Thermoreversible gelation and scaling laws for graphene oxide-filled κ-carrageenan hydrogels

Abstract

The effects of graphene oxide (GO) and ammonia functionalized graphene oxide (AGO) on microstructure, thermoreversible and critical gelation properties of κ-carrageenan in aqueous solution were studied by micro-differential scanning calorimetry (micro-DSC), field emission scanning electron microscopy (FESEM) and rheology. The presence of GO or AGO was able to decrease the gel formation and melting temperatures and reduce gel strength. The effect of AGO was more evident than that of GO. It was observed by FESEM that κ-carrageenan gel was induced by the formation of a fibrillar network, and fibrillar size and density decreased with increasing GO or AGO content. The viscoelastic properties at the gel point were examined using the Winter-Chambon criterion to obtain the critical relaxation exponent n and the critical gel strength Sg. n increased but Sg decreased with increasing GO or AGO content. The more obvious increase in n and decrease in Sg were observed in the AGO/κ-carrageenan solution, which further demonstrated the stronger weakening effect of AGO. A schematic diagram has been proposed to explain the effect of AGO on the gelation of κ-carrageenan, where AGO sheets attracted a number of κ-carrageenan chains through hydrogen bonding and electrostatic interaction between sulfate groups of κ-carrageenan and amine groups of AGO. As a result, the number of free κ-carrageenan chains in solution decreased and the coil-helix transition and aggregation of helices of κ-carrageenan were affected negatively.