Montmorillonite–levan nanocomposites with improved thermal and mechanical properties

Highlights

• Bacillus sp. levan has a well-defined solution structure and ∼10 MDa molecular weight.

• Good quality MMT–levan nanocomposites can be prepared by controlling T and RH.

• 1 wt% MMT–levan composites have improved thermal stability and sharply increased T_g.

• 5–10 wt% MMT–levan composites show 450% increases in tensile moduli and yield stress.

• The onset of an MMT isotropic–nematic transition may rationalize observed properties.

Abstract

This work reports on the structure and properties of novel nanocomposites composed of exfoliated montmorillonite clay blended with levan, a polysaccharide produced by Bacillus sp. Dry levan is very brittle, making it difficult to obtain stand-alone films. MMT–levan composites were prepared by solution blending in water, coating on plastic surfaces, partial drying at 50 °C, and conditioning in air at 50–60% relative humidity. This process results in freestanding, transparent, and flexible films of pure levan and MMT–levan composites plasticized by 10–15 wt% water. XRD patterns from levan–MMT composites indicate an MMT interlayer spacing 0.62 nm greater than that of the starting MMT, suggesting re-stacking of MMT platelets coated by adsorbed, uncoiled levan molecules. FTIR results suggest that levan adheres to MMT via water-mediated hydrogen bonding between the levan's hydroxyl groups and MMT surface oxygens. MMT–levan composites have improved thermal stability and a well-defined glass transition temperature that increases with MMT loading. The tensile moduli of levan–MMT composites increase by as much as 480% relative to pure levan. The XRD and mechanical property results suggest that MMT reinforces levan through a filler network structure composed of MMT platelets bridged by adsorbed levan molecules, enhanced when the MMT loading becomes high enough (5–10 wt% MMT) to induce an isotropic–nematic transition in MMT platelet orientation.

Introduction

Polysaccharides offer considerable promise as sustainable, biodegradable materials for packaging and other applications (Johansson et al., 2012, Majeed et al., 2013, Tang et al., 2012). Polysaccharides extracted from plants, such as cellulose, hemicellulose, starch, pectin, and chitin, have received much attention (Chivrac et al., 2009, Rhim and Ng, 2007, Sinha Ray and Bousmina, 2005, Sorrentino et al., 2007, Yu et al., 2006), perhaps because their source materials are widely available in mass quantities and at low cost. Exopolysaccharides secreted by microbes often have well-defined structure and high molecular weight (French, 1989), but they have received less attention because they have not been readily available in large quantities and high purity. Advances in microbial fermentation have resulted in increasing availability of high purity, well characterized biopolymers, especially polyhydroxyalkanoates (Johansson et al., 2012, Sinha Ray and Bousmina, 2005, Sorrentino et al., 2007, Tang et al., 2012, Yu et al., 2006). Among the exopolysaccharides, levan has been produced by large-scale fermentation (Bodie et al., 1985, Han and Watson, 1992, Keith et al., 1991, Kim et al., 2005, Küçülaşik et al., 2011, Liu et al., 2010, Poli et al., 2009, van Dyk et al., 2012).

Levan and inulin are fructans, fructose-based polysaccharides produced by many plants and microorganisms (French, 1989). Levan consists of d-fructofuranosyl monomers joined by β(2 → 6) linkages and branched via β(2 → 1) linkages (Supporting information, Fig. S1). Plant fructans have relatively low degrees of polymerization (<100 fructofuranose monomers, or residues) with molecular weights on the order of 10⁴ Da. Microbial levans, on the other hand, typically have higher degrees of polymerization (∼10,000 residues) and molecular weights (∼10⁵–10⁷ Da). Chemical analyses in conjunction with ¹³C NMR indicate that branching occurs on up to 30% of the residues in levan (Seymour et al., 1979, Simms et al., 1990). Electron microscopy images (Ingelman and Siegbahn, 1944, Newbrun et al., 1971), data from light scattering and sedimentation (Bahary et al., 1975, Stivala et al., 1975), small-angle X-ray scattering (Khorramian and Stivala, 1982, Stivala and Khorramian, 1982), and viscometry (Arvidson et al., 2006, Kasapis et al., 1994) all agree that levan molecules in aqueous solutions do not undergo gelation with increasing solution concentration and have a compact, globular structure (spheroidal or ellipsoidal).

Levan's high molecular weight and water solubility make it attractive for various industrial applications, including cosmetics, pharmaceutical coatings, and adhesives (Combie, 2006, Kim et al., 2005, Kang et al., 2009) There have been only a few studies aimed at developing levan as a bio-based plastic for packaging applications. Barone and Medynets (2007) used compression molding and melt extrusion to prepare cohesive, pliable levan films plasticized by glycerol. Levan films containing less than 10 wt% glycerol were too brittle to be characterized by tensile testing. Films containing 10–35% glycerol had tensile moduli less than 0.1 GPa and glass transition temperatures (T_g) ranging from 60 °C (10 wt%) down to 0 °C (30 wt%). These property values are too low to permit the use of levan/glycerol films in packaging or structural applications. Manadhar, Vidhate, and D'Souza (2009) prepared levan fibers by electrospinning from concentrated solutions (60 wt%), but fiber mechanical properties were not reported. The poor mechanical properties of pure levan films are probably related to its molecular structure: despite its high molecular weight, levan's highly branched, compact globular structure does not permit significant intermolecular entanglement. This leads to brittleness in neat levan, and low tensile modulus in glycerol-plasticized levan.

Recent reviews (Chivrac et al., 2009, Johansson et al., 2012, Majeed et al., 2013, Rhim and Ng, 2007, Sinha Ray and Bousmina, 2005, Sorrentino et al., 2007, Tang et al., 2012, Yu et al., 2006) demonstrate that, in many cases, biopolymer nanocomposites have properties superior to those of the corresponding pure biopolymer. Examples of recent work in this vein are studies of xylan (Unlu et al., 2009, Viota et al., 2010) or xyloglucan (Kochumalayil et al., 2013) reinforced with MMT. The present work explores the possibility of improved thermal and mechanical properties for nanocomposites of exfoliated montmorillonite (MMT) dispersed in levan. Improved properties may extend levan's range of applications to include barrier packaging with unique features, such as edible packaging or accelerated biodegradability.