Orientation Measured by Broad-Line NMR

In NMR experiments, molecular mobility leads to narrowing of the resonance lines. Conversely, restricted molecular motion, as occurs in crystalline phases, causes line-broadening. Until the advent of magic angle spinning and related techniques, tliis was a hindrance to NMR studies of solid polymers. We have used it to advantage in following the orientation effects in solid PTFE. 

Broad-line NMR derivative spectra were obtained using a Brucker HFX-90 spectrometer to record the resonance at 84.67 MHz. The specimens, made by compacting granular PTFE into preforms, sintering at 380°C, and cooling slowly at a rate of 0.02 deg/min had a specific gravity of 2.205. The second moment of tire NMR line shape is of interest because the fourth moment of the orientation distribution function is proportional to it. 

Specimens were elongated the indicated amount, then released from the grips of the tensile machine, cut parallel to the draw direction, and placed in NMR tubes. From the derivative NMR spectra, the second moments of die orientation distribution were measured at temperatures of 158 and 345°K. 

Elongation, by definition, is X = Z/7o. These specimens had been drawn at room temperature to elongations X of 1.0, 2.0, 3.0, and 4.0 (strains of 0, 100, 200, and 300%). At 345°K, the main contribution to the second moment is from the XL phase while at 158°K it is from the NXL phase. It is observed that the second moment increases for more highly elongated specimens, i.e., the line broadens as the NXL molecules are oriented and are thereby restricted in mobility. 

Studies on PTFE by F-NMR at temperatures of 35 to 40°C have been performed by Brandolini et al. in order to measure an order parameter. Results fit a theoretical model but the unrealistic assumption is made that the amorphous regions are unoriented... . 

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