Infrared and raman spectroscopy of stressed polyethylene

Abstract

Polyethylene films were stressed in uniaxial tension at room temperature while the infrared (FTIR) or Raman spectra were simultaneously measured. The purpose of these experiments was to gain an understanding of the molecular origins of the stress‐induced vibrational frequency shifts Δν and determine the mechanisms of molecular deformation and fracture of PE molecules with stress σ. The PE samples consisted of ultradrawn, high‐density, ultrahigh‐molecular‐weight, solid‐state‐extruded films and the stress was applied parallel to the nearly perfectly oriented orthorhombic crystal c‐axis direction. The experimental frequency shifting coefficients α = (∂ν/∂σ)σ→0 were compared with calculated α values determined from a vibrational analysis of an isolated PE chain using conformational energy minimization methods in which both harmonic and anharmonic (Morse) potential energy functions were used for the CC stretching modes in the valence force field. The following α values were obtained and are compared with theory (anharmonic case in parentheses, in cm−1/GPa): CC symmetric stretch, α(1127) = −5.9 (−5.3); CC asymmetric stretch, α(1059) = −11.2 to −5.7 (−5.8); CH2 rock, α(730/720) = −2.0 to −3.0 (−2.2); CH2 scissors, α(1472/1462) = −1.0 to −1.2 (−1.1); CH2 twist, α(1295) ≃ 0(0.8); CH2 wag, α(1370) undetermined owing to weak bands (−4.6); CH2 stretch α(3000) ≈ 0. The largest negative frequency shifts were observed for the CC stretching modes, in agreement with the calculations using the Morse potential. The harmonic potential for CC stretching resulted in small positive α values for both the CC stretching and CH2 wagging modes, while the other modes were unaffected by this choice of potential. The relative contribution of CC bond stretching and CĈC valence angle bending to the c‐axis strain was about 1:1, in agreement with experiment. The Young's tensile modulus of a PE chain was calculated as Ec = 267 GPa.

Rheo-optical Raman study of microscopic deformation in high-density polyethylene under hot drawing

“…Under large deformation (ε ≈ 10), the value of ∆ν s is appreciably larger than that of ∆ν as . This might be because of formation of fibrillar structures, where ∆ν s is twice as large as ∆ν as [17].…”

“…Although IR spectroscopy has been applied to hot drawing of thin films [15,16], this method has several intrinsic disadvantages, such as most IR active modes being mainly ascribed to vibrations of side chains, and only the orientation parameter 〈P 2 〉, which is a measure of the average orientation degree, can be determined. However, Raman spectroscopy has several advantages over IR spectroscopy, such as the skeletal C-C stretching vibration being strongly Raman active, load sharing on the polymer chains can be directly observed [17], and the higher-order orientation parameter 〈P 4 〉 can be estimated. Then, by using Raman spectroscopy, we can discuss the molecular orientation more precisely [18][19][20][21].…”

Bond Scission in a Perfect Polyethylene Chain and the Consequences for the Ultimate Strength

“…This is in close agreement with the experimental values from Raman spectroscopy, which are 1061 and 1128 cm -1 , respectively. 34 We have calculated the spectra for the transition state in the supercell with L ) 14.57 Å and the corresponding uniformly strained chain. Both are shown in Figure 7.…”

Stress-augmented thermal activation: Tribology feels the force

“…This has already been done in some solid-state systems, e.g., Ref. [148], but is technically difficult for very thin films because of the limited number of active centres, and thus the very weak signal. Another possible avenue would be the application of nano-manipulation methods, already developed for chemical studies [109], to investigate the properties of single lubricant additive molecules on surfaces, such as the strength of metal-ligand bonds.…”