Wheat-Gluten-Based Natural Polymer Nanoparticle Composites Xiaoqing Zhang,* My Dieu Do, Katherine Dean, Pam Hoobin, and Iko M. Burgar Commonwealth Scientific and Industrial Research Organization Manufacturing & Materials Technology, Private Bag 33, Clayton South MDC, Clayton South, Victoria 3169, Australia Received September 28, 2006; Revised Manuscript Received November 8, 2006 A series of wheat-gluten-based nanocomposites were produced by dispersing Cloisite-30B nanoclay particles into plasticized wheat gluten systems under thermal processing conditions. The exfoliation of the nanoparticles as confirmed by wide-angle X-ray diffraction and transmission electron microscopy has resulted in significant enhancement of the mechanical properties for both deamidated proteins and vital gluten systems under 50% relative humidity (RH). Such strength improvement was also pronounced for wheat gluten (WG) systems under a high humidity condition (RH ) 85%). A similar level of further strength enhancement was obtained for the WG systems that had been strengthened by blending with poly(vinyl alcohol) (PVA) and cross-linking with glyoxal. Although the nanoclay modifier, a quaternary ammonium, caused an additional plasticization to the materials, the interactions between the gluten matrix and the nanoparticles were predominant in all of these nanocomposites. A solid-state NMR study indicated that the polymer matrix in all of these nanocomposites displayed a wide distribution of chain mobilities at a molecular level (less than 1 nm). The interactions between the nanoparticles and the natural polymer matrix resulted in motional restriction for all components in the mobile phases including lipid, plasticizers, and plasticized components, although no significant influence from the nanoparticles was obtained in the mobility of the rigid phases (unplasticized components). On a scale of 20-30 nm, the deamidated protein systems tended to be homogeneous. The small domain size of the matrix resulted in modifications of the spin- lattice relaxation of these systems via spin diffusion. The residual starch seemed to remain in a relatively larger domain size in WG systems. The nanoparticles could enhance the miscibility between the starch and the other components in the WG nanocomposite, but such miscibility enhancement did not occur in the WG/PVA blend and the cross-linked system. These polymer matrixes were still heterogeneous on a scale of 20-30 nm. 1. Introduction Development of renewable polymer materials from agriculture feedstocks has become a great challenge for material scientists due to increasing environmental concerns and diminishing petrochemical resources. Wheat gluten is one of the most important natural polymer resources due to its good viscoelastic properties, strong tensile strength, excellent gas barrier proper- ties, low price, and constant quality with large-scale availability. Thermal processing is an efficient method to produce polymer materials; however, it is normally difficult to achieve for wheat gluten due to strong inter- and intramolecular interactions in conjunction with some extent of cross-linking among the protein macromolecules. A large amount of plasticizers is usually required in the process to reduce the strong inter-/intramolecular interactions and increase the mobility of the protein chains for reaching sufficient flexibility and extensibility of the materials. 1-8 Such plasticization significantly weakens the strength of the materials; 9-12 therefore, enhancement of the mechanical performance of plasticized wheat-gluten-based polymer materials plays a key role in extending the application of the materials. Polymer/layered clay nanocomposites have attracted great interest from academic research and industries because signifi- cant improvement in many aspects of material performance can be obtained with a very low dosing amount of nanoparticles. 13-16 The main reason for such property improvement as compared to pure polymers or conventional composites (micro- or macrocomposites) is due to the significantly strengthened interfacial interactions between the polymer matrix and the nanoparticles (layer thickness on the order of 1-2 nm, diameter of 30-2000 nm) when such particles are well dispersed in the polymer matrix. Layered silicates such as montmorillonite, hectorite, and saponite are frequently used. Isomorphic substitu- tions in these layered clay sheets lead to a net negative charge necessitating the presence of cationic counterions in the inter- sheet region or gallery spacing. These counterions are usually exchanged with organic alkyl ammonium modifiers that would enhance the exfoliation in a polymer matrix. 13,17 Generally two types of polymer/layered clay nanocomposites are achievable and described on the basis of the particle dispersion, intercalated and exfoliated nanocomposites, 14 although a flocculated model was also mentioned. 18 The methodology also has been used to thermally process natural polymers such as starch and soy proteins. 19-23 Those fully or partially exfoliated nanocomposites achieved a remarkable improvement in tensile strength, stiffness, toughness, modulus, and barrier properties. Ultrasonic energy was also applied to promote the intercalation and exfoliation of unmodified montmorillonite clays when melt blending nanoparticles with biodegradable polymers. 22 The focus of this work was to disperse a commercially available clay nanoparticle into plasticized wheat gluten/proteins to form nanocomposites and then to examine the enhancement of mechanical performance and the changes in phase structures of the materials. Alkyl-ammonium-modified montmorillonite clay (Cloisite-30B) 24 was used as the nanoparticle. The alkyl ammononium groups in the layered clay particles can improve the wetting characteristics of the polar protein chains, which * Author to whom correspondence should be addressed. Phone: +63 95452653. Fax: +613 95441128. E-mail: Xiaoqing.Zhang@csiro.au. 345 Biomacromolecules 2007, 8, 345-353 10.1021/bm060929x CCC: $37.00 © 2007 American Chemical Society Published on Web 01/19/2007