Chain-Folding Structure of a Semicrystalline Polymer in Bulk Crystals Determined by ¹³C–¹³C Double Quantum NMR

A unique approach using ¹³C–¹³C double quantum (DQ) NMR combined with selective ¹³C isotope labeling is proposed to investigate the chain trajectory of the synthetic polymer in bulk crystals. Since the DQ buildup curve highly depends upon coupled spin number, topology, and internuclear distance, which originated from the chain trajectory of selectively ¹³C-labeled polymers, the adjacent re-entry site and fraction under finite chain-folding number can be determined

Elucidation of the Chain-Folding Structure of a Semicrystalline Polymer in Single Crystals by Solid-State NMR

Despite tremendous efforts over the last half-century to elucidate the chain-folding (CF) structure of semicrystalline polymers, the re-entrance sites of folded chains, the successive CF number <i>n</i>, and the adjacent re-entry fraction <i>F</i> have not been well characterized due to experimental limitations. In this report, ¹³C–¹³C double-quantum (DQ) NMR was used to determine for the first time the detailed CF structure of ¹³C CH₃-labeled <i>isotactic</i> poly­(1-butene) (<i>i</i>PB1) in solution-grown crystals blended with nonlabeled <i>i</i>PB1 across a wide range of crystallization temperatures (<i>T</i>_cs). Comparison of the results of DQ experiments and spin dynamics simulations demonstrated that the majority of individual chains possess completely adjacent re-entry structures at both <i>T</i>_c = 60 and ∼0 °C, as well as indicated that a low polymer concentration, not kinetics, leads to cluster formations of single molecules in dilute solution. The changes in crystal habits from hexagonal shapes at <i>T</i>_c = 60 °C to rounded shapes at ∼0 °C (kinetic roughness) are reasonably explained in terms of kinetically driven depositions of single molecule clusters on the growth front

Chain Trajectory and Crystallization Mechanism of a Semicrystalline Polymer in Melt- and Solution-Grown Crystals As Studied Using ¹³C–¹³C Double-Quantum NMR

The re-entrance sites, successive chain-folding number ⟨<i>n</i>⟩, and chain-folding fraction ⟨<i>F</i>⟩ of the chain-folding (CF) structure of ¹³C CH₃-labeled <i>isotactic</i> poly­(1-butene) (<i>i</i>PB1) with an weight-averaged molecular weight (⟨<i>M</i>_w⟩ = 37 K g/mol) in solution- and melt-grown crystals as a function of crystallization temperature (<i>T</i>_c) were determined using solid-state (SS) NMR. The solution- and melt-grown crystals possessed adjacent re-entry structures between the right- and left-handed stems along the (100) and (010) planes, which were invariant as a function of <i>T</i>_c. The adjacent re-entry structures in the former exhibited long-range order (⟨<i>n</i>⟩ ≥ 8) compared with that in the latter (⟨<i>n</i>⟩ ≥ 1.7–2). These results indicated that the concentration and entanglement of polymers play significant roles in the CF process and structural formation during the initial stage of crystallization, whereas kinetics does not. Transmission electron microscopy (TEM) revealed well-defined hexagonal and circular crystals grown from the solution state at <i>T</i>_c = 60 and ∼0 °C, respectively. The morphological and molecular-level structural data demonstrated that kinetics influences the structural formations of polymers differently at different length scales during crystallization. Moreover, SS-NMR, small-angle X-ray scattering (SAXS), and atomic force microscopy (AFM) indicated that the crystallinity (χ_c) and lamellar thickness (⟨<i>l</i>_c⟩) of the melt-grown crystals are highly dependent on <i>T</i>_c, whereas in the solution-grown crystals, these parameters are independent of <i>T</i>_c. The experimental results and molecular dynamics, as reported in the literature, indicated that both χ_c and ⟨<i>l</i>_c⟩ are primarily determined by the molecular dynamics of the stems after deposition of the chains on the growth front (late process)

Chemical Reactions and Their Kinetics of <i>atactic</i>-Polyacrylonitrile As Revealed by Solid-State ¹³C NMR

Inter- and intramolecular chemical reactions and their kinetics for ¹³C-labeled <i>atactic</i>-polyacrylonitrile (<i>a</i>PAN) powder heat-treated at 220–290 °C under air and vacuum were investigated by various solid-state nuclear magnetic resonance (ssNMR) techniques. By applying ¹³C direct polarization magic angle spinning (DPMAS) as well as through-bond and through-space double quantum/single quantum ssNMR techniques, it was concluded that <i>a</i>PAN heat-treated under air at 290 °C for 300 min adopted the ladder formation, namely, conjugated six-membered aromatic rings with partially cross-linked and oxidized rings and polyene components. In contrast, <i>a</i>PAN heat-treated under vacuum at the same condition thermally decomposed into oligomeric chains that were mainly composed of isolated aromatic rings connected by alkyl segments. Furthermore, early stages of the chemical reactions were investigated by ¹³C cross-polarization (CP) and DPMAS spectra. The latter provided quantitative information regarding the kinetics of the chemical reactions. As a result, it was shown that chemical reactions under oxygen occurred homogeneously with a higher activation energy (<i>E</i>_a) of 122 ± 3 kJ/mol compared to that of vacuum at 47 ± 2 kJ/mol. By comparing both chemical structures and kinetics under two different conditions, the chemical reaction mechanisms of <i>a</i>PAN will be discussed in detail

One-Pot, Room-Temperature Conversion of CO₂ into Porous Metal–Organic Frameworks

The conversion of CO2 into functional materials under ambient conditions is a major challenge to realize a carbon-neutral society. Metal–organic frameworks (MOFs) have been extensively studied as designable porous materials. Despite the fact that CO2 is an attractive renewable resource, the synthesis of MOFs from CO2 remains unexplored. Chemical inertness of CO2 has hampered its conversion into typical MOF linkers such as carboxylates without high energy reactants and/or harsh conditions. Here, we present a one-pot conversion of CO2 into highly porous crystalline MOFs at ambient temperature and pressure. Cubic [Zn4O­(piperazine dicarbamate)3] is synthesized via in situ formation of bridging dicarbamate linkers from piperazines and CO2 and shows high surface areas (∼2366 m2 g–1) and CO2 contents (>30 wt %). Whereas the dicarbamate linkers are thermodynamically unstable by themselves and readily release CO2, the formation of an extended coordination network in the MOF lattices stabilizes the linker enough to demonstrate stable permanent porosity

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

Polymer Chains Fold Prior to Crystallization

There are long-standing debates in crystallization mechanism of polymer chains at the molecular levels: Which comes first, chain folding or lamellae formation during crystallization? In this study, we report the local chain trajectory of 13C-labeled semicrystalline polymer in an extreme case of rapidly quenched glassy state as well as thermodynamically stable crystals formed via different pathways from glass and melt. Magnetically dipole interactions do not require a long-range order of molecular objects and thus enable us to trace the local chain trajectory of polymer chains even in a glassy state. To accurately characterize the local chain trajectory of polymer glass, the natural abundance effect on 13C–13C double-quantum (DQ) nuclear magnetic resonance (NMR) signal is re-examined using extended chain conformation. As results, it is found that glassy chains adopt the same adjacent re-entry structure (adjacent re-entry number, n = 1) with the melt- and cold-grown crystals. From these results, it is concluded that (i) folding occurs prior to crystallization and (ii) melt and cold crystallization do not induce additional folding but proceed with rearrangements of polymer chains in the existing templates

Molecular Structural Basis for Stereocomplex Formation of Polylactide Enantiomers in Dilute Solution

Poly­(l-lactide) (PLLA) and poly­(d-lactide) (PDLA) alternatively pack with each other and form stereocomplex crystals (SCs). The crystal habits of SCs formed in the dilute solution highly depend on the molecular weight (⟨<i>M</i>_w⟩). In this study, we investigated chain-folding (CF) structure for ¹³C labeled PLLA (<i>l</i>-PLLA) chains in SCs with PDLAs that have either high or low ⟨<i>M</i>_w⟩s by employing an advanced Double Quantum (DQ) NMR. It was found that the ensemble average of the successive adjacent re-entry number ⟨<i>n</i>⟩ for the <i>l</i>-PLLA chains drastically change depending on ⟨<i>M</i>_w⟩s of the counter PDLA chains in the SCs. It was concluded that the CF structures of <i>l</i>-PLLA depending on ⟨<i>M</i>_w⟩s of PDLA determine the crystal habits of SCs