A look inside the extruder: evolution of chemistry,morphology and rheology along the extruder axis during reactive processing and blending

A simple device was recently developed for fast sampling (within a few seconds) of representative melt samples (about 2 g) on a running extruder. An array of such devices has been mounted on a twin‐screw extruder. The goal of this study was to de‐black‐box reactive processing of polymers by studying some typical examples. ‐ Processing of polyolefins in the presence of peroxides: when the polymer is molten and the melt temperature is sufficiently high branching/cross‐linking of PE and degradation of PP occurs; the conversion follows a convex profile along the screw axis, which profile is similar to the exponential profile calculated for peroxide decomposition. ‐ Free‐radical grafting of maleic anhydride (MA) onto polyolefins: MA grafting onto PE and PP also follows a convex profile with branching/cross‐linking as parallel side reaction for PE and degradation for PP; for PE degradation of the formed grafted/cross‐linked gel is observed at the end of the extruder. ‐ Reactive blending of PA‐6 with EPM‐g‐MA: within a few seconds the in‐situ compatibilization reaction, resulting in PA‐6/EPM graft copolymers, is completed and the degree of rubber dispersion has changed from the mm to the sub‐μm range, regardless of the MA content of EPM‐g‐MA and the EPM‐g‐MA content of the blend; PA degradation occurs along the whole extruder.     

A Complete Active Space Self‐Consistent Field (CASSCF) Study of the Reaction Mechanism of the α‐Alkynone Rearrangement

The pyrolytic rearrangement of α‐alkynones has been discovered by Karpf and Dreiding [1] in 1979. The mechanism of this reaction, which involves an acetylene‐vinylidene rearrangement followed by cyclization of the intermediate vinylidene carbene by insertion into a β‐C−H bond, has been debated in a couple of theoretical investigations. Restricted Hartree‐Fock (RHF) and single‐point Møller‐Plesset 2 (MP2) calculations at the RHF geometries apparently indicate the carbene cyclization to be the rate‐determining step, contrary to chemical intuition. However, larger‐scale correlated calculations with completely optimized molecular geometries ((8,8) CASSCF/6‐311G**), augmented with a perturbative account for the dynamic correlation contribution to the electronic energy, show vanishing energy barriers to the cyclization step and large activation energies for the acetylene‐vinylidene rearrangement, which is thus confirmed as the rate‐determining step of the title reaction.     

New crystals in the lithium sulfate family

The preparation and structures of caesium lithium sulfate, Cs_1.15Li_2.85(SO₄)₂, and caesium lithium rubidium sulfate, Cs_0.90Li_2.88Rb_0.22(SO₄)₂, are described and discussed in the context of simple and double sulfate polymorphism. The latter structure is related to the former through the substitution of Rb for Cs. In both crystals, the sulfate ions occupy two non‐equivalent sites, but the ions are disordered in Cs_1.15Li_2.85(SO₄)₂.     

Novel blends from agave fibers and poly(methyl methacrylate)

Cellulose-rich fibers were isolated from Agave lechuguilla (AL) and Agave fourcroydes (AF) growing in the Mexican northeast. These fibers are a valuable feedstock for the preparation of blends with synthetic polymers like poly(methyl methacrylate), PMMA. Blends of different types of agave fibers (dewaxed, mercerized, and grafted) and PMMA were prepared and investigated by means of tension measurements and dynamic mechanical analysis. The fiber-containing blends are more stable than the plain PMMA. Surprisingly, the mechanical stability of the blends is practically independent of the pretreatment of the fibers. Methyl methacrylate (MMA) was grafted onto the biopolymer fibers initiated by the cerammonium nitrate redox initiator. Grafting yields of 26.5% were realized with fibers from AL while up to 75.8% MMA was grafted onto fibers from AF. The materials were characterized by means of FTIR spectroscopy and DSC.