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, | G *|, on oscillation frequency, ω as | G *| ≈ ω 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 S q 2 t ) + B exp(− D L q 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 D S ), with R S varying from ∼20 nm (for sol) to 250 nm (at gelation point). The second hydrodynamic radius (from D L ) obtained was R L in the range of ∼200−500 nm (for sol) to ∼1000 nm (at 38 °C, gelation point). These data could be universally fitted to R S ≈ ε -3/5 and R L ≈ ε -1/3 (ε = ( T / T g − 1), T > T g ). The S ( q, t ) behavior close to the gel transition point ( T g ≈ (38 ± 3 °C determined from rheology) followed a stretched exponential function: S ( t ) = A exp(− t / t s ) β . The β factor increased from 0.25 to 1 as the gel temperature approached 25 °C from T g, and relaxation time, t s, showed a peak at T ≈ 30 °C. The SLS data (in the sol state) suggested the scaling of scattered intensity, I s ( q ) ≈ ε -γ (ε = ( T / T g − 1), T > T g ) with γ = 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 T g (+2 °C), I S ( q ) scaled with q as I s ( q ) ≈ q -α with α = 2.2 ± 0.2, which decreased to 1.4 ± 1 just below T g (−2 °C), implying a coil−helix transition for 0.2% (w/v) and 0.3% (w/v) samples. For a 0.01% sample, α = 3.5 ± 0.5 which indicated the presence of spherical microgels.