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 (Formula presented.) 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 (Formula presented.), 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.