Ionizing radiation: a versatile tool for nanostructuring of polymers

Ion track-etched membranes

Scissioning (controlled degradation) of polymer chains by high energy radiation is a typical case for top-down manufacturing process in nanostructuring of polymers. The most widely used application of controlled degradation in lithography has already been mentioned briefly in the Introduction part. When polymeric thin films are irradiated with energetic heavy ions, zones of high density excitations and ionizations are created as they pass through the materials leaving a straight track of radiation damage. These damaged zones are called latent tracks and they can be converted into well defined cylindrical or conical ion track pores by chemical etching, as shown in Fig. 1.

Fig. 1:

Schematic presentation of formation of latent track (right) showing damaged zone upon heavy ion irradiation and cylindrical ion track pore after chemical etching of latent track.

The unirradiated parts of the polymer is chemically stable. During chemical etching the damaged zone is removed and transformed into a hollow cylindrical channel. Etching conditions and procedures determine the size and shape of the pores [5]. Polycarbonate, poly(ethylene terephtalate) and polyimides are the most preferred polymers whose films are used for the preparation of cylindrical ion-track pores with uniform sizes. Swift heavy ion beams in the 1–10 MeV/u energy range have been used. Ion-track membranes are precision bore films with very narrow pore-size distribution. The pore diameters can be from 10 nm to tens of microns and pore density can vary from 1 to 10¹⁰/cm². Track-etched membranes are commercially available for process filtration, cell culture and laboratory filtration. Purification of deionized water in microelectronics, filtration of beverages, separation and concentration of various suspensions can be listed as other so-called passive applications of these membranes [6]. The cylindrical nanochannels of ion track membranes are often used fort he template synthesis of nanostructures [7].

The use of ion track membranes are further enhanced by grafting of pores with stimuli-responsive polymers thus converting passive membranes into responsive membranes [8]. While the flow through passive membranes is ruled by the applied pressure difference, the flow through responsive membranes is controlled by temperature, pH, chemical composition of the solution and electrical potential. When inside walls of the pores are grafted with polymers such as poly(N-isopropyl acrylamide) [9, 10] or poly(acryloil N-proline methyl ester) [11] the pores acquire temperature responsiveness whereas with poly(acrylic acid) pH responsiveness. The chains of former thermo-responsive polymers swell in water, expand and fill the pores below about 30°C (near the lower critical solution temperature of grafted polymers) and collapse and open the gates above this temperature thus acting as on-off devices or valves. Changing the environmental factors controls the flow rate and the transmittance of small and large molecules through these membranes. It has been shown that opening and closing of pores can be cycled many times over long time periods by varying the temperature [9]. The recent progress in methods and applications of radiation-induced grafting inside the confined spaces of nanoporous materials has been outlined in a review article [12].

Fuel cell membranes

The use of radiation-induced grafting in preparation of proton-exchange membranes based on fluorine-containing polymers is an attractive technique that have received intensive research efforts [13]. This method allows introduction of graft chains carrying ionic groups to a preformed polymer film by grafting of a vinyl monomer and subsequent functionalization reaction [14]. Compared to conventional preparation methods of ionic membranes, radiation-induced grafting has the advantages of simplicity, low cost, close control over reaction parameters and ability to tune the content as well as the distribution of the grafted chains in the membranes.

The pore-filled membranes prepared based on track-etched substrates have advantageous properties compared to corresponding membranes with substrates having random pores. The cylindrical pores in the track-etched membranes provide short pathways for the proton transfer. Grafting of polyelectrolytes from the inner surfaces of nanopores provides strong covalent bonding between the polymer base and grafted chains. This approach increases durability of the pore-filled membrane by eliminating the risk of leaching of physically trapped polyelectrolytes during application.

Ion track grafting has been shown to be a promising way of preparing highly conductive membranes for fuel cell applications. Proton conducting membranes were prepared in four steps by γ irradiation grafting of styrene onto Xe ion irradiated ETFE film by: (1) Xe ion irradiation to form the latent tracks and chemical etching, (2) γ-ray pre-irradiation to generate radicals, (3) styrene grafting from radical sites to introduce aromatic rings and (4) sulfonation of aromatic rings to introduce sulfonic acid groups [15]. These membranes showed high proton conductivity depending on the degree of grafting. A maximum proton conductivity of 0.1 S/cm was achieved at a degree of grafting of 50% and water content of 40%.

By a similar approach proton conducting individual channels have been created in poly(vinyledene fluoride) PVDF films using ion track grafting technique [16]. Polystyrene was radiation grafted and later sulfonated leading to formation of poly(styrene sulfonate) chains inside the tracks. The cylindrical pores filled with conductive polymers allow proton conduction when used in fuel cells. The optimum degree of grafting has been shown to be 140%. These membranes assembled in fuel cells showed conductivities between 50 and 80 mS/cm depending on the operating conditions which are values close to that of Nafion used under similar conditions.

Radiation-induced grafting method was also employed for the preparation of micro-patterned proton-exchange membranes. It is known that mechanical properties of a graft polymer film may deteriorate, limiting the use of resultant membranes in fuel cells. One of the approaches used to overcome this problem is partial grafting of a substrate only in defined areas of the film. Patterning with lateral dimensions as small as 100 nm can be achieved by means of either electron beam or lithographic X-ray exposures by using masks [17, 18]. Subsequently, an irradiated substrate is exposed to a monomer solution to initiate graft polymerization. During grafting, growth of the polymer occurs only on the irradiated area and the polymer chains form “brush-like” structures on the surface of the substrate. As a final step, sulfonation is carried out to introduce functionality to the membrane. Consequently, the grafted areas can provide the required ionic conductivity while the remaining non-irradiated areas maintain the mechanical stability of pristine film.

The amount and distribution of proton conducting sulfonic acid groups are among the most critical factors affecting the performance of PEMs. In radiation-induced grafting method these parameters are directly related with the graft characteristics, e.g. lengths of the grafted chains, their distributions, uniformities, etc. By conventional radiation-induced grafting technique, controlling/tailoring of such properties is not achievable. The grafting is totally a random process in conventional radiation-induced grafting and neither the molecular weigth nor its distribution can be controlled. The control over free radical polymerization has been achieved during last two decades by reversible activation–deactivation of polymerization kinetics [19]. One of these techniques applicable under irradiation conditions is called reversible addition-fragmentation chain transfer (RAFT) polymerization. The control of chain growth has been made possible by employing special chain transfer agents of thionyl thio compounds [20].

Novel membranes based on ethylene-tetrafluoroethylene (ETFE) copolymer for fuel cell applications have been synthesized by RAFT-mediated radiation-induced graft polymerization of styrene followed by sulfonation as depicted in Fig. 2 [21]. Frontal development of grafting process across the membrane thickness resulted with homogeneous distribution of graft chains with uniform chain lengths hence increasing the compactness of the membranes. The introduction of RAFT-mediated graft polymerization enhanced the structural uniformity and showed a significant increase in terms of proton conductivity compared to conventional grafting.

Fig. 2:

General procedure of radiation-induced RAFT-mediated controlled grafting of styrene from ETFE films and sulfonation for preparing fuel cell membranes [21].

Sensors/detectors

Another interesting field of application of ion track membranes is in the development of sensors and detectors. Ion tracks are particularly suitable in biosensing applications because they have nanoscale dimensions. Ion tracks can confine chemical reactions in well-defined pre-determined locations ensuring the high enrichment of reaction products locally. If membranes containing such etched, modified tracks are placed in the path of current flows in a vessel, the ions would be forced to pass through the nanopores. The changes occuring in the electrical resistance of the pores would help sensing of chemical reactions. The advantages of using conically etched tracks were shown by Siwy et al. [22, 23] where conically etched tracks were developed by etching the polymer foil from one side only. In a recent work, temperature dependent ionic transport through the nanochannels functionalized by amine-terminated poly(N-isopropylacrylamide) brushes has been achieved. The effective pore diameter is tuned by manipulating the temperature via swelling/shrinking behavior of brushes grafted to the nanopore walls, controlling the ionic transport across the membrane [24].

The controlled grafting on wall surfaces of cylindirical nanochannels in track-etched membranes imparts almost endless properties and consequent applications of these membranes. One such recent development is the preparation of polymer thin film electrodes based on radiation modification of poly(vinyledene fluoride) (PVDF) membranes. Poly(acrylic acid) was grafted inside the cylindirical nanopores of track-etched PVDF membranes with 75 nm diameters in a controlled manner by RAFT polymerization [25]. It was observed that the pore diameter decreased steadily with the degree of grafting and pores started to be filled by PAA graft chains beyond ~40% grafting as determined from AFM measurements. These membranes were transformed into very sensitive polymer thin film electrodes by gold sputtering onto both surfaces of the membrane. These polymer thin film electrodes were found to be very sensitive to sub-ppb concentrations of Pb²⁺ ions in square-wave anodic stripping voltammetry measurements [26]. Controlling of graft chain lengths by RAFT mechanism was found to increase the sensitivity of membrane electrodes almost three times compared to grafting by conventional free radical polymerization in sub-ppb concentrations of Pb²⁺. Figure 3 shows schematically the formation of graft polymers inside the cylindrical nanochannels of PVDF track-etched membranes and cross-sectional view of a nanopore with grafted PAA capturing lead ions.

Fig. 3:

Diagram showing controlled radiografting of polymer chains on pore walls by RAFT polymerization (a and b) and a single pore from a gold coated 9 micron thick PAA grafted track-etched PVDF membrane with lead ions absorbed from water [25, 26].

The track-etched poly(ethylene terephthalate) (PET) membranes can be used as templates for effective radiochemical synthesis of metallic nanoparticles in the interior of cylindrical nanochannels of the membrane. Radiation-induced synthesis of copper nanoparticles was achieved first by functionalization of interior surface of the nanochannels through successive oxidation and graft polymerization of acrylic acid [27], which allowed the capture of Cu²⁺ ions by the carboxylate groups and at the same time stabilizing the nanoparticles on the surface and inside the pores of PET membranes. The conversion of Cu²⁺ ions into metallic copper was straightforward by the reaction of extremely reactive hydrated electrons generated in the aqueous irradiation medium. The reaction scheme is shown below [28]:

H 2 O → radiolysis e aq − ,  H 3 O, H ⋅ , H 2 ,  OH ⋅ ,  H 2 O 2 H ⋅ ( O H ⋅ ) + C H 3 CH 2 OH → H 2 ( H 2 O ) + C H 3 C ˙ HOH Cu 2+ → e aq − Cu + → e aq − Cu 0 → Cu + Cu 2 + → Cu m n + → Cu agg

In the radiolysis of water mainly hydrated electrons, and OH radicals are generated and with equal efficiency. Although hydrated electrons are the strongest reducing agents in aqueous systems, OH radicals on the other hand are the strongest oxidizing species in water. In order to eliminate possible oxidation of radiation synthesized metallic copper nanoparticles to corresponding oxides, OH radical scavengers like ethyl alcohol is added to reaction medium converting the oxidizing species to strong reducing agents like ethyl alcohol radicals thus providing an all reducing reaction medium [29].

The results showed an almost complete reduction of copper ions at a dose of 250 kGy to form metallic copper nanoparticles with an average size of 70 nm [28]. Electron microscopy revealed a good distribution of nanoparticles throughout the length of the nanochannels, as seen from Fig. 4. Track-etched PET membranes with nanometallic copper loaded nanochannels showed promising results for catalytic and sensor applications.

Fig. 4:

SEM microphotographs of cross-sectional view of PAA-g-PET nanochannels with (right) and without (left) Cu nanoparticles deposited after γ-irradiation to a dose 98 kGy [28].

Cell culture dishes

When tissue engineering was first proposed in the 1980s the key materials used were biodegradable polymeric scaffolds. By combining preformed polymeric scaffolds and specific cell types, various tissues have been reconstructed. After transplantation of tissue-engineered constructs into hosts the scaffolds degrade over weeks or months. The space formerly occupied by the scaffolds is filled with proliferated cells and deposited extracellular matrix (ECM) such as collagen. However, ECM deposition in tissues often results in fibrosis and some properties of the scaffolds can be undesired in some tissues. This gave rise to the development of Cell Sheet Engineering in order to find solutions to avoid these limitations.

In tissue reconstruction cell sheets are used not single cells. Cell sheets are prepared by using temperature responsive culture dishes. Imparting thermoresponsive properties to the surfaces of culture dishes is best achieved by radiation-induced grafting of a thermoresponsive polymer called poly(N-isopropylacrylamide) (PNiPAAm) on the surfaces where cells will be grown. For cell culturing commercially available polystyrene dishes are usualy used for grafting of PNiPAAm. Okano and his group for the first time prepared PNiPAAm grafted cell culture dish surfaces by electron beam irradiation [30]. The cell sheets grown at 37°C with the graft polymer in fully hydrophobic state are detached simply and easily by lowering the temperature below 32°C with polymer transformed into hydrophobic state as described schematically in Fig. 5 [31]. Very thin layer of radiation-grafted PNiPAAm offers non-invasive control of cell attachment and detachment.

Fig. 5:

Cell sheet harvesting from PNiPAAm grafted PS surfaces. Cell sheets grown on hydrophobic surface at 37°C (shown on the left) can be detached at temperatures below 32°C from the hydrophilic surface (lower right). Enzymatic treatment would destroy the cell-cell connections as well as deposited ECM [31].

The thickness of the PNiPAAm layer is a key factor affecting the hydration features and hence the cell sheet recovery characteristics. For a PNiPAAm layer with a thickness higher than the desired level, poor hydrophobicity and a non-cell adhesive character are reported as the dehydration promoted by the PNiPAAm chains at the substrate interface cannot efficiently reach the PNiPAAm chains at the outermost surfaces. The optimum thickness of PNiPAAm layer has been found to be 20–30 nm [32]. Fine controlling of the thickness of PNiPAAm within the desired limits is best possible by controlling the free-radical graft polymerization of PNIPAAm from PS petri dish via reversible addition fragmentation chain transfer (RAFT) polymerization already mentioned in Fuel Cell Membranes section. By using this technique in surface initiated grafting it is possible to prepare polymer brushes with well defined, predetermined molecular weights, thus bringing a full control over the thickness of the thermoresponsive graft layer [33].

A rapid cell sheet recovery is desirable for clinical applications. To accelerate the cell-sheet detachment process, hydration of PNiPAAm chains should occur in a relatively short time. In order to facilitate the hydration of PNiPAAm layer, grafting a layer of hydrophilic poly(acrylamide) (PAAm) prior to modification of cell culture dishes with PNIPAAm functionalities enhances the cell recovery features [34]. In an ongoing study from author’s lab PAAm component as a model hydrophilic polymer has been grafted from electron beam (EB) irradiated surface of PS petri dishes via RAFT polymerization. PAAm grafts were further extended by PNIPAAm chains based on the living features provided by the RAFT chain-end moieties of PAAm.

The application of cell sheet technology based on thermosensitive polymer grafting has been extended to include corneal epithelial, esophageal ulceration, cardiac tissue, periodontal tissue, sealing of lung air leak, pancreatic islet tissues [35].

Polymer thin films containing metal nanoparticles

Polymer films embedded with metal nanoparticles have shown great potential in various applications and industries such as, electronics, catalysis, chemical sensors/detectors, semiconductors and antibacterial surfaces. The size of the metal nanoparticles and the distance between them in a polymer matrix are known to have great influence on their electronic, magnetic and optical properties. Therefore control over size, dispersion, shape and homogeneity is very important for developing new devices with advanved capabilities [55].

Metal nanoparticles stabilized in a host polymer matrix can be synthesized by various routes all based on the reduction of precursor metal ions. The reduction of metal ions into their metallic state with the formation of nanoclusters can be achieved by; (a) using different chemical reducing agents, (b) photoreduction, (c) sonochemical, and (d) using ionizing radiation [56]. The unique advantage of radiolytic synthesis lies in the fact that the strongest reducing agent in aqueous environment namely, hydrated electron {E^o [H₂O/e⁻(aq)]=–2.87 V} is generated in-situ in de-aerated solutions with very high efficiency. This enables reduction of any metal ion including noble metals to their zero valence. The metal nanoparticles thus obtained are pure and very stable. γ irradiation of aqueous metal salt solutions for the synthesis of metal nanoparticles offers unequaled advantages of simplicity, reproducibility, absence of impurities and application at room temperature. By using X-rays or accelerated electrons with a range of dose rates it is also possible to have a control on the size and size distribution of metal nanoparticles. The higher the dose rate the higher will be the concentration of hydrated electrons hence increased nucleation rates of particle formation leading to the formation of nanoparticles with smaller sizes [29].

The stability of synthesized metal nanoparticles has been achieved by using proper capping agents or polymers appropriate for the specific metal ions. Radiation synthesized noble metal nanoparticles have been found to be relatively stable while stability of copper nanoparticles has been a challenge due to their easy oxidation [57]. It has been shown that when copper ions complexed with interpolyelectrolyte complexes of poly(acrylic acid) and poly(allyl amine), copper nanoparticles with very high stability can be obtained by γ or x-ray irradiations of films in the presence of water. The mechanism of reduction of Cu²⁺ under irradiation in aqueous environment has already been explained in the subsection titled Sensors before. It was shown that nano particles having sizes in the range of 3–4 nm and 5–10 nm were obtained when X-ray and γ irradiation were used, respectively. Controlling of nanoparticle sizes was achieved by using different radiation source, radiation dose and polymer/metal ion ratio. The metallic copper nanoclusters were observed to be stable when imbedded in polymer films for several months [58]. Figure 8 shows the TEM images of copper nanoparticles embedded in polymer blend films together with their electron diffraction patterns. The stability of copper nanoparticles thus synthesized against oxidation showed a good prospect for their use as sensors and catalysts.

Fig. 8:

TEM images of blend films composed of Poly(acrylic acid)/Poly(allyl amine)/Cu(II) in 2/2/1 molar ratio irradiated to 140 kGy dose at pH=8.6. The average particle sizes of copper nanoparticles are 6 nm [58].

Polymer/clay nanocomposites

Polymer/Clay Nanocomposites were first developed in 1985 in Toyota Central Research Laboratories and commercially applied to passanger cars in 1989 [59]. When a few percent of silicate layers of clay are randomly and homogeneously dispersed in polymer matrix superior mechanical, thermal and barrier properties are imparted to host polymers. Commodity or engineering polymers have long been reinforced with particulate or fibrous fillers such as talc, calcium carbonate, carbon black, glass or carbon fibers, typical content of a filler being usually between 20 and 40%. Polymers and filler are not mixed homogeneously on a microscopic level limiting the interaction between the matrix and the filler. If mixing could be achieved on molecular level, the interface would be increased and interaction much improved.

According to the International Union of Pure and Applied Chemistry, a nanocomposite is a composite in which at least one of the phase domains has at least one dimension in the order of 1–100 nm [60]. Montmorillonite (MMT), one of the most abundant clay minerals has been shown to be a very good nanofiller since the layered silicates are 1 nm thick and have a cross-sectional area of 100 nm². Polymer/Clay nanocomposites can be prepared either by polymerizing a monomer in the presence of a clay or by solution or melt mixing of a polymer with the clay. Since clays are hydrophilic substances and most of the matrix polymers hydrophobic, either the clay or the polymer should be rendered more compatible with each other. This problem has been solved to a great extent by using commercially available organo-modified montmorillonites. Nylon6, polyimides, polypropylene, polyethylene, ethylene-vinyl acetate copolymers and rubber have been extensively investigated as the matrix polymers for polymer/clay nanocomposites. Ionizing radiation can be used in different ways for the preparation of polymer/clay nanocomposites. It can be used to initiate in-situ free radical polymerization inside the clay structure and depending on the chemical structure of monomer, simultaneous crosslinking of liquid vinyl monomers mixed with 1–5% clay mineral thus forming the nanocomposite. Electron-beam induced reactive in-situ modification of unmodified MMT in polypropylene has been achieved. The effect of different electron-beam parameteres such as electron energy and absorbed dose on intercalation/exfoliation of MMT in PP was investigated [61]. Recently RAFT-mediated grafting of polystyrene from the radicals confined on the surface of clay was investigated [62]. Polyolefines can be irradiated in air to control radiation-induced oxidation to impart polar and hydrophilic properties to enhance their compatibility with the clay. γ irradiated polypropylene was used as a compatibilizer in the preparation of short carbon fiber reinforced polpropylene composites. Strong compatibilizing effect of irradiated PP (radiation degraded and oxidized) on the mechanical properties of the composite was observed [63]. Another approach can be irradiation of compounded polymer/clay system to introduce crosslinks within the matrix to enhance mechanical and barrier properties.