Stabilization of <i>Atactic</i>-Polyacrylonitrile under Nitrogen and Air As Studied by Solid-State NMR

Solid-state (ss) NMR spectroscopy was applied to study the stabilization process of 30 wt % ¹³C-labeled <i>atactic</i>-polyacrylonitrile (<i>a</i>-PAN) heat-treated at various temperatures (<i>T</i>_s) under nitrogen and air. Direct polarization magic-angle spinning (DP/MAS) ¹³C NMR spectra provided quantitative information about the functional groups of stabilized <i>a</i>-PAN. Two dimensional (2D) refocused ¹³C–¹³C INADEQUATE and ¹H–¹³C HETCOR NMR spectra gave through-bond and through-space correlations, respectively, of the complex intermediates and final structures of <i>a</i>-PAN stabilized at different <i>T</i>_s values. By comparing 1D and 2D NMR spectra, it was revealed that the stabilization process of <i>a</i>-PAN under nitrogen is initiated via cyclization, while the stabilization under air proceeds via dehydrogenation. Different initial processes lead to the isolated aromatic ring and ladder formation of the aromatic rings under nitrogen and air, respectively. Side reactions and intermediate structures are also discussed in detail. Through this work, the stabilization index (SI) was defined on the basis of the quantified C-1 and C-3 DP/MAS spectra. The former reached 0.87 at <i>T</i>_s = 370 °C, and further higher <i>T</i>_s values did not affect SI; however, the latter continuously increased up to 0.66 at <i>T</i>_s = 450 °C. All of the experimental results indicated that oxygen plays a vital role on the whole reaction process as well as the final products of stabilized <i>a</i>-PAN

Solid-State NMR Study of the Chain Trajectory and Crystallization Mechanism of Poly(l‑lactic acid) in Dilute Solution

The nucleation and growth mechanisms of semicrystalline polymers are a controversial topic in polymer science. In this work, we investigate the chain-folding pattern, packing structure, and crystal habits of poly­(l-lactic acid) (PLLA) with a relatively low molecular weight, ⟨<i>M</i>_w⟩ = 46K g/mol, and PDI = 1.4 in single crystals formed from dilute amyl acetate (AA) solution (0.05 or 0.005 wt %) at a crystallization temperature (<i>T</i>_c) of 90, 50, or ∼0 °C. The crystal habits drastically changed from a facet lozenge shape at <i>T</i>_c = 90 °C to dendrites at ∼0 °C, whereas the chains adopt a thermodynamically stable α packing structure at both 90 and 0 °C. Comparing the experimental and simulated ¹³C–¹³C double quantum (DQ) buildup curves of ¹³C-labeled PLLA chains in crystals blended with nonlabeled chains at a mixing ratio of 1:9 indicates that the PLLA chains fold adjacently in multiple rows when the <i>T</i>_c ranges from 90 to ∼0 °C. The results at different length scales suggest that (i) a majority of the chains self-fold in dilute solution and form baby nuclei (intramolecular nucleation) and (ii) the intermolecular aggregation process (secondary nucleation), which is dominated by kinetics, results in morphological differences

Stoichiometry and Packing Structure of Poly(lactic acid) Stereocomplex as Revealed by Solid-State NMR and ¹³C Isotope Labeling

Poly­(l-lactic acid) (L)/poly­(d-lactic acid) (D) blends form a stereocomplex (SC) at a mixing ratio of 7/3–3/7. The stoichiometry and packing structure of L/D in the SC are controversial topics because the SC is semicrystalline and because the enantiomeric pair has the same chemical structure. In this study, both the stoichiometry and packing structure of 33% ¹³C CH₃-labeled (<i>l</i>) L/nonlabeled D blends at mixing ratios of 7/3–3/7 were investigated by using solid-state (SS) NMR. The ¹³C CO signals in natural abundance provided the fractions of the SC (Φ_SC), α, and amorphous regions of <i>l</i>-L/D blends. Moreover, the 33% ¹³CH₃-labeled signals could determine the fraction of only <i>l</i>-L in the SC (Φ_L) and amorphous region. These two data sets allowed us to determine the stoichiometry of <i>l</i>-L/D in the SC (Φ_L‑SC/Φ_D‑SC) to be 1/1. ¹³C–¹³C double-quantum (DQ) buildup curves of <i>l</i>-L in the SC followed one universal curve even at different mixing ratios. Comparison of the experimental and simulated DQ curves led to the conclusion that all SC crystals adopt a regular packing structure at varied mixing ratios

Unfolding of <i>Isotactic</i> Polypropylene under Uniaxial Stretching

Despite numerous investigations on polymer processing, understanding the deformation mechanisms of semicrystalline polymer under uniaxial stretching is still challenging. In this work, ¹³C–¹³C Double Quantum (DQ) NMR was applied to trace the structural evolution of ¹³C-labeled <i>isotactic</i> polypropylene (<i>i</i>PP) chains inside the crystallites stretched to an engineering strain (<i>e</i>) of 21 at 100 °C. DQ NMR based on spatial proximity of ¹³C labeled nuclei proved conformational changes from the folded chains to the locally extended chains induced by stretching. By combining experimental findings with literature results on molecular dynamics, it was concluded that transportation of the crystalline chains plays a critical role to achieve large deformability of <i>i</i>PP

Folding of Polymer Chains in the Early Stage of Crystallization

Understanding the structure formation of an ordered domain in the early stage of crystallization is one of the long-standing issues in polymer science. In this study, we investigate the chain trajectory of <i>isotactic</i> polypropylene (<i>i</i>PP) formed via rapid and deep quenching, using solid-state NMR spectroscopy. Comparisons of experimental and simulated ¹³C–¹³C double quantum (DQ) buildup curves demonstrated that individual <i>i</i>PP chains adopt adjacent re-entry sequences with an average folding number ⟨<i>n</i>⟩ = 3–4 in the mesomorphic form, assuming an adjacent re-entry fraction ⟨<i>F</i>⟩ of 100%. Therefore, long flexible polymer chains naturally fold in the early stage of crystallization, and folding-initiated nucleation results in formation of mesomorphic nanodomains

Balancing Charge Injection via a Tailor-Made Electron-Transporting Material for High Performance Blue Perovskite QLEDs

One of the great challenges in perovskite quantum dot light-emitting diodes (Pe-QLEDs) is the unbalanced charge injection that significantly hinders the device performance and stability. Herein, we tailor-made a high mobility electron-transporting material (ETM), named B2, to balance the carrier injection in blue Pe-QLEDs. B2 with a tailored asymmetric anthracenyl structure exhibits a promising electron mobility of 2.7 × 10–4 cm2·V–1·s–1, which is almost 20 times higher than the commonly used ETM-TPBi (1.1 × 10–5 cm2·V–1·s–1). Subsequently, sky blue (490 nm) Pe-QLED with B2 as the ETM presented a remarkably high external quantum efficiency (EQE) of 13.17% and a low turn-on voltage of 2.2 V, which is much better than that of the TPBi-based device (EQE of 8.31% and Vturn‑on of 3.2 V). In addition, B2 also demonstrated a universal application in green and deep blue Pe-QLEDs. This work provides an important guidance to rational design of high electron mobility ETMs for high-performance LEDs