Direct observation of taut tie molecules in high-strength polyethylene fibers by Raman spectroscopy

The Raman spectra of four gel-spun high-strength polyethylene fibers were recorded as a function of stress, with detailed study of the 1063 cm^?1 band. The change in the peak position of this band was linear at low stresses, but there was little change at high stresses. The center of mass of the band moved linearly with stress until fracture, and the shift per unit stress was almost the same for all the fibers studied at 4.0 ± 0.5 cm^?1/GPa. The line shape is symmetric at low stresses, and the full width at half maximum increases with stress. When plastic deformation begins the band develops a broad low intensity tail extending to 1000 cm^?1. This corresponds to stresses of up to 15 GPa, and the tail contains up to 18% of the band intensity when the maximum stress is applied. Equivalent wide-angle x-ray studies showed no such tail, so the highly stressed material is not crystalline, although it must be in the all-trans form to contribute to the Raman peak. Such load-bearing extended-chain-disordered material is described as taut tie molecules. The amount is much larger than that usually derived from mechanical modelling, it is essentially all the amorphous material if the DSC crystallinity of 85% is correct. It is thought to be interfibrillar, stressed when fibrils slide past each other in creep. On unloading the fiber, the band became more symmetrical again but did not always return to its initial position at zero stress. The peak moved to a wave number greater than its initial value on unloading and then over time relaxed to its original value. Thus tensile stresses in the disordered material, balanced by compression in the crystals, are slow to relax. Lower molecular weight fibers (M_n = 8 × 10⁵) had small anelastic effects, probably because they relaxed within the time of the experiment.