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

The absence of any critical effects around c in the diffusional behav iour of a range of molecular weights of narrow fraction polystyrene in tetrachloromethane has been re-examined. Analysis of the molecular we ight dependent concentration coefficients k(f) of the concentration de pendence shows sharp transitions between the dilute and semi-dilute be haviour. Spin-spin relaxation time measurements have also been perform ed and yield a value for c. These values and other calculated and mea sured values are discussed in terms of the spectrum of motions present in a polymer system.