A dielectric relaxation study of starch–water and starch–glycerol films

Abstract

The use of starch as a solid polymer electrolyte (SPE) has gained interest recently due to its desirable qualities that include biodegradability, low production cost, and good physical and chemical properties [1, 2]. Starch is a natural polymer that is present in plants in the form of granules. It is found in the form of granules that vary in size (1– 100 μm). Starch consists of amylose and amylopectin. Amylose is a linear or lightly branched (1→4)-linked αglucan of MW 10–10 while amylopectin is a highly branched molecule of MW 10–10 containing numerous (1→4)-linked α-glucan chains linked (1→6) [3]. Linear polymers typically crystallize more readily than branched polymers [4]. As starch consists of both amylose and amylopectin, with only amylose contributing to the crystallinity, it becomes very essential to understand the mechanisms that govern the dynamics of the polymer molecules along with the role of plasticizers that contain charged groups. Additionally, the presence of networks in the films can contribute significantly to important physical properties like electrical conductivity. Dielectric relaxation spectroscopy (DRS) has been found to be a very useful tool in studying the polymer dynamics of polysaccharides [5, 6]. Dielectric spectroscopy can help us understand better the dynamics of the molecules and the mechanisms that occur at the interphase of crystalline and amorphous regions which cannot be explored by other techniques. Dielectric relaxation spectroscopy is a useful way to study the changes in the molecular dynamics of polysaccharides influenced by its interactions with water [7–11]. The dielectric characteristics of starch have been shown to be highly influenced by the presence of water [12, 13]. Butler et al. have identified different relaxations in starch by studying solid starch and dextran [13]. They have shown that at low temperatures (from −120 to −90 °C), a relaxation (γ1) obeying Arrhenius behavior is observed and it was attributed to motions of small parts of amylose, amylopectin, and dextran. The typical activation energies lay from 50 to 65 kJ mol. At temperatures lower than that, another relaxation (γ2) exists and was observed only in dry starch [13]. Its presence was attributed to the rotation of methylol group present on the amylose and amylopectin molecules in starch. At temperatures between 0 and 50 °C, another local relaxation (β) was shown to exist that was highly dependent upon water content. Its presence was attributed to the increase in the mobility of large segments of amylose and amylopectin molecules. Further, Butler et al. have shown the presence of another relaxation (α) between 60 and 80 °C in granular starch and was attributed to the increase of mobility of amylose and amylopectin molecules upon swelling of and release of starch molecules during gelatinization. Starch films made from using water are usually very brittle. Glycerol is one commonly employed plasticizer [14–18]. Addition of glycerol as a plasticizer greatly reduces the Tg of the films, making it Electronic supplementary material The online version of this article (doi:10.1007/s11581-011-0636-1) contains supplementary material, which is available to authorized users.