Gelation under stress: impact of shear flow on the formation and mechanical properties of methylcellulose hydrogels

• Published in: Soft matter Vol. 18; no. 7; pp. 1554 - 1565
• Main Authors: Nelson, Arif Z, Wang, Yilin, Wang, Yushi, Margotta, Anthony S, Sammler, Robert L, Izmitli, Aslin, Katz, Joshua S, Curtis-Fisk, Jaime, Li, Yongfu, Ewoldt, Randy H
• Format: Journal Article
• Language: English
• Published: England Royal Society of Chemistry 16.02.2022
• Subjects: Gelation; Gravity; High temperature; Hydrogels; Mechanical properties; Methylcellulose; Modulus of elasticity; Polymers; Rheological properties; Rheology; Shear flow; Temperature
• ISSN: 1744-683X; 1744-6848
• DOI: 10.1039/d1sm01711j

We demonstrate that small unidirectional applied-stresses during temperature-induced gelation dramatically change the gel temperature and the resulting mechanical properties and structure of aqueous methylcellulose (MC), a material that forms a brittle gel with a fibrillar microstructure at elevated temperatures. Applied stress makes gelation more difficult, evidenced by an increased gelation temperature, and weakens mechanical properties of the hot gel, evidenced by a decreased elastic modulus and decreased apparent failure stress. In extreme cases, formation of a fully percolated polymer network is inhibited and a soft granular yield-stress fluid is formed. We quantify the effects of the applied stress using a filament-based mechanical model to relate the measured properties to the structural features of the fibril network. The dramatic changes in the gel temperature and hot gel properties give more design freedom to processing-dependent rheology, but could be detrimental to coating applications where gravitational stress during gelation is unavoidable. Small stresses (<1 Pa) can dramatically disrupt the gelation of aqueous methylcellulose, causing the gel temperature to shift and a softer viscoelastic gel to form. A filament network model relates the measured properties to microstructural features.