Oxygen transport and free volume in cold-crystallized and melt-crystallized poly(ethylene naphthalate)

In the present study we examined the oxygen-transport properties of poly(ethylene naphthalate) (PEN) isothermally crystallized from the melt (melt crystallization) or quenched to the glass and subsequently isothermally crystallized by heating above the glass transition temperature (cold crystallization). The gauche/trans conformation of the glycol linkage was determined by infrared analysis, and the crystalline morphology was examined by atomic force microscopy (AFM). Explanation of the unexpectedly high solubility of crystallized PEN required a two-phase transport model consisting of an impermeable crystalline phase of constant density and a permeable amorphous phase of variable density. The resulting relationship between oxygen solubility and amorphous-phase density was consistent with free volume concepts of gas sorption. Morphological observations provided a structural model for solubility and permeability. The model consisted of a permeable amorphous matrix of constant density containing dispersed spherulites of lower permeability. The spherulites themselves were composites of impermeable crystallites and permeable interlamellar amorphous regions of lower density than the amorphous matrix. Dedensification of the interlamellar amorphous phase was due to the constrained nature of amorphous chains anchored to crystallites.