Equilibration of melts of initially un-entangled polymers

通过特殊催化剂的控制结晶，已有可能获得含有单链的聚合物晶体。当这种长链聚合物的晶体被加热到熔点以上时，就形成了最初未纠缠的聚合物的熔体。随着时间的推移，在熵力的驱动下，纠缠密度逐渐增大，最终形成完全纠缠链的平衡熔体。瞬间的纠缠产生了关联，这种关联相对于微观时间来说是非常持久的。它们随着脱离时间的延长而衰减，而脱离时间与摩尔质量密切相关。本项目的目的是用场循环(FC)核磁共振弛豫法研究最初未纠缠的长链聚合物熔体以及向纠缠熔体平衡的过程。FC核磁共振提供自旋-晶格弛豫速率的频率依赖关系，通常在10 kHz到20 MHz(质子)的频率范围内。在过去的几年里，我们已经建立了被称为分子流变学的方法。与流变学实验类似，在不同温度下测量的弛豫谱可以合并为覆盖所有相关松弛过程的主曲线。纠缠的存在导致了低频下较强的色散效应。使用超高相对分子质量聚乙烯(UHMW PE)，可以选择足够长的解离时间，以监测从非纠缠熔体到纠缠熔体的平衡过程。无纠缠UHMW-PE将由S.Rastogi集团(英国拉夫堡大学)提供。作为参考和项目的开始，我们将研究缠绕UHMW PE和不同M的标准PE，以便在分子水平上检验管状爬行模型。

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.