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Dynamics of Crystalline and Amorphous PolytetrafluoroethyleneStudied by Multiple Quantum NMR

Published online by Cambridge University Press:  15 February 2011

David A. Lathrop  and
Karen K. Gleason
David A. Lathrop
Affiliation:
Department of Chemical Engineering, MIT, Cambridge, MA 02139.
Karen K. Gleason
Affiliation:
Department of Chemical Engineering, MIT, Cambridge, MA 02139.
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Abstract

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

Type
Research Article
Information
MRS Online Proceedings Library (OPL) , Volume 321: Symposium E – Crystallization and Related Phenomena in Amorphous Materials—Ceramics, Metals, Polymers, and Semiconductors , 1993 , 155
Copyright
Copyright © Materials Research Society 1994

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References

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