Spinodal Decomposition and Phase Separation Kinetics in Nanoclay-Biopolymer Solutions

Intensity of light, I(q,t), scattered from homogeneous aqueous solutions, of nanoclay (Laponite) and protein (gelatin-A), was studied to monitor the temporal and spatial evolution of the solution into a phase-separated nanoclay-protein-rich dense phase, when the sample temperature was quenched below spinodal temperature, T-s (=311 +/- 3 K). The zeta potential data revealed that the dense phase comprised charge-neutralized intermolecular complexes of nanoclay and protein chains of low surface charge. The early stage, t < 500 s, of phase separation could be described adequately through Cahn-Hilliard theory of spinodal decomposition where the intensity grows exponentially, I(q, t) = I-0 exp.(2R(q)t). The wave vector, q dependence of the growth parameter, R(q) exhibited a maxima independent of time. Corresponding correlation length, 1/q(c) = xi(c) was found to be approximate to z75 +/- 5 nm independent of quench depth. In the intermediate regime, anomalous growth described by I(q, t) similar to t(alpha) with alpha = 0.1 +/- 0.02 independent of q was observed. Rheological studies established that there was a propensity of network structures inside the dense phase. Isochronal temperature sweep studies of the dense phase determined the melting temperature, T-m = 312 +/- 4 K, which was comparable with the spinodal temperature. The stress-diffusion coupling prevailing in the dense phase when analyzed in the Doi-Onuki model yielded a viscoelastic correlation length, xi(v) determined from low-frequency storage modulus, G'(0) approximate to k(B) T/xi(3)(v), which was xi(v) approximate to 35 +/- 3 nm indicating 2 xi(v) approximate to xi(c). It is concluded that the early stage of phase separation in this system was sufficiently described by linear Cahn-Hilliard theory, but the same was not true in the intermediate stage. (C) 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 555-565, 2010