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Isothermal Crystallization Kinetics of α‑Olefin Molecular Bottlebrushes

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posted on 21.08.2020, 06:30 authored by Carlos R. López-Barrón, John R. Hagadorn, Joseph A. Throckmorton
The isothermal crystallization kinetics of a series of poly­(α-olefin) bottlebrushes, with side-chain lengths (Nsc) ranging from 10 to 16 carbons, was systematically investigated via differential scanning calorimetry (DSC), small- and wide-angle X-ray scattering (SAXS/WAXS), small-angle light scattering (SALS), and polarized optical microscopy (POM). Analysis of the DSC data with the Avrami model reveals that crystallization proceeds via one-dimensional growth of rod-shaped crystallites (fibrils), which is unprecedented in semicrystalline polymers. Both the nucleation and crystallization rates, as well as the equilibrium melting temperature (obtained by the linear Hoffman–Weeks extrapolation method), are increasing functions of Nsc, which results from the decreasing energy barrier for crystallization as the side chains are longer. Analysis of the crystallization kinetics with the Lauritzen–Hoffman theory revealed two stages of crystallization. The transition from regime I to regime II occurs at undercooling values that decrease with Nsc and range from 10 K, for poly­(1-octadecene), to 45 K, for poly­(1-dodecene). Analysis of the crystal structure (via SAXS/WAXS, SALS, and POM) during isothermal crystallization showed an increase in crystallinity and a decrease in interfibril distance as undercooling deepens, as well as the development of bicontinuous crystal domains.