Two Chain-Packing Transformations and Their Effects on the Molecular Dynamics and Thermal Properties of α‑Form Isotactic Poly(propylene) under Hot Drawing: A Solid-State NMR Study

The chain packing, crystal thickness, molecular dynamics, and melting temperature of α-form <i>isotactic</i> polypropylene (<i>i</i>PP) drawn uniaxially at high temperatures of 100–150 °C were investigated using solid-state (SS) NMR and DSC. Two types of <i>i</i>PP samples with disordered (α₁) and relatively ordered (α₂-rich) packing structures were prepared via different thermal treatments and drawn up to an engineering strain (<i>e</i>) of approximately 20. High-resolution ¹³C NMR detected continuous α₂ → α₁ transformations in the original α₂-rich samples over the entire deformation range at all drawing temperatures (<i>T</i>_ds). A sudden α₁ → α₂ transformation was found to occur in the original α₁ sample in the small <i>e</i> range of approximately 3–7 at <i>T</i>_d = 140 °C. Then, in the late stage, the newly grown α₂ structure reversely transformed into α₁ structure with further increase in <i>e</i>, as observed in the original α₂-rich sample. These results indicate that at least two different processes are involved in large deformations. On the basis of crystallographic constraints, the continuous α₂ → α₁ transformation over the entire deformation range is attributed to molecular-level melting and recrystallization facilitated by chain diffusion. The steep α₁ → α₂ transformation in the smaller <i>e</i> range is assigned to isotropic melting and recrystallization induced by stress. After the large deformations (<i>e</i> ≈ 20) of the original α₂-rich and α₁ samples at <i>T</i>_d = 150 and 140 °C, respectively, ¹H spin diffusion verified increases in the crystal thickness in both the former (14.1 at <i>e</i> = 0 → 20.1 nm at <i>e</i> = 20) and the latter (9.2 → 17.0 nm). Centerband-only detection of exchange (CODEX) NMR at 120 °C demonstrated that the correlation time (⟨τ_c⟩) of the helical jump for the former drastically decreased from ⟨τ_c⟩ = 52.4 ± 5.2 at <i>e</i> = 0 to 9.3 ± 1.8 ms at <i>e</i> = 20 but slightly increased from 4.2 ± 1.3 to 7.1 ± 0.9 ms for the latter. Additionally, DSC indicated that the melting temperature (<i>T</i>_m) for the former decreased considerably from 173 °C at <i>e</i> = 0 to 165 °C at <i>e</i> = 20, whereas the melting temperature (<i>T</i>_m) remained nearly invariant at 163 °C for the latter. On the basis of these findings, we conclude that the local packing structure plays a crucial role in determining the molecular dynamics of the stems and <i>T</i>_m of largely deformed <i>i</i>PP materials. The established relations among the structures, the dynamics, and the thermal properties provide a useful guide to achieving improved properties of <i>i</i>PP materials under processing

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

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

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 Characterization of the Chemical Defects and Physical Disorders in α Form of Isotactic Poly(propylene) Synthesized by Ziegler–Natta Catalysts

The order–disorder phenomenon and spatial heterogeneity of chain packing, partitions of stereodefects, and molecular dynamics of α form of isotactic polypropylene (<i>i</i>PP) samples, which are synthesized by Zieglar–Natta catalysts, are investigated by solid-state (SS) NMR. High-resolution ¹³C NMR under high-power TPPM decoupling at field strengths of 110 kHz allows observation of the order–disorder phenomenon in the chain-packing structures of α form. High isotacticity samples (isotacticity at pentad level, ⟨<i>mmmm</i>⟩ = 99.4%) give a maximum ordered packing (α₂) fraction of 66% at crystallization temperature (<i>T</i>_c) of 155 °C while low stereoregularity samples (⟨<i>mmmm</i>⟩ = 91.0%) have only 47% at the same <i>T</i>_c. However, <i>M</i>_w (58.7–982 kg/mol) does not play a significant role in ordered packing formation. Using ¹³C-labeled CH₃ of <i>i</i>PP, direct spatial correlations between the α₂ and α₁ structures are investigated by ¹³C detection of two-dimensional (2D) ¹H–¹H spin-diffusion (CHHC) experiments. The time dependence of the spin-diffusion polarization transferred signal intensities determines the average domain size of the α₁ and α₂ structures of <i>i</i>PP crystallized at 150 °C, which was found to be 40 nm under an assumption of 2D spin diffusion. Additionally, the ¹³C filter CPMAS NMR spectrum on ¹³C CH₃-labeled <i>i</i>PP demonstrates that chemical defect is almost excluded from the crystalline region at <i>T</i>_c = 150 °C (defect free crystal) while ca. 2% is in melt quench sample. Moreover, ¹³C centerband-only detection of exchange experiments on α₂-rich sample with highest ⟨<i>mmmm</i>⟩ = 99.4% indicate that crystalline dynamics follows a single Arrhenius plot with an activation energy of 116 kJ/mol across reported order–disorder transition temperatures (157–159 °C)

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

Composition and Function of Spider Glues Maintained During the Evolution of Cobwebs

Capture silks are an interesting class of biological glues that help spiders subdue their prey. Viscid capture silk produced by the orb web spiders is a combination of hygroscopic salts that aid in water uptake and interact with adhesive glycoproteins to make them soft and sticky. The orb was a stepping stone to the evolution of new web types, but little is known about the adhesives in these webs. For instance, cobweb spiders evolved from orb-weaving ancestors and utilize glue in specialized sticky gumfoot threads rather than an elastic spiral. Early investigation suggests that gumfoot adhesives are quite different viscid glues because they lack a visible glycoprotein core, act as viscoelastic fluids rather than solids, and are largely invariant to humidity. Here, we use spectroscopic and staining methods to show that the gumfoot silk produced by Latrodectus hesperus (western black widow) is composed of hygroscopic organic salts and water insoluble glycoproteins, similar to viscid silk, in addition to a low concentration of spider coating peptides reported before. Our adhesion studies reveal that the organic salts play an important role in adhesion, similar to that seen in orb web spiders, but modulating function at much lower humidity. Our work shows more similarities in the viscid silk produced by orb web and cobweb spiders than previously anticipated and provide guidelines for developing synthetic adhesives that can work in dry to humid environments