Wheat Gluten-Thiolated Poly(vinyl alcohol) Blends with Improved Mechanical Properties Rebecca M. Dicharry, Peng Ye, Gobinda Saha, Eleanor Waxman, Alexandru D. Asandei, and Richard S. Parnas* University of Connecticut, Institute of Materials Science, 97 North Eagleville Road U-3136, Storrs, Connecticut, 06269-3136 Received May 4, 2006; Revised Manuscript Received August 3, 2006 A multifunctional macromolecular thiol (TPVA) obtained by esterification of poly(vinyl alcohol) (PVA) with 3-mercaptopropionic acid was characterized by a combination of NMR, IR, transmission electron microscopy (TEM), and differential scanning calorimetry (DSC), and was used as a wheat gluten (WG) reactive modifier. The effect of TPVA molecular weight (M w ) 2000, 9500, 50 000, and 205 000) and blend composition (5, 20, and 40% w/w TPVA/WG) on the mechanical properties of compression-molded bars indicates that TPVA/WG blends increase the fracture strength by up to 76%, the elongation by 80%, and the modulus by 25% above WG. In contrast, typical WG additives such as glycerol and sorbitol improve flexibility but decrease modulus and strength. Preliminary investigations of suspension rheology, water uptake, molecular weight distribution and electron microscopy of TPVA/WG and PVA/WG blends illustrate the different protein interactions with PVA and TPVA. Further work is underway to determine whether TPVA and WG form protein conjugates or microphase-separated morphologies. Introduction Commodity materials made from naturally occurring biopoly- mers can be both a cost-effective and environmentally sound alternative to traditional petroleum-based polymers. Wheat gluten (WG), when molded into plastic, has high stiffness in the range of epoxy (E ∼ 1 GPa 1 and 3.5 GPa 2 ) and reasonable strength (20-35 MPa 1 and 50 MPa 2 ) in comparison with other bioplastics. 3 However, WG is quite brittle, leading to this and previous investigations into additives to improve its ductility. In particular, protein-based materials are particularly attractive because of their low cost and wide variety of functional groups associated with the amino acid residues. Structural modifications via physical (temperature or pressure treatment), enzymatic (hydrolysis, attachment of amines and deamidation), or chemical (cross-link) methods 4 are often used to tailor protein properties for different applications. Among them, chemical modification of the amino acid functional groups is a very powerful and versatile tool to improve protein properties, such as water resistance and mechanical performance. Since molded WG is brittle, hydrophilic plasticizers such as water, glycerol, and sorbitol are used to screen noncovalent interactions. Plasticizers decrease the glass transition temperature and increase the flexibility of WG. Water is the most effective plasticizer, and its effects on WG are well-known. 5-17 While glycerol is an effective plasticizer, it migrates to the surface during storage, and thus WG films lose flexibility within a few days. 4,18 Sorbitol is less effective but is retained during storage. 19 Diethanolamine and triethanolamine can also plasticize WG, 20 and the plasticizing effect of a series of saturated fatty acids was approximated 21,22 to be intermediate between water and glycerol. Plasticized WG has high flexibility, but very low modulus and strength. Thus, to increase strength, proteins can be cross- linked by several methods. Chemical and radiation treatments were applied to both the film-forming solution (pretreatment) and on the film (posttreatment). 23-28 Formaldehyde is a well- known cross-linking agent for many types of proteins derived from pea, 24 cottonseed, 25 corn zein, 29 and wheat, 45-47 and it generally decreases the elongation and increases the tensile strength. However, because of its toxicity, its use is limited. Ultraviolet and γ radiation can cross-link proteins, but the efficiency of the radiation in the cross-linking reaction depends on the protein source; soy protein is rich in tyrosine and phenylalanine and is thus sensitive to UV radiation, 23 which is not the case for pea 24 and gluten films. 30 In addition to the use of plasticizers, various polymers were also blended with WG. This includes aliphatic polyesters, 31 poly- (hydroxy ester ether), 32 maleic anhydride-modified polycapro- lactone, 33 poly(ethylene-co-vinyl acetate)/poly(vinyl chloride), 34 and cassava starch. 35 The blends were mixed using a screw extruder. Mechanical properties were investigated on bars formed by injection molding of the extrudate pellets. In most cases, the mechanical properties were satisfactory, but the elongation was reduced compared with that of the pure polymer additive. To circumvent these problems, other methods of improving the mechanical properties of WG are investigated herein. Since the disulfide bond plays a central role in the wheat protein network, chemical modification causes significant changes in the overall properties of wheat protein. This is evidenced by our recent work of modifying wheat protein with a low molecular weight (M w ) 1247) three-arm, thiol-terminated poly- (ethylene oxide). 1 However, custom-designed additives are very expensive, and the cost is a concern when making a competitive alternative to commodity plastics. In this work, we present a new polythiol additive based on thiolated poly(vinyl alcohol) (TPVA) via its esterification with 3-mercaptopropionic acid. The mechanical properties of TPVA/ WG and PVA/WG blends were studied. Evidence of the chemical interaction of the TPVA with WG was revealed by * Corresponding author. E-mail: rparnas@ims.uconn.edu. 2837 Biomacromolecules 2006, 7, 2837-2844 10.1021/bm060432n CCC: $33.50 © 2006 American Chemical Society Published on Web 09/19/2006