Polymers Under Uniaxial Stress

Local order and dynamics in polymers under mechanical stress is studied by low-field NMR. Permanent magnets in a Halbach arrangement permit NMR investigation without the limits present in high-field NMR. In particular the confined stray field permit the application of benchtop NMR in a stretching apparatus and a rheometer.

The major drawback of low-field NMR analyzer, the lack of chemical shift resolution, is not a problem, because in the study of known materials properties other than their chemical composition are of interest.  Mechanical stress on elastomers results in partial chain ordering and consequently reduced chain mobility.

The resulting stronger residual dipolar couplings are manifested in the stronger buildup of double quantum coherences and in a shortening of the slower component of the transverse relaxation time. After releasing the load the return to the dipolar couplings and the relaxation times of the unextended sample is followed on a time constant of tens of minutes The crystalline and amorphous fractions of semicrystalline polymers are distinguished by their transverse relaxation times.

To localise the stress effect in the rf coil, the diameter of the rod under study is reduced in the portion located in the rf coil. Under mechanical load there is a significant shortening of the transverse relaxation time as well as an increase in the residual dipolar coupling which are determined from the build up of double quantum intensities.

The shortened relaxation times return to values close to those found in unloaded samples, when the load is kept constant. The time constant of this relaxation appears to be longer than that found in mechanical stress relaxation experiments.

The interaction with paramagnetic moieties in the fillers in polymer nanocomposites has a strong impact on the longitudinal relaxation time. Delaminating filler particles under mechanical stress results in a shorter T1 of the protons in the polymer, because the contact area between the filler and the polymer increases.

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