Three-Dimensional Conformation of Folded Polymers in Single Crystals

The chain-folding mechanism and structure of semicrystalline polymers have long been controversial. Solid-state NMR was applied to determine the chain trajectory of ${}^{13}\mathrm{C}$ ${\mathrm{CH}}_3$-labeled isotactic poly(1-butene) ($i\mathrm{PB}1$) in form III chiral single crystals blended with nonlabeled $i\mathrm{PB}1$ crystallized in dilute solutions under low supercooling. An advanced ${}^{13}\mathrm{C}\text{−}{}^{13}\mathrm{C}$ double-quantum NMR technique probing the spatial proximity pattern of labeled ${}^{13}\mathrm{C}$ nuclei revealed that the chains adopt a three-dimensional (3D) conformation in single crystals. The determined results indicate a two-step crystallization process of (i) cluster formation via self-folding in the precrystallization stage and (ii) deposition of the nanoclusters as a building block at the growth front in single crystals.

Figure 1

Schematic of polymer crystallization. (a) One-step: subsequent stem deposition on the surface of existing crystals. (b) Two-step: cluster formation via folding during the precrystallization stage and deposition of the clusters on the crystal surface. The ${}^{13}\mathrm{C}$-labeled chains and atoms are colored blue and red, respectively.

Figure 2

(a) AFM amplitude error image of $i\mathrm{PB}1$ form III crystallized at $50°\mathrm{C}$. (b) ${}^{13}\mathrm{C}$ $T_{1\rho \mathrm{H}}$-filtered DQ (green) and SQ (black) spectrum of 35% ${}^{13}\mathrm{C}$-labeled $i\mathrm{PB}1$ form III measured at $-20°\mathrm{C}$. Chain-packing structure of $i\mathrm{PB}1$ form III on the (001) plane and ${}^{13}{\mathrm{CH}}_3\text{−}{}^{13}{\mathrm{CH}}_3$ internuclear distances at (c) the $B$ and (d) $A$ sites colored blue and red, respectively. The unit for all distances is Å. The numbers in parentheses indicate the distance between the overlapping carbons with different $z$ coordinates on the (001) plane. (e) Experimental (open circles) $\xi$ curves of 35% ${}^{13}\mathrm{C}$-labeled $i\mathrm{PB}1$ form III at $-20°\mathrm{C}$ and simulation curves (solid lines) with the shortest ${}^{13}\mathrm{C}\text{−}{}^{13}\mathrm{C}$ internuclear distances of 3.8 Å between the neighboring stems at both the $A$ (red) and $B$ sites (blue) and exponential $T_2$ values of 8.2 and 8.0 ms, respectively. (f) 2D DQ/SQ NMR spectrum of 35% ${}^{13}\mathrm{C}$-labeled $i\mathrm{PB}1$ with ${\tau }_{\text{ex}}=7.06\mathrm{ms}$ at $-20°\mathrm{C}$.

Figure 3

Various CF models as 2D clusters and the corresponding simulated DQ NMR of $i\mathrm{PB}1$ form III at $T_{\mathrm{c}}=50°\mathrm{C}$: (a) CF0, (b) CFA, (c) CFB, and (c) CFAB models. The red ($A$ site) and blue ($B$ site) solid curves are the calculated results of (a) CF0 and are based on $⟨n⟩=13$ and (b) CFA with $⟨F_{\mathrm{CFA}}⟩=100\%$, (c) CFB with $⟨F_{\mathrm{CFA}}⟩=100\%$, and (d) CFAB with $⟨F_{\mathrm{CFA}}⟩=90\%$ under the assumption of CF0 contributing to DQ efficiency as the remaining fraction $100\text{−}⟨F⟩\%$. The red ($A$ site) and blue ($B$ site) open circles are the experimental DQ buildup curves of ${}^{13}\mathrm{C}$-labeled $i\mathrm{PB}1$ chains blended with nonlabeled chains ($1/9$).

Figure 4

(a) 2D DQ/SQ NMR spectrum of ${}^{13}\mathrm{C}$-labeled iPB1 blended with nonlabeled chains ($1/9$) with ${\tau }_{\text{ex}}=7.06\mathrm{ms}$; (b) and (c) show cluster models and DQ buildup curves of 10 wt % ${}^{13}\mathrm{C}$-labeled iPB1 blend in single crystals at $T_{\mathrm{c}}=50°\mathrm{C}$. The red ($A$ site) and blue ($B$ site) open circles are the experimental curves. The red ($A$ site) and blue ($B$ site) solid lines were calculated on the basis of (b) $⟨F⟩=70\%$ and $⟨n_{\text{stem}}⟩=9$ and (c) $⟨F⟩=70\%$ and $⟨n_{\text{stem}}⟩=14$. (d) 2D molecular views of the $⟨n_{\text{stem}}⟩=9$ and 14 cluster models. (e) Mixture model of 2D (CFAB with $⟨F⟩=15\%$ and $⟨n⟩=13$) and 3D ($⟨F⟩=55\%$ and $⟨n_{\text{stem}}⟩=12$, 3 layers) clusters, including CF0 with $⟨F⟩=30\%$. The illustration represents a 2D and a 3D cluster.