pH Effect on the Mechanical Performance and Phase Mobility of Thermally Processed Wheat Gluten-Based Natural Polymer Materials

Abstract

The mechanical properties, phase composition, and molecular motions of thermally processed wheat gluten- (WG-) based natural polymer materials were studied by mechanical testing, dynamic mechanical analysis (DMA), and solid-state NMR spectroscopy. The performance of the materials was mainly determined by the denaturization and cross-linking occurring in the thermal processing and the nature or amount of plasticizers used. The pH effect also played an important role in the materials when water was used as the only plasticizer (WG-w). Alkaline conditions modified the chemical structure of WG, possibly via deamidation; enhanced the thermal cross-linking of WG macromolecules to form a more stable aggregation structure; and promoted intermolecular interactions between water and all components in WG (proteins, starch, and lipid), resulting in a strong adhesion among different components and phases. The saponification of lipid under alkaline conditions also enhanced the hydrophilicity of lipid and the miscibility among lipid, water, and WG components. However, when glycerol was used with water as a plasticizer (WG-wg), the phase mobility and composition of the materials mainly depended on the content of glycerol when the water content was constant. During thermal processing under either acidic or alkaline conditions, glycerol was unlikely to thermally cross-link with WG as suggested previously. The advanced mechanical performance of the WG-wg materials was attributed to the nature of hydrogen-bonding interactions between glycerol and WG components in the materials. This caused the whole material to behave like a strengthened "cross-linked" structure at room temperature due to the low mobility of glycerol. The pH effect on phase mobility and compositions of WG-wg systems was not as significant as that for WG-w materials.

Formation of Wheat‐Protein‐Based Biomaterials through Polymer Grafting and Crosslinking Reactions to Introduce New Functional Properties

“…Figure 5 shows the 13 C CP/MAS and SPE spectra of WP and WP/AG materials. The WP component was mainly observed in the CP/MAS spectra while lipid and some mobile protein components were detected in the SPE spectra, the same as those reported previously 31–37. Note that the AG segments (e.g.…”

“…Figure 6 shows the MAS 1 H NMR spectra of WP and WP/AG samples measured via CPMG pulse sequence at two typical τ times. Mainly a broad peak of water at 4.8 ppm and a group of sharp lipid signals were detected for WP when a short τ time (40 µs) was used, the same as those reported previously for plasticized WP systems 15, 34–36. For the WP/AG systems, the broad intensity of AG was overlapping with the water resonance and the intensity around 4.1 ppm became more pronounced as the AG content increased in the systems.…”

“…The DMA results of the thermally processed WP/AG samples are shown in Figure 4 and some DMA key data are listed in Table 2. The general trends of the DMA results were similar to those of plasticized WP‐based materials 15, 31–37. Note that all E' data of WP/AG materials were relatively lower than those of WP, indicating the reduction of rigidity due to the contribution of AG segments in WP/AG networks.…”

“…Different AG content in the systems could also result in different effects on the network structures, molecular motions of each component and phase structures of the whole materials. Solid‐state NMR technique has provided a powerful tool to examine these behaviours of the materials as demonstrated in our previous work on various natural polymer systems 31–37…”

Lower critical solution temperatures of thermo‐responsive poly(N‐isopropylacrylamide) copolymers with racemate or single enantiomer groups

“…Smart materials that can respond to environmental stimuli such as temperature,1–8 pH,9–12 ions or molecules,8, 13–15 light16–19 and electric fields20–22 have attracted increasing interest over the last decade. Among smart materials, thermo‐responsive chiral recognition materials1–5 have a wide variety of potential applications, including in chiral separation membranes, biomimetic membranes, chromatographic systems and so on.…”