Equilibration of melts of initially un-entangled polymers

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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.