Polymer–monomer binary mixtures. I. Eutectic and epitaxial crystallization in poly(ϵ-caprolactone)–trioxane mixtures

Binary mixtures of a linear polyester (poly(?-caprolactone)) and a crystallizable monomer (trioxane) have been investigated by means of differential scanning calorimetry and optical and electron microscopy. The phase diagram indicates the existence of a eutectic at a temperature T_mE = 314°K and for a polymer volume fraction ?_2E = 0.70, values close to those predicated by the Flory–Huggins theory (using χ = 0.3). Microscopic studies reveal unusual morphologies: (1) In hypoeutectic mixtures, at room temperature, the solvent crystallizes as highly ramified or branched needles. When the remaining melt reaches the eutectic composition, transcrystallization of the polymer is induced by the epitaxial deposition (as established by electron diffraction) of polycaprolactone on the existing trioxane crystals, and leads to highly ordered structures. (2) In hypereutectic mixtures a predominantly spherulitic texture is observed. Blends of trioxane and several other linear polyesters are found to exhibit similar behavior.