Sacran is a supergiant cyanobacterial exopolysaccharide with an extremely high absolute molecular weight over 10 Mg/mol (molecular length: over 30 um) and a very high rigidity parameter, r, of 3.8 to exhibit a liquid crystalline (LC) phase at an extremely low concentration of 0.3 wt%. From this, it is found that sacran formed rigid rods with an extremely high aspect ratio of X~1600 (from Flory’s lattice theory). When sacran LC aqueous solution is cast and annealed at a temperature range of 70-140 degC, anisotropic hydrogels are formed by immersion in water, where the thickness change of the film by swelling is 70 fold higher than the width change. Increase in the film thickness or decrease in sacran chain length reduced the swelling anisotropies, indicating that LC chains with micrometer-scale size comparable with a thickness of self-standing film showed cooperative effects to align along the substrate surface during drying. Thus uniaxially-swelling hydrogels are efficiently created.<br/>Additionaly we find Divinylsulfone (DVS) works as a chemical cross-linker of sacran chains in a dilute aqueous solution to form hydrogels, but some of the added DVS remains in the hydrogel without cross-linking. The remaining DVS cross-links further with the pre-formed networks of sacran chains in LC state during slow drying to produce in-plane oriented xerogels. The xerogels show heterogeneous anisotropy in the successive swellings steps; the linear swelling ratio in the thickness direction is 10,000-40,000 fold higher than that in the width direction, due to the molecular orientation of the sacran hydrogels. X-ray diffraction imaging of the hydrogels reveal not only the orientation of the xerogel films but also the unusual orientation of water molecules binding to sacran networks in the hydrogel state.

Supergiant cyanobacterial exopolysaccharides, sacran, having anomalous gel properties

254th National Meeting (2017)

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