Frequency shifting and asymmetry in infrared bands of stressed polymers

Abstract

The effect of stress and temperature on infrared frequency shifting and band asymmetry in polymers is investigated. Valence coordinate deformation of individual polypropylene chains accounts for the observed frequency shifting in samples of oriented isotactic polypropylene. Each chain may be modeled as a one-dimensional enharmonic system, whose wave functions and energies are obtained in the quasi-harmonic approximation. The theoretical stress-induced frequency shifting coefficient is related to the Gmneisen parameter and is expressed by ac = -a0 -CTosc, where a0 and C are positive constants and Toer is the temperature of the oscillators in equilibrium with the radiation field. Two mechanisms of band broadening are considered: The first is due to the presence of an average interchain perturbing force. The second is due to the existence of anisotropic crystal field forces. Each description predicts symmetric broadening. Therefore, the development of an observable asymmetry in a stressed infrared band may be associated with a nonuniform molecular stress distribution. Only the crystal field broadening mechanism, however, is consistent with the assumptions required for calculation of the linear ac(T) relation, which agrees well with experiments on oriented polypropylene. The anisotropic broadening mechanism is also investigated for oriented polyethylene.

Twinkling fractal theory of the glass transition

“…We use the Morse potential or its quasiharmonic equivalent to generate quantitative data later, but any anharmonic interatomic potential function, such as the Lennard-Jones potential, will suffice. The anharmonicity controls the vibrational frequencies x, with temperature T, hydrostatic pressure P and stress r, 19 and the thermal expansion coefficient a L via the mean bond displacement hXi (Fig. 2).…”

Probing deformation of double-walled carbon nanotube (DWNT)/epoxy composites using FTIR and Raman techniques

“…In our FTIR measurements, attention was focused on both the aliphatic and aromatic ether C-O bonds associated with the ether group (C-O-C) that lies in the epoxy backbone. The rationale was that stretching would cause an increase in atomic separation, effectively leading to a decrease in frequency as shown by Bretzlaf and Wool [18]. Fig.…”

“…Polypropylene appears to be the most widely studied material, where the wavenumbers for both stretching and rotation modes are found to decrease with applied tensile stress [14][15][16][17][18]. However, there are a few exceptions, where the shift is positive with tensile stress, and the difference has theoretically been attributed to stretching of the harmonic versus the anharmonic potential energy functions for C-C stretching component [15].…”

Dynamic rheo-optical characterization of uniaxially oriented polyamide 11

“…This should be justified for small deformations in the oriented systems. 20 It must be said that the reorientation of functional groups, the deformation of molecular internal coordinates, and the thickness change of the film during the stretch cycle are always coupled to each other. The relative contributions to the signal changes depend on the vibrational mode and the morphology of the sample.…”

“…The relative contributions to the signal changes depend on the vibrational mode and the morphology of the sample. 20,21 For simplicity, the relative contributions of these components are considered to be independent of each other, which is helpful for qualitative analysis of the changes of the infrared signals under small-amplitude oscillatory strain. Therefore, the change of dynamic infrared intensity at wavenumber v can be expressed as follows:…”