Elsevier

European Polymer Journal

Volume 108, November 2018, Pages 406-411
European Polymer Journal

Macromolecular Nanotechnology
Structure, morphology and annealing behavior of ion tracks in polycarbonate

https://doi.org/10.1016/j.eurpolymj.2018.09.025Get rights and content

Highlights

  • Ion track formation in polycarbonate is characterized by a damage zone of ∼2.5 nm radius with a density deficit of ∼5%

  • The stoichiometry of the damage track shows equal loss of C and O and increased H loss.

  • Annealing leads to track recovery by diffusion predominately from a track halo area of ∼5 nm radius.

Abstract

Ion tracks created in polycarbonate foils by irradiation with 2.2 GeV Au ions were characterized using a combination of small-angle x-ray and neutron scattering (SAXS/SANS) and Fourier transform infrared spectroscopy (FTIR). The ion tracks were found to consist of a cylindrical damage core with a radius of ∼2.5 ± 0.2 nm and a relative density approximately 5% below that of the pristine polycarbonate. Upon exposure to thermal annealing between 100 and 200 °C, the tracks were observed to double in size. Simultaneously, this led to a recovery in the density of the ion track, reaching a value just below that of the pristine polymer. A mechanism is proposed that explains this behavior by diffusion of radiolysis products/material flow into the under-dense track core from the surrounding region. Treatment of the tracks with UV radiation has shown no significant change in the track structure and size.

Introduction

In a wide range of materials, the irradiation with swift heavy ions leads to the formation of ion tracks [1]. These damage trails, formed along the trajectory of each individual ion, result from a complex interaction process initially between the ion projectile and the electrons of the target material followed by rapid defect creation within the atomic system [2]. Ion tracks are typically a few nanometers in diameter and up to tens of micrometers in length. Insulators are generally the most susceptible materials to ion track formation with organic materials being most sensitive. In polymers, ion tracks were found to consist of a narrow ‘core’ of a few nanometers in diameter, which is characterized by severe damage and a noticeable change in density. This core is surrounded by a much larger halo (∼100 nm) consisting of modifications of the polymer structure such as cross-linking and chain-scission [3]. There exists a specific interest in tracks in polymers driven by the fact that tracks can be converted to open channels by selectively removing the damaged material in a chemical etching process [2], [4]. In this procedure, the etchant (usually an alkaline hydroxide solution) first dissolves the highly damaged core region and subsequently the surrounding halo and undamaged material. This results in cylindrical or conical pores with diameters ranging from a few nanometers to several micrometers. The size and shape of the channels can be controlled by adjusting suitable etching parameters such as temperature, concentration, additives and etching duration [5], [6]. Such ‘track-etched’ membranes can be utilized as micro/nano-porous filters [7] and are commercially available from a number of companies such GE Healthcare Life Sciences and Oxyphen. Furthermore, they can act as a host matrix for many secondary materials, leading to applications in nano- and microelectronics as templates for nanowire growth [8], micro-capacitors [9], diodes and sensors [10]. However, the success of track etching mainly relies on empirical parameters; in addition, the fundamental formation process and damage structure of ion tracks in the pre-etching stage is still not fully understood [11]. Improvements in the understanding of unetched (latent) tracks are relevant to better control the details of the etching process and create smallest channels for nanofluidic elements, ionic diodes, sensors, or model systems for ion-channels in bio-membranes [12], [13], [14], [15]. Moreover, even unetched tracks in polymers show potential applications in ion selective filter membranes [16] or to create electronically active elements of nanometric dimensions [10], [17], [18] by using the change in structure and/or chemical composition.

Due to the small size of ion tracks, imaging them requires high resolution techniques such as transmission electron microscopy [11], [19]. Alternatively, ion tracks in polymers can be analyzed indirectly through the study of bulk degradation of the material by means of infrared and mass spectrometry [20] or x-ray diffraction when exposed to swift heavy ion irradiation [11]. By using small-angle x-ray and neutron scattering, however, it is possible to characterize the average track diameter with very high precision [11], [21], [22].

Here, we present a characterization of ion tracks in polycarbonate focusing on the size and changes in the mass density with respect to the surrounding host material, using a combination of small-angle scattering with x-rays (SAXS) and neutrons (SANS) as well as Fourier Transform Infrared Spectroscopy (FTIR). SAXS and SANS probe the mean size and structure of track ensembles, while FTIR provides direct information on the ion-beam induced damage and changes of molecular bonds. Thermal annealing is shown to gradually reduce the density difference between track and host material while simultaneously enlarging the radius of the tracks. A mechanism is proposed to understand the origin of the size increase caused by annealing.

Section snippets

Experimental section

Stacks of commercial polycarbonate (PC) foils (Makrofol N, Bayer AG) with stoichiometric composition C16O3H14 were irradiated with 2.2 GeV Au ions at the linear accelerator UNILAC (GSI Helmholtzzentrum Darmstadt, Germany) at a fluence of 5×1010ions/cm2. Each stack consisted of three 30-µm thick foils. Makrofol N is a predominantly amorphous cast film produced with no stretching and thus no significant fraction of semicrystallinity. The ion energy was high enough to completely penetrate the

Track morphology

Fig. 1 shows the detector images from the SAXS and SANS measurements of PC foil #1. Similar images are obtained for foil #2 (not shown). The non-irradiated, pristine PC foils (a, d) show no detectable scattering anisotropy. The scattering patterns for the ion-irradiated samples (b, e) are characterized by two slightly bent streaks with oscillating intensity. This anisotropy is a consequence of the tilt between the incident X-ray beam and the parallel, aligned cylindrical tracks. Fig. 2 shows

Conclusion

Ion tracks in polycarbonate were characterized by complementary SAXS/SANS and FTIR measurements. The SAXS/SANS measurements revealed a cylindrical region with a typical radius of ∼2.5 nm and a mass density about 5% lower than that of the undamaged surrounding matrix. FTIR spectroscopy measurements have identified a decrease in CH3 bonds, attributed to the reduction in mass density and damage formation. Exposing the ion tracks to UV radiation has no observable effect on the size and density of

Data availability

The raw/processed data required to reproduce these findings cannot be shared at this time due to technical or time limitations.

Acknowledgments

Part of the research was undertaken on the SAXS/WAXS beamline at the Australian Synchrotron, part of ANSTO. We acknowledge the support of the Australian Centre for Neutron Scattering, Australian Nuclear Science and Technology Organisation, in providing the neutron research facilities used in this work. P. K. acknowledges the Australian Research Council for financial support. P.M.S acknowledges the CONACyT (Mexico) for financial support.

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