Salt-induced LCST-type thermal gelation of methylcellulose: quantifying non-specific interactions via fluctuation theory

Elucidation of the Specific Ion Effects and Intermediate Structures of Cellulose Fibers Swollen in Inorganic Salt Solutions via In Situ X-ray Diffraction

The solubilities of many substances are significantly affected by specific ions, as demonstrated by the Hofmeister series of proteins. Cellulose has a resistant fibrillar structure that hinders its swelling and dissolution. Certain inorganic salt solutions are effective swelling agents and solvents for cellulose. However, the precise effects of these ions on cellulose are not fully understood. In this study, we studied the intermediate structures of cellulose fibers during their swelling process in ZnCl2 and LiBr solutions via in situ X-ray diffraction. Two swollen phases with characteristic morphologies were observed for both salt treatments. Only the surfaces of the fibers were swollen in ZnCl2, whereas the ions penetrated the fibers and formed complexes with cellulose while the morphology of the fibers was maintained in LiBr. Our findings clarify the reasons that ZnCl2 has been used as an excellent swelling agent, whereas LiBr has been used as a good solvent for cellulose.

Polyacrylamide-Based Block Copolymer Bearing Pyridine Groups Shows Unexpected Salt-Induced LCST Behavior

Thermal-responsive block copolymers are a special type of macromolecule that exhibit a wide range of applications in various fields. In this contribution, we report a new type of polyacrylamide-based block copolymer bearing pyridine groups of polyethylene glycol-block-poly(N-(2-methylpyridine)-acrylamide; Px) that display distinct salt-induced lower critical solution temperature (LCST) behavior. Unexpectedly, the phase-transition mechanism of the salt-induced LCST behavior of Px block copolymers is different from that of the reported LCST-featured analogues. Moreover, their thermo-responsive behavior can be significantly regulated by several parameters such as salt species and concentration, urea, polymerization degree, polymer concentration and pH values. This unique thermal behavior of pyridine-containing block copolymers provides a new avenue for the fabrication of smart polymer materials with potential applications in biomedicine.

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

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.