In situ block copolymer formation during solid-state shear pulverization: An explanation for blend compatibilization via interpolymer radical reactions

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 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.