Intermolecular magnetic dipole-dipole-interaction in 1H NMR of polymer melts and intermolecular dynamic correlations in polymer chains

聚合物在熔体中的动力学几十年来一直被两个互补的模型--Rouse ansatz模型和Reptation模型所解释。他们分别描述了在短链和长链的极限情况下的相互作用，其中转变发生在纠缠开始时。单个节段的均方位移(MSD)经常被用作这些动力学的合适参数，节段本身被假设为抽象单元--内部旋转和振动发生在短得多的时间尺度上，并且不被上述模型所覆盖。MSD可以通过例如核磁共振场梯度方法或中子散射直接获得，并且理论预测，即特征时间相关性，被反复确认，因此Rouse和Retation模型被认为是令人满意的描述。此外，还有一些间接的方法来确定相关的量，首先需要提到的是在聚合物熔体研究中成功应用了大约30年的核磁共振弛豫。直到最近几年，人们才开发出一种方法，能够利用氚聚合物中的同位素稀释来分离链状分子的分子内和分子间的偶极相互作用。虽然这种方法最初的目的只是为了将MSD从松弛数据中分离出来，但它确实包含了关于熔体中长期动态关联的有价值的信息，这是经典模型所不能描述的。在这些研究的背景下，已经发现了与旋转模型的预测的系统偏差，首先是在核磁共振弛豫测量法中，后来也通过替代方法，特别是通过分析本研究项目第一阶段内的回声衰减函数。具体地说，我们发现，对于三种化学上不同的高分子量聚合物，分子间相互作用的贡献与旋转理论明显不同，这表明各向同性动力学模型的有效性，而不是假设a-在某个时间标度-刚性管上存在的旋转。在这个项目的计划继续中，我们希望涵盖聚氧乙烷作为合适的模型聚合物的一系列分子量和温度以及边界条件(限制)的组合；通过这种方法，我们想要验证我们早期的发现并加深我们对聚合物动力学的理解。为此，我们还采用了氢气和可能的碳弛豫研究。该项目将由Nail Fatkullin教授陪同，他将进一步完善理论描述--例如，通过具体计算链端的动力学，或通过解决核磁共振实验编码期间的时间演变--并将支持选择优化的实验参数，以允许区分并发模型方法。

The dynamics of polymers in the melt has been explained for several decades by two complementary models, the Rouse ansatz and the reptation model. They describe the interactions in the limiting cases of short and long chains, respectively, where the transition occurs at the onset of entanglements. The mean squared displacement (MSD) of individual segments is frequently employed as a suitable parameter for these dynamics, with the segments themselves being assumed as abstract units – internal rotations and vibrations occur on much shorter timescales and are not covered by the mentioned models.The MSD is directly accessible by, for instance, NMR field gradient methods or neutron scattering, and the theoretical predictions, i.e. the characteristic time dependences, were confirmed repeatedly, so that Rouse and reptation models are considered as satisfactory descriptions. In addition, indirect methods exist which determine related quantities, first and foremost one needs to mention NMR relaxation which has successfully been employed in polymer melt studies for about 30 years. Only during recent years methods have been developed that are able to separate between intra- and intermolecular dipolar interactions of the chain molecules by making use of isotopic dilution in deuterated polymers. While this approach initially served only the purpose to isolate the MSD from relaxation data, it does contain valuable information about long-range dynamic correlations in the melt which is insufficiently described by the classical models. In the context of these studies, systematic deviations from the predictions of the reptation model have been found, first in NMR relaxometry, later also by alternative methods, in particular by analysis of echo decay functions within the first period of this research project. Specifically, we have found, for three chemically different polymers of high molecular weight, a contribution of the intermolecular interaction that is clearly at variance with reptation theory and suggests the validity of isotropic dynamic models rather than reptation which assumes the existence of a – on a certain timescale – rigid tube. In the planned continuation of this project, we would like to cover a broad range of combinations of molecular weights and temperatures as well as boundary conditions (confinement) for poly (ethylene oxide) as a suitable model polymer; with this approach we want to verify our earlier findings and deepen our understanding of polymer dynamics. To this end, we also employ deuteron and possibly carbon relaxation studies. The project will be accompanied by Prof. Nail Fatkullin who will further refine the theoretical description – for instance by specifically computing the dynamics of chain ends, or by addressing the temporal evolution during encoding periods of NMR experiments – and who will support the choice of optimized experimental parameters for allowing discrimination between concurring model approaches.