NMR studies of molecular mobility in uniaxially stretched oriented polymers

✍ Scribed by Egorov, E. A. ;Zhizhenkov, V. V.

• Publisher: Wiley (John Wiley & Sons)
• Year: 1982
• Tongue: English
• Weight: 943 KB
• Volume: 20
• Category: Article
• ISSN: 0098-1273
• DOI: 10.1002/pol.1982.180200701

✦ Synopsis

Abstract

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 studies 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. The only exception is polypropylene, in which the reverse, i.e., enhancement of molecular mobility, can be observed in a certain temperature range. In the spectra of polymers stretched above the glass transition temperature the narrow component decreases, thus indicating inhibition of micro‐Brownian motion, a phenomenon we call “mechanical vitrification.” Such mechanical vitrification is proved to result from reduction in the number of possible tie‐chain conformations in the non‐crystalline regions and not from closer packing of chains. In discussing the results we use the experimental data on the reduction of the number of gauche isomers under tension (on average, one transition of a gauche link to the trans state causes at least five methylene groups in the main chain to become immobile). The results of studies of molecular mobility in stretched polymers are used for more accurate definition of the mechanisms of molecular motion at different temperatures. A method for evaluation of the energy of intermolecular interactions which hinder small‐scale motion at low temperature is suggested.