Sub-micron dispersed-phase particle size in polymer blends: overcoming the Taylor limit via solid-state shear pulverization

A comparison was made of the fineness of dispersion in immiscible polymer blends achieved by a continuous mechanical alloying technique, solid-state shear pulverization, relative to that achieved by melt mixing. Two polymer blend systems were investigated. A polystyrene (PS)/polyethylene (PE) wax blend was studied because, based on a classic analysis by G.I. Taylor, melt mixing was expected to yield a number-average dispersed-phase domain size, D_n, well above 1 μm. A PS/high density polyethylene (HDPE) blend was also studied because it was known to produce a sub-micron number-average dispersed-phase particle size when mixed by twin-screw extrusion. In the case of the PS/PE wax blend at compositions ranging from 1 to 15 wt% polyethylene wax, pulverization resulted in nearly identical D_n values (typical value of 0.7 μm) independent of minor-phase content; these D_n values were an order of magnitude smaller than the anticipated Taylor limit for melt-mixed blends. In contrast, PS/PE wax blends made by batch, intensive melt mixing yielded D_n values between ∼3 μm at both 1 and 5 wt% minor-phase content and 17.5 μm at 15 wt% minor-phase content. The increase in D_n with increasing dispersed-phase content in the melt-mixed blend is a consequence of coalescence present during melt processing; such effects are disallowed in the pulverization process occurring in the solid state. Scanning electron microscopy of a 95/5 wt% PS/HDPE blend provided D_n values of 500 and 270 nm in the twin-screw extruded and pulverized samples, respectively. Fractionated crystallization studies further corroborated the ability of pulverization to result in a finer, nanoscopic dispersion of the minor phase as compared to extrusion.