Dynamic Fourier Transform Polarized Raman Spectroscopy of Polyethylene Fibers under a Modulated Strain

The response of materials under stress is a critical aspect of the in-use performance of fibers and films. Dynamic infrared measurements have been shown to be very informative and have provided information about both chain reorientation and conformational state change during deformation. However, the use of infrared transmission techniques necessarily confines the measurements to thin films. Polymeric fibers present a real challenge to this approach. Raman scattering is extremely well suited for fibers. There is no real constraint on sample size, shape, or thickness. Dynamic measurements on fibers under deformation have been extended using a Raman probe. Step-scan FT-Raman spectroscopy has allowed the decoupling of the sample strain frequencies from the Fourier frequencies, and the sensitivity is sufficient to observe the small changes associated with an elastic tensile deformation. For polyethylene fibers, the dynamic spectra exhibit both stress-induced frequency shifts and intensity changes due to chain reorientation. Vibrational modes that are coupled to chain backbone motions are found to exhibit the strongest stress-induced frequency shifts, while decoupled motions, such as C–H stretching modes, exhibit the effects of chain reorientation.