Phase separation and the kinetics of phase coarsening in commercial impact polypropylene copolymers

The phase segregation and subsequent minor phase coarsening of a commercial impact polypropylene copolymer was studied. The major components of the impact polypropylene copolymer studied were 82.4 wt % polypropylene homopolymer and 17.6 wt % ethylene-propylene rubber (EPR). The system was artificially manipulated to ensure homogeneity by precipitation from solution with a nonsolvent. This ensured that the initial system did not exhibit large-scale phase segregation. The homogeneous initial system was subjected to storage in the melt at 193°C for a series of times. The two-phase morphology of commercial impact polypropylenes was generated in the melt state by storage in the melt for various periods of time from 5 s to 1 h. Small nuclei of particles appeared at short time and increased in volume with increasing time in the melt state. The coarsening of the minor phase EPR component was shown to follow the theoretically predicted d ～ t^1/3 and N ～ t^?1 (where d = diameter, N = number of particles, and t = time in the melt) relationships to a close approximation in accord with Ostwald ripening theory. At short times these relationships were not obeyed. The indication was that the long-time coarsening regime was not entered until several minutes elapsed in the melt state. The particle size distribution was initially quite narrow and exhibited a trend of broadening at longer times of coarsening. This may be due to a shift from the short-time regime to the long-time coarsening regime. The initial polymer, which was precipitated from solution, was shown not to have undergone large-scale phase segregation in that it exhibited a one-phase morphology (i.e., no particles with > 0.1 μm diameter) as determined by ¹²⁹Xe NMR spectroscopy. The precipitated blend produced incipient particle nuclei (> 0.1 μm diameter) after a very short time (5 s) in the melt state. © 1994 John Wiley & Sons, Inc.