Hierarchical structures in agar hydrogels

Small angle neutron scattering experiments were performed on agar solutions and gels to explore their differential microscopic structures. In solution state, the wave vector, q, dependence of static structure factor, I(q), could be described by I(q)=Igexp(−q2Rg2/3)+IRq−α. Statistical analysis gave: Rg = 18 nm and α = 0.85 ± 0.07 indicating the existence of rod-like rigid structures of length, L=12Rg ≈ 63 nm. In gels, I(q)=IGexp(−q2Ξ2)+IFq−β+(IP/q)exp(−q2Rc2/2) which had discernible Gaussian, power-law and Kratky-Porod regimes in the low, intermediate and high-q regions. Regression analysis yielded a characteristic length, Ξ = 3.3 − 4 nm for gels with agar concentration, c = 0.1 − 0.3% (w/v). The exponent β = 1.2 ± 0.2 and the cross-sectional radius of cylindrical fibres, Rc = 1.5 ± 0.3 nm remained invariant of agar concentration. This assigned a value 5 nm to the persistence length of the fibres in the solution phase that reduced to 3 nm in the gel phase indicating differential hydration of the fibres.

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