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)

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