Creation of Ion-Selective Membranes from Polyethylene Terephthalate Films Irradiated with Heavy Ions: Critical Parameters of the Process

Abstract

A combination of a long exposure to ultraviolet (UV) radiation and the extraction of radiolysis and photolysis products from tracks makes it possible to create ion-selective membranes from polyethylene terephthalate (PET) films irradiated with heavy ions. These membranes exhibit high selectivity for singly charged cations and high transport characteristics in the electrodialysis mode. The aim of this study is to analyze the mechanisms of the transformation of latent tracks into a system of through pores of the subnanometer range in more detail. Polyethylene terephthalate films are irradiated with accelerated Xe and Bi ions with energy losses in the polymer of 11 and 18 keV/nm, respectively. The evolution of the free volume and the accumulation of carboxyl groups in the irradiated films at different stages of the treatment are studied using gravimetry, IR and UV spectroscopy, conductometry, and electron microscopy methods. It is found that the properties of the resulting membranes depend on several critical parameters, which include, in addition to temperature during extraction, the energy loss of the bombarding ion, the pH of the solution used for extraction, and ion fluence. Dramatic changes in the membrane properties are observed at ion fluences at which individual tracks begin to overlap.

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.…”

“…These estimates are rather higher than the estimate of 3 nm for U ions reported in [44], but broadly align with those in [45,46], which were derived from measurements of the radial etching rate of latent tracks after irradiation with heavier ions (U, Au, Xe) of generally higher energies (in the range of 1 to 11.6 MeV/u), which found radii of up to 100 nm. The X-ray diffraction pattern for the lowest initial ion charge (Kr 13+ ) (Figure 2a, red lines Figure 3a below) exhibits the same qualitative features as those seen for Ar 8+ 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, Figures 4 and 5: Fluences of 7.5 × 10 10 ions/cm 2 and 1 × 10 11 ions/cm 2 respectively At both these fluences, the X-ray diffraction patterns for Kr 13+ ions show a clear anisotropy in the intensity of the peak at 2θ = 26 • , with a greater anisotropy at the higher fluence (see also Figure 3b,c, red lines). Using the same assumptions and approach as previously, this leads to an estimate of 18 nm for the value of the radius of latent tracks for Kr 13+ ions.…”

“…As shown in References [18,19] the key factors affecting the high selectivity and permeability of PET films are the increased density of carboxyl units on the inner walls of the latent track and the reduced density of the material in the core. 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.…”

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

“…We would like to emphasize that, as in [20], we clearly see the reversible nature of this additional ordering, in contrast to the irreversibility of the processes of radiation crosslinking caused by δ-electrons and their cascades of secondary electrons, both immediately after irradiation and throughout the aging of irradiated polymer films [9,10,12].…”

“…Irradiation of polymers with swift ions leads to various irreversible effects, such as amorphization and destruction [4,5], surface modification [6][7][8], and chemical cross-linking of polymers [9][10][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][10][11][12][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][13][14], so cannot cause molecular ordering (which, as far as we are aware, has never been observed), either directly or through the aging processes in irradiated plastic films.…”