Microstructure and micromechanics of high-performance polymer fibers: A study by Raman spectroscopy and x-ray diffraction

Raman spectroscopy and x-ray diffraction were used to study the microstructure and micromechanics of high strength polymer fibers. Two different types of polymer fibers were used in this study--(1) polyethylene, an extended chain flexible polymer, and (2) poly(p-phenylene terephthalamide) (PPTA), a semi-rigid rod polymer.; Properties which depend on molecular vibrations, such as the peak positions of Raman spectra, generally alter with changes in unit cell dimensions caused by temperature or stress and can therefore be used to describe the state of strain in the fiber. In all-trans chains there is a linear relationship between the local strain in the molecular chain direction and the vibrational frequency corresponding to the polymer backbone. The line shape of the Raman band is then directly related to the strain distribution in the sample.; Similarily the chain axis reflection in wide angle x-ray diffraction also shifts and broadens when the fiber is loaded in tension. The high brightness of the synchrotron x-ray radiation has allowed x-ray diffraction patterns to be obtained from single fiber samples and unlike fiber bundle samples one can be sure that the loading at the fiber level is uniform. Broadening of the chain axis reflection then indicates inhomogeneities within the fiber. Deconvolution with the line profile of the fiber at zero stress gives the distribution of crystal strains.; In polyethylene the peak position of the Raman band was linear at low stresses, but there was little change at high stresses. The line shape was symmetric at low stresses and the full width at half maximum increased with stress. At high stresses when plastic deformation begins the band became asymmetric and developed a low intensity tail towards lower wavenumbers. Equivalent wide angle x-ray diffraction studies showed no such tail so the highly stressed material is not crystalline, although it has to be in the all-trans form to contribute to the Raman band. This result has been explained by considering the polyethylene fibers to have two load bearing phases, the crystalline phase, and the "disordered" phase primarily made up of taut tie molecules. At low stresses the crystalline phase carries most of the load. At high stresses the load on this phase no longer increases and all the extra load is now carried by taut tie molecules which is seen in the asymmetric tail of the Raman band.; In PPTA the peak position the Raman band shifted to lower wavenumbers and broadened with stress. The Raman band shift was constant till fracture and the peak shift per unit stress was the same for PPTA fibers with different crystallite orientation. A simple one phase paracrystalline model is used to explain the observed shifts in PPTA fibers.