Stability and phase behavior of the poly(ethylene oxide)-urea complexes prepared by electrospinning

The poly(ethylene oxide)-urea binary system forms a stable (α) complex that can be melt-quenched into a mixture of urea and a second (β) complex with a different stoichiometry. Our results confirm that this mixture is metastable, but that the β-α solid-solid transition is much slower than previously reported. In contrast with the melt-quenched mixture, we observe for the first time that the pure β complex, prepared by electrospinning, is thermodynamically stable up to its melting temperature. This melting is incongruent and leads to the formation of the crystalline α complex and liquid PEO. A phase diagram is drawn over the complete composition range and allows interpreting the formation of the various out-of-equilibrium mixtures observed experimentally. This work also demonstrates an interesting new possibility of electrospinning: the direct preparation of thermodynamically stable compounds that are otherwise inaccessible because of the formation of kinetically frozen products.