2002/01/26 by Qiu, M. D. Ediger
Chemical Engineering · Materials Science · #Rheology and Fluid Dynamics Studies #Material Dynamics and Properties #Polymer crystallization and properties
paper · doi:10.1021/ma010597i
13 C NMR T 1, NOE, and T 1 ρ were measured for two entangled polyethylene melts ( M = 16K and 37K) at 5 and 75 MHz 13 C Larmor frequencies from the melting point up to 530 K. Two models for entanglement dynamics are tested against the experimental data. This comparison shows that entanglements not only stretch out those relaxation times longer than the entanglement time τ e but also have a spatial effect in restricting the motion of entangled points on the time scale of τ e . For polyethylene, the amplitude of residual orientation on the time scale of τ e is lower than the theoretical prediction for a Gaussian chain of the entanglement length with both ends fixed. The opposite conclusion was drawn from recent double quantum NMR experiments on polybutadiene. The magnetization decay curve for the T 1 ρ experiment is nonexponential, indicating that the loss of orientation on long time scales depends on position along the chain backbone.