THE CRITICAL OVERLAP CONCENTRATION MEASURED BY PULSED-FIELD GRADIENT NUCLEAR-MAGNETIC-RESONANCE TECHNIQUES

The absence of any critical effects around c* in the diffusional behaviour of a range of molecular weights of narrow fraction polystyrene in tetrachloromethane has been re-examined. Analysis of the molecular weight dependent concentration coefficients k(f) of the concentration dependence shows sharp transitions between the dilute and semi-dilute behaviour. Spin-spin relaxation time measurements have also been performed and yield a value for c*. These values and other calculated and measured values are discussed in terms of the spectrum of motions present in a polymer system.