Sol/gel transition of chitosan solutions

Area of Science:

• Materials Science
• Polymer Chemistry
• Rheology

Background:

• Chitosan (CS) is a versatile biopolymer with applications in various fields.
• Understanding the sol/gel transition and rheological properties of CS is crucial for its effective utilization.
• Previous studies have explored CS gelation, but specific behaviors under varying conditions require further investigation.

Purpose of the Study:

• To investigate the sol/gel transition and gel rheology of chitosan solutions.
• To determine the influence of pH, molecular weight (Mw), and temperature on CS gel properties.
• To elucidate the uncommon aging behavior observed in CS gels near the sol/gel point.

Main Methods:

• Oscillatory shear rheometry using a Rheometrics SR-5 rheometer.
• Employing Couette and parallel plate geometries for small amplitude oscillatory shear tests.
• Systematic variation of experimental conditions including pH, molecular weight, and temperature.

Main Results:

• Sol/gel transition concentration and elastic modulus (G") of CS gel decrease with increasing pH and Mw, up to a certain threshold.
• Ionization constant (Kp) affects transition concentration, with formic acid solutions showing higher concentrations than acetic acid.
• CS gel elastic modulus (G") increases with temperature, consistent with rubber elastic theory, and exhibits unusual aging where properties decline over time.

Conclusions:

• Chitosan gelation and rheology are significantly influenced by pH, Mw, and temperature.
• The observed temperature dependence of G" aligns with classical rubber elastic theory.
• The study details an unusual aging phenomenon in CS gels, characterized by property decline and reversion to the sol state.