Formation of Gels in Ethylcellulose Solutions. An Interpretation from Dynamic Viscoelastic Results

Dynamic viscoelastic measurements of the systems formed on cooling from solutions of ethylcellulose (EC) in three diester phthalates and one monoester phthalate are analyzed. Data obtained in the linear viscoelastic region indicate that diester solvents develop thermoreversible gels. We analyze the thermomechanical stability of the gels in terms of gel−sol transition temperature, T gs, and breaking stress for rupture of the structure at constant temperature, σ c . A maximum in tan δ has been detected below T gs on heating, leading us to define what we call the T i transition. Elastic modulus, T gs, σ c, and T i increase as the size of the alcohol in diesters is increased. The increase of solvent molar volume and C O polarization with the size of the alcohol leads to the hypothesis of a competition between polymer−polymer direct hydrogen bonding (involving ethylcellulose OH groups at the C-6 position) and polymer−solvent electrostatic interaction involving the same OH groups. In diester phthalates the presence of two carbonyl (C O) for each molecule allows the solvent to act as a bridge between EC chains, in a way similar to that envisaged for PVC/diester systems.