Phase structure of impact-modified polypropylene blends

A morphological study of polypropylene/ethylene-propylene-diene terpolymer (PP/EPDM) and polypropylene (PP)/polyolefin thermoplastic rubber blends was conducted. Dispersion of the impact modifier in the blend was investigated by transmission and scanning electron microscopy (TEM and SEM). It was established that number-average particle size ($\text{D}\text{?}{}_{\mathrm{n}}$) of the EPDM impact modifier increased with its melt viscosity. The differences in melt viscosities of the blended components were characterized by the phase viscosity ratio (μ). The course of $\text{D}\text{?}{}_{\mathrm{n}}$ vs. log μ function was qualitatively in agreement with the Rayleigh—Taylor—Tomotika theory. Accordingly, high degree of dispersion of the impact modifier can be achieved if melt viscosities of the blended components are very closely matched, i.e. if $\text{μ ? 1}$. It was concluded from SEM results that, below an impact modifier content of 20%, the modifier formed the dispersed phase in the continuous PP matrix. In blends containing 50% of impact modifier, the latter may also form continuous phase depending on its type and μ value beside the still continuous PP phase (co-continuous network structure).