Novel synthesis of branched polypropylene via solid-state shear pulverization

As synthesized by Ziegler-Natta or metallocene catalysis, commercial polypropylene (PP) is linear and has low melt strength. A common approach for improving melt strength is to incorporate long-chain branches (LCBs). We describe the discovery of a novel approach to prepare LCB PP by subjecting linear PP to solid-state shear pulverization (SSSP) with benzoyl peroxide (BPO) as the lone additive. Depending on BPO content, LCB PP can be formed by radical reactions during SSSP or post-SSSP melt extrusion. Using shear rheology, we demonstrated that LCB PP was formed during SSSP of samples with 4 and 6 wt% BPO. Relative to linear PP, the post-SSSP sample (purified of residual BPO) made with 6 wt% BPO exhibited enhanced shear thinning behavior, a decreased dependence of storage modulus (G′) on frequency (ω) at low ω, and a deviation from linear polymer behavior in its van Gurp-Palmen curve. For samples that were prepared with low levels of BPO (0.5-1.5 wt%), LCBs were not formed (within error) during SSSP; instead, LCBs were formed during post-SSSP melt extrusion (with residual BPO). While a sample with 1.5 wt% BPO showed no deviation from linear polymer behavior when tested immediately after SSSP, the same sample after post-SSSP melt extrusion demonstrated enhanced shear thinning behavior, decreased dependence of G′ on ω at low ω, deviation from linear polymer behavior in its van Gurp-Palmen plot, and improved crystallization and tensile properties (as is expected for branched PP). We also showed that the LCB formation is achieved by taking advantage of the near-ambient temperature conditions associated with SSSP and is unattainable via conventional melt processing of PP.