As part of a long-term international scientific program for studying the survival of various life forms in open space, a series of experiments was carried out on board of the International Space Station in ‘Expose-R’, a multi-user exposure facility provided by the European Space Agency (ESA) attached to the external surface of the Russian segment (ISS(RS)) (Rabbow et al. Reference Novikova, Deshevaya, Levinskikh, Polikarpov, Poddubko, Gusev and Sychev2015). In the experiment conducted by the Institute of Biomedical Problems (IBMP) of the Russian Academy of Sciences, samples of dormant forms of microorganisms, fungi and seeds of higher plants were placed in heat-sealed polymer bags and left in sample compartments connected to a core facility, which was equipped with electronics and heaters (Novikova et al. Reference Novikova, Deshevaya, Levinskikh, Polikarpov, Poddubko, Gusev and Sychev2015) and communicated also with outer space through venting lines and venting holes. The bags were exposed to solar irradiation in a space vacuum through magnesium fluoride (MgF₂) optical windows (transmission threshold 110 nm) for 22 months during years 2009–2011. After sample exposure, it was found that the inner surface of the windows in many cases was contaminated with brownish deposits (thicknesses 5–10 nm). Their structure and chemical composition were studied by different methods, including infrared (IR) spectroscopy, and the presence of CH₂, CH₃, NH, OH, C═O, Si–CH₃ as most probable groups was demonstrated (Demets et al. Reference Demets2015). Contamination deposits on MgF₂ windows were also observed in the experiment AMINO after irradiation of empty sample compartments. Their IR spectra contain the bands for most probable chemical groups CH₂, CH₃, Si–CH₃, Si–O–Si. Possible sources of contamination were discussed in details to eliminate them in future experiments. The conclusion was made that the main source of contamination is outgassing of polymeric materials in the core facility, which was connected with both sample compartments and outer space. It was suggested that the products of outgassing contain compounds, which can attach to the surface of the window and form brown polymeric deposits as a result of the action of the vacuum ultraviolet (VUV) component of Solar radiation. To reduce formation of contamination deposits, a prolonged vacuumization of sample compartments and core facility was proposed in advance of solar exposure.

The goal of the present paper is to consider an important role of two other sources of contamination.

1. 1. As was mentioned earlier, vacuum medium of sample compartments communicates not only with core facility but also with outer space through venting lines and venting holes. Previous studies demonstrated that, main components of contaminating deposits on outer surfaces of orbital stations contain the chemical groups SiO_x (x ≤ 2) and carbon-containing groups (identified by X-ray photoelectron spectroscopy (XPS)) formed from volatile organosilicone compounds of intrinsic outer atmosphere (Skurat et al. Reference Skurat2001, Reference Skurat, Leipunsky, Volkov, Pshechenkov, Beriozkina and Gatsenko2011, Reference Skurat2013). Their origin and chemical composition are not completely understood and therefore are the objects of research. One study demonstrated that volatile organosilicone compounds could be polydimethylsiloxane and tetra-n-propylsilicate, common components of adhesives and additives in polymeric materials (Hanrahan & Ianno Reference Hanrahan and Ianno2014). These volatile organosilicone compounds may permeate sample compartments from intrinsic outer atmosphere and from core facility. Participation of these components in the window deposit formation may be ascertained by detection of their constituents, chemical groups Si–CH₃, Si–O–Si, CH₂, CH₃. Besides polydimethylsiloxane and tetra-n-propylsilicate, many other additives to polymeric materials may contain CH _n (n = 1–3) groups. In addition, contaminations on the outer surfaces of the station ‘Mir’ and ISS (RS) contain oxygen-containing groups. Possibly, they are O–H and C═O groups described in the recent study (Demets et al. Reference Demets2015).

Let us discuss this topic in more detail.

Orientation of ISS changed many times during the experiment ‘Expose-R’. According to Rabbow et al. (Reference Rabbow2015), outer gas pressure in the zone of venting holes of ISS can vary from 5 × 10⁻⁴ Pa around venting holes when they are oriented to meet the running gas flow (ram direction) to 10⁻⁷ Pa around venting holes oriented in the opposite, or wake, direction. Pressure inside of sample compartments and core facility also changes with some delay when station orientation changes. Therefore, due to changes of pressure inside of sample compartments and core facility, the components of intrinsic outer atmosphere (including volatile organosilicone compounds) may permeate sample compartments not only by simple diffusion and thermodiffusion, but also as a result of molecular motion in preferential direction along the pressure gradient, induced by a difference in pressure (up to 5 × 10⁻⁴ Pa) between intrinsic outer atmosphere and inner gaseous medium in sample compartments. When the venting holes change the orientation to wake direction, pressures in sample compartments and core facility become higher than outer pressure and molecular motion is directed from sample compartments and core facility to outside, carrying out the outgassing products in intrinsic outer atmosphere. Therefore, it is important to search common precursors in formation of contamination deposits on the outer surfaces of the station and inside of sample compartment. Possibly, volatile organosilicone compounds-derived SiO_x and carbon-containing groups on the outer surfaces of ISS and groups Si–CH₃, Si–O–Si, CH₂ and CH₃ in sample compartments have a common origin. During 22-month flight, mixing of intrinsic outer atmosphere and sample compartments gas medium may occur. It would explain some similarity between contaminations on the outer surfaces of ISS and on the windows of sample compartments. However, contaminations are not identical, since outer surfaces are intensively bombarded by atomic oxygen from residual Earth atmosphere in addition to Solar irradiation.

1. 2. Chemical groups OH and C═O in deposits could originate from intrinsic outer atmosphere. However, the NH group, which was also found in deposits had no nitrogen-containing analogues on outer surfaces. Nitrogen-containing groups on outer surfaces were observed only in zones of direct impact of exhausted gases from orientation thrusters (Skurat et al. Reference Skurat2013). Ignition and cease of burning, which could happen up to 1000 times per year, cause an incomplete combustion of propellants, 1, 1-dimethylhydrazine and nitrogen tetraoxide. Both compounds participate in the formation of nitrogen-containing deposits in affected zones of outer surfaces in the form of non-volatile nitrate (or nitrite) of 1,1-dimethylhydrazinium as demonstrated by XPS. No nitrogen was found in deposits outside of these zones. Therefore, an alternative mechanism for formation of nitrogen- and carbon-containing deposits should exist to explain the results obtained in the IBMP-conducted study.

We propose that nitrogen-containing groups originated from polymer bags, which were used as containers for biological samples. Solar VUV radiation initiated photopolymerization of nitrogen-containing caprolactam released from the surface of polymer bags.

The bags that were used in the experiment (Novikova et al. Reference Novikova, Deshevaya, Levinskikh, Polikarpov, Poddubko, Gusev and Sychev2015) were produced by Cosmo Bio Co., Ltd., (Japan) (Cosmo Bio Co) (personal communication from S.V. Poddubko (2015), one of the authors of the study). Each bag contains two layers (total thickness 65 µm ± 10%): inner layer, linear polyethylene of low density and outer layer made of Nylon 6. Additionally, both layers contain additives of antioxidants and slip agents (communication from the Company) but chemical identity and volatility of these additives are not known. Therefore, presence of volatile organosilicone and nitrogen-containing compounds cannot be excluded. Possibly, they are involved in the formation of deposits. However, their content in the polymer is low.

Based on analogy with gamma-irradiation, we can expect that solar VUV radiation would induce a number of photochemical reactions in outer polymer layer, leading to formation of H₂, CO, CO₂, CH₄, and to the generation of nitrogen-containing cyclic caprolactam, C₆H₁₁ON, which could be released into vacuum. Additionally, processes of chain scission and cross-linking of macromolecules might be initiated (Chang & La Verne Reference Chang and La Verne2002).

Caprolactam C₆H₁₁ON has a cyclic seven-membered molecular structure and in the process of polymerization it forms linear chains of Nylon 6 (C₆H₁₁ON)_n. This polymer always contains residual monomer, caprolactam (Park et al. Reference Park, Cho, Jeon, Kwak, Lee and Park2006; Araujo et al. Reference Araujo, Felix, Manzoni, Padula and Monteiro2008; Chitiri et al. Reference Chitiri, Goulas, Badeka, Riganakos, Petridis and Kontominas2008; Felix et al. Reference Felix, Manzoli, Padula and Monteiro2014; Heimrich et al. Reference Heimrich, Nicki, Bonsch and Simat2015) (for example, about 0.17 g monomer per 100 g polymer (Heimrich et al. Reference Heimrich, Nicki, Bonsch and Simat2015)). In the IBMP study, temperature in sample compartments can reach 49°C and caprolactam evaporates into vacuum from Nylon layers of bags, since saturated vapour pressure of caprolactam is equal to 0.0021 mm Hg at temperature 25°C (Heimrich et al. Reference Heimrich, Nicki, Bonsch and Simat2015). Additionally, Nylon 6 contains cyclic dimer of caprolactam and other oligomers. However, their ability to diffuse into vacuum has not yet been studied. Therefore, further discussion will focus on the behaviour of monomer, caprolactam.

Although the effect of VUV radiation on the release of caprolactam from Nylon 6 has not been described, the effects of ionizing radiation (gamma-rays, fast electrons) and gas-discharge plasma on this process were previously characterized. Indeed, alpha-line of Lyman series of atomic hydrogen (121.6 nm; photon energy 10.7 eV) dominates in the spectrum of Solar VUV radiation. Moreover, radiation with wavelength 121.6 nm induces photoionization of macromolecules with their subsequent transformations similar to those induced by other types of ionizing radiations (Dorofeev & Skurat Reference Dorofeev and Skurat1982; Barbashev et al. Reference Barbashev, Dorofeev and Skurat1992; Skurat & Dorofeev Reference Skurat and Dorofeev1994; Van Eesbeek et al. Reference Van Eesbeek, Levadou, Skurat, Dorofeev, Vasilets and Barbashev1994; Skurat et al. Reference Skurat, Barbashev, Budashev, Dorofeev, Nikiforov, Ternovoy, Van Eesbeek and Levadou1997; Skurat et al. Reference Skurat2001; Skurat & Samsonov Reference Skurat and Samsonov2001; Skurat Reference Skurat2003). However, a substantial difference of absorbed radiation doses is observed in gamma-radiolysis, bombardment by fast electrons and VUV irradiation (see, for example, (Skurat Reference Skurat2003)). Therefore, the similarity is not very strong.

While the absorption coefficients for Nylon 6 in VUV region are not known, the absorption spectrum of Nylon 6 in the wavelength range 110–250 nm was described (Onari Reference Onari1970). It has the highest radiation absorption near 121.6 nm.

Since Nylon 6 is used for hermetic sealing of foods with subsequent sterilization by radiation, the effects of ionizing radiation on the polymer were well characterized (Chang & La Verne Reference Chang and La Verne2002; Park et al. Reference Park, Cho, Jeon, Kwak, Lee and Park2006; Araujo et al. Reference Araujo, Felix, Manzoni, Padula and Monteiro2008; Chitiri et al. Reference Chitiri, Goulas, Badeka, Riganakos, Petridis and Kontominas2008) and special attention was paid to radiation-induced release of monomer caprolactam.

Gamma irradiation of various films of Nylon 6 in sealed ampoules with doses 3, 7 and 17 kGy followed by liquid extraction of caprolactam led to either increase or decrease in monomer content depending on the manufacturer of the polymer. Interestingly, correlation between the dose of radiation and the yield of monomer was not observed (Araujo et al. Reference Araujo, Felix, Manzoni, Padula and Monteiro2008). Gamma-irradiation of Nylon 6 with doses 5–200 kGy caused increase in concentration of caprolactam from 70.76 ppm to 122–164 ppm, and the effect was not dose-dependent (Chitiri et al. Reference Chitiri, Goulas, Badeka, Riganakos, Petridis and Kontominas2008).

The release of caprolactam into vacuum was observed by mass spectrometry in studying the effect of the plasma of radio-frequency discharge in helium at low pressure on Nylon 6 film (Nelea et al. Reference Nelea, Vasilets, Skurat, Truica-Marasescu and Wertheimer2010). Mass spectrum of the products contained the line with mass number m/z = 113 corresponding to molecular mass of caprolactam.

Thus, the results of several studies indicate that Nylon 6 contains caprolactam, which could be released into vacuum and that ionizing radiation induces the formation of caprolactam.

The Fourier Transform Infra-Red (FTIR) spectra of deposits were identical for all windows and thus not dependent on the specific identity of the biological samples (three very different types: larvae of the midge, seeds of the tomato plant, seeds of the radish plant). This observation pleads for the bags, and against the biological samples as sources of nitrogen-containing contamination.

Next, we show that caprolactam, released from Nylon 6 film under Solar VUV radiation, could be fixed on the inner window surfaces followed by photopolymerization giving Nylon 6. Further, phototransformations of this new-formed Nylon 6 take place under solar radiation. Typical IR spectrum of the deposit on the MgF₂ window contains five identified bands, including O–H (near 3500 cm⁻¹), N–H (near 3300 cm⁻¹), CH₂ and CH₃ (near 2940 cm⁻¹), C═O (1700 cm⁻¹), Si–CH₃ (near 3300 cm⁻¹) and a few non-identified bands (Demets et al. Reference Demets2015). Bands of O–H and Si–CH₃ groups are not present in IR spectrum of Nylon 6 (Bruker Optics Inc.). They correspond to components, which come from intrinsic outer atmosphere (see before).

In the paper (Chang & La Verne Reference Chang and La Verne2002), the characteristic bands of IR spectrum of Nylon 6 are given: wave numbers 2867 cm⁻¹ (or 2860 cm⁻¹) and 2937 cm⁻¹ (or 2930 cm⁻¹) correspond to aliphatic symmetric and antisymmetric C–H stretching vibrations respectively; 3300 cm⁻¹ to N–H stretching vibration; 1640 cm⁻¹ to C═O stretching vibration; 1540 cm⁻¹ to C–N stretching vibration. Gamma irradiation with doses up to 240 kGy causes appearance of new IR bands around 3380 cm⁻¹ (associated with NH₂ vibrations in the new chemical compound RCONH₂) and bands 1600, 1470, 1380, 1300 cm⁻¹, associated with N-alkylamines RNH₂, R₁NHR₂, RCONH₂. Some of them were found in the IR spectrum of contamination deposits and were referred to unidentified bands. The band 1300 cm⁻¹ was identified as band of Si–CH₃ group (Demets et al. Reference Demets2015).