Effects of process method and quiescent coarsening on dispersed-phase size distribution in polymer blends: comparison of solid-state shear pulverization with intensive batch melt mixing

We compare how solid-state shear pulverization (SSSP), batch melt mixing (BMM), and static melt-state annealing affect the morphology of an immiscible blend of polypropylene (PP) and ethylene-α-olefin copolymer (EOC). For 85/15 wt% PP/EOC blends, SSSP and BMM led to log-normal distributions of dispersed-phase particle size. The SSSP blend had smaller average particle diameters (e.g., D _n = 0.24 μm) and a narrower particle size distribution (e.g., D _w/D _n = 1.17; D _v/D _n = 1.56) than the BMM blend (D _n = 0.28 μm; D _w/D _n = 1.25; D _v/D _n = 1.79). The fact that BMM is subject to thermodynamically and flow-induced coalescence while SSSP is not, can lead to smaller particle sizes and a narrower distribution by SSSP. Although annealing at 200 °C for 30 and 90 min led to continuous growth of average particle size in the BMM blend, the particle-size dispersity remained virtually unchanged. In contrast, after 30-min annealing at 200 °C, the SSSP blend showed less growth in \(D_{{_{n} }}^{{^{3} }}\) than the BMM blend but a dramatic increase in particle-size dispersity and a loss of the log-normal size distribution. Between 30 and 90 min, there was at most slight growth in \(D_{{_{n} }}^{{^{3} }}\), consistent with partial compatibilization caused by in situ block copolymer formation during SSSP, and major reductions in D _w/D _n and D _v/D _n close to those of the BMM blend and recovery of the log-normal size distribution. These results suggest that caution should be used in correlating immiscible blend properties to a particular average particle size as that value may not reflect possible complexity of the underlying size distribution.