NMR STUDIES OF MOLECULAR MOBILITY IN UNIAXIALLY STRETCHED ORIENTED POLYMERS.

In the reported experiments, the effect of elasticlike uniaxial tension on molecular mobility in polymers have been studied over a wide temperature range using the broad-line NMR technique. The stueies were carried out on oriented semicrystalline samples of nylon 6, poly(ethylene terephthlate), polypropylene, polyethylene, polyoxymethylene, poly(vinyl alcohol), and polytetrafluorethylene. Above the low-temperature transition the NMR spectra are reversibly transformed under tension. Increases in the second moments of the spectra are attributed to weaker molecular motion in stressed polymers. In the spectra of polymers stretched above the glass transition temperature the narrow component decreases, thus indicating inhibition of micro-Brownian motions, a phenomenon called ″mechanical vitrification.″ Such mechanical vitrification is proved to result from reduction in the number of possible tie-chain conformations in the noncrystalline regions and not from closer packing of chains. A method for evaluation of the energy of intermolecular interactions which hinder small-scale motion at low temperature is suggested.