Preparation and characterization of electrospun pullulan/montmorillonite nanofiber mats in aqueous solution

Abstract

Pullulan (PULL)/montmorillonite clay (MMT) nanofiber blend mats with various weight ratios have been fabricated by the electrospinning technique in aqueous solution. The PULL/MMT nanofiber mats are characterized by X-ray diffraction (XRD), differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA), field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), Fourier transform infrared (FT-IR) and mechanical measurements. The study shows that the introduction of MMT results in improvement in tensile strength, and thermal stability of the PULL matrix. XRD patterns and TEM micrographs suggest the coexistence of intercalated MMT layers over the studied MMT contents. XRD analyses also reveal an increase of the crystallinity of the blend nanofiber mats with addition of MMT fillers. Moreover, FT-IR divulges that there might be possible interaction occurred between the MMT clay and PULL matrix.

Introduction

Polymer/MMT clay hybrid nanocomposites have attracted great interest due to MMT filled polymer composites often exhibit remarkable improvement in material properties with only a low percentage of MMT fillers added. One of the major findings that have stimulated the interest in MMT filled nanocomposites is the work by Okada and coworkers (1990). They reported that with only a small amount of layered silicate (MMT) added into nylon-6, pronounced improvements in thermal and mechanical properties can be obtained. Subsequently, Vaia and his group (Vaia, Ishii, & Giannelis, 1993) reported that it is possible to melt-mix polymers with layered silicates, without the use of organic solvents. Today, efforts are being conducted globally; using almost all types of polymer matrices to produce MMT based nanocomposites. The main advantages of these nanocomposites are improved thermal and mechanical properties, reduced flammability and better barrier properties comparing to unfilled polymer. The composite studies focus on the method of their preparation, structure characterization, mechanical and thermal properties as well as processing.

Pullulan is an extracellular microbial polysaccharide produced by the fungus-like yeast, Aureobasidium pullulans (Kachhawa et al., 2003, Yuen, 1974). It is a neutral glucan (like amylose, dextran, cellulose), with a chemical structure somewhat depending on carbon source, producing microorganism, fermentation conditions. The basic structure is a linear α-glucan one, made from three glucose units linked α-(1,4) in maltotriose units which are linked in a α-(1,6) way. The three glucose units in maltotriose are connected by an α-(1,4) glycosidic bond, whereas consecutive maltotriose units are connected to each other by an α-(1,6) glycosidic bond. The regular alternation of (1 → 4) and (1 → 6) bonds results in two distinctive properties of structural flexibility and enhanced solubility (Leathers, 1993). The unique linkage pattern also endows pullulan with distinctive physical traits along with adhesive properties and its capacity to form fibers, compression moldings and strong, oxygen impermeable films. The α-(1,6) linkages that interconnect the repeated maltotriose units along the chain are responsible for the flexible conformation and the ensued amorphous character of this polysaccharide in the solid state (Gidley, Cooke, & Ward-Smith, 1993). Pullulan’s solubility can be controlled or provided with reactive groups by chemical derivatization. Due to its excellent properties, pullulan is used as a low-calorie ingredient in foods, gelling agent, coating and packaging material for food and drugs, binder for fertilizers and as an oxidation-prevention agent for tablets. Other applications include contact lenses manufacturing, biodegradable foil, plywood, water solubility enhancer and for enhanced oil recovery (Israilides et al., 1998, Leathers, 2003, Schuster et al., 1993). It is water soluble, insoluble in organic solvents and non-hygroscopic in nature. Its aqueous solutions are stable and show a relatively low viscosity as compared to other polysaccharides. It decomposes at 250–280 °C. It is moldable and spinnable, being a good adhesive and binder. It is also non-toxic, edible and biodegradable.

Electrospinning is a very simple and effective approach to produce nanofibers, including aligned nanofibers and crossbar structures with the diameters ranging from micrometers to few nanometers scale, which may have be found attractive for various applications in biomedical engineering, filtration, protective clothing, catalysis reaction and sensors (Li and Xia, 2004, Reneker and Chun, 1996). In a typical electrospinning process, a high voltage is applied to create electrically charged jets of polymer solutions. The jets dry and form nanofibers, which are collected on a target as non-woven mat. The principle of the electrospinning method is quite simple; the electrostatic field stretches the polymer solution into fibers at the same time as the solvent evaporates. However, the process is difficult to control and several variables have an influence on the properties of the end product. Furthermore, the quality of the fibers is typically inconsistent, for example, the fiber deposition may be uneven or the distribution of fiber diameter may be large. Research on PULL/MMT nanofiber mats by electrospinning technique has not been a focus in enormously. To the best of our knowledge, no reports are available on the morphology and crystalline structure of PULL/MMT nanofiber mats by electrospinning technique.

In this study, we have demonstrated for the first time, ultrafine PULL/MMT nanofiber mats can be fabricated by using the electrospinning technique. The PULL/MMT nanofiber mats are investigated using FE-SEM, TEM, FT-IR, XRD, DSC, TGA, mechanical measurements and the related characterizations are also discussed.