We report a new technique for probing polymer dynamics through the
refocussing of multiple quantum (MQ) nuclear magnetic resonance (NMR)
coherences. The MQ-NMR experiment follows the correlated behavior of
multiple spin-1/2 nuclei interacting through dipolar couplings. Motion which
modulates the dipolar coupling strengths on the same time scale as the
experiment (∼1 to 20 kHz) alters the intensity of the observed coherences.
Temperature dependent 19F data are presented on polytetrafluoroethylene
samples of varying crystallinity. For the as-polymerized 98% crystalline
PTFE sample, a sharp increase in MQ coherence refocussing occurs, centered
at -298 K. The 64% crystalline melt-quenched sample shows a increase at the
same temperature but which has a lower intensity. Thus, the ∼298 K peak is
most associated with motion in the crystalline phase. This temperature is
intermediate between the two first order transition at 293 and 303 K.
Oscillations in the refocussed fractions are observed from 208 to 230 K for
the 98% crystalline sample, while this ratio is constant over the same
temperature range for the 64% crystalline sample. These oscillations may be
associated with paracrystalline defects found only in the first sample.
Thus, the MQ refocussing experiment is able to clearly differentiate between
polymer samples which have different thermal histories. The sharpness of the
MQ refocussing features and their variations in magnitude, shape, and sign
with temperature are signatures of the molecular level details of the
underlying dynamics which produce them.