The fractionated crystallization phenomenon in poly(3hydroxybutyrate)/poly(ethylene oxide) blends

Differential scanning calorimetry and scanning electron microscopy were used to investigate the phenomenon of 'fractionated crystallization' exhibited in poly-d(--)(3-hydroxybutyrate)/poly(ethylene oxide) (PHB/PEO) blends. PHB and PEO, both crystallizable polymers, are compatible in the melt, where only one homogeneous phase is formed. However, complex phase separation takes place during subsequent crystallization: in particular, under non-isothermal conditions, after PHB crystallization, the PEO phase can give rise to fractionated crystallization, as it can crystallize in two steps at different supercoolings. It was observed that this phenomenon depends mainly on the crystallization conditions, the previous thermal history and the crystallization rate used to crystallize the PHB phase. The two different exothermic peaks relative to PEO crystallization were attributed to a homogeneous (high supercooling) and a heterogeneous (low supercooling) nucleation mode respectively. The addition of a nucleating agent, saccharin, supported such a hypothesis. These foreign particles act as heterogeneous nuclei, thus allowing the PEO phase to crystallize completely at lower supercooling according to a heterogeneous mode of nucleation.