Temperature and pH-sensitive chitosan hydrogels: DSC, rheological and swelling evidence of a volume phase transition

Summary

On heating, alkali chitin solutions undergo phase separation describing a characteristic “U-shaped” cloud point curve with a lower critical solution temperature (LCST) centered at ∼30 °C. The process is accompanied by gelation of the polymer-rich phase. A different strategy to induce alkali chitin phase separation/gelation is by applying vacuum to the solution at room temperature during aprox. 72 h. Once washed to neutrality, chitin gels had a degree of acetylation of ∼30–40% (i.e. they were converted into chitosan). On cooling, these gels exhibit an exothermic peak in micro-DSC and a depression in G^′′ and tan δ traces, evidencing a volume phase transition centered at ∼20 °C. This transition is observed only within a narrow range of pH ∼7.3–7.6. Variation in the mechanical response as a result of cyclic stepwise changes in temperature between 50 and 0 °C at pH values from 7.3–7.6, revealed that the G’ modulus of the gels increases on heating and decreases on cooling, a behavior that persists over at least four cycles of temperature change. Only marginal changes in G’ at pH 8.0 and not at all at pH 12.0 are observed. By contrast, the variation of G^′′ persists throughout the range of pH. This behavior is rationalized in terms of the existence of a fine balance between hydrophobic and hydrophilic interactions at varying temperature and pH, thus effectively controlling swelling and shrinking states of the gel network. The degree of swelling at pH 7.6 reaches a minimum at ∼22–25 °C.