(433f) Role of Hydrophobe Distribution on the Thermal Gelation of Biopolymers--Experiments and Simulations

Hydrophobically modified cellulose polymers in dilute aqueous solution undergo the unusual behavior of gelling as the temperature is raised. We have studied the phenomena experimentally using turbidity measurements during temperature cycling, and we have performed Monte Carlo simulations to understand the role of the uniformity of substitution or blockiness of substitution on the phase transitions. Four derivatized cellulose polymers are used that have different molecular weights, levels of methyl or hydroxypropyl substitution and uniformity of substitution. The shape of the turbidity curve depends on the uniformity of the distribution of hydrophobic sites whereas the temperature for the onset of turbidity depends on the total substitution level. At constant levels of substitution and uniformity of substitution, increasing molecular weight increases the turbidity but neither shifts the temperature for onset nor the shape of the curve. The simulations show changes in the phase diagrams that explain the experimental observations.