Imaging and thermal studies of wheat gluten/poly(vinyl alcohol) and wheat gluten/thiolated poly(vinyl alcohol) blends.

The morphology of wheat protein (WG) blends with polyvinyl alcohol (PVA) and respectively with thiolated polyvinyl alcohol (TPVA) was investigated by atomic force (AFM) and transmission electron microscopy (TEM) as well as by modulated dynamic scanning calorimetry (MDSC). Thiolated additives based on PVA and other substrates were previously presented as effective means of improving the strength and toughness of compression molded native WG bars via disulfide-sulfhydryl exchange reactions. Consistent with our earlier results, AFM and TEM imaging clearly indicate that the addition of just a few mole percent of thiol to PVA was sufficient to dramatically change its compatibility with wheat protein. Thus, TPVA is much more compatible with WG and phase separates into much smaller domains than in the case of PVA, although there are still two phases in the blend: one WG-rich phase and another TPVA-rich phase. The WG/TPVA blend has phase domains ranging in size from 0.01 to 0.1 microm, which are roughly 10 times smaller than those of the WG/PVA blend. MDSC further illustrates the compatibilization of the protein with TPVA via the dependence of the transition temperatures on composition.

Wheat gluten-thiolated poly(vinyl alcohol) blends with improved mechanical properties.

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.