The structure of agarose gels prepared in aqueous binary solvents has been studied by means of X-ray and neutron small-angle scattering. The scattering curves are interpreted by considering a random assembly of straight fibers displaying cross-section polydispersity. The cross-section polydispersity is well accounted for with a distribution function of the type w(r) ∼ r -λ bounded by two radii, r min and r max, and where λ depends upon both the nature of the binary solvent and the agarose concentration. Departure from the Porod regime at wide scattering angles suggests the existence of free and/or dangling chains that do not participate in the network elasticity (loose chains). The fraction of loose chains gradually vanishes upon increasing the agarose concentration. Experimental elastic modulus−concentration relations obtained on the same type of samples point to the occurrence of entropic elasticity once analyzed in light of the existence of loose chains. Entropic elasticity is rather consistent with disorganized gel junctions.

The thermoreversible gelation of agarose has been investigated in four different aqueous binary solvents: water/dimethyl sulfoxide, water/N,N-dimethylformamide, water/N-methylformamide, and water/formamide. The thermal behavior through the gelation/melting temperatures and the gelation/ melting enthalpies has been studied, and phase diagrams have been subsequently established as a function of agarose concentration and solvent composition. The elastic modulus has been determined as a function of the same parameters. The results are discussed by considering different possible models. It is suggested that agarose/water/cosolvent ternary complexes can account for the experimental results.

The behavior of chiral-nematic and isotropic phases of helical κ-carrageenan in aqueous solution of sodium iodide was compared with that of the anisotropic biphasic phase that contains both these phases. On the basis of birefringence, rheology, chemical analysis, average molecular weight, and polydispersity index measurements, we derived a number of characteristic differences as well as similarities between these phases, over a range of polysaccharide concentrations obtained by the dilution of each phase. For example, we assessed the critical concentration of an isotropic-anisotropic transition (C i ), the temperature of the anisotropic-isotropic phase shift during thermal heating-cooling cycles, and the viscosity changes due to the phase shift and due to the diminishing of the helical conformation. We also demonstrated how the different phases and their dilutions behave under the effect of shear and frequency of oscillation and how the viscoelastic properties vary in each phase and discussed the isotropic and anisotropic liquid crystal controlling behavior mechanisms. From a theoretical point of view, we propose to combine the wormlike chain model for semiflexible polyelectrolytes interacting via both hard-core and electrostatic repulsion to assess the concentration of isotropic-nematic transition, to assess the coexistence concentration range, and to determine the effects of charge by applying the effective diameter and a twisting effect. IntroductionThe carrageenans are linear, sulfated polygalactans extracted from various species of marine red algae, (i.e., Eucheuma cottoni, Eucheuma spinosum, Gigartina acicularis). 1,2 The primary structure is based on a repeating disaccharide sequence of 1,3-linked -D-galactopyranose and 1,4-linked 3,6-anhydro-D-galactopyranose residues. κ-and ι-carrageenan are the two best known gelling varieties.Rigid and semirigid macromolecules can form gels, but there is also a possibility to form ordered phases (nematic, chiral nematic, smectic, hexatic) or both gels and liquid crystalline phases. 3 The liquid crystalline phase is caused by restrictions on rotation, which in turn are caused by volume exclusion for the polymers of high axial ratios. Most experimental investigations on κ-carrageenan (KC) have dealt with conditions where gel formation or aggregations of the helices occur, therefore preventing the development of longrange liquid crystalline order. However, binding of iodide to the (negatively charged) KC helix increases the charge density, thus preventing aggregation and further gelation of the helices. 4,5 In fact, this was demonstrated for sodium iodide concentration of 0.1 M for polymer concentrations up to about 1 g/L. For higher values of sodium iodide concentration, aggregation does indeed take place, nevertheless, at a reduced extent. 6 The binding of iodide explains why the KC solutions require sodium iodide as a solvent to give a nematic liquid crystalline phase. 4 These solutions show a macroscopic phase separation into one anisotropic bottom phase and one isot...