Shape of nanopores in track-etched polycarbonate membranes

Highlights

• Shape, size, and the size distribution of the track-etched nanopores are studied.

• First study to quantify the taper nanopore shape using small-angle X-ray scattering.

• Nanopore shape was found to be predominantly cylindrical with tapered ends.

• The surface radius is ~8 nm smaller than the inner radius of the nanopore.

• Nanopore shape impacts the transport and sensing abilities of membranes.

Abstract

High aspect-ratio nanopores of nearly cylindrical geometry were fabricated by irradiation of 20 μm thick polycarbonate (PC) foils with Pb ions followed by UV sensitization and etching in 5 M NaOH at 60 °C. Synchrotron-based small-angle X-ray scattering (SAXS) was used to study the morphology and size variation of the nanopores as a function of the etching time and ion fluence. The shape of the nanopores was found to be consistent with cylindrical pores with ends tapering off towards the two polymer surfaces in the last ~1.6 μm. The tapered structure of the nanopores in track-etched PC membranes was first observed more than 40 years ago followed by many other studies suggesting that the shape of nanopores in PC membranes deviates from a perfect cylinder and nanopores narrow towards both membrane surfaces. It was also reported that the transport properties of the nanopore membranes are influenced by the tapered structure. However, quantification of the shape of nanopores has remained elusive due to inherent difficulties in imaging the pores using microscopy techniques. The present manuscript reports on the quantitative measurement of the tapered structure of nanopores using SAXS. Determination of this structure was enabled by obtaining high quality SAXS data and the development of appropriate fitting models. The etch rates for both the radius at the polymer surface and the radius of the pore in bulk were calculated. Both etch rates decrease slightly with increasing fluence. This behavior is ascribed to the overlap of track halos which are characterized by cross-linking of the polymer chains. The halo radius was estimated to be approximately 120 nm. The influence of the observed nanopore shape on the pore transport properties was estimated and found to have a significant influence on the water flow rates compared to cylindrical pores. The results enable a better understanding of track-etched membranes and facilitate improved pore design for many applications.

Introduction

When a material is irradiated with heavy ions in the MeV-GeV range, the ions can create a narrow damage trail along their path governed by the interaction with the materials' electrons. In particular, in polymers, these so-called ‘ion tracks’ are significantly more susceptible to chemical etching than the undamaged material and can thus be used to fabricate nanopores. By selecting suitable irradiation and etching conditions, the size and to some extent the shape of the pores can be controlled. The first track-etched membrane was fabricated from a polycarbonate film in 1963 [1], and production of such membranes, using fission fragments as bombarding particles, under the trademark “Nuclepore®” began soon after [2]. Since the 1970s, track-etched membranes have become an indispensable tool for several applications, including laboratory filtration, water filtration, cell culture growth, and environmental studies [2,3]. In the 1980s, advances in heavy-ion accelerator technology resulted in the replacement of the fission-fragments with high-energy charged ions [4,5]. Nowadays, several facilities provide ion beams for the commercial production of ion-track membranes. The tracks and consequently the pores are aligned parallel and are randomly distributed over the irradiation surface. The track density is easily adjusted between a single ion per sample and up to 10¹⁰ ions cm^-2 or more by adjusting the ion fluence. Under suitable etching conditions, the tracks are converted into uniform nanopores with an extremely narrow size distribution. These properties make the membranes interesting for specific filtration applications and for the transport of liquids, gases, particles, solutes, and electrolytes. Also, research on electromagnetic waves in restricted volumes and selective interaction processes of molecules and ions with the chemically or physically modified nanopore surface has attracted great interest due to emerging applications in life science and nanotechnology (see examples of historical and recent reviews [[2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12]] and some landmark original papers [[13], [14], [15], [16], [17], [18], [19], [20], [21], [22]]).

The ion track technology is usually applied to few polymers, including polyethylene terephthalate (PET), polycarbonate (PC), and polyimide [5]. One of the most versatile and intensively studied materials for nanopore fabrication is polycarbonate because of its excellent mechanical and chemical properties and because the pores are well defined and have a very small size distribution [[23], [24], [25]].

It is generally assumed that nanopores created through the track-etch technology in PC membranes are cylindrical; however, this is only a crude approximation of the actual pore structure. In the past [14,[26], [27], [28], [29], [30], [31]], it has been reported that the structure of the pores is tapered towards the surface. D. Cannell and F. Rondelez [14] were the first to report this feature for PC nanopore membranes. They measured the diffusion of polystyrenes dissolved in ethyl acetate through nanopore membranes. To characterize the nanopores, the authors measured the flow of water and ethyl acetate through the nanopores and calculated the pore size using Poiseuille's equation. They concluded that the pores have ‘small lips’ at the surface with an opening smaller in diameter than the inner pore. A. Hernandez et al. [26] investigated the porosity of several Nuclepore® polycarbonate membranes employing a pycnometric method (utilizing the mass difference between a membrane inside the pycnometer with pure water and a dry membrane). They found that the porosity calculated from microscopy images was different and implied that the pores are not cylindrical but have a ‘barrel-like’ geometry. The shape of the nanopores can also be inferred by using the membranes as templates for growing nanowires. This technique delivered similar results. C. Schönenberger et al. [27] observed that the nanowires grown are not cylindrical but are tapered towards the ends and suggested that the pores have a ‘cigar-like’ structure. E. Ferain and R. Legras [28] obtained similar results using nanopores in PC to grow cobalt nanowires describing the shape as ‘toothpick’ like observing that the grown nanowires are tapered towards the ends. J. Duchet et al. [29] synthesized polypyrrole (PPy) in the pores of PC. They also found that the PPy tubules have a “cigar-like” shape resulting from the non-cylindrical shape of the nanopores in the PC membrane. Cornelius et al. [32] compared SAXS data with SEM data and found systematic smaller pore radii for SEM measurements. However, no quantitative measurements of this structure of the pores and how it evolves with etching time and ion fluence have been reported to date.

In this paper, we report the characterization of the nanopore shape and size in PC membranes as a function of the irradiation fluence with high precision using synchrotron-based small-angle X-ray scattering (SAXS) complemented by scanning electron microscopy (SEM). SAXS is based on the elastic scattering of monochromatic X-rays that result from density fluctuations on the nanometer to micrometer scale [[33], [34], [35], [36]]. Ion tracks or track-etched nanopores embedded in the polymer matrix act as scattering objects of reduced density. SAXS allows the accurate determination of their size and shape in a non-destructive manner in particular for parallel and monodisperse structures [[37], [38], [39], [40]]. SAXS can provide accurate and statistically reliable information because the measured scattering intensity results from hundreds of thousands of pores. To derive the nanopore morphology from the scattering curves, detailed modelling of various contributions to the scattering intensity needs to be done, and a suitable real space model assumption is used to develop the fitting model. The quality of the scattering data also determines the complexity of the model that can be resolved. Several previous studies that utilize SAXS to study nanopores in PC considered the pores to be cylindrical and have employed a cylinder model for the analysis [32,[40], [41], [42]]. We demonstrate here that the data obtained from our SAXS experiments showing particularly well-defined oscillations at high scattering vectors q, allows us to reliably determine the more complex structure of the pores and distinguish it from a simple cylinder. We have developed a form-factor model that describes the cylindrical nanopores with tapered ends towards the membrane surfaces as observed earlier [14,[26], [27], [28], [29], [30], [31]] and consistent with SEM analysis.

Reliably determining the pore structure is essential for the exploitation of track-etched membranes in advanced applications. Track-etched polycarbonate membranes are widely available as commercial products. They are used for size-selective filtration and separation in medical and biochemical applications [[43], [44], [45], [46], [47], [48], [49]], biological and chemical sensors [[50], [51], [52], [53], [54], [55], [56], [57], [58], [59], [60], [61], [62], [63], [64]], electrophoresis [65], optical sensors [66], and templates for the growth of hydrogels and nanowires of metals, semiconductors, and insulators [27,[67], [68], [69], [70], [71], [72], [73], [74], [75]]. Most of these applications are linked to the transport properties of various particles (ions, molecules, cells, etc.) through the pores. Hence the quantification of the tapered structure presented in this study will directly influence future studies involving track-etched polycarbonate membranes. P. Ramírez et al. [76] have shown, using theoretical models employing a continuum approach based on Nernst-Plank equations, that the transport properties such as conductance, I–V characteristics, rectification ratio, and ion/molecular selectivity through the nanopores are mainly dictated by the shape of the opening of the nanopore. The pore shape, size and size distribution influences the ionic current rectification, electroosmotic flow rectification and the permeability properties of the membrane [49,[76], [77], [78], [79]] and also impact the fluid flow through the pores [80]. Our results quantify the dimensions of the nanopore opening, which is consistent with a truncated cone, and estimate the influence on the transport properties of water in comparison to cylindrical pores. Our results also allow for a better fabrication of single nanopore membranes through careful choice of etching conditions, which are readily utilized for advanced applications, including bio-molecular sensing, DNA sequencing, and water desalination [50,51,56]. Thus, the pore shape quantified in the present study plays a vital role in assessing the properties of and designing new functionalities for track-etched PC membranes.