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-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

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

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

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

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

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

Supporting Information from Elucidation of the hierarchical structure of natural eumelanins

Eumelanin is one of the most ubiquitous pigments in living organisms and plays an important role in colouration and UV protection. Because eumelanin is highly cross-linked and insoluble in solvents, the chemical structure is still not completely known. In this study, we used atomic force microscopy, X-ray photoelectron spectroscopy and solid-state NMR to compare intact eumelanosomes (pigment granules mostly made of eumelanin) from four phylogentically distant species: cuttlefish (<i>Sepia officinalis</i>) inks, black coloured fish crow (<i>Corvus ossifragus</i>) feathers, iridescent wild turkey (<i>Melleagris gallopavo</i>) feathers and black coloured human hair. We found that eumelanosomes from all four species are composed of subunit nanoparticles with a length of 10–60 nm, consistent with earlier observations in eumelanosomes from the sepia ink and human hair. The solid-state NMR results indicate the presence of quinone methide tautomers in all four eumelanins. We also found clear differences in the UV absorbance, the ratio of 5,6-dihydroxyindole-2-carboxylic acid/5,6-dihydroxyindole and protonated aryl carbon ratios in sepia eumelanin relative to the other three. This comparison of natural eumelanin across a phylogenetically broad group of organisms provides insights into the change in the eumelanin structure over the evolutionary history and enables the production of synthetic eumelanin with properties that are similar to natural eumelanin