Rheology and structural changes of plasticized zeins in the molten state

• Published in: Rheologica acta 2017-11, Vol.56 (11), p.941-953
• Main Authors: Chaunier, Laurent, Della Valle, Guy, Dalgalarrondo, Michèle, Lourdin, Denis, Marion, Didier, Leroy, Eric
• Format: Article
• Language: English
• Subjects: Characterization and Evaluation of Materials; Chemical and Process Engineering; Chemical Sciences; Chemistry and Materials Science; Complex Fluids and Microfluidics; Crosslinking; Electrophoresis; Engineering Sciences; Equilibrium flow; Extrusion; Food Science; Fused deposition modeling; Gelation; Materials Science; Mechanical Engineering; Melts; Molecular structure; Original Contribution; Polymer melts; Polymer Sciences; Polymers; Proteins; Rheological properties; Rheology; Rheometry; Shear thinning (liquids); Size exclusion chromatography; Sodium dodecyl sulfate; Soft and Granular Matter; Viscoelasticity; Viscosity; Zein
• ISSN: 0035-4511; 1435-1528
• DOI: 10.1007/s00397-017-1045-9

Zeins, storage proteins from maize, are suitable for making biobased thermoplastic materials. The rheological behavior of a commercial zein plasticized with 20 w% glycerol was studied in the molten state by steady-state flow experiments in extrusion conditions and oscillatory rheometry. For low residence times, a shear-thinning viscoelastic behavior was observed, with G ″ exceeding G ′. After 300 s at 130 °C, the complex viscosity | η ∗ | = 7 × 10 3 ω −0.46 was found to be similar to that of thermoplastic polymer melts used in fused deposition modeling. However, the ratio between the exponents of the power laws describing G ′( ω ) and G ″( ω ) did not meet the typical value of 2 for entangled polymer melts. Moreover, for longer residence times, the viscosity increased and a gelation phenomenon was observed with a crossing over of G ′( ω ) and G ″( ω ). Gel times ranged from 6000 s at 120 °C to 1700 s at 150 °C. The evolution of the macromolecular structure assessed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and high-performance size exclusion chromatography suggested that this gelation phenomenon involves various types of covalent and non-covalent cross-links. Disulfide bonds played a significant role in gelation kinetics despite a very low cysteine residue content in the protein primary structure (about 1 mol%). These results suggested that plasticized zeins initially behave like a low-viscosity non-entangled polymer melt, before cross-linking progressively led to a continuous network.