A facile method to synthesize strong salt-enhanced hydrogels based on reversible physical interaction

Area of Science:

• Materials Science
• Polymer Chemistry

Background:

• Hydrogels often suffer from reduced mechanical properties in saline environments.
• Developing salt-tolerant and mechanically robust hydrogels is crucial for various applications.

Purpose of the Study:

• To synthesize a novel poly(hexafluorobutyl methacrylate-acrylamide) [P(AAm-co-HFBMA)] hydrogel with enhanced mechanical strength.
• To investigate the salt-enhancement effect on the hydrogel's mechanical properties and understand the underlying mechanism.

Main Methods:

• Copolymerization of acrylamide (AAm) and hexafluorobutyl methacrylate (HFBMA) using N,N'-methylene bisacrylamide (NMBA) as a cross-linker.
• Preparation of the hydrogel in a dimethylformamide (DMF)/aqueous solution followed by DMF replacement with water.
• Mechanical testing (tensile fracture stress) and Differential Scanning Calorimetry (DSC) analysis.

Main Results:

• The P(AAm-co-HFBMA) hydrogel exhibited significantly improved tensile fracture stress compared to pure polyacrylamide (PAAm) hydrogel.
• Tensile fracture stress increased with higher HFBMA content, reaching 2.34 MPa with 25 mol% HFBMA.
• Soaking in NaCl solutions enhanced mechanical strength to 3.50 MPa (2 M NaCl), demonstrating a unique salt-enhancement trend.

Conclusions:

• The P(AAm-co-HFBMA) hydrogel shows remarkable salt-enhanced mechanical properties.
• The enhancement is attributed to the formation of dynamic cross-linking junctions via switchable hydrophobic interactions between C-F groups in the presence of salt.
• This mechanism provides an effective dynamic energy dissipation pathway, leading to improved toughness.