Preparation and properties of biodegradable starch–clay nanocomposites

Abstract

Well-dispersed starch–clay nanocomposites were prepared by adding a dilute clay dispersion to a solution of starch followed by coprecipitation in ethanol. The clay didn’t significantly influence the type of crystalline structure of starch molecules although the amount of crystallinity appears to be somewhat lower in the nanocomposites. The nanocomposites show improved modulus and strength without a decrease in elongation at break. The increase in modulus and strength is 65% and 30%, respectively for the nanocomposite containing 5 wt.% clay compared to the unfilled starch materials. Further increases in clay result in deterioration in properties most likely due to poorer clay dispersion and lower polymer crystallinity. As the amount of water increases, the modulus of both pure starch and starch nanocomposites decreases, although the change is less pronounced in the nanocomposites suggesting that the addition of clay to form nanocomposites can improve the stability of starch-based products during transportation and storage.

Introduction

Most synthetic polymers are produced from petrochemicals and are harmful to nature. Their synthesis produces hazardous waste and these materials are not easily degradable, causing environmental problems. Plants are potential sources for a wide variety of polymers which are renewable and ecologically friendly. The agro-based biopolymers are edible, biocompatible and biodegradable, which make them superior to synthetic polymers and particularly useful in disposable plastics, food, and medicine applications. Starches, the dominant carbohydrate reserve materials of higher plants, are abundant and relatively inexpensive biopolymers. They contain stored energy from sun and can be produced steadily without fears of exhaustion. Furthermore, in the presence of plasticizers, starches can be processed using conventional thermoplastic techniques. However, the mechanical properties of starch-based materials are often sensitive and responsive to the changes in the environment such as humidity, temperature, and pH. This drawback has to be overcome to obtain high performance biomaterials.

Biopolymer–clay nanocomposites are a new class of materials with potentially improved mechanical properties. These composites are prepared by addition of low amounts of clay to the biopolymer matrix (Zhao, Torley, & Halley, 2008). The main challenge for preparing nanocomposites is the nanoscale dispersion of clay in the biopolymer matrix. Montmorillonite is the most commonly used natural clay and has been successfully applied in numerous nanocomposite systems (Giannelis, 1996, Paul and Robeson, 2008, Pavlidou and Papaspyrides, 2008, Raquez et al., 2008, Ray and Okamoto, 2003). However, most of the reported starch–clay nanocomposites suffer from poor dispersion, which is required for obtaining high performance materials (Bagdi et al., 2006, Chiou et al., 2006, Pandey and Singh, 2005, Park et al., 2003, Park et al., 2002, Wilhelm et al., 2003a, Wilhelm et al., 2003b). To improve the dispersion, organic cations such as stearyl dihydroxyethyl ammonium chloride (Bagdi et al., 2006), distearyl dimethyl ammonium chloride (Bagdi et al., 2006) and quaternary ammonium-modified starches (Chivrac et al., 2008) were used to exchange with the sodium ions residing in the interlayer of pristine montmorillonite. The more the modifier is compatible with starch, the more it facilitates clay dispersion (Chivrac et al., 2008). However, miscibility is still an issue and clay dispersion remains a challenge (Bagdi et al., 2006, Chiou et al., 2006, Park et al., 2002, Park et al., 2003).

Plasticizer is another important factor that influences the properties of starch–clay nanocomposites. The melting temperature of a starch granule is close to its degradation temperature. Hence, plasticizers are required to destroy the inter-molecular hydrogen bonding in the crystalline regions of starch granules and decrease the melting temperature during thermoplastic processing. Small molecules such as glycerol and water that can form hydrogen bonds with starch can serve as plasticizers in starch-based materials. A part of the plasticizers can potentially be absorbed on the clay surface, influencing the interfacial strength and stress transfer between the polymeric matrix and clay (Chivrac, Pollet, Schmutz, & Averous, 2008). Depending on the relative humidity, the moisture content (plasticizer concentration) of the starch–clay nanocomposites will change and consequently influence the Young’s modulus of the materials (Avella et al., 2005, Chivrac, Pollet et al., 2008, Huang et al., 2006, Ma et al., 2007, Perez et al., 2008).

In this study, we prepared well-dispersed starch–clay nanocomposites with three different kinds of clay: montmorillonite, chitosan-modified montmorillonite, and laponite. Chitosan-modified montmorillonite is of interest because chitosan is a natural polysaccharide that is compatible with the starch matrix as well as being ion-exchanged in the clay (Darder et al., 2003, Kampeerapappun et al., 2007). Laponite is a synthetic silicate clay with a smaller aspect ratio (20–30 nm in the planar dimension and ∼1 nm in the thickness) and purer than montmorillonite. Well-dispersed nanocomposites were produced, and the silicate dispersion from nanometer to micrometer scales was characterized using XRD, TEM, and SEM. Furthermore, the mechanical properties of starch–montmorillonite nanocomposites with various clay concentrations were investigated. In order to understand the interactions of starch matrix with plasticizers and clay, mechanical properties of the nanocomposites were evaluated with respect to relative humidity (0–75% RH) which also provides an insight to their mechanical response to the environmental changes.