Protection in an aggressive environment

All polymer materials continuously degrade under environmental action [1–6]. The factors causing destruction under normal environmental conditions are sunlight, water (moisture), and atmospheric oxygen. In some applications, polymer materials are used in the presence of particularly aggressive media, such as solvents, chemically active gases or liquids, high energy irradiations such as UV and Vacuum Ultraviolet (VUV) light, electron and ion fluxes, and X- and γ-rays. The stabilization of polymer materials against degradation is very important in the polymer industry.

Ion beam implantation changes the structure of the polymer surface layer. In cases where breaching of the surface layer is responsible for the degradation process in aggressive environments, ion beam implantation can increase the resistance of polymers against degradation by aggressive environmental agents. Despite the effectiveness of ion beam treatment for polymers, we know only of a few applications where ion beam implantation is used for improving the degradation resistance of polymer materials.

One important application of stabilized polymers is in greenhouses, where a polymer film is exposed to sunlight (including low- and high-energy UV light), oxygen and ozone active gases, gaseous acidic and alkaline industrial and natural pollutants, and water and snow. Such outdoor ageing is particularly intense in regions with high sunlight irradiation levels, such as deserts [4]. The problem of polymer stability in aggressive environments is usually solved by the application of special additives, such as antioxidants, fillers, and inhibitors of ageing. But antioxidant and inhibitor activity is limited, and sometimes the lifetime is reduced under strongly aggressive environmental agents, like the intense short-wave UV light in Australian deserts. In these cases, other methods of preventing degradation processes become necessary.

Polyolefins are sensitive to UV irradiation from the sun (Figure 10.1). Under sunlight, the polymer loses its strength, becomes brittle, and, for some kinds of polyolefin, depolymerization occurs. The mechanism of ageing under UV light includes the excitation of macromolecules upon photon absorbance and the breaking of chemical bonds, resulting in the formation of free radicals. These radicals start a chain of free radical reactions, which cause cross-linking, depolymerization, and molecular structure transformations. In the presence of air, oxygen molecules take part in free radical reactions, resulting in the formation of oxygen-containing groups. The oxidation processes accelerate the ageing and depolymerization of the polymers, because free radical reactions with oxygen occur more quickly and cause further breakage of macromolecules. In the presence of oxygen without UV light, polyolefins are sufficiently stable.

Modern antioxidants absorb the free radicals that are generated by exposure to UV light, and the stability of polyolefin is increased. There are usually special molecular traps for free radicals that prevent free radical reactions. Another stabilization method is applying a resistant coating to important parts of polymer devices exposed to sunlight. However, sometimes, coatings cannot be applied because they degrade the functional properties. In these special cases, ion beam treatment can help.

In our experiments 0.05 mm low-density polyethylene films containing N-phenylnaphthalene-1 as an antioxidant were treated with nitrogen ions at an energy of 20 keV. This film is usually used for greenhouse covers. The stability of the cover is about 1 year in an outdoor environment. After ion beam treatment, the polyethylene films were exposed under real environmental conditions, including sunlight, day and night cyclic variations in temperature, rain and snow, and moisture variations (Figure 10.2). The films were exposed to open air in the Mendeleevo village, Perm region, Russia. The distance from industrial centers was about 100 km; the town does not have any industrial pollution and car pollution is minimal. The samples were exposed for 6 months from July to January; the average solar irradiation level was 85 kkal/cm², the average annual temperature was +2°C, seasonal temperature variations were from −40°C to +30°C, and the average annual precipitation level was 450–500 mm, including 60–80 mm of snow [7].

The ageing of untreated polyethylene is observed by changes in its FTIR spectrum by the appearance of a carbonyl ν(CO) band as the result of oxidation of the polyethylene macromolecules under sunlight in atmospheric oxygen. The band is wide and corresponds to overlapping carbonyl, carboxyl, and aldehyde group vibration lines. These groups appear in the polyethylene macromolecules as a result of ageing and destruction. The intensity of these lines is attributed to the concentration of oxygen-containing groups, which increases with an increase in the degree of ageing.

The wide band can be fitted with individual lines attributed to different chemical groups. The fitting is based on well-known literature data on carbonyl group spectra, and many polymer oxidation investigations. The FTIR spectrum of aged polyethylene contains the 1708 cm⁻¹ carboxyl group line, the 1724 cm⁻¹ ketone group line, the 1740 cm⁻¹ aldehyde group line, and the 1750–1770 cm⁻¹ ester and γ-lactone group lines (Figure 10.3). This set of vibrational lines corresponds to the well-known spectrum of polyethylene aged under UV light [1,6,8]. The band in the spectrum of polyethylene after ion implantation is quite similar, because oxidation occurs after ion penetration as well (Figure 10.4). However the intensity distribution and, therefore, the corresponding relative concentrations of the oxygen-containing groups are quite different. The spectrum of polyethylene after outdoor ageing contains a more intense low-frequency shoulder attributed to carboxyl and ketone groups. The spectrum of polyethylene after ion implantation contains a more intense high-frequency shoulder attributed to aldehyde and ester groups. The difference is explained not only by different oxidation processes, but also by differences in the structure of the surface layer of the polyethylene. The surface layer of aged polyethylene contains only oxidized macromolecules, such that the structure of the polyethylene macromolecule is retained. The surface layer of ion-treated polyethylene also contains highly carbonized structures, which can become oxidized. In such structures, the oxidation process takes another path and the oxygen-containing groups created have different neighboring groups. After ageing of the ion-modified polyethylene, the carbonyl band in its FTIR spectrum becomes closer to the spectrum of untreated aged polyethylene (Figure 10.5). The high-frequency shoulder decreases and the low-frequency shoulder increases. This corresponds to oxidation processes in the unmodified bulk region of the polyethylene film.

The intensity of the carbonyl band increases with ion fluence, as shown in Chapter 4, but the intensity of the carbonyl line for ion-modified polyethylene is lower than for aged polyethylene. The difference between oxidation of ion-implanted polyethylene and polyethylene aged in an outdoor environment gives an oxidation index. The oxidation index shows the amount of oxygen adsorbed and bonded to polyethylene macromolecules during ageing.

The oxidation index as a function of the ion implantation fluence of the polyethylene film aged for 6 months is shown in Figure 10.6. The normalized oxidation index is determined by the intensity of the carbonyl group line and decreases with fluence. The rate of oxidation as determined by the oxidation index of polyethylene, ion implanted with a fluence of 10¹⁴ ions/cm², is six times lower than that for an untreated polyethylene film.

The mechanical strength of the polyethylene films decreases with the ion treatment from 14.4 MPa for unmodified films, to 10.8 MPa for films modified with a fluence of 10¹⁴ ions/cm². This is caused by a change in the character of the stress/strain curve. A more detailed analysis of the strength changes is presented in Chapter 4.

After ageing for 6 months in an outdoor environment, the strength of the polyethylene film decreases from 14.4 to 7.9 MPa. The polyethylene film becomes brittle, and elongation at breakage decreases from 400% for unmodified films, to 63% for films aged for 6 months in an outdoor environment.

In contrast, the strength of the ion-modified film decreases from 10.8 to 10.2 MPa after outdoor ageing for 6 months, and the elongation at breakage does not change with ageing. The strength degradation rate decreases with increasing ion fluence (Figure 10.7). At a fluence of 10¹⁴ ions/cm², the mechanical properties of the polyethylene film remain the same, despite exposure of the film to the outdoor environment.

The mitigation of ageing by ion treatment can be explained by the absorption of UV light in the carbonized surface layer of modified polyethylene (Figure 10.1B). Strong UV absorption in the ion-modified layer is observed even after low-fluence treatments (see Chapter 4). Degradation typically commences from the surface layer. Since the surface layer of the ion-modified polyethylene film is carbonized and its structure is more stable under destructive environmental conditions, surface layer degradation cannot propagate into the bulk. Therefore, free radical chain reactions do not propagate into the bulk to cause destruction of the polymer film. The modified layer works as a barrier to the destruction process in the bulk polymer. Therefore, ion treatment causing the carbonization of the surface layer of polyethylene prevents the degradation of the polymer in outdoor environments.

Desirable functional properties of polymers can also degrade in aggressive liquid environments. For example, the deformation of polymers is observed in aggressive (for polymers) solvents. Typically, a polymer material can be dissolved in a number of organic solvents. The solubility of the polymer depends on the intermolecular interactions between the molecules of the solvent and the macromolecules of the polymer. One way of characterizing the solubility is the Flory–Huggins parameter for solvents and polymers [9]. The Flory–Huggins parameter characterizes the cohesion energy of a substance. Good solubility of a polymer in a solvent is indicated by similar Flory–Huggins parameters of the polymer and solvent.

For a polymer to be stable in a solvent, the Flory–Huggins parameters of the solvent and the polymer must be different. If the parameters are similar, the polymer will dissolve or swell in the solvent. This causes deformation of the polymer. Such deformation is especially critical for polymer coatings, where the deformation stresses can peel the polymer coating off the substrate.

A way to mitigate dissolving and swelling effects is by cross-linking the areas of the polymer that will be exposed to the aggressive solvents. For example, this strategy is used for rubber materials with plasticizers. In the absence of vulcanizing agents, the rubber materials can even be dissolved in certain plasticizers. But vulcanized (cross-linked) rubbers have good mechanical properties and operate in the presence of active liquids, such as lubricants or oil.

In specific cases when vulcanization cannot be applied to the polymer materials, ion implantation can be used to cross-link the polymer coating instead. We demonstrated this with the example of a thin polystyrene coating. A polystyrene coating of 100 nm in thickness was spincoated onto a silicon wafer. Toluene is an excellent solvent for polystyrene, and the toluene takes only a few seconds after its application to completely remove the polystyrene coating from the substrate. After ion beam implantation with a low fluence, the coating becomes cross-linked. Toluene could not remove any part of the polystyrene from the silicon wafer; instead, the film swelled and became wrinkled after drying due to the evaporation of toluene (Figure 10.8). At such low fluences, the density of cross-linking is low and the polystyrene macromolecules are mobile enough for the toluene to penetrate. After a high-fluence ion implantation, the polystyrene film does not change its structure at all in toluene. The mass, thickness, and surface morphology of the film remain unchanged after the application of toluene. Ion beam implantation cross-links the polystyrene macromolecules densely enough to prevent its swelling in toluene. Such a treatment can be used for coatings if other methods of vulcanization or cross-linking cannot be applied or if these are more complicated and expensive.

Ion beam modification can also be useful for polymers used in the presence of chemically aggressive media. In this case, the modification of a thin surface layer by ion implantation cannot prevent complete degradation of the polymer, but some negative effects following the degradation process can be prevented.

An example is the case of biodegradable polymers, which are used for drug delivery systems. The polymer contains a drug as a filler, or in an encapsulated volume. Over time, the polymer degrades by hydrolysis reactions with water in the organism and the drug is released into the organism inducing pharmaceutical effects.

Here we consider the degradation process of poly(lactic-co-glycolic acid) (PLGA), a biodegradable polymer with many applications in medicine [10–15]. PLGA degrades in living organisms through a hydrolysis reaction in which degradation products, such as lactic and glycolic acids, are released:

The organism metabolizes these reaction products. Due to its safe biodegradability, drugs encapsulated in the PLGA are gradually and locally released during the biodegradation process. PLGA can be used as a therapeutic agent in humans, in the form of drug delivery systems, tissue engineering implants, microspheres, microparticles, nanoparticles, and coatings.

PLGA swells in water solution prior to degradation. The swelling increases the dimensions of the PLGA film. If the PLGA coating has insufficiently strong adhesion to the substrate, then the PLGA film ruptures and forms separate drops on the substrate surface (Figure 10.9). The process occurs over a number of days. After some hours in water, the surface develops hills and valleys. The valleys become deeper with time, eventually reaching the substrate surface. The valleys spread until they overlap and separate drops of PLGA appear. This process is called dewetting. Dewetting depends on the nature of the interaction of the polymer film with the substrate, the viscosity of the film, and the presence of structural defects (rupture centers). The dewetting process is quicker than the degradation process. After dewetting, the PLGA drops degrade as described previously by hydrolysis. If dewetting of the PLGA film occurs in an organism, the kinetics of drug release become uncontrolled, and the therapeutic effect of the drug becomes unpredictable.

We used PIII modification of PLGA films to eliminate the dewetting effect of the PLGA coating in water solution. The surface morphology of the film does not change after ion implantation treatment: the surface roughness remains the same. Visual inspection of modified PLGA showed color changes: PLGA films subjected to high-fluence PIII become milky white when observed in reflection, and light brown in transmission. The structure of the PLGA film was modified by the implanting ions, as observed in FTIR spectra of the treated PLGA coating (Figure 10.10). The absorbance in the 1600–1700 cm⁻¹ region of the spectra increases with the ion fluence. Using ellipsometry, the refractive index of the unmodified PLGA was found to be in the range of 1.47–1.49 over the 400–1000 nm spectral interval. After ion implantation, the refractive index increased to 1.6–1.7. At short wavelengths, the refractive index for the modified samples is considerably higher than at long wavelengths, because of dispersion due to optical absorbance at short wavelengths. The short wavelength absorbance is caused by the appearance of unsaturated carbon structures in the ion-modified PLGA (Figure 10.11). The spectral changes are associated with carbonization of the ion-implanted surface layer. The thickness of the PLGA film decreases due to etching under the ion beam. The etching rate depends on the energy and fluence of the ion beam implantation: at high fluence, carbonization of the modified layer decreases the etching rate. The PLGA film after modification becomes cross-linked and it cannot be dissolved in previously effective solvents, such as acetone. The FTIR spectra of the insoluble PLGA film show that the macromolecular structure of the PLGA remains largely intact after the ion implantation.

Cross-linking of the PLGA decreases the mobility of the macromolecules, and the surface layer becomes much more stable. The modified film does not rupture in water solution, and the degradation propagates in the film (Figure 10.12). The final stage of degradation depends on the ion fluence. At high fluence (10¹⁵ ions/cm² and higher), the PLGA film is cross-linked and highly carbonized, forming a stable nondegradable structure. It is no longer a biodegradable polymer, but a carbonized coating on the substrate. At low fluence (10¹⁴ ions/cm² and lower), the degrees of cross-linking and carbonization are not so high and the film remains biodegradable. In our experiment, we observed a 90% biodegradable fraction.

In all cases after ion implantation with low and high fluences, the dewetting effect is not observed for thin films, when the thickness of the film is close to the ion penetration depth. For thick films, when the thickness is much greater than the ion penetration depth, the dewetting process shows complex characteristics: the top layer of the film is cross-linked and forms a continuous film (Figure 10.13). The bottom, unmodified layer is not cross-linked, and so this layer dewets under the cross-linked layer. We observed such underlayer dewetting for PLGA films of 1000 nm thickness, when the ion penetration depth was 40 nm. Modification of a thick film requires higher energy ions that create a modified layer with a depth close to the film thickness for complete cessation of the dewetting process.

Here, we considered only convenient ion modification of polymers without deposition techniques. PIII and ion beams can also be used to assist in the deposition of inorganic layers on top of the polymers to create protective coatings against aggressive media. Some literature on the degradation of ion beam modified polymers in the space environment can be found in Refs. [16,17].