA and cellulose nanomaterials (Sain et al., 2011; Ahmad et al., 2007).
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Figure 8 show the optical images of the of PMMA and cellulose nanocomposites. In neat PMMA, very few partially dissolved granules were observed in Figure 8. It was apparent from the optical micrograph of PMMA/cellulose nanocomposite that some cellulose nanomaterials might have reaggregated among themselves, adhered to the polymer surface and distribution was not uniform. A few potential scenarios may be envisaged. (1) During the reaction, the hydrophilic cellulose nanomaterials migrated to the polymer water interface, which might be 10 Page 10 of 22
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because of either Van der Waal’s attraction or strong hydrogen bonding interaction among themselves. (2) Agglomeration occurred due to the incompatibility between the hydrophilic cellulose surface and hydrophobic nature of the PMMA (Sain et al., 2011; Ahmad et al., 2007).
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The republic of Turkey, The Scientific and Technological Research Council of Turkey (TUBITAK) is greatly acknowledged for support of the scholarship of the researcher Esra Erbas Kiziltas to do this study at the University of Maine. The authors would like to acknowledge the contributions of Justin Crouse, Dr. Jason Bolton, Alex Nash, Donald Gjeta, Dr. Sanjeev Kumar Kandpal, Connie Young Johnson and Chris West whose hard work made this paper possible. The authors would also like to thank U.S. Army Corps of Engineers, Engineer Research and Development Center project 912HZ-07-2-0013 and Maine Agricultural and Forest Experiment Station (MAFES) project ME09615-08MS and the Wood Utilization Research Hatch 2007-2008 project.
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4. Conclusions PMMA-based transparent nanocomposites were successfully prepared by solution casting with the reinforcement of cellulose nanofibers. The transmittance of PMMA/cellulose nanocomposites was reduced with increased loading of cellulose nanofibrils. The UV-visible light transmittance of the nanocomposites decreased by 9% and 27% with the addition of 0.25 wt. % CNC and NDC respectively at 600 nm. Thermogravimetric analysis indicated retained thermal stability of the transparent composites. The maximum decomposition temperature was increased 20ºC with an NFC content of 0.25 wt. % in the nanocomposites. The maximum residual mass of the NDC was about 35 % obtained at a maximum temperature of 600 °C. The DSC measurements also indicated that the presence of cellulose nanofibers slightly changed the Tg of the PMMA matrix. The storage modulus of the PMMA/CNC nanocomposite containing 0.25 and 0.5 wt. % cellulose increased about 46% and 260% compared to that of the pure PMMA at the glass transition temperature, respectively. The flexural modulus of the nanocomposites reinforced with NDC at the 0.5 wt.% loading level was 2.56 GPa, which was 23% greater than the pure PMMA (2.07 GPa). The NFC composites showed enhanced flexural strength of around 60 MPa compared to those comprised of CNC and NDC. These results imply that at even low cellulose nanomaterials content enhanced thermal and mechanical properties of the rigid-thermoplastic PMMA can be obtained and these serve as promising reinforced nanomaterials for the production of transparent nanocomposites.
5. References Ahmad S., Ahmad S. & Agnihotry S.A. (2007) Synthesis and characterization of in situ prepared poly(methyl methacrylate) nanocomposites. Bull. Mater. Sci., 30(1): 31–35. Althues H., Henle J. & Kaskel S. (2007) Functional inorganic nanofillers for transparent polymers. Chemical Society Reviews 36: 1454-1465. Barud H. S., Regiani T., Marques R. F. C., Lustri W. R., Messaddeq Y. & Ribeiro S. J. L. (2011) Antimicrobial bacterial cellulose-silver nanoparticles composite membranes. Journal of Nanomaterials http://dx.doi.org/10.1155/2011/721631. 1-8pp.
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Table 1. Dimensions and mechanical properties of nanoscale cellulose fibers compared to microcrystalline cellulose. Table 2. UV-visible transmittances of the neat PMMA and cellulose nanocomposites at 400 nm, 600 nm and 800 nm. Table 3. Glass transition temperature (Tg), tan δ maximum peak values (tan δ max. peak), storage modulus at room temperature (SM at RT) and glass transition temperature (SM at Tg) for neat PMMA and cellulose nanocomposites.
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Table 1. Dimensions and mechanical properties of nanoscale cellulose fibers compared to microcrystalline cellulose. Material BC
CNC CNF MCC
Length (nm) >1000
100-600 >1000 >1000
Diameter (nm) 24-84
2-20 10-40 >1000
Aspect Ratio (l/d)
Tensile Strength (GPa) 15-35
Modulus of Elasticity (GPa) 114-300
Density (g/cm3) 1.25
7.5 3 -
120-143 138-141 25
1.5 1.45
∼50 10-100 100-150 ∼1
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Table 2. UV-visible transmittances of the neat PMMA and CNF nanocomposites at 400 nm, 600 nm and 800 nm. Sample Name Transmittances (%) 400 nm 600 nm 800 nm Neat PMMA 90 79 81 0.25 %NFC 22 21 29 0.25 %CNC 74 72 86 0.25 %NDC 50 58 75 0.5 %NFC 8 4.5 7 0.5 %CNC 56 55 69 0.5 % NDC 15 15 20
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Table 3. Glass transition temperature (Tg), tan δ maximum peak values (Tan δ max. peak), storage modulus at room temperature (SM at RT) and glass transition temperature (SM at Tg) for neat PMMA and cellulose nanocomposites. Group Name Tg (°C) Tan δ max. peak SM at RT (GPa) SM at Tg (GPa) PMMA 71.87 (0.76) 0.99 (0.02) 2.73 (0.20) 0.023 (0.004) 0.25% NFC 75.51 (1.89) 0.96 (0.01) 2.77 (0.07) 0.024 (0.001) 0.25% CNC 78.67 (1.40) 0.81 (0.05) 2.80 (0.08) 0.034 (0.002) 0.25%NDC 74.35 (1.90) 0.96 (0.02) 3.04 (0.09) 0.030 (0.001) 0.5%NFC 72.25 (1.26) 0.86 (0.02) 2.97 (0.04) 0.032 (0.001) 0.5%CNC 76.41 (0.68) 0.65 (0.04) 2.88 (0.15) 0.060 (0.006) 0.5%NDC 72.72 (1.91) 0.97 (0.03) 2.91 (0.11) 0.030 (0.002) Parenthesis indicates standard deviation.
Figure 1. Photographs of the pure PMMA and CNF nanocomposites placed on a background paper for demonstrating transparency. Figure 2. UV–visible transmittance spectra of the neat PMMA and CNF nanocomposites. Figure 3. TGA curves of the PMMA, cellulose nanofibers, and CNF nanocomposites. Figure 4. a) The temperature at 10% and 50% M.L. and b) mass loss at 600 °C for neat PMMA, cellulose nanofibers and CNF nanocomposites from their TGA curves, respectively. Figure 5. DSC thermograms of the neat PMMA and CNF nanocomposites. Figure 6. Stress-strain behavior and flexural strength (bars) and flexural modulus of elasticity (lines) of neat PMMA and CNF nanocomposites, respectively. Figure 7. FT-IR spectra of the neat PMMA and cellulose nanocomposites. Figure 8. Optical micrographs of the neat PMMA and cellulose nanocomposites.
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Figure 1. Photographs of the pure PMMA and CNF nanocomposites placed on a background paper for demonstrating transparency.
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Figure 2. UV–visible transmittance spectra of the neat PMMA and CNF nanocomposites.
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Neat PMMA NFC CNC NDC 0.25% NFC 0.25% CNC 0.25% NDC 0.5% NFC 0.5% CNC 0.5% NDC
40
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Temperature (°C) Figure 3. TGA curves of the PMMA, cellullose nanofibers, and CNF nanocomposites.
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Group Name
Figure 4. a) The temperature at 10% and 50% M.L. and b) mass loss at 600 °C for neat PMMA, cellulose nanofibers and CNF nanocomposites from their TGA curves, respectively.
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Figure 5. DSC thermograms of the neat PMMA and CNF nanocomposites.
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Flexural Strength (MPa)
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Flexural Modulus of Elasticity (GPa)
Stress (Pa)
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Group Name Figure 6. Stress-strain behavior and flexural strength (bars) and flexural modulus of elasticity (lines) of neat PMMA and CNF nanocomposites, respectively.
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Figure 7. FT-IR spectra of the neat PMMA and cellulose nanocomposites.
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Figure 8. Optical micrographs of the neat PMMA and cellulose nanocomposites. 22 Page 22 of 22
S0144-8617(15)00232-5 http://dx.doi.org/doi:10.1016/j.carbpol.2015.03.029 CARP 9771
To appear in: Received date: Revised date: Accepted date:
9-11-2014 5-3-2015 7-3-2015
Please cite this article as: Kiziltas, E. E., Kiziltas, A., Bollin, S. C., and Gardner, D. J.,PREPARATION AND CHARACTERIZATION OF TRANSPARENT PMMACELLULOSE-BASED NANOCOMPOSITES, Carbohydrate Polymers (2015), http://dx.doi.org/10.1016/j.carbpol.2015.03.029 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Transparent PMMA/cellulose nanocomposites were successfully prepared by a solution casting method which included a combination of ultrasonication and mechanical stirring processes using acetone as the solvent. Thermogravimetric analysis (TGA) indicated retained thermal stability of the transparent nanocomposites. The transmittance of PMMA/cellulose nanocomposites was reduced with increased loading of cellulose nanofibrils. No significant differences were observed between the FTIR spectra of pure PMMA and cellulose nanocomposites. Optical micrograph of PMMA/cellulose nanocomposite that some cellulose nanomaterials might have reaggregated among themselves. CNF can serve as promising reinforced nanomaterials for the production of PMMA nanocomposites in diverse applications such as: packaging, flexible screens, optically transparent films and light-weight transparent materials.
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Highlights
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PREPARATION AND CHARACTERIZATION OF TRANSPARENT PMMA-
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CELLULOSE-BASED NANOCOMPOSITES
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Esra Erbas Kiziltas1, 2†*, Alper Kiziltas1, 3 , Shannon C. Bollin4 and Douglas J. Gardner 1
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Advanced Structures and Composites Center, University of Maine, Maine, 04469, USA
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The Scientific and Technological Research Council of Turkey (TUBİTAK), Tunus Cad, Kavaklıdere, Ankara, 06100, TURKEY
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Research and Advanced Engineering, Ford Motor Co. Dearborn, Michigan 48124, USA
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University of Bartin, Bartin, 74100 TURKEY
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Department of Forest Industry Engineering, Faculty of Forestry,
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†: Corresponding author.
E-mail: [email protected]
Tel: +1-207- 249-9346 Fax: +1-207-581-2074
*: First author.
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E-mail: [email protected]
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Tel: +1-207- 249-9346 Fax: +1-207-581-2074
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Abstract
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Nanocomposites of polymethylmethacrylate (PMMA) and cellulose were made by a solution casting method using acetone as the solvent. The nanofiber networks were prepared using three different types of cellulose nanofibers: (i) nanofibrillated cellulose (NFC), (ii) cellulose nanocrystals (CNC) and (iii) bacterial cellulose from nata de coca (NDC). The loading of cellulose nanofibrils in the PMMA varied between 0.25 and 0.5 wt. %. The mechanical properties of the composites were evaluated using a dynamic mechanical thermal analyzer (DMTA). The flexural modulus of the nanocomposites reinforced with NDC at the 0.5 wt. % loading level increased 23% compared to that of pure PMMA. The NFC composite also exhibited a slightly increased flexural strength around 60 MPa while PMMA had a flexural strength of 57 MPa. The addition of NDC increased the storage modulus (11%) compared to neat PMMA at room temperature while the storage modulus of PPMA/CNC nanocomposite containing 0.25 and 0.5 wt.% cellulose increased about 46% and 260% to that of the pure PMMA at the glass transition temperature, respectively. Thermogravimetric analysis (TGA) indicated that there was no significant change in thermal stability of the composites. The UV-vis transmittance of the CNF nanocomposites decreased by 9% and 27% with the addition of 0.25 wt. % CNC and NDC, respectively. This work is intended to spur research and development activity for application of CNF reinforced PMMA nanocomposites in applications such as: packaging, flexible screens, optically transparent films and light-weight transparent materials for ballistic protection.
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Keywords: Nanocellulose; Nanocomposites; Thermal analysis; Transparency; PMMA
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1. Introduction It is well known that cellulose, a linear homopolymer composed of β-1, 4-linked glucose molecules, is the most abundant biopolymer on earth (Pecoraro et al. 2008). Characteristic behavior of cellulose mostly depends on its unique structural hierarchy and its biological origin, which provides excellent properties including; high mechanical properties, such as elastic modulus of the crystalline region of cellulose I is ~138 GPa, biodegradability, high strength, high aspect ratio, high specific surface area, low thermal expansion, and low density (Tanpichai et al. 2012; Nishino et al., 1995). The current demand in materials research is to develop multifunctional materials which comprise excellent features such as enhanced mechanical properties and thermal stability, biodegradability, being eco-friendly, and low-cost. Thus, cellulose nanomaterials from different sources have been widely used as a reinforcing phase in nanocomposites to produce innovative products for science, and technology especially in medicine, electronics or energy production (Vitta et al. 2012).
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Nanoscale cellulose fibers can be obtained from four different methods: (1) microfibrillated/nanofibrillated cellulose plant cell fibers, (2) cellulose whiskers or cellulose nanocrystals, (3) bacterial cellulose nanofibers (BC) and (4) cellulose nanofibers by electrospinning (Gardner et al. 2008). Nanofibrillated cellulose (NFC) is typically a fibrous component of cellulose fibers that have nanoscale (less than 100 nm) diameter and lengths up to several micrometers (Stelte et al. 2009). Using NFC as a reinforcement agent in polymer composite materials has gained increasing attention because of its excellent properties such as large surface area, water retention value, transparency, sustainability, and its unique features such as high strength and stiffness, low weight and biodegradability (Turbak et al. 1983; Nair et al. 2013). Cellulose nanocrystals (CNCs), rod-like or whisker shaped crystalline particles isolated from cellulose, have gained wide attention over a range of research areas (Habibi et al. 2010 and Klemn et al. 2011) attributed to its good mechanical properties, unique optical and self assembly properties (Wang et al., 2013). These excellent material properties play a significant role in developing functionalized optically transparent materials and lightweight nanocomposites for many applications. On the other hand, bacterial cellulose, a microbial polysaccharide, has gained a great deal of attention owing to its impressive physico-mechanical properties being obtained through a bottom up process which is biosynthesis of a class of acetic acid producing bacteria that includes that includes Acetobacter xylinus (Ha et al., 2011; Hestrin & Schramm, 1954; Shaha et al., 2013). Although bacterial cellulose (BC) has the same molecular formula as plant cellulose, it exhibits a unique three-dimensional micro and nano-porous network structure that provides high purity, a high degree of polymerization, high crystallinity (of 70–80%), high water content to 99%, and high mechanical stability (Barud et al., 2011). In recent years there has been considerable interest in using bacterial cellulose as a reinforcement nanomaterial in the preparation of optically transparent materials (Yano et. al.2005 and Nogi et. al.2005). Its nanosized fiber structure, typically a width of 50-80 nm, and a thickness of 3-8 nm (Tabuchi, 2007), enables the reduction of light scattering. BC also has a low coefficient of thermal expansion, which is more important for reinforcement fillers in optoelectronic devices (Nogi et. al., 2008). The dimensions and mechanical properties of nanoscale cellulose fibers compared to microcrystalline cellulose are summarized in Table 1. (Edge et al., 2000; Chauhan and Chakrabart 2012; Vitta and Thiruvengadam 2012; Yano 2010; Eichorn and Young 2001; Lee et al., 2009). Although the diameters of cellulose nanomaterials are less than 100 nm in Table 1, purity, surface energy (NFC: 41mN/m, BC: 57mN/m and CNC: 69mN/m) and crystal structure
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of the cellulose nanomaterials are different. In this regard, it is important to compare the reinforcing efficiency of cellulose nanomaterials in polymer composites (Lee et al., 2012; Yuwawech et al, 2015).
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In the past years, many transparent polymers including poly (methyl methacrylate), polystyrene, and polycarbonate have gained great interest because of their excellent optical clarity and low density. Poly (methyl methacrylate) (PMMA), a glassy polymer with excellent transparency and good processing ability is also used as a model polymer for making nanofillerreinforced transparent nanocomposites. Despite its huge potential, there are some certain drawbacks including mechanical-dynamical properties (low strength, impact resistance and storage modulus etc.), which limit its efficient use in engineering applications (Liu et. al. 2010; Li et. al., 2013). PMMA are also often used in place of glass in certain applications in which both high mechanical-dynamical properties and optical transparency are required. However, mechanical strength of PMMA still does not have sufficient for many current applications (Day et al., 1997). These drawbacks can be overcome via reinforcements with nano- and microfillers (Nussbaumer et. al., 2003; Tang et.al. 2006; Chen et.al. 2009). In the Liu et al. study, transparent polymethylmethacrylate (PMMA) composites were fabricated using freeze dried cellulose nanocrystals (CNCs) through a solution casting method. Their research showed that transparent composite sheets had better mechanical properties and their thermal stability seemed retained with respect to the matrix polymer (Liu et al. 2010). A general need also exists for increasing the mechanical strength and stiffness of PMMA composites while still retaining their good optical transparency (Day et al., 1997).
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In cellulose-based transparent composites, it is important to maintain the optical properties. Because of this cellulose nanofibrils need to have a certain cross sectional size such as less than half of the shortest wavelength of visible light (380–570 nm) to have reduced interfacial light scattering (Althues et al. 2007). However, dispersion is an important factor in terms of optical properties. Poor dispersion of nanofillers in polymeric matrices, lead to composites will have inadequate transparency even at very low loadings (Xu et. al., 2013). With the homogenous dispersion of nanofillers in polmer matrices, nanocomposites continue to maintain optical properties (Ben Mabrouk et. al., 2014). Processing methods also play an important role on the fabrication of composite films because the ultimate properties depend on morphological and cellulose-polymer interactions. Thus, the solvent casting technique is a favorable processing method to synthesize cellulosic nanocomposites films because of its slow solvent evaporation that provides better nanofiller arrangement and also gives enough time for the cellulose nanofibrils to form a percolation network (Dufresne et. al., 2013). In the Hajji et al. study, the influence of three processing methods was compared and the solvent evaporation method had pronounced success in enhancing the mechanical properties of cellulose nanowhisker-based nanocomposites. Nanofiber-network reinforced optically transparent composites with improved properties have also been reported (Nogi et al. 2005 and Okahisa et al., 2011). Hence, PMMA and cellulose nanofiber reinforced PMMA-based nanocomposites were fabricated by the solvent casting technique in the present study.
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It has been recently reported in the literarture (Jonoobi et al. 2013 and Wang et al.2013) that mechanical, thermal and optical properties were improved with cellulose nanomaterials. However, it has been also reported that cellulose nanomaterials loading may often deteriorate 5 Page 5 of 22
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these properties. In this respect, the effects of cellulose nanomaterials in polymers are not yet fully understood. Therefore, the objective of this work is to elucidate the differences or similarities among cellulose nanomaterials (NFC, CNC and NDC from BC) in terms of mechanical, thermal and optical properties in PMMA matrix. This study also compared their origin (bacteria versus trees), morphologies and dispersion properties in PMMA matrix, interactions with matrix, and the resulting reinforcing effects on the matrix polymer. Thermal, mechanical and optical properties of nanocomposites were evaluated by differential scanning calorimetry (DSC), dynamic mechanical thermal analysis (DMTA), thermogravimetric analysis (TGA), and UV–visible spectroscopy, respectively. These results should allow us to further investigate the industrial application areas for certain types of cellulose nanofibers.
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2. Material and Methods 2.1. Materials Nanofibrillated Cellulose (NFC) suspension at 3.5 wt. % was kindly obtained from the University of Maine Chemical and Biological Engineering Department. Cellulose Nanocrystals (CNC) were provided by the United States Department of Agriculture (USDA) Forest Service, Forest Products Laboratory. Solids content of CNC was 6.5 wt. %. Bacterial Cellulose (NDC) was extracted from Nata de coco which is a food product from the fermentation of coconut milk using bacteria Acetobacter xylinum. PMMA (OPTIX ® CA – 75 CLEAR) was supplied from Plaskolite and acetone, (Purity ≥ 98%) was purchased from Sigma Aldrich, respectively.
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2.2. Bacterial Cellulose Production from Nata De Coco BC was extracted from 10 jars (450gr) of Nata-de-coco. First, Nata-de-coco was rinsed with de-ionized water (3×10 dm³) to wash away the sugar syrup. The washed Nata-de coco then was blended for 1 min using a laboratory blender. This BC blend was then homogenized for 5 min and centrifuged at 14,000 g to remove the excess water. To further purify the BC, the centrifuged material was redispersed in deionized water (10 dm³). Sodium hydroxide (40 g, 1 mol) was added into this mixture and heated to 80˚ C for 60 min, while stirring to remove any soluble polysaccharides. The purified BC was then successively centrifuged and homogenized back to neutral pH using deionized water (Juntaro 2009).
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2.3. Production of Transparent Composites PMMA (20g) was dissolved in 100 ml of acetone at 50°C. The maximum temperature for the curing process was selected as 50 °C because the boiling point of acetone is 56 °C (Kim et al. 2005). PMMA-cellulose nanomaterial blends were prepared by dissolving different amounts of cellulose nanofibers (0.25% and 0.5% by weight) in the PMMA-acetone solution under continuous stirring for 1 hour followed by ultra-sonication for several hours at 50°C. The content of PMMA was kept constant for all samples. Composites were prepared by casting this solution in a glass Petri Dish and allowed to evaporate the solution at 50°C in oven overnight and then in vacuum oven for two days. After this, the films were peeled from the glass plate using a doctor blade to obtain free-standing films (Tomar et al. 2011). 2.4. Characterization of the Composites The optical transmittance of the PMMA and transparent nanocomposites were measured at wavelengths from 400 to 1000 nm using an ultraviolet (UV)–visible spectrophotometer (HP8453). Transmission spectra were measured using air as a reference. Thermogravimetric 6 Page 6 of 22
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Analysis (TGA) measurements were carried out using a Mettler Toledo analyzer on samples of about 8 mg. Each sample was scanned over a temperature range from ambient temperature to 600 °C at a heating rate of 10°C/min under nitrogen with a flow rate 20 ml/min to avoid sample oxidation. The samples used for the TGA measurement were randomly picked 5 individual samples from ground samples (Kiziltas et al., 2014; Ozen et al., 2013). Differential scanning calorimetry (DSC) experiments were used to determine glass transition temperature (Tg) of the neat PMMA and nanocomposite materials using a TA Q2000 differential scanning calorimeter (TA Instrument Inc., New Castle, DE, USA). The samples were heated from room temperature to 165°C to remove any possible solvent residues, cooled to 0°C, and then heated to 200° C at the same heating ramp of 10° C /min under a nitrogen atmosphere. The data were collected on the second heating ramp. At least three specimens were tested for each composition, and the results are presented as an average for tested samples. The viscoelastic properties of the composites were determined with a Rheometric Scientific DMTA IV. The experiments were conducted in tensile mode under isochronal conditions at a frequency of 10 Hz. The strain amplitude was fixed at 0.01% to be in the domain of the linear viscoelasticity of the composites. The temperature range was from 0 to 100°C at a scanning rate of 5°C /min. The storage modulus (E’), loss modulus (E’’) and loss factor (tan δ) of the samples were measured as a function of temperature. At least three specimens were tested for each composition, and the results are presented as an average for tested samples. DMTA was also used to investigate flexural modulus of elasticity and flexural strength of neat PMMA and nanocomposites. All specimens were subjected to a three point bending at a test temperature of 21.7±0.3°C with a constant strain rate of 1x10-4 mm/s. At least three specimens were tested for each composition, and the results are presented as an average for tested samples. Attenuated Total Reflectance Fourier-Transform InfraRed (ATR-FTIR) spectroscopy analysis was carried out on the neat PMMA and cellulose nanocomposite samples on a Perkin Elmer Spectrum One FTIR spectrometer (Wellesley, MA, USA), equipped with a Universal ATR accessory, using 200 scans and a resolution of 4 cm−1, over the range 4000–400 cm−1. Before the analysis, samples were dried at 90 °C under vacuum for 24 h (Erbas Kiziltas et al., 2015). Optical microscopy samples were prepared with a Leica Ultracut Microtome, using a diamond blade at room temperature. Sections were cut to a 5 micron thickness and floated in oil between a glass slide and coverslip. The samples were viewed in transmission on a Nikon FX upright microscope.
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3. Results and Discussion Figure 1 shows images of the neat PMMA and nanocomposite sheets with 0.25 wt% NFC, CNC, NDC and 0.5 wt. % NFC. The patterns and letters in the background indicate that the nanocomposite sheets are transparent. It can be seen from Figure 1 that the CNC reinforced nanocomposites exhibited better transparency compared to the NFC and NDC reinforced nanocomposites at the 0.25 wt. % loading level. This result indicated the light transmittance was less affected by the cellulose nanocrystals at low loading levels because of their relatively small size and homogenous dispersion (Liu et al. 2010). Consequently, an increased cellulose nanomaterials loading level (0.5wt. %) became increasingly opaque because of CNF agglomeration in the polymer matrix (Liu et al. 2010). Similar results were also observed for PMMA/NFC composites with a loading level of 0.5 wt. % (Littunen et al. 2013). Figure 2 shows quantitatively measured profiles of light transmittance versus the wavelength of visible light for the pure PMMA matrix and cellulose nanocomposite sheets 7 Page 7 of 22
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examined by a UV–visible spectrometer at a visible wavelength range of 400– 800 nm. The percent transmittances at 400, 600 and 800 nm are also tabulated in Table 2. The pure PMMA sample transmitted about 90% of the incident light. Figure 2 also showed that the UV-vis spectra of PMMA nanocomposites reinforced with 0.25 wt. % CNC, NDC and 0.5 wt. % CNC reinforced transparent composites offered 72%, 58% and 55% light transmittance respectively at a wavelength of 600 nm. However, because of the heterogeneous nature of the composites, the transmittance of nanocomposite was reduced with the increase of the loading content of cellulose nanofibers. It has been shown that the optical transmittance of the CNF composites is largely dependent on the dispersion of CNFs in the polymer matrix (Liu et al. 2010). The decrease in light transmittance by adding NDC (81%) and NFC (94%) at a percentage of 0.5% was found to be relatively greater compared to NDC (26.5%) and NFC (73%) with a percentage of 0.25% compared to pure PMMA at 600 nm. These results suggest that inadequate dispersion and agglomeration of cellulose nanofibers in the nanocomposites lead to reduced transparency. On the other hand, further addition of NFC generated a significant decrease of composite transparency although NFC exhibited better dispersion in the composites. This could probably be attributed to NFCs relatively large lengths. However, high transparency and uniform dispersion can be achieved by filtering cellulose suspensions prior to solvent casting (Nogi et al. 2009).
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Figure 3 shows the thermal degradation behavior of PMMA, cellulose nanomaterials, and PMMA/cellulose nanocomposites. The initial weight loss below 200 °C observed for CNFs attributed to removal of moisture from the cellulose. The thermal degradation that occurred between temperatures of 200−380 °C is attributed to the depolymerization of hemicellulose (for NFC) and cleavage of glycosidic linkages of cellulose for all cellulose nanofibers (Lee et al., 2012). Figure 3 also shows that thermal degradation behavior of CNC, NFC and NDC were very similar, whereas the onset degradation temperature of CNC was higher than those of NFC and NDC. The degradation onset temperatures corresponding to CNC, NFC and NDC were 287°C, 273°C and 260°C, respectively. The PMMA and cellulose nanocomposites underwent thermal degradation at a higher temperature than that of cellulose nanofibers. This could be explained via hydrogen bonding interactions between CNC hydroxyl groups and carbonyl groups on the PMMA matrix to facilitate miscibility of polymer blends (Kuo et al., 2008; Dong et al., 2012) where hydrogen bonding had a significant effect on the thermal properties of polymer blends. However, there was no significant difference found in the degradation temperatures of PMMA and cellulose nanocomposites with different loadings probably because of the low contents of cellulose nanofibrils.
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The PMMA began thermal degradation at 305 °C with a mass loss of 10% which indicated its onset decomposition temperature whereas the cellulose nanocomposites degradation peaks varied in the range of 292–325 °C as shown in Figure 4.a. The maximum decomposition temperature of the cellulose nanocomposites was increased 5 to 20ºC with the variation of cellulose nanofibers content whereas CNC (0.25 wt.) and NDC (0.5 wt. %) showed lower decomposition temperature compared to other cellulose nanocomposites. Sample preparations (size, morphology and homogeneity) may influence heat transfer within the samples and thus influence the course of reaction and the thermogravimetric measurements (Bottom 2008). In NDC production, sodium hydroxide was added into this mixture and heated to 80˚ C for 60 min, while stirring to remove any soluble polysaccharides. Residues of the polysaccharides remaining after processing of NDC could be the reason for the lower thermal stability. The thermal stability 8 Page 8 of 22
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The Tg of the cellulose composites was determined as the inflection point of the specific heat increment at the glass-rubber transition in DSC experiments. Figure 5 shows the effect of cellulose nanofibers on the Tg of the composites measured by DSC. It can be seen that incorporation of low contents of cellulose nanomaterials (0.25% and 0.5%) slightly increased the glass transition temperature of PMMA. (~ 80°C and nanocomposites ~ 84°C). The trend of the glass transition shifting to a slightly higher temperature suggests a restriction of the mobility of polymer chains (Kuo, 2008). This result was expected since cellulose nanomaterials’ hydroxyl groups could interact with the ester functional group (COOCH3) of the PPMA polymeric matrix in its side chain and therefore hinder the rotation of polymeric chains (Kuo, 2008; Jonoobi et al., 2010). A Similar result was reported for cellulose nanoparticle-reinforced PMMA composites in the literature (Han et al. 2014).
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The mechanical properties of the PMMA and the nanocomposites were also studied to examine the influence of cellulose nanofibers on the deformation and viscoelastic behavior of the nanocomposites. Figure 6 shows representative stress–strain curves of neat PMMA and cellulose nanocomposites with varying cellulose nanomaterial contents from DMTA. The nanocomposites with different loading content of CNC and NDC possessed slightly decreased values of flexural strength attributed to higher surface tension for CNC (69 mN/m), NDC (57 mN/m) compared to NFC (41 mN/m). However, NFC had a slight increase in the flexural strength of around 60 MPa while the neat PMMA had a flexural strength of 57 MPa as presented in Figure 6. This could be attributed to PMMA (41 mN/m) has better compatibility with the reinforcing material with lower surface energy (Oporto et al., 2011). The flexural strength values decreased with respect to the strength of the PMMA matrix between approximately 7-13%. Figure 6 indicates that the flexural modulus of the nanocomposites reinforced with NDC at the 0.5 wt. % loading level was 2.56 GPa, 23% greater than to that of the pure PMMA (2.07 GPa). The values of the flexural modulus of elasticity for PMMA, PMMA/NDC (0.25%), PMMA/CNC (0.25%), PMMA/NFC (0.25%), PMMA/NDC (0.5%), PMMA/CNC (0.5%) and PMMA/NFC (0.5%) were 2.07, 2.15, 2.05, 2.13, 2.56, 2.19 and 2.35 GPa, respectively. It can be seen that the NDC reinforced nanocomposites showed better flexural modulus of elasticity compared to NFC and CNC reinforced composites. This increase in flexural modulus of elasticity indicates that the NDC acted as reinforcement in the polymer matrix by transferring load from the polymer matrix to the NDC. It was also observed that from testing, PMMA/NFC composites did not have greater strain to failure than PMMA/CNC and PMMA/NDC nanocomposites.
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of cellulose nanocrystals which were prepared by sulfuric acid hydrolysis was lower than that of the original cellulose because of the presence of acid sulfate groups which decreased the thermal stability by a dehydration reaction (Roman and Winter 2004; Wang et al. 2007). The maximum decomposition temperature increased 20ºC with the NFC (0.25 wt. %) in the nanocomposites. Figure 4.b. shows mass loss at 600°C. The maximum residual mass at 600 °C was obtained from the NDC (about 35 %). There were no residual mass obtained for neat PMMA and the cellulose nanocomposites at 600 °C attributed to the degradation of PMMA starting slowly at 220°C and completely degraded at a temperature higher than 305°C (Pielichowski et al. 2005).
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The storage moduli (E′) at room temperature and glass transition temperatures of the PMMA and nanocomposites with different cellulose nanofiber loadings are shown in Table 3. The glass transition of PMMA was detected at around 75°C, the glass transition temperature 9 Page 9 of 22
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values were not significantly changed as the cellulose nanomaterials content was increased since the viscoelastic properties of the composite are strongly influenced by the matrix polymer. The PMMA composites had glass transition temperatures between 73 and 76° C. The storage moduli of the nanocomposites slightly increased with cellulose nanofibers at loading contents of 0.25 wt. % and 0.5 wt. % at room temperature. At low temperatures, the PMMA matrix was very rigid, and therefore no vigorous reinforcing effect from cellulose nanomaterials was detected at the magnitude of the storage moduli of nanocomposites (Han et al. 2014). The storage modulus of neat PMMA at room temperature was 2.73 GPa. However, the addition of 0.25 wt. % of NDC had a positive effect and the storage modulus of the composite (3.04 GPa) increased 12% compared to neat PMMA. This result could be attributed to the higher crystallinity of 71% of the NDC compared to NFC (41%) (Lee at al. 2012). The storage moduli (E′) at the glass transition temperature and the tan δ maximum peak values of the neat PMMA and nanocomposites with different cellulose nanomaterial loadings are shown in Table 3. The storage modulus of the nanocomposites was greatly increased with cellulose nanofibers at loading contents of 0.25 wt. % and 0.5 wt. % at the glass transition temperature. The reinforcement effect of the cellulose nanomaterials was clear at the glass transition temperature because the PMMA matrix became softened and the CNFs restricted the motion of the PMMA chains which promoted the rigidity of the PMMA (Han et al. 2014). The magnitude of the tan δ peak values of the CNC reinforced nanocomposites were significantly decreased around the glass transition temperature. This can probably be attributed to restriction in chain segment mobility in the amorphous region of the polymer (Kiziltas et al. 2011). The highest moduli of the PMMA/CNC nanocomposites containing 0.25 and 0.5 wt. % cellulose were about 46% and 260% of that of pure PMMA at the glass transition temperature. This result confirmed the creation of enhanced reinforcement with the addition of cellulose nanocrystals into the PMMA matrix.
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Figure 7 depicts the FT–IR spectra of neat PMMA and cellulose nanocomposites. It is evident that all the four spectra are similar except for a few changes in the spectra of the nanocomposites. No significant differences were observed between the FTIR spectra of pure PMMA and cellulose nanocomposites. The fingerprint characteristic vibration bands of PMMA appear at 1728 cm-1 C=O stretching mode. The bands at 3100 and 2800 cm–1 correspond to the methylene C–H stretching while the bands at 1350 and 1450 cm–1 are associated with C–H symmetric and asymmetric stretching modes, respectively. The 1240 cm–1 band is assigned to antisymmetric C-C-O stretch and the 1150 cm–1 band corresponds to skeletal vibrations coupled to C-H deformations, while C–C stretching bands are at 1000 and 800 cm–1. Absence of any additional bands by the presence of cellulose nanomaterials other than those of PMMA in the spectrum of PMMA and further they remaining unperturbed in all the four spectra indicate (1) the purity of the polymer obtained, (2) formation of the nanocomposites and (3) no chemical interaction or chemical bond formation between PMMA and cellulose nanomaterials (Sain et al., 2011; Ahmad et al., 2007).
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Figure 8 show the optical images of the of PMMA and cellulose nanocomposites. In neat PMMA, very few partially dissolved granules were observed in Figure 8. It was apparent from the optical micrograph of PMMA/cellulose nanocomposite that some cellulose nanomaterials might have reaggregated among themselves, adhered to the polymer surface and distribution was not uniform. A few potential scenarios may be envisaged. (1) During the reaction, the hydrophilic cellulose nanomaterials migrated to the polymer water interface, which might be 10 Page 10 of 22
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because of either Van der Waal’s attraction or strong hydrogen bonding interaction among themselves. (2) Agglomeration occurred due to the incompatibility between the hydrophilic cellulose surface and hydrophobic nature of the PMMA (Sain et al., 2011; Ahmad et al., 2007).
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The republic of Turkey, The Scientific and Technological Research Council of Turkey (TUBITAK) is greatly acknowledged for support of the scholarship of the researcher Esra Erbas Kiziltas to do this study at the University of Maine. The authors would like to acknowledge the contributions of Justin Crouse, Dr. Jason Bolton, Alex Nash, Donald Gjeta, Dr. Sanjeev Kumar Kandpal, Connie Young Johnson and Chris West whose hard work made this paper possible. The authors would also like to thank U.S. Army Corps of Engineers, Engineer Research and Development Center project 912HZ-07-2-0013 and Maine Agricultural and Forest Experiment Station (MAFES) project ME09615-08MS and the Wood Utilization Research Hatch 2007-2008 project.
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4. Conclusions PMMA-based transparent nanocomposites were successfully prepared by solution casting with the reinforcement of cellulose nanofibers. The transmittance of PMMA/cellulose nanocomposites was reduced with increased loading of cellulose nanofibrils. The UV-visible light transmittance of the nanocomposites decreased by 9% and 27% with the addition of 0.25 wt. % CNC and NDC respectively at 600 nm. Thermogravimetric analysis indicated retained thermal stability of the transparent composites. The maximum decomposition temperature was increased 20ºC with an NFC content of 0.25 wt. % in the nanocomposites. The maximum residual mass of the NDC was about 35 % obtained at a maximum temperature of 600 °C. The DSC measurements also indicated that the presence of cellulose nanofibers slightly changed the Tg of the PMMA matrix. The storage modulus of the PMMA/CNC nanocomposite containing 0.25 and 0.5 wt. % cellulose increased about 46% and 260% compared to that of the pure PMMA at the glass transition temperature, respectively. The flexural modulus of the nanocomposites reinforced with NDC at the 0.5 wt.% loading level was 2.56 GPa, which was 23% greater than the pure PMMA (2.07 GPa). The NFC composites showed enhanced flexural strength of around 60 MPa compared to those comprised of CNC and NDC. These results imply that at even low cellulose nanomaterials content enhanced thermal and mechanical properties of the rigid-thermoplastic PMMA can be obtained and these serve as promising reinforced nanomaterials for the production of transparent nanocomposites.
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Jonoobi M., Aitomäki Y., Mathew A.P., & Oksman K. (2013). Thermoplastic polymer impregnation of cellulose nanofibre networks: Morphology, mechanical and optical properties. Composites: Part A 58:30-35. Juntaro J. (2009). Environmentally friendly hierarchical composites. PhD Thesis, London, England: Department of Chemical Engineering and Chemical Technology. University of London, London. Kim H.C., Lee S.E., Kim C.G. & Lee J.J. (2005) Mechanical improvement of multi-walled carbon nanotube /poly (methyl methacrylate) composites. Key Engineering Materials 297-300: 2545-2550. Kiziltas A., Gardner D.J., Han Y., Yang H-S. (2011) Dynamic mechanical behavior and thermal properties of microcrystalline cellulose (MCC)-filled nylon 6 composites Thermochimica Acta 519: 38-43. Erbas Kiziltas E., Kiziltas A. & Gardner D.J. (2015) Synthesis of bacterial cellulose using hot water extracted wood sugars. Carbohydrate Polymers, 124:131-138. Kiziltas, A., Nazari, B., Gardner, D.J. & Bousfield, D.W. (2014). Polyamide 6–cellulose composites: effect of cellulose composition on melt rheology and crystallization behavior. Polymer Engineering & Science, 54, 739-746. Klemm D., Kramer F., Moritz S., Lindstrom T., Ankerfors M., Gray D. & Dorris A. (2011) Nanocelluloses: a new family of nature-based materials Angewandte Chemie International Edition 50:5438-5466. Kuo S. W. (2008). Hydrogen bonding in polymer blends. Journal of Polymer Research 15:459486. Lee K.-Y., Blaker J.J. & Bismarck A. (2009). Improving the properties of nanocellulose / polylactide composites by esterification of nanocellulose. Can it be done? In 17th International Conference on Composite Materials, Edinburgh, UK. Lee Koon-Yang., Tammelin Tekla., Schulfter K., Kiiskinen H., Samela J. & Bismarck A. (2012) High performance cellulose nanocomposites: comparing the reinforcing ability of bacterial cellulose and nanofibrillated cellulose. ACS Applied Materials & Interfaces 4:4078-4086. Littunen K., Hippi U., Saarinen T. & Jukka Seppälä (2013). Network formation of nanofibrillated cellulose in solution blended poly (methylmethacrylate) composites. Carbohydrate Polymers 91:183-190. Liu H.Y., Liu D.G., Yao F. & Wu, Q.L. (2010) Fabrication and properties of transparent polymethylmethacrylate/cellulose nanocrystals composites. Bioresource Technology 101:56855692. Nair S.S., Zhu J.Y., Deng Y. & Ragauskas A.J. (2014) Hydrogels prepared from cross-linked nanofibrillated cellulose. ACS Sustainable Chemistry & Engineering DOI: 10.1021/sc400445t. Nishino T., Takano K. & Nakamae K. (1995) Elastic modulus of the crystalline regions of cellulose polymorphs. Journal of Polymer Science Part B- Polymer Physics 33:1647-1651. Nogi M. & Yano H. (2008) Transparent nanocomposites based on cellulose produced by bacteria offer potential innovation in the electronics device industry. Advanced Materials 20:1849-52. Nogi M., Handa K., Nakagaito A.N. & Yano, H. (2005) Optically transparent bionanofiber composites with low sensitivity to refractive index of the polymer matrix. Applied Physics Letters 87:243110-243112. Nogi M., Iwamoto S., Nakagaito A. N. & Yano H. (2009) Optically transparent nanofiber paper. Advanced Materials 21:1595-1598.
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605 606 607 608 609 610 611 612 613 614
Table 1. Dimensions and mechanical properties of nanoscale cellulose fibers compared to microcrystalline cellulose. Table 2. UV-visible transmittances of the neat PMMA and cellulose nanocomposites at 400 nm, 600 nm and 800 nm. Table 3. Glass transition temperature (Tg), tan δ maximum peak values (tan δ max. peak), storage modulus at room temperature (SM at RT) and glass transition temperature (SM at Tg) for neat PMMA and cellulose nanocomposites.
an
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586 587 588 589 590 591 592 593 594 595 596 597 598 599 600 601 602 603 604
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LIST OF TABLES:
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Table 1. Dimensions and mechanical properties of nanoscale cellulose fibers compared to microcrystalline cellulose. Material BC
CNC CNF MCC
Length (nm) >1000
100-600 >1000 >1000
Diameter (nm) 24-84
2-20 10-40 >1000
Aspect Ratio (l/d)
Tensile Strength (GPa) 15-35
Modulus of Elasticity (GPa) 114-300
Density (g/cm3) 1.25
7.5 3 -
120-143 138-141 25
1.5 1.45
∼50 10-100 100-150 ∼1
615 616 617 15 Page 15 of 22
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Table 2. UV-visible transmittances of the neat PMMA and CNF nanocomposites at 400 nm, 600 nm and 800 nm. Sample Name Transmittances (%) 400 nm 600 nm 800 nm Neat PMMA 90 79 81 0.25 %NFC 22 21 29 0.25 %CNC 74 72 86 0.25 %NDC 50 58 75 0.5 %NFC 8 4.5 7 0.5 %CNC 56 55 69 0.5 % NDC 15 15 20
625 626 627 628
an
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cr
618 619 620 621 622 623 624
630
LIST OF FIGURES:
631 632 633 634 635 636 637 638 639 640 641 642
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Table 3. Glass transition temperature (Tg), tan δ maximum peak values (Tan δ max. peak), storage modulus at room temperature (SM at RT) and glass transition temperature (SM at Tg) for neat PMMA and cellulose nanocomposites. Group Name Tg (°C) Tan δ max. peak SM at RT (GPa) SM at Tg (GPa) PMMA 71.87 (0.76) 0.99 (0.02) 2.73 (0.20) 0.023 (0.004) 0.25% NFC 75.51 (1.89) 0.96 (0.01) 2.77 (0.07) 0.024 (0.001) 0.25% CNC 78.67 (1.40) 0.81 (0.05) 2.80 (0.08) 0.034 (0.002) 0.25%NDC 74.35 (1.90) 0.96 (0.02) 3.04 (0.09) 0.030 (0.001) 0.5%NFC 72.25 (1.26) 0.86 (0.02) 2.97 (0.04) 0.032 (0.001) 0.5%CNC 76.41 (0.68) 0.65 (0.04) 2.88 (0.15) 0.060 (0.006) 0.5%NDC 72.72 (1.91) 0.97 (0.03) 2.91 (0.11) 0.030 (0.002) Parenthesis indicates standard deviation.
Figure 1. Photographs of the pure PMMA and CNF nanocomposites placed on a background paper for demonstrating transparency. Figure 2. UV–visible transmittance spectra of the neat PMMA and CNF nanocomposites. Figure 3. TGA curves of the PMMA, cellulose nanofibers, and CNF nanocomposites. Figure 4. a) The temperature at 10% and 50% M.L. and b) mass loss at 600 °C for neat PMMA, cellulose nanofibers and CNF nanocomposites from their TGA curves, respectively. Figure 5. DSC thermograms of the neat PMMA and CNF nanocomposites. Figure 6. Stress-strain behavior and flexural strength (bars) and flexural modulus of elasticity (lines) of neat PMMA and CNF nanocomposites, respectively. Figure 7. FT-IR spectra of the neat PMMA and cellulose nanocomposites. Figure 8. Optical micrographs of the neat PMMA and cellulose nanocomposites.
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643 PMMA+0.5wt% NFC
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PMMA+0.25wt% NFC
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PMMA Sheet
PMMA+0.25wt% CNC 644 645 646 647
PMMA+0.25wt% NDC
Figure 1. Photographs of the pure PMMA and CNF nanocomposites placed on a background paper for demonstrating transparency.
17 Page 17 of 22
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Figure 2. UV–visible transmittance spectra of the neat PMMA and CNF nanocomposites.
M
648 649 650 651
d
100
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60
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Mass Loss (%)
80
Neat PMMA NFC CNC NDC 0.25% NFC 0.25% CNC 0.25% NDC 0.5% NFC 0.5% CNC 0.5% NDC
40
20
0
100
652 653 654
200
300
400
500
600
Temperature (°C) Figure 3. TGA curves of the PMMA, cellullose nanofibers, and CNF nanocomposites.
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380
(a)
ip t
340
320
cr
300
260
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0.5% NFC
M
0.255 NDC
d 0.5% CNC
(b)
0.5% NFC
0.255 NDC
te 0.25% NFC
NDC
PMMA
50
CNC
60
NFC
70
0.25% CNC
90
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Mass Loss at 600°C (%)
100
656 657 658 659
0.25% CNC
0.25% NFC
NDC
CNC
NFC
PMMA
Group Name
655
0.5% NDC
Temp. at 50% M.L. Temp. at 10% M.L.
240
80
us
280
0.5% NDC
Temperature (°C)
360
Group Name
Figure 4. a) The temperature at 10% and 50% M.L. and b) mass loss at 600 °C for neat PMMA, cellulose nanofibers and CNF nanocomposites from their TGA curves, respectively.
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Figure 5. DSC thermograms of the neat PMMA and CNF nanocomposites.
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660 661 662 663 664 665 666 667 668
20 Page 20 of 22
6e+7
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4e+7
cr
3e+7 Neat PMMA 0.25% NFC 0.25% CNC 0.25% NDC 0.5% NFC 0.5% CNC 0.5% NDC
2e+7
us
0 0
2
4
8
Strain (%)
10
d
2.5
2.0
1.5
1.0
0.5
0.5% NDC
0.5% CNC
0.0 0.5% NFC
0.25% NDC
PMMA
0
0.25% CNC
20
0.25% NFC
40
3.0
te
60
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Flexural Strength (MPa)
M
669
670 671 672 673 674
6
an
1e+7
Flexural Modulus of Elasticity (GPa)
Stress (Pa)
5e+7
Group Name Figure 6. Stress-strain behavior and flexural strength (bars) and flexural modulus of elasticity (lines) of neat PMMA and CNF nanocomposites, respectively.
21 Page 21 of 22
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Figure 7. FT-IR spectra of the neat PMMA and cellulose nanocomposites.
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675 676 677 678
679 680 681
Figure 8. Optical micrographs of the neat PMMA and cellulose nanocomposites. 22 Page 22 of 22
