Formation of gels in ethylcellulose solutions. An interpretation from dynamic viscoelastic results

Dynamic viscoelastic measurements of the systems formed on cooling from sol utions of ethylcellulose (EC) in three diester phthalates and one monoester phthalate are analyzed. Data obtained in the linear viscoelastic region in dicate that diester solvents develop thermoreversible gels. We analyze the thermomechanical stability of the gels in terms of gel-sol transition tempe rature, T-gs, and breaking stress for rupture of the structure at constant temperature, sigma(c). A maximum in tan delta has been detected below T-gs on heating, leading us to define what we call the T-i transition. Elastic m odulus, T-gs, sigma(c), and T-i increase as the size of the alcohol in dies ters is increased, The increase of solvent molar volume and C=O polarizatio n with the size of the alcohol leads to the hypothesis of a competition bet ween polymer-polymer direct hydrogen bonding (involving ethylcellulose OH g roups at the C-6 position) and polymer-solvent electrostatic interaction in volving the same OH groups. In diester phthalates the presence of two carbo nyl (C=O) for each molecule allows the solvent to act;as a bridge between E C chains, in a way similar to that envisaged for PVC/diester systems.