Highly durable hydrophobicity in simulated space environment

Abstract

By infusing perfluoropolyether lubricating liquids into hierarchical micro- and nano-structures, a highly durable hydrophobic material was obtained for resisting low earth orbit space irradiations such as atom oxygen, ultraviolet, proton, and electron irradiations.

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Introduction

Increasing bionic research, inspired by several natural superhydrophobic surfaces, has developed enormous artificial superhydrophobic surfaces.^1–6 These surfaces could find extensive applications in self-cleaning, anti-fouling, anti-icing, drag reduction, and water/oil separation, etc.;^7,8 however, they are seriously hindered by their poor durability. Unlike in natural plants, where superhydrophobicity can be preserved very well because of the unique regeneration ability of living species,⁹ artificial surfaces can permanently lose superhydrophobicity because of damages to either rough structures or low surface energy materials on the top of the surface. Especially in the case of the latter, they can be damaged by mechanical wear or decomposed due to sunlight. Recently, several methods have been employed to improve the resistance of artificial superhydrophobicity to environments such as the use of anti-friction materials,^10,11 easily repairable materials,^12–18 and controlled-release materials.^19–23

However, the durability of superhydrophobicity could still be a concern when these surfaces are exposed to extremely rigorous environments such as aerospace in low earth orbit (LEO), where plasma, light, and electromagnetic radiations (including atomic oxygen, ultraviolet, protons, and electrons) are extremely harsh toward almost all the polymers used in spacecraft.^24,25 Because of high chemical reactivity and high impinging energy (∼5 eV) of atomic oxygen (AO), especially considering very high relative velocity between the surface of a spacecraft and the AO, it can erode all kinds of organic materials. As a matter of fact, the monolayer of fluorocarbon polymers that are commonly used in artificial superhydrophobic surfaces could withstand only seconds of AO irradiation before eroding completely.^26,27 This may result in serious loss of superhydrophobicity or hydrophobicity of artificial materials. Therefore, developing durable hydrophobic surfaces under extremely rigorous space environment is a highly challenging task.

In this work, a new material to address the abovementioned challenge is introduced using a facile fabrication method, i.e., liquid perfluropolyether (PFPE), which fills up the micro- and nano-scale hierarchical structure of anodic aluminum oxide (AAO). PFPE is commonly used as a lubricant in aerospace vehicles because of its high stability with low volatility and high temperature resistance in space environment. Especially, it is a liquid at room temperature with very low surface energy, and thus can be conveniently imbued into the large inner space of hierarchical micro- and nano-scale structures. A unique property of this material is that micro- and nano-scale structures can be used as a reservoir to store liquid PFPE. Even though the outermost PFPE molecules of PFPE-filled AAO are eroded by space irradiations, stored PFPE molecules in the bottom gaps of AAO can creep and spread all over the top surface of AAO to maintain the material hydrophobic. To verify this hypothesis, samples of PFPE-filled AAO were prepared and subjected to various vacuum irradiation tests in simulated space environment such as AO, ultraviolet (UV), proton, and electron irradiations. The exposed samples were subjected to surface morphology and chemical composition characterizations to investigate the effects of various irradiations.

Experimental

Materials and methodology

Micro- and nano-structure anodic alumina oxide was prepared by unconventional two-step anodization according to a procedure reported in the literature.²⁸ Once prepared, the samples were immersed into a 2.5% (v/v) PFPE (Sinopec Lubricant Company) solution in 1,1,2-trichlorotrifluoroethane (Sigma-Aldrich) for 24 h to load PFPE, followed by rinsing with acetone. Finally, PFPE-filled AAO were prepared by heating at 100 °C for 60 min to completely remove the solvent that remained in PFPE and promote molecular rearrangement. Two samples were fabricated for controlled experiments. One was prepared by spin-coating a droplet of pure PFPE liquid onto a silicon surface at 1000 rpm for 2 min, and another was prepared by spin-coating PFPE solution onto an AAO surface at 3000 rpm for 60 s. Both the controlled samples were treated by heating at 100 °C for 60 min in an oven. The PFPE-filled AAO was then put into a chamber of a space environment simulation facility at Lanzhou Institute of Chemical Physics to conduct AO irradiation at 4.0 × 10⁻³ Pa. The flux of AO is 6.73 × 10¹⁵ atoms per cm² per s. Exposure times of 0.5 h, 1.5 h, 2.5 h, 3.5 h and 5 h were selected to investigate the time independence of the wettability of PFPE-filled AAO. Meanwhile, we also carried out other irradiations such as UV exposure, proton irradiation, and electron irradiation. The UV exposure experiment was carried out under excimer light with wavelengths of 200–450 nm under a pressure of 4.0 × 10⁻³ Pa using a mercury Xenon lamp, the intensity of which was determined by a UV monitor to be 15.8 mW cm⁻²; the proton irradiation experiment was conducted under a pressure of 3.0 × 10⁻³ Pa. For electron irradiation, the energy of the electron beams was 28 keV and its power flux was 50 μA cm⁻². Note that we labeled the samples as irradiation-XX h according to the irradiation type, for example, AO-0.5 h represents the sample exposed to AO irradiation for 0.5 h.

Characterizations

Contact angles (CA) were acquired using a DSA-100 optical contact-angle meter (Krüss Company, Ltd., Germany) at ambient temperature (25 °C) by injecting 5 μL of testing liquids onto the samples, and the CA value was determined automatically using the Laplace–Young fitting algorithm. Average CA values were obtained by measuring the sample at five different positions, and images were captured with a digital camera (Sony, Ltd., Japan). Surface morphologies were observed using a field-emission scanning electron microscope (SEM, JSM-6701F, Japan) at 5–10 kV. Changes in the chemical structure of the PFPE-filled AAO were recorded by Fourier transform infrared spectra (FTIR) on a TENSOR 27 instrument (Bruker, KBr disks for compounds and polymers, whereas ATR was used for modified surfaces). Chemical composition information was obtained by X-ray photoelectron spectroscopy (XPS), which was carried out on a PHI-5702 multi-functional spectrometer using Al Kα radiation, and the binding energy was referenced to the C1s line at 284.6 eV from adventitious carbon.

Results and discussion

To investigate the wettability of PFPE-modified AAO after AO irradiation, contact angles of water were measured. Before irradiation, the CA of PFPE-filled AAO was ∼72° when a 5 μL water droplet was dropped onto its surface. Fig. 1a shows CA dependence on irradiation time and storing time after irradiation, and it can be easily seen that the CA varied from 135°, 138°, 114°, 83°, to 0° when prolonging the irradiation time from 0.5 h, 1.5 h, 2.5 h, 3.5 h, to 5 h, respectively. However, the CA of all the exposed samples surprisingly maintained (AO-0.5 h and AO-1.5 h) or returned back (AO-2.5 h, AO-3.5 h and AO-5 h) to >130° after being stored for 3 days at atmosphere, which was 138°, 139°, 149°, 137°, and 132°, respectively. On increasing storage time to 30 days and 130 days, corresponding CA increased at first and subsequently decreased slightly. Therefore, regardless of how long PFPE-filled AAO samples were irradiated by AO, they were always hydrophobic after they were stored over a period of time in air. Furthermore, we tested repeatability by investigating if the samples could endure multiple cycles of AO exposure. Herein, heating and pumping were applied to speed up the healing of hydrophobicity. From Fig. 1b, we can see that PFPE-filled AAO retained excellent hydrophobicity (CA ∼ 139°) via several cyclic tests of irradiating–healing.

Fig. 1 Durable hydrophobicity of PFPE-filled AAO irradiated by AO irradiation. (a) Dependence of contact angle of water droplet on PFPE-filled AAO surface on exposure time and storing time before and after irradiation. (b) Repeatability of enhanced reversible transition between hydrophilicity and hydrophobicity by heating (top) and pumping (bottom) after irradiation. Different-shaped symbols represent the storing time at atmosphere after irradiation (square: 0 day; circle: 3 days; upper triangle: 10 days; lower triangle: 130 days).

To investigate the reason for the CA changes of PFPE-filled AAO under AO irradiation, surface morphologies, chemical structures, and chemical compositions were characterized by SEM, FTIR, and XPS before and after AO irradiation.

Surface morphologies of PFPE-filled AAO are shown in Fig. 2a. It is obvious that PFPE completely covered the rough and hierarchical AAO surface. Although there are some rugged areas, the surface is relatively flat compared with bare AAO (Fig. S1†). After AO irradiation, PFPE on the AAO surface was clearly reduced with increasing irradiation time, which is clearly seen in Fig. 2b–d for sequentially corresponding samples irradiated for 0.5 h, 2.5 h and 5 h. Hierarchical AAO structures were recovered after irradiation for 2.5 h and 5 h, even nanowire structures can be seen clearly in the inset of Fig. 2c and d, compared with the samples with shorter irradiation time of 0.5 h. We can conclude increasing irradiated duration has a greater exposure of hierarchical structures.

Fig. 2 SEM surface morphologies of PFPE-filled AAO before AO irradiation (a) and after irradiation for (b) 0.5 h, (c) 2.5 h, and (d) 5 h. The inset of (c) and (d) are corresponding magnified views.

Surface chemical structures before and after irradiation are shown in the FTIR spectra of Fig. 3. Non-irradiation PFPE-filled AAO consisted of C–O–C stretching vibration mode at 981 cm⁻¹ and 1121 cm⁻¹, CF stretching vibration mode at 1182 cm⁻¹, CF₂ stretching vibration mode at 1228 cm⁻¹, and CF₃ stretching vibration mode at 1306 cm⁻¹. All these peaks could be assigned to the PFPE molecular structure. Meanwhile, it is clear that after AO irradiation the intensities of the CF₂ and CF₃ peaks were significantly reduced, and positions shifted to 1222 cm⁻¹ and 1297 cm⁻¹ (inset of Fig. 3), respectively. This reduction in intensity is attributed to the loss of PFPE by AO erosion and hierarchical structure exposure because of the PFPE loss, which led to a weak contact between the sample and the ATR probe of the FTIR instrument during characterization. The reason for peak shift is still unclear. It might have been caused by the large roughness of the surface or other unknown materials.

Fig. 3 FTIR spectra of PFPE-filled AAO before and after AO irradiation for 0.5 h and 5 h. Inset: zoom-in view of the FTIR spectra of samples after AO irradiation from 1100 to 1340 cm⁻¹.

The chemical compositions of PFPE-filled AAO before and after irradiation were measured by XPS (Fig. 4). Note that the irradiated samples were stored for 10 days in the atmosphere before XPS measurement. It was found that all the samples exhibited F, O, and C peaks. It is interesting that for the samples with 2.5 h, 3.5 h, and 5 h of AO irradiation Al peaks emerged, which indicates that most of the outermost PFPE was scissored and bottom Al₂O₃ was exposed roundly with an increase in irradiation time. These results are consistent with previous SEM morphology change. The fine spectra of C1s can display some change in information of chemical compositions in Fig. 4b–f. Before AO irradiation, there were strong C1s peaks localized at 293.4 eV and 291.4 eV, which are attributed to CF₂–CF₂–O and CF₂–CF₂–CF₂, respectively, from PFPE,²⁹ and two other weak peaks localized at 284.6 eV and 282.6 eV. After increasing the irradiation time from 0.5 h to 5 h, intensity of CF₂–CF₂–O and CF₂–CF₂–CF₂ decreased gradually due to erosion of main chain backbones. However, all the samples had CF₂–CF₂–O and CF₂–CF₂–CF₂ signals, indicating that PFPE molecules still existed on sample surfaces. Two other peaks clearly arose at binding energies of 284.6 eV and 282.6 eV, which may have resulted from C–C (C–H) bond and Al–O–C (or Al–C-like species), respectively.^30–32 Peak intensity of AO-0.5 h to AO-2.5 h and AO-3.5 h located at 284.6 eV increased while that of AO-5 h reduced. The going up may have been because numerous dangling bonds were formed by the breaking of carbon chain to absorb significant amounts of water or carbonaceous molecules at atmosphere or reacted to form a cross-linked polymer on the surface; the going down was because 5 h of AO irradiation clear surface PFPE completely leading to significant loss of active sites and reduced formation of C–C (C–H). Al–O–C (or Al–C-like species) located at 282.6 eV could be formed because of annealing at 100 °C. Several studies have reported that Al₂O₃ promoted PFPE decomposition from the C–O bonds of C–C–O as a Lewis acid at above ∼200 °C.^33,34 It is deduced that the decomposition products (such as C–C- or –O–C–C) of PFPE may react with active Al₂O₃, forming C–C–Al or Al–O–C at local areas (temperature could go over 200 °C due to local overheating). Its intensity increased relatively with irradiation time due to the reduction of PFPE molecules. All these indicate that AO irradiation is the principal cause of breaking the main carbon chains of PFPE.

Fig. 4 XPS survey spectra (a) of PFPE-filled AAO before and after AO irradiation and storing for 10 days. C1s fine spectra of PFPE-filled AAO before irradiation (b), and after irradiation for 0.5 h (c), 2.5 h (d), 3.5 h (e), and 5 h (f). All the irradiated samples were stored for 10 days.

It is well-known that a hydrophobic surface is composed of a rough structure and a low surface energy material. The loss or recovery of hydrophobicity can be derived from surface damage or recovery. Combining the previously mentioned SEM morphologies, FTIR spectra analysis, and XPS composition characterization, we can effectively deduce the wettability behaviors of PFPE-filled AAO during AO irradiation with the schematic illustration of Fig. 5. The hierarchical micro- and nano-structure AAO was immersed in PFPE solution, allowing PFPE solution to fill up the hierarchical structures and form a thick and plain film on the top surface. Although there were numerous CF₂ groups, the flat surface exhibited hydrophilic behavior with CA of ∼72°, which is similar to the spin-coated PFPE film on Si-wafer (∼77°) in Fig. S2.† After AO irradiation with a kinetic energy of about 5 eV, a pure physical sputtering of surface atoms in the absence of any chemical changes was observed. The PFPE main carbon chain backbones were scissored and several small molecules volatilized (CF₂O, CF₂, and CF₃) at the same time, which resulted in the PFPE film gradually becoming thinner with irradiation time. After irradiation for 0.5 h, some PFPE was removed, resulting in the surface becoming rougher so that the CA value increased to 135°. With increasing irradiation time to 2.5 h, PFPE main carbon chains were further significantly reduced, which led to exposures of a few protrusions of hierarchical micro- and nano-structures of AAO with oxygen-containing hydrophilic groups due to AO erosion. Therefore, water contact angle decreased slightly to 114° compared to 135° of AO-0.5 h. On further increasing irradiation time to 5 h, PFPE molecules on the surface, where AO can reach, were removed completely such that the surface became completely wettable. However, it is very surprising that AO-2.5 h and AO-5 h samples recovered their hydrophobicity after storing at atmosphere for 3 days, and CA could recover to 149° and 132°, respectively. It may be noted that AO exposure had no significant impact on the underlying PFPE that were deeply embedded in the bottom gap of AAO, and therefore residual PFPE molecules could creep out from the bottom and spread all over the hierarchical surface at atmosphere, forming a PFPE monolayer and producing a hydrophobic surface again, which agrees well with the XPS spectra of AO-2.5 and AO-5 h. To further demonstrate that PFPE monolayer on AAO surface can act as repulsing water, we prepared PFPE-coated AAO surface by spin-coating and obtained a CA of ∼145° (Fig. S2†). The abovementioned deduction can also be verified by the cyclic tests of irradiating–healing (by pumping and heating), shown in Fig. 1b. Low pressure caused by pumping could result in rapid molecule diffusion due to an increase of a molecular free path, and therefore recovery time of hydrophobicity was reduced from several days to less than 5 h. For the heat treatment, on the one hand, high temperature could help PFPE molecules to rapidly diffuse from AAO bottom to AAO top surface; on the other hand, high temperature could increasingly rearrange PFPE molecules on AAO surface. Therefore, recovery time was also reduced from several days to ∼3 h.

Fig. 5 Schematic illustration of the surface changes of PFPE-filled AAO via AO irradiation.

In addition to the effect of AO irradiation on the wettability of PFPE-filled AAO, we also investigated the effects of other LEO space irradiations including UV, proton, and electron irradiations. It was found that PFPE-filled AAO retained hydrophobicity with three types of irradiation to some extent. Fig. 6a shows the change in wettability under UV irradiation for 4 h, 6 h, 8 h, 10 h, and 12 h. It is clearly shown that CA changed from 117° to 127°, 127°, 132°, and 133°, respectively. The CA values of samples stored at atmosphere for 3 and 30 days after irradiation were measured to investigate hydrophobic durability; their CA changed slightly compared with that of the non-stored samples. After proton irradiation, water CA values were 129° for 3 min, 134° for 6 min, 127° for 9 min, and 132° for 12 min. After storing for 3 or 10 days, all these samples experienced some increase in CA, ranging from 4° to 13°. After electron irradiation, CA values roughly increased from 104° to 146°, corresponding to increasing irradiation time from 5 to 25 min, respectively. Similarly, the CA reached above 120° and even 146° after storing at atmosphere for 3 or 10 days. For proton irradiation and electron irradiation, less than 30 min of irradiation time was selected. This is because these forms of radiation possess very high energy for corroding PFPE molecules compared with AO irradiation. Even during short irradiation time, numerous volatile molecules containing fluorine can accumulate and absorb onto a vacuum chamber, resulting in chamber contamination. This can be verified by observing that vacuum pressure rapidly increased from 10⁻³ Pa to 10⁻² Pa within 30 min.

Fig. 6 Durable hydrophobic property of PFPE-filled AAO via UV, proton, and electron irradiations. (a) CA of PFPE-filled AAO before and after UV irradiation for 4 h, 6 h, 8 h, 10 h, and 12 h; CA of samples stored at atmosphere for 3 and 10 days after irradiation. (b) CA of samples before and after proton irradiation for 3 min, 6 min, 9 min, and 12 min; CA of samples stored at atmosphere for 3 and 10 days after irradiation. (c) CA of PFPE-filled AAO before and after electron irradiation for 10 min, 15 min, 20 min, and 25 min; CA of samples stored at atmosphere for 3 and 10 days after irradiation (square: 0 day; circle: 3 days; triangle: 10 days).

To investigate hydrophobic mechanism, surface morphologies of PFPE-filled AAO after UV, proton, and electron irradiations were characterized by SEM. For UV irradiation, surface morphologies of samples irradiated for 6 h and 12 h are shown in Fig. 7a(i) and (ii), respectively. It can be seen that chemical materials still remained on the AAO surface even after 12 h of irradiation, which may be due to the fact that the PFPE molecule was crosslinked rather than etched, and thus the surface was always hydrophobic. This can be seen in the FITR spectrum of samples irradiated for 6 h and 12 h (Fig. 8). Their peak positions and intensity are similar to the non-irradiated sample. For proton and electron irradiation, surface morphologies of samples can be seen in Fig. 7b and c. All the surface micro- and nano-structures of AAO were clearly observed, which are similar to pristine AAO structures, indicating that PFPE polymer on surface were almost removed completely even though experiencing very short irradiation time of 3 min (proton) and 10 min (electron). It is deduced that electron or proton irradiation induced the decomposition of PFPE decomposition. However, irradiated samples were found to be always hydrophobic, which is different from atomic oxygen irradiation samples. This was attributed to dissociative fluorine species absorbing onto the sample surface during irradiation, which can be confirmed by the abovementioned side observation that the vacuum pressure rapidly increased from 10⁻³ Pa to 10⁻² Pa within 30 min.

Fig. 7 SEM morphologies of PFPE-filled AAO irradiated by UV irradiation for 6 h (a (i)) and 12 h (a (ii)), proton irradiation for 3 min (b (i)) and 9 min (b (ii)), and electron irradiation for 10 min (c (i)) and 25 min (c (ii)).

Fig. 8 FTIR spectra of PFPE-filled AAO after UV irradiation for 6 h and 12 h.

Conclusions

In conclusion, we developed a highly durable hydrophobic surface by using liquid perfluropolymer PFPE, which is usually used as a lubricant in space aircraft to fill up hierarchical micro- and nano-scale AAO. This material, PFPE-filled AAO, always maintained hydrophobicity, irrespective of AO irradiation in simulated space. By SEM, FTIR, and XPS characterization and analysis, we observed that PFPE molecules with low surface energy always remain on AAO surface because: (1) short time irradiation can not completely clear PFPE molecules; (2) long-time irradiation can remove PFPE molecules on sample surface but PFPE embedded in AAO bottom will diffuse to AAO surface at atmosphere. We also found that PFPE-filled AAO could independently retain hydrophobicity via other simulated space irradiation such as UV, proton, and electron irradiations. Because of its especially durable hydrophobicity, we believe that PFPE-filled AAO can be applied in some special environments. Simulated experiments of LEO space irradiations are accelerated tests: irradiation doses are far more than real LEO space irradiation^27,35,36 and the vacuum degree that influences recovery speed of hydrophobicity is much lower than that of real LEO space. Therefore, we believe that it is possible that a balance may be maintained between the irradiation damage and self-healing of hydrophobicity in LEO space. Furthermore, if it is not put in an LEO space environment, we believe PFPE-filled AAO could also remain hydrophobic in a special environment such as plasma irradiation (similar to AO irradiation) and mechanical wear (if the surface chemical is damaged, embedded PFPE would diffuse onto the top surface to recover hydrophobicity), etc. Finally, we predict that other simple materials with similar AAO structure and a low surface energy chemical can be easily fabricated and applied in a special environment.

Acknowledgements

This work is financially supported by the NSFC (21303233, 51203173, 21173243) and 973 project (2013CB632300).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Full experimental details, SEM images, photographs. See DOI: 10.1039/c4ra02552k

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