Montmorillonite–levan nanocomposites with improved thermal and mechanical 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 10–15wt% water. XRD patterns from levan–MMT composites indicate an MMT interlayer spacing 0.62nm 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 (5–10wt% MMT) to induce an isotropic–nematic transition in MMT platelet orientation.