NMR studies of molecular mobility in uniaxially stretched oriented polymers

Abstract

SynopsisThe 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(ethy1ene terephthlate), polypropylene, polyethylene, polyoxymethylene, poly(viny1 alcohol), and polytetrafluorethylene. Above the lowtemperature 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 noncrystalline 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.

Drawing and extrusion of semi-crystalline polymers

“…A great deal of information about drawing has been obtained from electron microscopy (EM) [52][53][54][55][56][57][58][59][60][61], the formation of kink bands after drawing [62], the small angle neutron scattering (SANS) [63], the birefringence of oriented chains in crystals and in amorphous layers [64][65][66], the increased chain orientation and reduced mobility by wide line nuclear magnetic resonance (NMR) [67,68], and the ultimate mechanical properties of the drawn material. The formation of radicals at the fracture of polymer chains detected by electron spin resonance [69], of microcracks during drawing and fracture derived from small angle X-ray scattering (SAXS) [70], and the calculation of the energy and fracture process during the rupture of a single chain [71][72][73] are helpful in the study of the drawing process [74].…”

Mechanical properties and molecular mobility in oriented polyolefines

“…This leads to lower intensity of segmental movements, as shown by NMR spectra detected on polymers containing straightened chains [7,8]. Therefore, a connection should exist between the mechanical properties of an oriented crystalline polymer and the dynamics of molecular movements therein.…”

“…All the types of holding chains are characterized by ordered, high-density packing, which [2] enables them to resist concentrated/-/NO 3 analogously to the molecules in the crystallites. Holding chains are almost straight, oriented along the axis of stretching [3,4], and their flexibility depends on conformation [5][6][7].…”

Structural Basis of High‐Strength High‐Modulus Polymers

“…By changing the drawing temperature and rate, one can suppress or stimulate the mechanical destruction. An increase of the drawing rate or a decrease of the drawing temperature suppresses the micro-Brownian mobility of the chain segments in the disordered regions, which results in their vitrification and more probable rupture [114].…”