Gelation mechanism and network structure in gels of carrageenans and their mixtures viewed at different length scales – A review

Studies on the gelation mechanism and network structures of κ-(KC) and ι-(IC) carrageenan gels and their mixtures at different length scales from macroscopic to microscopic and molecular levels are reviewed. Studies on pure carrageenan gels revealed the gelation mechanism to induce different gelation abilities. Particle tracking methods characterized the local network structure of weak carrageenan gels. Nuclear magnetic resonance (NMR) studies revealed the aggregation behavior of carrageenans, suggesting a preferential aggregation for longer carrageenan chains in the network. This review also addressed whether the mixtures of KC and IC formed a phase-separated or interpenetrating network structure. Macroscopic measurements accompanied by a theoretical model suggested the formation of a phase-separated network structure. Studies on the self-diffusion of carrageenan chains and the diffusion of probe polymer by NMR, however, proposed two possible structures, interpenetrated network or phase-separated network with a domain size of less than 450 nm. Fluorescence-labeled carrageenan viewed by using confocal laser scanning microscopy showed a homogenous network of KC in the mixtures of KC/IC compared to pure KC suggesting possible co-aggregation or interpenetrated network of KC and IC chains. Conversely, microscopic characterization using particle tracking revealed a bimodal distribution in diffusivity with local and diffusion mode characteristics. This result implied the formation of a phase-separated network structure comprised of KC-rich and IC-rich domains. Nevertheless, despite the progress made to understand the interactions of carrageenans in the mixtures, further studies are required to clarify the gelation mechanism and network structures in the mixed carrageenan gels. [Display omitted] •Particle tracking revealed the microscopic network structures in carrageenan gels.•NMR measurements for relaxation times and diffusion on carrageenan gels were reviewed.•Phase transition among continuous/filler, bicontinuous, filler/continuous phase is suggested.•Particle tracking revealed the heterogeneity in the mixed KC/IC and KC/LC gels.•Microrheology provides complementary information to macroscopic measurements.