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 (l) 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 l-L/D blends. Moreover, the 33% ¹³CH₃-labeled signals could determine the fraction of only l-L in the SC (Φ_L) and amorphous region. These two data sets allowed us to determine the stoichiometry of l-L/D in the SC (Φ_L-SC/Φ_D-SC) to be 1/1. ¹³C–¹³C double-quantum (DQ) buildup curves of l-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.

中文翻译：

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固态 NMR 和 13C 同位素标记揭示的聚乳酸立体络合物的化学计量和堆积结构

聚 ( l-乳酸) (L)/聚 ( d-乳酸) (D) 共混物以 7/3–3/7 的混合比形成立体络合物 (SC)。SC 中 L/D 的化学计量和堆积结构是有争议的话题，因为 SC 是半结晶的并且因为对映体对具有相同的化学结构。在这项研究中，使用固态 (SS) NMR 研究了混合比为 7/3–3/7的 33% ¹³ C CH ₃标记 ( l ) L/未标记 D 共混物的化学计量和堆积结构。自然丰度的^{13 C CO 信号提供了 SC (Φ} _SC )、α 和l -L/D 混合物的无定形区域的分数。此外，33% ¹³CH ₃标记的信号可以确定在 SC (Φ _L ) 和非晶区中只有l -L 的分数。这两个数据集使我们能够确定 SC 中l -L/D 的化学计量比 (Φ _L-SC /Φ _D-SC ) 为 1/1。即使在不同的混合比下，SC中l -L 的¹³ C– ¹³ C 双量子 (DQ) 累积曲线也遵循一条通用曲线。比较实验和模拟的 DQ 曲线得出结论，所有 SC 晶体在不同的混合比下都采用规则的堆积结构。