Abstract
We report here experimental results investigating the influence of the initial swift heavy ion charge on the structure of polyethylene terephthalate (PET) film after irradiation, using a structurally sensitive X-ray diffraction method. Kr ions with an energy of 100 MeV and charges of 13+, 14+, and 15+ were each used at irradiation fluences of 5 × 1010, 7.5 × 1010, 1 × 1011, 2.5 × 1011 and 5 × 1011 ions/cm2. At constant energy and irradiation fluence, the post-irradiation structural changes in PET film show a clear dependence on the initial ion charge. As either the fluence or ion charge increase, the latent tracks begin to overlap, leading to cross-linking of PET chain molecules to form rotational isomers (rotamers). We use the fluence corresponding to the onset of overlapping to estimate the size of latent tracks for different ion charges. At the highest fluences, the latent tracks become entirely overlapped, and the interchain cross-linking extends throughout the whole film. Since this cross-linking is due to the dipole–dipole interaction of subunits of repeat units of PET chain molecules, it is reversible, in contrast to the well-known chemical cross-linking of polymer chain molecules under irradiation.
Highlights
Polyethylene terephthalate (PET) films have been the subject of much study for a long time, both in their applied [1,2,3,4,5,6,7,8,9,10] and theoretical aspects [11,12,13,14,15,16,17]
In Reference [22] we reported the emergence of regions of induced ordering in latent tracks in PET films after irradiation with swift heavy ions, as a result of rotation of benzene-carboxyl subunits of repeat units of chain molecules in the amorphous part of PET under the influence of the residual electric field of the latent track
The X-ray diffraction pattern for the lowest initial ion charge (Kr13+ ) (Figure 2a, red lines Figure 3a below) exhibits the same qualitative features as those seen for Ar8+ ions at the lower two fluences used in [22]: a meridian reflection in the region 2θ = 5–12◦ showing induced ordering in the amorphous phase and an azimuthally isotropic diffraction ring at 2θ = 26◦ in the crystalline phase, indicating that each latent track is isolated from all others
Summary
Polyethylene terephthalate (PET) films have been the subject of much study for a long time, both in their applied [1,2,3,4,5,6,7,8,9,10] and theoretical aspects [11,12,13,14,15,16,17]. In Reference [22] we reported the emergence of regions of induced ordering in latent tracks in PET films after irradiation with swift heavy ions, as a result of rotation of benzene-carboxyl subunits of repeat units of chain molecules in the amorphous part of PET under the influence of the residual electric field of the latent track. Since the residual electric field has cylindrical symmetry, and dipoles always align with the field gradient, this rotation leads to the initially random distribution of carboxyl ions relative to the axis of the chain molecule in the amorphous phase of the film being replaced by an ordered distribution of carboxyl dipoles in the radial direction This results in an increased density of carboxyl units on the inner walls of the latent track, without the irradiated film being introduced into water or an aqueous electrolyte. We report the results of our study
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