Solid state NMR studies of the location, immobilization and conformation of short chain branches in semicrystalline polyethylene-hexene copolymers

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2014-01-01
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White, Allison
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Patricia Thiel
Klaus Schmidt-Rohr
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Chemistry

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Distinct localization of butyl branches in high- and linear low-density polyethylenes (HDPEs and LLDPEs) has been detected by solid-state 13C NMR (ssNMR). Several important mechanical properties of polyethylene materials, such as their deformation in response to stress, depend on the composition and connectivity of the noncrystalline interlamellar phase. Even with spectral editing, the 13C NMR signals from segments in the crystalline-noncrystalline interfacial region, or generally with intermediate mobility, usually overlap with signals from either the noncrystalline or the crystalline segments. We have introduced double inverse filtering as a systematic, robust approach to selectively observe the signals from these intermediate-mobility segments.

This approach is applied to a set of HDPE and LLDPE copolymers with 0.35-3.3 mol% hexene. Two branch types are delineated: (i) mobile amorphous branches with faster 13C spin lattice relaxation and more motional averaging of NMR interactions, and (ii) trans-rich limited-mobility branches with slower 13C spin lattice relaxation and less motional averaging. Using 1H spin-diffusion experiments and T1rH measurements, it is also shown that the limited-mobility butyl branches accumulate near the crystalline-noncrystalline interface. Their number is shown to remain approximately constant at about 0.5 mol% for the range of hexene content covered by this study. This roughly matches one branch immobilized at every point where a chain emanates from the crystal; for an HDPE with less than 0.5 mol% hexene, nearly all branches are found at the crystal surface.

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Wed Jan 01 00:00:00 UTC 2014