From miscible to immiscible polycarbonate/poly(ε-caprolactone) blends

Dielectric relaxations of polycarbonate (PC) and poly(epsilon-caprolactone) (PCL) blends that were thought to be completely homogeneous in the amorphous phase from many previous calorimetric studies are performed by using the thermally stimulated depolarization currents technique. Samples kept for 18 months at room temperature were compression-molded and then either quenched or slowly cooled. Quenched samples evidenced a phase separation for the 20/80 PC/PCL composition and the existence of continuous concentration fluctuations for other blend compositions. The slowly cooled samples from the melt exhibited an increase of the crystallinity degree for both. blend components, together with a segregation of a predominantly rich PCL phase and the appearance of high-temperature relaxations attributed to interfacial polarization. These findings are confirmed by new calorimetric results on the same blends and by transmission electron microscopy. The spherulitic growth rate also points to the coexistence of two phases which are not completely immiscible but remain separated, each one being rich in one of the blend components. The phase separation of the PC/PCL blends increases with long storage times at room temperature. The driving force behind this segregation process seems to be the slow crystallization of the PCL component during storage. Once the blends have undergone phase separation, only extrusion in the melt or solution mixing can increase their miscibility.