Preparation and characterization of transparent PMMA–cellulose-based nanocomposites
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, Manzani, Messaddeq and Ribeiro, 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 (Nishino, Takano, & Nakamae, 1995; Tanpichai et al., 2012). 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).
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, Oporto, Mills, & Samir, 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 & Sanadi, 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 (Nair et al., 2014, Turbak et al., 1983). Cellulose nanocrystals (CNCs), rod-like or whisker shaped crystalline particles isolated from cellulose, have gained wide attention over a range of research areas (Habibi, Lucia, & Rojas, 2010; Klemm et al., 2011) attributed to its good mechanical properties, unique optical and self assembly properties (Wang et al., 2013a, Wang et al., 2013b. 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 and 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 (Nogi et al., 2005; Yano et al. 2005). Its nano-sized 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 & Yano, 2008). The dimensions and mechanical properties of nanoscale cellulose fibers compared to microcrystalline cellulose are summarized in Table 1. (Chauhan and Chakrabart, 2012, Edge et al., 2000, Eichorn and Young, 2001, Lee et al., 2009, Vitta and Thiruvengadam, 2012a, Vitta and Thiruvengadam, 2012b; Yano, 2010). Although the diameters of cellulose nanomaterials are less than 100 nm in Table 1, purity, surface energy (NFC: 41 mN/m, BC: 57 mN/m and CNC: 69 mN/m) and crystal structure 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).
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 nanofiller-reinforced 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 (Littunen et al., 2013, Liu et al., 2010). 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, Stoffer, & Barr, 1997). These drawbacks can be overcome via reinforcements with nano- and microfillers (Chen et al., 2009, Nussbaumer et al., 2003, Tang et al., 2006). 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).
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, Henle, & Kaskel, 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 polymer 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, 2011). In the Hajji, Cavaille, Favier, Gauthier, and Vigier (1996) 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, Handa, Nakagaito, & Yano, 2005; Okahisa et al., 2011). Hence, PMMA and cellulose nanofiber reinforced PMMA-based nanocomposites were fabricated by the solvent casting technique in the present study.
It has been recently reported in Jonoobi, Aitomäki, Mathew, and Oksman, (2013) and Wang et al., 2013a, Wang et al., 2013b that mechanical, thermal and optical properties were improved with cellulose nanomaterials. However, it has been also reported that cellulose nanomaterials loading may often deteriorate 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–vis spectroscopy, respectively. These results should allow us to further investigate the industrial application areas for certain types of cellulose nanofibers.
Section snippets
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
Results and discussion
Fig. 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 Fig. 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
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–vis 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
Acknowledgements
The republic of Turkey, The Scientific and Technological Research Council of Turkey (TUBİTAK) 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.
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