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

1. Correct.

2. Figure 1 shows X-ray diffraction patterns of the studied sample in the pristine state and after irradiation. A full description of the structural characteristics of the pristine sample was presented in [20] and can be summarised by the presence of the following characteristic areas: a halo at 2θ=5-15° indicating the presence of an amorphous zone in the PET film under study; two isotropic azimuthal ring diffraction reflections with maxima at 2θ=23° and 26° that belong to the PET crystalline phase [25]; and a transition area in the angular range 2θ=15-20° (see Figure 1a).

Figure 1. X-ray diffractograms of the PET sample: a) pristine (X-ray diffractograms for the pristine sample were taken from [20] for comparison); b) after irradiation with a dose of 2x10¹² ions/cm² in the geometry of φ=0-2π; c) temporal evolution of the diffraction spectra of the irradiated PET sample.

3. According to our X-ray data for the pristine film [20], the ratio of the crystalline and amorphous parts of PET was 43:57, which corresponds well with the manufacturer's passport data and with the results of fundamental work on the study of PET structures [16]. The method for calculating this ratio via analysis of X-ray diffractograms was taken from [22].

4. The subject, experimental equipment, and techniques for studying structural changes in a sample of PET film of thickness 12 microns (HOSTAPHAN® polyester films, Mitsubishi Polyester Film GmbH) irradiated with Ar⁸⁺ ions with an energy of 70 MeV were described in [20].  In this study we used a flux of 2x10¹² ions/cm². Irradiation was carried out in a vacuum at room temperature. The angle of ion incidence was 42° with respect to the normal line to the surface. The direction of irradiation of the film coincided with the direction of motion of the film through the irradiation chamber. We used angled irradiation in order to maximise the electrical interaction of the irradiating ions along the polymer molecular chains, since it is well known that PET films and fibres have a strongly pronounced molecular texture in the direction of motion during their production [22,23].

5. Excess charge in the core of the latent track was also found in the work of Wang et al. [16], but with opposite polarity. This discrepancy is one of many examples showing that radiation effects in latent tracks are complex and remain insufficiently understood.  Whatever the polarity, both these experiments show that there is a residual radial electric field in latent tracks after irradiation.  The stability of  this electric field over time is determined by the properties of the irradiated material and can be very long in the case of electrets [17].  The electric field can be significantly affected by UV illumination, as shown by Wang et al. [16], who invented a special technique for processing polyethylene terephthalate (PET) film known as the ‘track-UV technique’. This technique is very close to the technology used to prepare ion irradiated films for etching [18] and is based on the general sensitivity of PET to UV illumination [19].

1. The authors took into account the comments of the reviewer and expanded the introduction.

Over the last three decades, the heavy ion irradiation of solid materials including polymers has been  the subject of extensive research, studying both the fundamental interaction mechanisms and the opportunities for real-world applications. This is evidenced by a large number of articles summarised both in reviews [1,2] and major monographs [3]. Irradiation of polymers with swift ions leads to various irreversible effects, such as amorphisation and destruction [4-5], surface modification [6-8], and chemical cross-linking of polymers [9-11]. The irreversibility of these effects is due to them being caused by highly energetic ‘δ-electrons’ knocked out from the track core by the irradiating ions, and cascades of secondary electrons formed in turn by these δ-electrons [9-13].  The stochastic nature of these processes means that these changes in the molecular structure of irradiated polymers inside the latent tracks are also stochastic [12-14].

Ordered changes have, however, been observed in the radial electron density of the latent track. In the experiments of Abu Saleh et al. [15], it was found that the radial electron density increased with increasing  radius, with the density in the inner core of the latent track (at radii less than 3 nm) being only 20% of that in the shell of the latent track, which means a relative excess of positive charge in the core of the latent track compared to its shell. It follows from Maxwell's equations that there is a radially inhomogeneous electric field across the latent track.

Excess charge in the core of the latent track was also found in the work of Wang et al. [16], but with opposite polarity. This discrepancy is one of many examples showing that radiation effects in latent tracks are complex and remain insufficiently understood.  Whatever the polarity, both these experiments show that there is a residual radial electric field in latent tracks after irradiation.  The stability of  this electric field over time is determined by the properties of the irradiated material and can be very long in the case of electrets [17].  The electric field can be significantly affected by UV illumination, as shown by Wang et al. [16], who invented a special technique for processing polyethylene terephthalate (PET) film known as the ‘track-UV technique’. This technique is very close to the technology used to prepare ion irradiated films for etching [18] and is based on the general sensitivity of PET to UV illumination [19].

The presence of a residual radial electric field in the latent track in PET is the cause of the induced ordering of benzene-carboxyl subunits of repeat units of polymer molecules seen in [20]. It is well-known that these subunits have the ability to rotate relative to the backbone of the molecule:  the rotational properties of molecular subunits forms the basis of the field of stereochemistry [see e.g. 21].  

 In this paper, we report a novel ordering effect in PET film irradiated with swift Ar⁸⁺ ions at a higher  flux (of 2x10¹² ions/cm² ) than used in [20]. This new structural effect is expressed in sharp X-ray diffraction peaks, at 2θ=9-10° and 2θ=19°, with variable azimuthal intensity.  As previously, the increase in flux causes a reduction in the average intensity of the crystalline phase diffraction reflections at 2θ=26° and 2θ=23°. At this higher flux, however, both of these previously isotropic azimuthal distributions now exhibit non-isotropic behaviour.  In the amorphous phase, we again see an area of area of induced ordering along the direction of the molecular texture of the polymer with an intensity that coincides with that seen at the lower flux of 1x10¹² ions/cm². This new effect goes beyond those seen at lower fluxes, but since these higher fluxes are achieved by longer periods of exposure to the same incident beam, these effects build cumulatively on those of lower fluxes.

2. Figure 1. X-ray diffractograms of the PET sample: a) pristine (X-ray diffractograms for the pristine sample were taken from [20] for comparison); b) after irradiation with a dose of 2x10¹² ions/cm² in the geometry of φ=0-2π; c) temporal evolution of the diffraction spectra of the irradiated PET sample.

3. Ordered changes have, however, been observed in the radial electron density of the latent track. In the experiments of Abu Saleh et al. [15], it was found that the radial electron density increased with increasing  radius, with the density in the inner core of the latent track (at radii less than 3 nm) being only 20% of that in the shell of the latent track, which means a relative excess of positive charge in the core of the latent track compared to its shell. It follows from Maxwell's equations that there is a radially inhomogeneous electric field across the latent track.

Excess charge in the core of the latent track was also found in the work of Wang et al. [16], but with opposite polarity. This discrepancy is one of many examples showing that radiation effects in latent tracks are complex and remain insufficiently understood.  Whatever the polarity, both these experiments show that there is a residual radial electric field in latent tracks after irradiation.  The stability of  this electric field over time is determined by the properties of the irradiated material and can be very long in the case of electrets [17].  The electric field can be significantly affected by UV illumination, as shown by Wang et al. [16], who invented a special technique for processing polyethylene terephthalate (PET) film known as the ‘track-UV technique’. This technique is very close to the technology used to prepare ion irradiated films for etching [18] and is based on the general sensitivity of PET to UV illumination [19].

The presence of a residual radial electric field in the latent track in PET is the cause of the induced ordering of benzene-carboxyl subunits of repeat units of polymer molecules seen in [20]. It is well-known that these subunits have the ability to rotate relative to the backbone of the molecule:  the rotational properties of molecular subunits forms the basis of the field of stereochemistry [see e.g. 21].   

 In this paper, we report a novel ordering effect in PET film irradiated with swift Ar⁸⁺ ions at a higher  flux (of 2x10¹² ions/cm² ) than used in [20]. This new structural effect is expressed in sharp X-ray diffraction peaks, at 2θ=9-10° and 2θ=19°, with variable azimuthal intensity.  As previously, the increase in flux causes a reduction in the average intensity of the crystalline phase diffraction reflections at 2θ=26° and 2θ=23°. At this higher flux, however, both of these previously isotropic azimuthal distributions now exhibit non-isotropic behaviour.  In the amorphous phase, we again see an area of area of induced ordering along the direction of the molecular texture of the polymer with an intensity that coincides with that seen at the lower flux of 1x10¹² ions/cm². This new effect goes beyond those seen at lower fluxes, but since these higher fluxes are achieved by longer periods of exposure to the same incident beam, these effects build cumulatively on those of lower fluxes.

4. The authors took into account the comments of the reviewer and expanded the introduction.

References

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17. Sessler, Gerhard Martin. "Physical principles of electrets." Electrets. Springer, Berlin, Heidelberg, 1980. 13–80.

18. Crawford, Wayne T., James S. Humphrey Jr, and Warren De Sorbo. "Method for making visible radiation damage tracks in track registration materials." U.S. Patent No. 3,612,871. 12 Oct. 1971.

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20. Zdorovets, M. V., et al. "Induced ordering in polyethylene terephthalate films irradiated with Ar ions with an energy of 70 MeV." Surface and Coatings Technology (2020): 125490.

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1. Correct.

We previously conducted a systematic study of the effects of irradiation on the structure of PET polymer films, using swift Ar⁸⁺ ions with an energy of 70 MeV and fluxes of 2x10¹⁰, 5x10¹¹, 1x10¹² ions/cm² [20].

2. The authors thank the referee for the question. Below is a detailed answer to it.

Over the last three decades, the heavy ion irradiation of solid materials including polymers has been  the subject of extensive research, studying both the fundamental interaction mechanisms and the opportunities for real-world applications. This is evidenced by a large number of articles summarised both in reviews [1,2] and major monographs [3]. Irradiation of polymers with swift ions leads to various irreversible effects, such as amorphisation and destruction [4-5], surface modification [6-8], and chemical cross-linking of polymers [9-11]. The irreversibility of these effects is due to them being caused by highly energetic ‘δ-electrons’ knocked out from the track core by the irradiating ions, and cascades of secondary electrons formed in turn by these δ-electrons [9-13].  The stochastic nature of these processes means that these changes in the molecular structure of irradiated polymers inside the latent tracks are also stochastic [12-14].

Ordered changes have, however, been observed in the radial electron density of the latent track. In the experiments of Abu Saleh et al. [15], it was found that the radial electron density increased with increasing  radius, with the density in the inner core of the latent track (at radii less than 3 nm) being only 20% of that in the shell of the latent track, which means a relative excess of positive charge in the core of the latent track compared to its shell. It follows from Maxwell's equations that there is a radially inhomogeneous electric field across the latent track.

Excess charge in the core of the latent track was also found in the work of Wang et al. [16], but with opposite polarity. This discrepancy is one of many examples showing that radiation effects in latent tracks are complex and remain insufficiently understood.  Whatever the polarity, both these experiments show that there is a residual radial electric field in latent tracks after irradiation.  The stability of  this electric field over time is determined by the properties of the irradiated material and can be very long in the case of electrets [17].  The electric field can be significantly affected by UV illumination, as shown by Wang et al. [16], who invented a special technique for processing polyethylene terephthalate (PET) film known as the ‘track-UV technique’. This technique is very close to the technology used to prepare ion irradiated films for etching [18] and is based on the general sensitivity of PET to UV illumination [19].

The presence of a residual radial electric field in the latent track in PET is the cause of the induced ordering of benzene-carboxyl subunits of repeat units of polymer molecules seen in [20]. It is well-known that these subunits have the ability to rotate relative to the backbone of the molecule:  the rotational properties of molecular subunits forms the basis of the field of stereochemistry [see e.g. 21].  

 In this paper, we report a novel ordering effect in PET film irradiated with swift Ar⁸⁺ ions at a higher  flux (of 2x10¹² ions/cm² ) than used in [20]. This new structural effect is expressed in sharp X-ray diffraction peaks, at 2θ=9-10° and 2θ=19°, with variable azimuthal intensity.  As previously, the increase in flux causes a reduction in the average intensity of the crystalline phase diffraction reflections at 2θ=26° and 2θ=23°. At this higher flux, however, both of these previously isotropic azimuthal distributions now exhibit non-isotropic behaviour.  In the amorphous phase, we again see an area of area of induced ordering along the direction of the molecular texture of the polymer with an intensity that coincides with that seen at the lower flux of 1x10¹² ions/cm². This new effect goes beyond those seen at lower fluxes, but since these higher fluxes are achieved by longer periods of exposure to the same incident beam, these effects build cumulatively on those of lower fluxes.

The appearance of these new features in the X-ray diffractograms suggests that there has been a change in the nature of structural transformations in the polymer under the influence of this higher irradiation flux. In order to explore the nature of these structural transformations, and to confirm that the changes we observed are not an experimental artifact, we conducted a study of their evolution over time. After 3 months of observation, the changes in the structural characteristics provided clear evidence of the underlying mechanisms for their occurrence and temporal evolution. 

3. The authors thank the referee for the question. Below is a detailed answer to it.

The subject, experimental equipment, and techniques for studying structural changes in a sample of PET film of thickness 12 microns (HOSTAPHAN® polyester films, Mitsubishi Polyester Film GmbH) irradiated with Ar⁸⁺ ions with an energy of 70 MeV were described in [20].  In this study we used a flux of 2x10¹² ions/cm². Irradiation was carried out in a vacuum at room temperature. The angle of ion incidence was 42° with respect to the normal line to the surface. The direction of irradiation of the film coincided with the direction of motion of the film through the irradiation chamber. We used angled irradiation in order to maximise the electrical interaction of the irradiating ions along the polymer molecular chains, since it is well known that PET films and fibres have a strongly pronounced molecular texture in the direction of motion during their production [22,23].

According to our X-ray data for the pristine film [20], the ratio of the crystalline and amorphous parts of PET was 43:57, which corresponds well with the manufacturer's passport data and with the results of fundamental work on the study of PET structures [16]. The method for calculating this ratio via analysis of X-ray diffractograms was taken from [22].

4. The authors thank the referee for the question. Below is a detailed answer to it.

Irradiation of polymers with swift ions leads to various irreversible effects, such as amorphisation and destruction [4-5], surface modification [6-8], and chemical cross-linking of polymers [9-11]. The irreversibility of these effects is due to them being caused by highly energetic ‘δ-electrons’ knocked out from the track core by the irradiating ions, and cascades of secondary electrons formed in turn by these δ-electrons [9-13].  The stochastic nature of these processes means that these changes in the molecular structure of irradiated polymers inside the latent tracks are also stochastic [12-14].

Ordered changes have, however, been observed in the radial electron density of the latent track. In the experiments of Abu Saleh et al. [15], it was found that the radial electron density increased with increasing  radius, with the density in the inner core of the latent track (at radii less than 3 nm) being only 20% of that in the shell of the latent track, which means a relative excess of positive charge in the core of the latent track compared to its shell. It follows from Maxwell's equations that there is a radially inhomogeneous electric field across the latent track.

Excess charge in the core of the latent track was also found in the work of Wang et al. [16], but with opposite polarity. This discrepancy is one of many examples showing that radiation effects in latent tracks are complex and remain insufficiently understood.  Whatever the polarity, both these experiments show that there is a residual radial electric field in latent tracks after irradiation.  The stability of  this electric field over time is determined by the properties of the irradiated material and can be very long in the case of electrets [17].  The electric field can be significantly affected by UV illumination, as shown by Wang et al. [16], who invented a special technique for processing polyethylene terephthalate (PET) film known as the ‘track-UV technique’. This technique is very close to the technology used to prepare ion irradiated films for etching [18] and is based on the general sensitivity of PET to UV illumination [19].

5. The authors thank the referee for the question. Below is a detailed answer to it.

If the flux is high enough to cause overlapping of latent tracks, the behaviour of benzene-carboxyl subunits will be determined not only by the residual electric field of a single latent track but also by the fields of neighbouring tracks. Because of the cylindrical symmetry of the latent tracks, the individual electric fields in overlapping regions act in opposition to each other to reduce the effective electric field.  This, together with the fact that the benzene-carboxyl subunits are already ordered, allows dipole-dipole interactions between the subunits of neighbouring chain molecules, causing correlated rotations of these subunits. The asymmetric nature of the dipoles in the subunits, together with their rotations, is sufficient in this situation for the formation of a spiral conformation of the subunits in neighbouring chain molecules, as shown in [27]. It is known from the physics of the class of polymer electrets to which PET belongs that dipole-dipole interactions between neighbouring chain molecules have low energy (0.02-0.9 eV) [29], which leads to low temporal stability of dipole-dipole conformational structures, as observed here.

In the wider context, we can make two observations about the results here and in [20]. First, reversing the arguments above,  in this situation some information on the structure of the residual electric fields in the PET film is provided by the x-ray diffractograms, even though they only map positions of reflecting centres in the molecules, because these show the rotation of benzene-carboxyl units responding to a residual electric field. In other words, the benzene carboxyl subunits are acting as nanoindicators of the gradient of the electric field, but provide no information about its magnitude.  This information could be provided by optical studies of irradiated films, for example, using the Franz-Keldysh effect [30], which the authors plan to do in the future. Second, our results, together with those in [26] for a very different experimental situation, suggest that preordering is a very important, and perhaps essential, prerequisite for the emergence of spiralisation.