Dynamics of Crystalline and Amorphous Polytetrafluoroethylene Studied by Multiple Quantum NMR
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DAVID A. LATHROP AND KAREN K. GLEASON Department of Chemical Engineering, MIT, Cambridge, MA 02139.
ABSTRACT 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.
INTRODUCTION The bulk properties of polymers are greatly influenced by molecular motion occurring within these solids. While a wide range of techniques including mechanical loss, dielectric loss, and NMR relaxation have been developed to study polymeric motion, complemenary information is sought via a new technique which relies on the refocusing of multiple quantum nuclear magnetic coherences (MQ NMR).l MQ coherences can involve anywhere from 2 to >100 nuclei, allowing the correlated motion for atomic groups of various sizes to be probed, rather than tracking the motion of a single site. Over a wide range of temperatures, this single technique is able to provide information which is complementary to the previously developed diagnostic techniques. In addition, unique information about the molecular level detail of motion is obtained.
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Mat. Res. Soc. Symp. Proc. Vol. 321. 01994 Materials Research Society
Refocussing of MQ coherences can be carried out using any polymer containing hydrogen and/or fluorine since these nuclei give rise to strong homonuclear dipolar couplings. Polymeric motion modulates these dipolar couplings through variations in internuclear distances and/or the orientation of the internuclear vectors with respect to the applied magnetic field of the NMR spectrometer. Traditional MQ NMR experiments prepare these MQ coherences using a ser
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