Research ArticleMATERIALS SCIENCE

Immobilized 13C-labeled polyether chain ends confined to the crystallite surface detected by advanced NMR

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Science Advances  11 Sep 2020:
Vol. 6, no. 37, eabc0059
DOI: 10.1126/sciadv.abc0059
  • Fig. 1 Schematics of possible locations of chain ends (filled red circles) in the lamellar semicrystalline morphology.

    Mobile segments (assuming T >> Tg) are marked by a greater line thickness. (A) Chain ends as dangling cilia. The location of the chain ends in low-density regions deep in the amorphous layers would facilitate their large-amplitude motions. (B) Chain ends mostly confined to the crystallite surface, to reduce excess density (1) in the folds and loops near the crystalline-amorphous interface. (C) Chain ends inside the crystallite generating row vacancy defects. A pair of chain ends is shown in the center of the figure. (D) Chain ends inside the crystallite producing kink defects. The crystallite thickness is typically 5 to 30 nm (see also fig. S1).

  • Fig. 2 DP 13C NMR spectra of 13C-Ac2-POM.

    Top trace (red): Quantitative DP spectrum obtained with a 125-s recycle delay, with an integral ratio of 13COO: O-CH2-O = 1.04:1; note that the chain-end signal is enhanced 90-fold by isotope labeling. Lower trace (blue): Selective DP spectrum of fast-relaxing, mobile O-CH2-O segments obtained with a 0.5-s recycle delay.

  • Fig. 3 Immobilized chain ends identified by CSA recoupling.

    (A and B) 13C CSA powder spectra of ester groups obtained by SUPER 13C NMR after cross-polarization (CP; top traces, blue, solid-like) and 1-s DP (bottom traces, red). (A) Spectra of the backbone esters in PCL shown for reference, extracted at the crystalline (blue) and amorphous (red) resonance positions. (B) Spectra of the 13COO acetyl chain ends in 13C-Ac2-POM. (C to E) Separation of 13C NMR signals of immobile and mobile ester groups by a 3 + 2–pulse CSA filter (26) with a 32-μs delay between first and second π-pulse. Thick orange line: Spectrum S after the CSA filter, from mobile segments with a motionally averaged small CSA. Thin blue line: Full spectrum (S0) of all esters recorded with CSA dephasing. Dashed green line: Difference S0S, mostly of esters with limited mobility. (C) Spectra of crystalline PCL selected by CP. (D) Spectra of mobile, amorphous PCL selected by DP with a 2-s recycle delay. (E) Comprehensive spectra of chain-end 13COO groups in 13C-Ac2-POM with and without CSA filtering, after DP with 70-s recycle delay (fully relaxed).

  • Fig. 4 Selection of signals of immobilized segments by quasi-static 1H DQ filtering for 2 × 6 μs, followed by 0.4-ms CP to 13C.

    Spinning frequency: 4 kHz. The inset shows the pulse sequence; for details, see the Supplementary Materials. (A and B) Demonstration of successful suppression of signals of mobile amorphous segments in (A) PE with 2.4 mole percent ethyl branches and (B) PCL (O-CH2 resonance). In addition to an unselective spectrum with the same CP time (middle, blue trace), a quantitative DP or multi-CP spectrum is also shown (top, green trace in each figure). DQf, DQ filtered. (C) Application to 13C-Ac2-POM.

  • Fig. 5 13C-detected 1H spin diffusion from the amorphous regions in 13C-Ac2-POM.

    (A) Series of spectra of the COO and O-CH2-O signals as a function of spin diffusion time, after 200 μs of T2H filtering that removes the crystalline 1H magnetization and chemical shift filtering that suppresses the CH3 1H magnetization, at 10-kHz MAS. (B) Time evolution of the chain-end 13COO resonance (filled triangles, top trace) and crystalline O-CH2-O resonance (open circles, bottom trace) as a function of the square root of the 1H spin diffusion time. The curves show the simulated time dependences of magnetization in the amorphous (dashed), surface (red), and crystalline (blue) regions. The magnetization distribution at three time points is shown schematically in the insets, explaining the maximum of the surface magnetization at tSD = 1 ms.

  • Fig. 6 Close contacts between chain ends in 13C-Ac2-POM detected by CODEX 13C NMR with dipolar spin exchange.

    (A) Normalized CODEX 13C NMR signal (S/S0) as a function of the square root of the 13C spin-exchange mixing time (tm1/2), up to tm = 5 s. A good fit of the CODEX decay is achieved with ~30% of the chains on the crystallite surface terminated by 13C-Ac groups when the surface is a (21¯14) face of the POM crystal (see the Supplementary Materials for simulation details). Because there are nine distinct orientations of segments on the 95 helix of POM, the intensity at long tm approaches 1/9. (B) Schematic illustration of the distribution of chain ends (red) on a crystallite surface that is a (21¯14) plane in the crystal structure of POM. The chains in the crystallite are tilted relative to the normal of the crystallite surface (black outlined quadrangle). The chains continuing from the crystallite into the interfacial layer are indicated by green column segments close to the crystallite surface.

Supplementary Materials

  • Supplementary Materials

    Immobilized 13C-labeled polyether chain ends confined to the crystallite surface detected by advanced NMR

    Shichen Yuan and Klaus Schmidt-Rohr

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    • Sections S1 and S2
    • Table S1
    • Figs. S1 to S9
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