OBSERVATION OF SLOW ATOMIC MOTIONS CLOSE TO THE GLASS-TRANSITION USING 2-D SI-29 NMR

Experiments were performed to investigate atomic re-arrangements just above the glass transition in potassium tetrasilicate (K2Si4O9). These experiments employed two-dimensional NMR techniques which were developed to study chemical exchange in non-viscous liquids and organic polymers. The major feature of the 29Si 2-D NMR exchange spectra of potassium tetrasilicate above its glass transition was a slow exchange (<1 kHz) between the silicon Q3 and Q4 environments. A similar experiment performed just below the glass transition showed no such exchange, which suggests that this Q species exchange is the motion that is 'frozen out' as a viscous liquid cools through its glass transition. The results are consistent with a very local region of atomic re-arrangement and contradict a polymeric view of silicate liquids. © 1990.