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

An explanation of the varying (ultra)drawability of semicrystalline polymer s is proposed, based on NMR evidence of alpha(c)-relaxation-associated heli cal jumps and chain diffusion through the crystallites of polyethylene and several similarly "alpha(c)-mobile" polymers; these include isotactic polyp ropylene, poly(ethylene oxide), poly(oxymethylene), poly(tetrafluoroethylen e), poly(vinyl alcohol), and several others. The chain motions provide a me chanism by which hot drawing of these polymers can extend an initially form ed fiber morphology by an order of magnitude to draw ratios >30, without me lting. A second class of polymers, including nylons, poly(ethylene terephth alate), syndiotactic polypropylene, isotactic polystyrene, and isotactic po ly(1-butene) (form I) lack a crystalline alpha-relaxation and the associate d chain mobility. Therefore, these polymers are "crystal fixed" and drawabi lity is limited to draw ratios <14, arising mostly from break-up of crystal line lamellae and deformation of the amorphous regions. On this basis, we c an explain which polymers are drawable to high draw ratios, given a suffici ently low level of entanglement. The motion through the crystallites is the rmally activated and the applied stress only biases the direction of the ju mps; this explains the crucial role of temperature and rate in tensile draw ing and solid-state extrusion processes. The behavior of the crystal-fixed, poorly drawable polymers strongly suggests that melting, straight chain pu ll-out, and sliding on crystal planes are not significantly operative durin g ultradrawing, and that weak intermolecular forces are not a sufficient co ndition for ultradeformation. Various stages of drawing are distinguished a nd other models of ultradrawability are discussed critically.