Graf Robert, Ewen Bernd, Spiess Hans Wolfgang
Max-Planck-Institute for Polymer Research, P.O. Box 3148, 55028 Mainz, Germany.
J Chem Phys. 2007 Jan 28;126(4):041104. doi: 10.1063/1.2437166.
In view of the importance of molecular dynamics in condensed matter both time scale and geometry of such processes should be determined experimentally. Whereas many techniques are available for the former, only NMR spectroscopy and neutron scattering can provide detailed information on the latter. Because of the different time scales of the dynamics, which the two techniques can detect best, direct comparisons of probing the geometry of the dynamics in the same system are scarce. Here we present such a comparison for the complex rotational motion of the phenylene groups in amorphous polycarbonate based on published (2)H NMR and newly recorded (13)C NMR data covering a wide temperature range, and recent quasielastic neutron scattering (QENS) data. We show that the results of the two techniques are in remarkable agreement, provided the data are consistently analyzed. No evidence is found for additional motions characterized by 90 degrees flips recently deduced from QENS data alone. Instead, the phenylene motion in the glassy state displays a broad heterogeneous distribution of rotational angles, about 80 degrees in width, centered at a flip angle of 180 degrees , which stays essentially constant over a wide temperature range. Thus, the phenylene motion that can consistently be observed in NMR and neutron scattering experiments is sensitive to the local packing.
鉴于分子动力学在凝聚态物质中的重要性，此类过程的时间尺度和几何结构都应通过实验来确定。虽然有许多技术可用于确定前者，但只有核磁共振光谱法和中子散射能够提供关于后者的详细信息。由于这两种技术能够最佳检测的动力学时间尺度不同，在同一系统中对动力学几何结构探测进行直接比较的情况很少见。在此，我们基于已发表的氘核磁共振（²H NMR）和新记录的涵盖宽温度范围的碳-13核磁共振（¹³C NMR）数据以及近期的准弹性中子散射（QENS）数据，对非晶态聚碳酸酯中亚苯基的复杂旋转运动进行了这样的比较。我们表明，只要对数据进行一致的分析，这两种技术的结果就会显著一致。没有发现仅从QENS数据推断出的以90度翻转特征的额外运动的证据。相反，玻璃态下亚苯基的运动表现出旋转角度的广泛非均匀分布，宽度约为80度，以180度的翻转角度为中心，在宽温度范围内基本保持不变。因此，在核磁共振和中子散射实验中能够一致观察到的亚苯基运动对局部堆积敏感。
