Analysis of the density and the enthalpy of poly(epsilon-caprolactone)-polycarbonate blends: Amorphous phase compatibility and the effect of secondary crystallization

The dependency of the density of poly(epsilon-caprolactone)-polycarbonate (PCL-PC) blends on composition has been studied. PCL/PC blends are typical of miscible blends containing crystallizable components, and miscibility is therefore considered with respect to the amorphous phase. In literature, a single glass transition temperature is reported for the PCL-PC system; however, the exact dependency on amorphous phase composition is not clear. For quenched amorphous blends, we found the Fox equation to be appropriate in order to describe the glass transition temperature as a function of composition. For amorphous samples containing low amounts of PCL (less than or equal to 40 wt %), an increase in the density of the blend over that of a linear average of the densities of amorphous PC and PCL was observed experimentally. This is indicative of significant interactions in the blend. For samples containing greater than or equal to 50 wt % PCL, crystallization of PCL has to be accounted for in the analysis of the density. It is shown that the experimental density data can only be described satisfactorily by assuming that secondary crystallization of PCL does not lead to an increase of the overall blend density. This is attributed to the rigid and volume filling primary crystalline structure at room temperature, i.e., below the melting point of PCL. Excess density is present in the amorphous phase of the PCL/PC blend over the whole range of composition; therefore, specific interactions exist in the amorphous phase of the blend over the whole range of composition. (C) 1997 John Wiley & Sons, Inc