Induced Spirals in Polyethylene Terephthalate Films Irradiated with Ar Ions with an Energy of 70 MeV

Abstract

This paper presents the results of a study of the ordering in polyethylene terephthalate (PET) film induced by Ar8+ ions with an irradiation fluence of 2 × 1012 ions/cm2, and of the temporal stability of the induced ordering in the irradiated sample, over a three month period. Immediately after irradiation, sharp new reflections not seen at lower fluences were observed in X-ray diffraction patterns, with angular positions of 2 θ = 9–10° and 19° and variable azimuthal intensities. X-ray reflections, previously observed at lower fluences, were also seen: at 2 θ = 26° and 23°, associated with PET crystallites, and at 2 θ = 5–12°, associated with induced ordering in the amorphous zone. Aging of the irradiated sample led to significant growth of the ordering region in the amorphous zone for angles up to 2 θ < 15°, as well as to dissipation and blurring of the new diffraction reflections at 2 θ = 9–10° and 2 θ = 19° and the formation of a new diffraction ring reflection in the range 2 θ = 11–16°. The azimuthal distribution of diffraction reflection intensities immediately after irradiation displays a clear oblique cross located predominantly along lines at angles of π/4 with respect to the direction of the texture of the PET film, indicating the formation of spiral structures based on the molecular strands of PET. Our experimental results lead us to conclude that the formation of coherent scattering areas in the amorphous region at 2 θ < 15° is due to intra-chain rotations of benzene-carboxyl subunits of repeat units of the PET chain molecules interacting with the residual electric field of a single latent track; whereas the formation of spiral structures is due to the inter-chain interaction of these preordered asymmetric subunits under the influence of the electric fields from overlapping latent tracks.

Ion Charge Influence on the Molecular Structure of Polyethylene Terephthalate Films after Irradiation with Swift Heavy Ions

“…This X-ray diffraction picture also shows an increase in the size of the induced ordering zone, with the meridional reflection increasing in length (from 2θ = 5-12 • to 2θ = 5-14 • ) and intensity, for which we currently have no explanation but might be elucidated via experiments at lower fluences. The X-ray diffraction pattern for Kr 15+ ions (Figure 2c, Figure 3a green lines) shows a clear azimuthal anisotropy in the intensity of the diffraction reflection at 2θ = 26 • , and a return of the meridional reflection to values of 2θ angular size and intensity similar to those for Kr 13+. Comparing these results with the results for Ar 8+ , where anisotropy began to emerge at a fluence of 1 × 10 12 ions/cm 2 (see Figure 3, [22]) and was well established at a fluence of 2 × 10 12 ions/ cm 2 [30], we see that for Kr 14+ , anisotropy emerges at an irradiation fluence that is 20 times lower, indicating a notably stronger ordering effect for Kr 14+ ions compared to Ar 8+ ions.…”

“…In the polar coordinate system (2θ, φ) these form a cross, oriented broadly along the meridian and equator, signaling the presence of spiral atropisomers. As these are in the amorphous zone, they must lie within latent tracks, in contrast to those observed earlier in overlap areas on the periphery of single tracks [22,30]. These new X-ray patterns can be understood as resulting from tracks overlapping to such an extent that the whole film has become 'overlapped track', with interchain cross-linking occurring throughout the entire film.…”

“…Since we are interested in comparing the differences in these patterns, in the graphs in Figure 3 we have plotted the azimuthal intensity for each for the three ion charges (and the pristine film) at 2θ = 5 • and 2θ = 26 • , for each of the five fluences. [30], the onset of this anisotropy indicates the formation of spiral conformations in overlapping regions of the residual electric fields of latent tracks in PET. This X-ray diffraction picture also shows an increase in the size of the induced ordering zone, with the meridional reflection increasing in length (from 2θ = 5-12 • to 2θ = 5-14 • ) and intensity, for which we currently have no explanation but might be elucidated via experiments at lower fluences.…”

“…If we assume that the fluence at which we first see structural evidence of overlap is close to its actual onset, then the average distance between ion trajectories at this fluence provides an estimate of the transverse dimensions of the latent tracks. In the case of Ar 8+ , this assumption is supported by the fact that there was no evidence of overlap at two lower fluences [30]. For Kr ions, no experiments were performed for lower fluences so it is possible that the onset of overlapping starts at fluences lower than 5 × 10 10 ions/cm 2 , in which case this approach will underestimate the transverse dimensions of the latent tracks.…”

“…To test our hypothesis, we make use of our previous experimental results [30] showing that the overlapping of the residual electric fields of neighboring latent tracks leads to the formation of interchain spiral conformations. These interactions, and the spiral conformations they create, are sensitive to the degree of overlap of the residential electric fields and can, therefore, be used to investigate the characteristics of latent tracks and their dependence on external parameters including the initial charge of the irradiating ions.…”