Spontaneous Crystallization of the Planar Zigzag Form of Syndiotactic Polypropylene at 0 °C
Spontaneous Crystallization of the Planar Zigzag Form of Syndiotactic Polypropylene at 0 °C
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DOI：
10.1021/ma980032v
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发表时间：
1998-04
期刊：
影响因子：
5.5
通讯作者：
T. Nakaoki;Y. Ohira;H. Hayashi;F. Horii
中科院分区：
文献类型：
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作者：
T. Nakaoki;Y. Ohira;H. Hayashi;F. Horii
介绍。最近，Ewen等人合成了具有高度立体规整性的共规聚丙烯（sPP）。尽管使用较少共规性样品进行的早期研究在精确表征方面存在困难，但高度共规性聚合的成功使得对晶体结构的详细分析成为可能。对sPP的分子链构象和晶体中的分子堆积也有大量的研究报道。对于多晶型，已知有四种晶型：平面之字形、2-5 t2g2t6g2、6-8和2种t2g2构象。在这些晶体中，具有平面之字形构象的晶体被认为是在熔体淬火成冰水混合物后通过冷拔形成的。最近，我们发现平面之字形的结晶是在0℃下自发诱导的，没有任何机械应力，尽管生长速度非常缓慢。在这篇通讯中，我们简要地报告了首次证实这种自发结晶形式的结果，这些结果是通过CP/MAS 13C NMR和广角x射线散射方法获得的。实验部分。使用由住友化学株式会社提供的高度共规聚丙烯样品，其rr分数为0.96，无需进一步纯化。在静态磁场4.7 t的条件下，采用Bruker MSL 200光谱仪在室温下测量了CP/MAS 13C的核磁共振波谱，实现了3 kHz的魔角旋转。交叉极化过程的接触时间为1 ms，脉冲延迟时间为4 s。13C化学位移表示为相对于四甲基硅烷（Me4Si）的值，以甘氨酸晶体在176.03 ppm时的CO碳线为外部参考。在室温下，用自动衍射仪在ni过滤的Cu - KR辐射下获得了广角x射线衍射图。结果和讨论。图1是在室温下测量的sPP薄膜在170℃的冰水中淬火后的CP/MAS 13C NMR谱图。图中所示的时间表示每张胶片在冰水中保存的时间。如图1a所示，在冰水中淬火后立即取出的薄膜的光谱与先前报道的光谱相似；在47.7和39.1 ppm时，亚甲基碳的两条共振线清晰地出现。虽然47.7 ppm的下场线似乎是由包括非晶成分在内的多种成分组成的，但一条与39.1 ppm的上场线形成双线的线显然包括在47.7 ppm的线中。这种双重态分裂是由所谓的γ-间扭效应7引起的，在晶体区域的t2g2螺旋结构中具有不同环境的亚甲基碳。当样品在冰水中浸泡数小时后，光谱轮廓开始逐渐变化。与48.2、27.0和18.8 ppm下的新谱线相比，由于t2g2构象的信号强度随着在0℃下保温时间的增加而降低。根据前人对冷拔sPP样品的报道，5、21、22后的新线归因于具有平面之字形构象的晶体。此外，13C自旋-晶格弛豫时间（T1C）测量证实，在48.2和27.0 ppm时，谱线中含有T1C 30-50 s的组分，这可能与晶体组分相对应，同时也含有T1C 1-5 s的非晶组分，并且前者的比例随着保温时间的增加而增加。
Introduction. Recently, syndiotactic polypropylene (sPP) with high stereoregularity was synthesized by Ewen et al. 1 Whereas early investigations carried out by using less syndiotactic samples had difficulties in precise characterization, the success of highly stereoregular polymerization made the detailed analysis of the crystal structure possible. A large number of investigations on the molecular chain conformation and molecular packing in crystals have also been reported for sPP. As for the polymorphs, four crystalline forms are known: the planar zigzag, 2-5 t2g2t6g2, 6-8 and two forms with the t2g2 conformation. 9-22 Of these, the crystals with the planar zigzag conformation have been believed to be formed only by cold drawing after quenching from the melt into an ice-water mixture. 2-5 Very recently, we have found that the crystallization of the planar zigzag form is spontaneously induced at 0 C without any mechanical stress, although the growth rate is very slow. In this communication, we report briefly the first results to confirm such spontaneous crystallization of the form at 0 C, which have been obtained by CP/MAS 13C NMR and wide-angle X-ray scattering methods. Experimental Section. A highly syndiotactic polypropylene sample with an rr fraction of 0.96, provided by Sumitomo Chemical Co. Ltd., was used without further purification. CP/MAS 13C NMR spectra were measured at room temperature on a Bruker MSL 200 spectrometer under a static magnetic field of 4.7 T. Magic angle spinning at 3 kHz was achieved with the double air bearing system. The contact time for the cross polarization process was 1 ms and the pulse delay time was 4 s. 13C chemical shifts were expressed as values relative to tetramethylsilane (Me4Si) by using the CO carbon line at 176.03 ppm of glycine crystals as an external reference. Wide-angle X-ray diffraction patterns were obtained at room temperature on an automatic RIGAKU diffractometer with Ni-filtered Cu KR radiation.Results and Discussion. Figure 1 shows CP/MAS 13C NMR spectra of sPP films quenched in ice-water from the melt at 170 C, which were measured at room temperature. The times shown in this figure indicate the periods for which each film was kept in ice-water. As shown in Figure 1a, the film taken out immediately after quenching in ice-water gives a spectrum similar to the previously reported one; 23 two resonance lines assignable to the methylene carbons clearly appear at 47.7 and 39.1 ppm. Although the downfield line at 47.7 ppm seems to be composed of multiple components including a noncrystalline one, a line forming a doublet with the upfield line at 39.1 ppm is evidently included in the line at 47.7 ppm. 21, 22 Such doublet splitting is induced by the so-called γ-gauche effect7 for the methylene carbons with different environments in the t2g2 helical structure in the crystalline region. The spectral profile begins to change progressively when specimens are soaked in ice-water for a few hours. The signals due to the t2g2 conformation are reduced in intensity with increasing time for holding at 0 C in contrast to new lines at 48.2, 27.0, and 18.8 ppm which increase in intensity. According to the previous reports for the cold drawn sPP samples, 5, 21, 22 the latter new lines are ascribed to the crystals with the planar zigzag conformation. Moreover, 13C spin-lattice relaxation time (T1C) measurements have confirmed that the lines at 48.2 and 27.0 ppm contain the component with T1C) 30-50 s, which may correspond to the crystalline component, together with the noncrystalline contribution with T1C) 1-5 s and the former fraction increases with the time of holding at 0 C. It is, therefore, strongly …