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

The authors would like to thank the reviewer for the comment. We have tried to make the introduction and the experimental part much more detailed, with a large number of detailed explanations that can help a general reader perceive our problem statement and the experiment itself. Figure 1 is divided into component parts according to the text, and the scale is increased for better perception.

Our previous paper, which has the citation of [30] in the paper under discussion, was accepted for publication on 25.05.2020 with the title "Induced Spirals in Polyethylene Terephthalate Films Irradiated with Ar Ions with an Energy of 70 MeV" and the DOI number https://doi.org/10.3390/cryst10060427. In that paper, we reported the appearance of a new type of structural ordering of the spiral type with an increase in the irradiation exposure and the beginning of overlap of latent tracks along with cylindrical type ordering for single latent tracks.

In this paper, we used the found induced helical conformations as a tool for studying the effect of swift heavy ion charge on the post-radiation state of the molecular structure of the irradiated PET film, which is scientifically different from the purpose of the second work.

Here, anisotropy refers to the change in the intensity of the diffraction maximum at θ=26°, when sweeping in a circular azimuthal angle φ=0-2π.  In order to make the meaning of this term clearer, we have introduced Figure 1 in the paper, which shows the X-ray diffraction pattern for the pristine PET film. It shows that the peak intensity at θ=26° has a constant value at all angles φ=0-2π. In contrast to Figure 1, Figure 2C shows that the peak intensity at θ=26° changes depending on the azimuthal angle φ, which directly indicates anisotropy in the intensity of the reflection. 

In Line 117, we qualitatively summarized the results of our experiment, bearing in mind that the increase in fluence in general leads to an increase in azimuthal changes in the peak intensity at θ=26°, that is, its anisotropy. 

This is despite the increases in both ion charge and fluence, and consequent increase in energy density in the latent tracks. We deleted this phrase. Now this is: This is despite the increases in both ion charge and fluence.

The authors made changes to the text of the manuscript according to the reviewer's comment.

The authors would like to thank the reviewer for the question, the answer to which may become the goal of one of the works in future. 

Figure 1 is corrected. The reviewer's suggestion to use markers and arrows in the drawings in order to help a reader understand the material presented has been accepted where it was convenient.

2.3 Study of the PET structure

Studies of structural changes in the irradiated PET films were performed by X-ray diffraction (D8 Advance Eco, Bruker, Karlsruhe, Germany) in the Bregg-Brentano geometry (coupled TwoTheta/Theta) in the angular range of 2θ=3-30°, step 0.01° and azimuthal sweep φ=0-2π in increments of 10°, X-ray radiation Cu-Kα, λ = 1.54 Å, Voltage – 40 kV, Current – 20 mA, spectrum acquisition time 2 s. Each of the X-ray images presented in this work contains 37 slices where slice 1 coincides with slice 37. This φ overlap was performed to confirm the absence of changes in the molecular structure of the test sample during a 360-degree survey.

Analysis of the degree of crystallinity as well as the phase composition of the initial sample was determined using the TOPAS v.5.0 (Diffraction Suite, Bruker AXS GmbH 2014) software code based on the Rietveld method [41]. A schematic layout of the survey geometry is shown in Figure 1b. The X-ray diffraction pattern of the pristine PET sample in a φ=0-2π sweep is shown in Figure 1c, where two individual regions can be distinguished: the amorphous part of the PET at 2θ=3-15° and the crystalline part characterized by two diffraction reflections (-110) at 2θ=23° and (100) at 2θ=26°.

We note that the coordinate φ is polar: the constant intensity of the peak for all angles φ in Figure 1c demonstrates the polar isotropy of the test sample. The X-ray diffractometry of chain molecules has its own nomenclature: the direction along the texture is called the "meridian" and is indicated by a red line in X-ray diffraction images, while the direction perpendicular is called the "equator" and is indicated by a blue line [42]. For this reason, X-ray slices with angles 0º, 180º and 360º in Figure 1c are highlighted in red and coincide with the direction of the meridian, and X-ray slices with angles 90º and 270º are indicated in blue and coincide with the direction of the equator.

We believe that the material of the published article [30] provides an answer about the justification of the statement made on lines 37-39. 

The authors would like to thank the reviewer for the comment. Figure 1 is divided into component parts according to the text, and the scale is increased for better perception.

The authors would like to thank the reviewer for the comment. Figure 2 is brought to a single y-axis scale for a better understanding of structural changes.

X-ray diffraction, which we use to determine the post-radiation structure of the PET film, cannot give information about the magnitude of the active electric fields in the zone of latent tracks. The ordering of the carboxyl benzene subunits are acting only as nanoindicators of the gradient of the electric field.

Figure 8 shows schematically a visualisation of the process of induced ordering in the residual electric field. Thin blue horizontal lines indicate the axes of chain molecules, lying mainly along the direction of movement of the film during its production. As mentioned above (Section 2.1), in the amorphous phase of the pristine film, benzene-carboxyl units are oriented in a random manner around the axes of chain molecules, as shown in Figure 8a by different projections of benzene rings with carboxyl units.

Figure 8. Schematic representation of the orientation of benzene-carboxyl units of the amorphous phase of the PET film: a) pristine; b) in the residual electric field of the latent track.

Figure 8b shows the rotational ordering of benzene-carboxyl subunits under the influence of the cylindrical residual electric field E (red arrow) of the latent track. These units are bound within the chain molecule by covalent forces and are not able to make linear movements (except for slow creep-like movements [30]), but, being linked to the chain by σ-hinges, they can rotate around the axis of the molecule. The combination of kinematic restrictions and rotation leads to the benzene-carboxyl units tending to move so as to maximise the projections of the dipole moments of the carboxyl groups onto the direction of the electric field vector E. This results in the carboxyl groups becoming generally oriented in the same direction, as shown in Figure 8b.

The appearance of helicoid structures occurs through self-assembly of repeat units of PET [47]. Another example of self-assembly of chain molecules with the formation of nanopores is shown in [48]

The experimental error in the case of X-ray radiography of maximum intensities at 2θ=23° and 2θ=26° was no more than 100-150 cps, which is no more than 1%.