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Decreasing the Alkyl Branch Frequency in Precision Polyethylene: Effect of Alkyl Branch Size on Nanoscale Morphology

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journal contribution
posted on 21.02.2016 by Bora Inci, Ingo Lieberwirth, Werner Steffen, Markus Mezger, Robert Graf, Katharina Landfester, Kenneth B. Wagener
Synthesis and morphological characterization are reported for a series of 13 precision branched polyethylene structures, the branch being placed on every 39th carbon and varying in size from methyl to pentadecyl group. A recently established synthetic scheme for preparation of the symmetrical α,ω-diene monomer was employed to increase the number of methylene carbons between the branch points from 20 to 38, yielding polymers with 5.26 mol % α-olefin incorporation. The morphology of these polymers was investigated using differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD), and transmission electron microscopy (TEM). Methyl branching significantly reduces the melting point and single crystal lamellae thickness of unbranched polyethylene. On the other hand, all further branches from ethyl to pentadecyl produce polymers that have similar melting points and single crystal lamellae thicknesses. A clear change in the morphology of both solution and melt-grown crystals of these polymers was observed from a situation where the methyl branch is incorporated in the polymer′s unit cell to one where branches of greater mass are mostly expelled from the unit cell.