Discovery of Polyolefin Granule Products That Can Be Deconstructed into Individual, Fully Separable Grains Provides New Insights into Grain Growth and Material Design

Abstract

We have discovered a silica-supported metallocene catalyst that produces large, open, porous polyolefin granules that can be easily and quantitatively deconstructed into their constituent polyolefin grains (Gₚ) by lightly crushing any granule. Three such polymerization products have been examined as follows: two heterophasic, comprising isotactic polypropylene (iPP) and polyethylene (PE), and one monophasic, comprising iPP. Deconstruction of granules from each product was followed by quantitative examination by optical microscopy of hundreds of grains. These data have enabled the first statistically robust demonstrations of lognormal distributions of polymer grain size and sphericity. While the lognormal results are not surprising per se, they offer the first quantitative demonstration of such distributions, rather than the unimodal distribution used in physical models since the introduction of the multigrain model over 30 years ago. Three contributions to the lognormal size distribution of the polymer grains are identified: dominant among these is the distribution of simple spheroidal polymer grains. For these simplest of grains, we also give the special notation “primaries”. The two remaining more complex grain topologies are constructed from primaries. One of these topologies is created from two or more fused, or attached, polymer primaries. The third type of grain topology is constructed from three or more fused primaries where the primary aggregate contains at least one void within the grain. In all, the polymer grains range in average diameter from 40 to as large as 470 μm. The median Gₚ diameter measured for all grains in each of the three products ranges from 83 to 150 μm. Between 6 and 16% of polymer grains have optically detectable voids, and grains with voids have on average 40–100% larger diameters than both the aforementioned solid grains denoted primaries and fused primaries. A simple morphological model is proposed for the dispersion of size and shape observed. Based on the data and model, the smallest supported catalyst grain, from which the polymer primary is grown, is calculated independently for each polymer product. These are ∼4–7 nm in diameter, or within the range of basic SiO₂ building blocks reported or calculated for spray-dried silica. Polarized light microscopy examination of isolated spheroidal primaries shows that chains in the heterophasic products have a preferential orientation, with chains aligning more or less along the radius of each primary, whereas chains in the iPP product show no preferential orientation. The morphology, or texture, of iPP and high-density polyethylene (HDPE) in a polymer primary taken from one of the heterophasic granule products was examined by scanning transmission electron microscopy. This examination, together with another for a closely related heterophasic product made with the same catalyst, shows that these two normally immiscible polyolefins are dispersed or mixed via direct sequential polymerization on a ∼50-250 nm, and perhaps finer, scale. Thus, the results provide visual confirmation that, to accommodate the growth of the HDPE phase, the first-grown iPP phase fracture extends through the primary grains themselves as the grains expand with new polymer. We expect that the demonstration of nearly effortless quantitative production of small polyolefin spheres, potentially with controlled surface roughness, as well as the commingling of immiscible polyolefins achieved on this fine scale in the nascent granules, could spawn new creative opportunities for material design.