Universal growth of Microdomains and gelation transition in agar hydrogels

Investigations were carried out on aqueous sols and gels of agar (extracted from red seaweed Gelidiella acerosa) to explore the growth of microdomains en route to gelation. Isothermal frequency sweep studies on gel samples revealed master plots showing power-law dependence of gel elastic modulus, vertical bar G*vertical bar, on oscillation frequency, omega as vertical bar G*vertical bar omega(n), independent of temperature, with 0.5 <= n <= 1.4: Dynamic structure factor data from sol samples comprised of a double-exponential relaxation function, S(q,t) =A exp(-D(sq)(2)t) + B exp(-D(Lq)(2)t) where D-s and D-L are the two translational diffusion coefficients and q is the scattering wave vector. This yielded hydrodynamic radii (from Ds), with R-s varying from similar to 20 nm (for sol) to 250 nm (at gelation point). The second hydrodynamic radius (from DL) obtained was R-L in the range of similar to 200-500 nm (for sol) to similar to 1000 nm (at 38 C, gelation point). These data could be universally fitted to R-s approximate to epsilon(-3/5) and R-L approximate to epsilon(-1/3) (epsilon = (T/T-g - 1), T > T-g). The S(q,t) behavior close to the gel transition point (T-g approximate to (38 +/- 3 degrees C determined from rheology) followed a stretched exponential function: S(t) = A exp(-t/t(s))(beta). The beta factor increased from 0.25 to 1 as the gel temperature approached 25 C from Tg, and relaxation time, t(s), showed a peak at T approximate to 30 degrees C. The SLS data (in the sol state) suggested the scaling of scattered intensity, I-S(q) approximate to epsilon(-gamma) (epsilon = (T/T-g - 1), T > T-g) with gamma = 0.13 +/- 0.03, and the presence of two distinct domains characterized by a Guinier regime (low q) and a power-law regime (high q). Close to and above Tg (+2 C), Is(q) scaled with q as Is(q) approximate to q(-alpha) with a = 2.2 +/- 0.2, which decreased to 1.4 coproduct 1 just below Tg (-2 degrees C), implying a coil-helix transition for 0.2% (w/v) and 0.3% (w/v) samples. For a 0.01% sample, alpha = 3.5 +/- 0.5 which indicated the presence of spherical microgels.