Polymer ultradrawability: the crucial role of α-relaxation chain mobility in the crystallites

Abstract

An explanation of the varying (ultra)drawability of semicrystalline polymers is proposed, based on NMR evidence of α_c-relaxation-associated helical jumps and chain diffusion through the crystallites of polyethylene and several similarly “α_c-mobile” polymers; these include isotactic polypropylene, poly(ethylene oxide), poly(oxymethylene), poly(tetrafluoroethylene), poly(vinyl alcohol), and several others. The chain motions provide a mechanism by which hot drawing of these polymers can extend an initially formed fiber morphology by an order of magnitude to draw ratios > 30, without melting. A second class of polymers, including nylons, poly(ethylene terephthalate), syndiotactic polypropylene, isotactic polystyrene, and isotactic poly(1-butene) (form I) lack a crystalline α-relaxation and the associated chain mobility. Therefore, these polymers are “crystal fixed” and drawability is limited to draw ratios < 14, arising mostly from break-up of crystalline lamellae and deformation of the amorphous regions. On this basis, we can explain which polymers are drawable to high draw ratios, given a sufficiently low level of entanglement. The motion through the crystallites is thermally activated and the applied stress only biases the direction of the jumps; this explains the crucial role of temperature and rate in tensile drawing and solid-state extrusion processes. The behavior of the crystal-fixed, poorly drawable polymers strongly suggests that melting, straight chain pull-out, and sliding on crystal planes are not significantly operative during ultradrawing, and that weak intermolecular forces are not a sufficient condition for ultradeformation. Various stages of drawing are distinguished and other models of ultradrawability are discussed critically.