Equilibration of melts of initially un-entangled polymers

通过用专用催化剂控制结晶，已经可以获得含有单链的聚合物晶体。当长链聚合物的这种晶体被加热到其熔点以上时，形成最初未缠结的聚合物的熔体。随着时间的推移，在熵力的驱动下，纠缠密度不断增加，最终建立起具有完全纠缠链的平衡熔体。短暂的纠缠产生了相关性，这些相关性相对于微观时间来说是非常长寿的。它们随脱离时间衰减，脱离时间强烈依赖于摩尔质量。本项目的目的是调查最初未纠缠的长聚合物链的熔体以及通过场循环（FC）NMR弛豫法对纠缠熔体的平衡过程。FC NMR提供自旋-晶格弛豫速率的频率依赖性，通常在10 kHz至20 MHz的频率范围内（质子）。在过去的几年里，我们已经建立了所谓的分子流变学的方法。类似于流变实验，在不同温度下测量的弛豫谱可以合并以产生覆盖所有相关弛豫过程的主曲线。纠缠的存在导致在低频下的强色散效应。通过使用超高分子量聚乙烯（UHMW PE），其已经实现了未缠结的聚合物晶体的生产，可以选择足够长的脱离时间以允许监测从未缠结到缠结熔体的平衡过程。未缠结的聚乙烯将由S组提供。Rastogi（英国拉夫堡大学）。作为参考和项目的开始，将研究缠结的UHMW PE以及具有不同M的标准PE，以在分子水平上测试管蠕动模型。

By controlled crystallization with specialized catalysts, it has become possible to obtain polymeric crystals which contain single chains. When such crystals of long chain polymers are heated above their melting point, a melt of initially un-entangled polymers is formed. In the course of time, driven by entropic forces, the entanglement density grows and finally the equilibrium melt with fully entangled chains is established. The transient entanglements create correlations, which are extremely long-lived with respect to the microscopic time. They decay with the disengagement time which depends strongly on molar mass. The aim of the present project is the investigation of melts of initially un-entangled long polymer chains as well as the process of equilibration towards the entangled melt by means of field-cycling (FC) NMR relaxometry. FC NMR provides the frequency dependence of the spin-lattice relaxation rate, typically in the frequency range 10 kHz to 20 MHz (protons). In the last years we have established the method as what has been coined molecular rheology. In analogy to rheological experiments the relaxation spectrum measured at differrent temperatures can be merged to yield master curves which cover all relevant relaxation processes. The presence of entanglement leads to strong dispersion effects at low frequencies. By using ultra-high molecular weight polyethylene (UHMW PE), for which the production of un-entangled polymer crystals has been achieved, the disengagement time can be chosen sufficiently long for allowing to monitor the equilibration process from the un-entangled to the entangled melt. Un-entangled UHMW-PE will be provided by the group of S. Rastogi (University of Loughborough, UK). As reference and as start of the project entangled UHMW PE as well as standard PE with varying M will be studied in order to test the tube-reptation model on molecular level.