Kinetics of sol–gel transition in thermoreversible gelation of gelatin

Abstract

The sol–gel transition in dilute gels of gelatin are studied by differential scanning calorimetry (DSC) and static light scattering techniques. The sol–gel phase diagram clearly shows the existence of an upper critical solution temperature in this system. In addition, the DSC data conclusively exhibits two more phase curves in the sol state—one pertaining to the coexistence of monomers and aggregates and the second one separating the random coil and helix domains in the solution phase. The Ferry–Eldridge equation has been used to determine the enthalpy of the melting of the gel structure (ΔH_g) which is equal to (30±2.0) kcal mol⁻¹. The gelation temperature T_g is correlated to Flory’s statistical model of gelation, which gives the enthalpy of melting of pure gelatin crystallites (ΔH_v) as ΔH_v=(35±2.0) kcal cm⁻³ and crystallite melting temperature (T_m) as T_m=(588±20.0) K. The Flory–Huggins interaction parameter (χ) has been determined as χ=0.49±0.05. Interesting scaling behavior has been observed through light scattering measurements. The intensity of scattered light, I_s∼ε^−γ and I_s∼ε₀^−β; ε=(T/T_g−1); T≥T_g and ε₀=(t/t_g−1); t≥t_g, where t is the time and t_g is the gelation time for ε and ε₀≤0.5 with γ≂β≂0.02±0.005. The initial phase of cooling of the sol has been analyzed within the framework of Smoluchowski aggregation kinetics. On the basis of the above results it is being proposed that the sol–gel transition path in this polymer has three distinct steps—monomer aggregation, coil–single‐helix transition, and single‐helix–triple‐helix transition followed by gelation.