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Interpolymer radical coupling reaction leading to block copolymer formation has been demonstrated for the first time in the solid state and in the absence of diffusion using solid-state shear pulverization. Radical coupling is often referred to as a diffusion-controlled reaction as it lacks an activation barrier to reaction. However, pulverization can lead both to intimate mixing, creating large interfacial area between blend components, and to chain scission, yielding polymer radicals and accommodating interpolymer coupling in the solid state. Fluorescence-detection gel permeation chromatography (GPC) was used to detect interpolymer reaction in high-molecular-weight (MW) polystyrene (PS)/pyrene-labeled PS and high-MW poly(methyl methacrylate) (PMMA)/pyrene-labeled PS blends. The latter system was chosen as PMMA/PS blend compatibilization was recently achieved via pulverization; this compatibilization was hypothesized to originate from in situ block copolymer formation via interpolymer radical coupling. Proof of coupling was obtained in this study from pyrene fluorescence in pulverized blends at GPC elution times less than those of the original pyrene-labeled PS. With the high-MW PS/pyrene-labeled PS mixture, comparison of label on coupled chains to label on chains that underwent scission indicates that ∼5% of the pyrene-labeled chains undergoing scission were able to couple to radicals originating from the high-MW PS. The effect of MW, blend composition, and screw design on chain scission during pulverization was also studied.

A novel process called solid-state shear pulverization (S3P) has been developed at Northwestern University to recycle single or commingled postconsumer or preconsumer polymeric waste without sorting by type or color. This continuous, one-step process converts shredded plastic or rubber waste into controlled-particle-size powder ranging from coarse (10 and 20 mesh) to fine (80 mesh) or ultrafine (200 mesh). As a result, the pulverization product is usable in applications ranging from direct injection molding without prior pelletization, to rotational molding, to use in protective and decorative powder coatings, as well as to blending with virgin resins and compounding with additives. Scanning electron microscopy reveals that the fine particles have a unique elongated shape that is attributed to the high shear conditions occurring during the pulverization process. Injection-molded parts made from the powder product of the S3P process have mechanical and physical properties comparable to or better than the properties resulting from direct conventional processing of recycled single or commingled plastics. In addition, the parts made from the powder product of the S3P process are uniform in color, whereas parts injection-molded from multicolored recycled feedstock without prior pulverization via the S3P process are streaked, re- 446KHAlT AND TORKELSON ducing their commercial applicability. The improved mixing achieved via the S3P process is often accompanied by scission of the carbon-chain backbone of the polymers involved, as revealed by the generation of free radicals during S3P processing, associated mechanochemistry, and modification of the melt flow rate of the polymers by the S3P process. The implications of this chain scission process for in situ compatibilization of commingled plastic waste via S3P processing are discussed.

A novel, continuous process, solid‐state shear pulverization (S3P), efficiently mixes blends with different component viscosities. Melt mixing immiscible polymers or like polymers of different molecular weight often requires long processing times. With a batch, intensive melt mixer, a polyethylene (PE)/polystyrene (PS) blend with a viscosity ratio (low to high) of 0.019 required up to 35 min to undergo phase inversion. Phase inversion is associated with a morphological change in which the majority component, the high‐viscosity material in these blends, transforms from the dispersed to the matrix phase, and may be quantified by a change from low to high mixing torque. In contrast, such blends subjected to short‐residence‐time (∼3 min) S3P yielded a morphology with a PS matrix and a PE dispersed phase with phase diameters ≤ 1 μm. Thus, S3P directly produces matrix and dispersed phases like those obtained after phase inversion during a melt‐mixing process. This assertion is supported by the similarity in the near‐plateaus in torque obtained in the melt mixer at short times with the pulverized blend and at long times with the non‐pulverized blend. The utility of S3P to overcome problems associated with melt mixing like polymers of extreme viscosity ratio is also shown.