An NMR Investigation of a Polymer Melt under Shear

Mechanical shear is part of processing steps of polymers in the melt. Shear influences polymer dynamics and chain order in the melt. The effect of mechanical shear on the NMR relaxation behaviour of polymer melts has been studied. A dedicated probehead capable for high-temperature rheo NMR has been developed and applied. Both the transverse and the longitudinal relaxation time exhibit a strong temperature dependence. The prolongation of the longitudinal relaxation time T-1 with increasing temperature is attributed to slowed down spin diffusion as a function of the enhanced motion. The enhanced mobility of polymer segments between the entanglements is directly reflected in the prolongation of the transverse relaxation time T-2. Shearing the polymer sample results in an increase of the transverse relaxation time, indicating enhanced molecular mobility which implies, that as an effect of the shear a fraction of entanglements in the polymer have been lost, resulting in longer chain segments, these in turn exhibit an increased molecular mobility as manifested in a prolongated T-2.