Zwitterionic polymer brush coatings with excellent anti-fog and anti-frost properties

Abstract

The formation of fog and frost on transparent surfaces can lead to many problems in our daily life. To address these problems, polymer brushes based on two zwitterionic analogues, poly(sulfobetaine methacrylate) (pSBMA) and poly(sulfobetaine vinylimidazole) (pSBVI), have been prepared by surface-initiated atom transfer radical polymerization (SI-ATRP). Hydrophilic and superhydrophilic pSBMA and pSBVI polymer brushes were prepared by controlling the thickness of the coatings to study the effect of wettability on the anti-fog and anti-frost properties. X-ray photoelectron spectroscopy (XPS), ellipsometry and atomic force microscopy (AFM) were respectively used to determine the interfacial elemental composition, the thickness and the morphology of the brushes. The wettability of the polymer brushes was measured using a water contact angle goniometer. Their anti-fog and anti-frost capabilities were determined visually and tested quantitatively by UV-vis spectroscopy. The results indicated that the optical transmittance of substrates modified with superhydrophilic polymer coatings under both hot and cold fogging conditions was very high. Additionally, no visible frost was formed on the superhydrophilic substrates after storage in a freezer at −20 °C for the duration of the experiment. These results convincingly demonstrated that the resistance of the modified substrates to fogging and frosting is strongly correlated to the surface wettability. Moreover, there is no considerable difference in the performance of pSBMA and pSBVI polymer brushes, but the former is preferred because SBMA monomer is commercially available and requires short polymerization time to obtain superhydrophilicity.

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Introduction

Condensation of water droplets or ice crystallization on surfaces in a variety of environmental conditions causes fog and/or frost formation, which in turn interferes with the light path and reduces optical transmission. This may lead to many problems for the use of eyeglasses, windows, mirrors, goggles, and optical instruments.^1–4 Over the last few decades, an extensive research studies have been reported to prevent the formation of fog and frost on solid surfaces by different approaches including superhydrophobic coatings,^5–7 superhydrophilic coatings^8,9 and combinations of both hydrophilic and hydrophobic properties in the coating.^10–15 Hydrophilic surfaces that have a water contact angle of less than 5° exhibit excellent anti-fog properties because they allow water droplets to spread uniformly to form a thin water film, which reduces light scattering.^9,16,17 In general, superhydrophilicity can be obtained by various chemical and physical methods.¹⁸ One common approach is to coat hydrophilic inorganic nanoparticles such as SiO₂ and TiO₂.^12,19,20 However, the fabrication methods for these materials involve multiple steps including seed growth, one dimension nanomaterial prefabrication and post treatments.²¹ In addition, they typically require exposure to UV light to induce photocatalytic activity and superhydrophilicity.²²

On the other hand, hydrophilic polymers can be used as coating materials for the preparation of superhydrophilic surfaces. For instance, a polymer-based anti-fog coating covalently grafted onto glass surfaces by means of a multistep process that includes spin coating of poly(ethylene-maleic anhydride) (PEMA) and poly(vinyl alcohol) (PVA) layers has been reported.¹ The results showed that the PEMA/PVA coating not only prolonged the initial period required for fog formation, but also decreased the rate of light transmission decay. Additionally, these PEMA/PVA coatings remained stable and their anti-fog properties were preserved after immersion in water for 24 h. Moreover, Lee and co-workers demonstrated that a zwitter-wettable surface prepared by layer-by-layer (LbL) assembly of PVA and poly(acrylic acid) (PAA) functionalized with poly(ethylene glycol methyl ether) (PEG) segments afforded a significantly enhanced anti-fog and anti-frost properties.²³ Recently, we have developed a facile superhydrophilic surfaces with significant anti-fog and self-cleaning properties via silanization of zwitterionic sulfobetaine silane (SBSi) on oxidized surfaces.²⁴ These surfaces exhibited long-term stability under exposure to heat and UV irradiation. The SBSi glasses maintained high optical transmittance due to the rapid formation of coalesced water thin films on surfaces in contact with vapour and moisture.^25,26

Zwitterionic poly(sulfobetaine methacrylate) (pSBMA) and poly(sulfobetaine vinylimidazole) (pSBVI) are widely used as anti-fouling materials in marine and biomedical applications.^27–30 Sulfobetaine pendant groups have strong hydration capacity due to the ionic solvation.^31–34 The use of surface-initiated atom transfer radical polymerization (SI-ATRP) for the preparation of polymer brushes enables precise control of their architectural features, such as the grafting density, the thickness and the composition; therefore, the interfacial properties of polymer films can be tuned flexibly.^35,36

It is important to explore the structure–property relationship of sulfobetaine-based polymer brushes for better understanding of their correlation with the formation of fog and frost on these surfaces. In this study, pSBMA and pSBVI polymer brushes were prepared and the surface wetting properties were altered by controlling the film thicknesses. The tests for the fog and frost formation were performed under both hot and cold fogging conditions to verify the effect of the interfacial properties on water coalescence and frost densification. To the best of our knowledge, this is the first work to systematically investigate and employ the zwitterionic pSBMA and pSBVI as anti-fog and anti-frost coatings with an attempt to explore the structure–property relationship.

Experimental section

Materials

[2-(Methacryloyloxy)ethyl]dimethyl-(3-sulfopropyl)ammonium hydroxide (SBMA), N,N-dimethylformamide (DMF), copper(I) bromide (99.999%), and 2,2′-bipyridyl (Bpy) were purchased from Sigma-Aldrich. Tetrahydrofuran (THF, anhydrous) was obtained from TEDIA. Triethylamine (TEA) and 2-bromoisobutyryl bromide (BIBB) were purchased from Acros Organics. (3-Aminopropyl)triethoxysilane, 1-vinylimidazole, and 1,3-propanesultone were purchased from Alfa Aesar. All other chemicals were analytical grade reagents and used without any further purification. Standard glass microscope slides (Green cross medical) and silicon wafers (Semiconductor Wafer Inc., Taiwan) were cut into 2.5 cm × 2.5 cm pieces before use.

Surface immobilization of ATRP initiator

The slides were cut into 2.5 cm × 2.5 cm, and then sonicated in ethanol and water for 15 min each and treated with piranha solution (70 : 30 H₂SO₄ : H₂O₂) for 30 min at 100 °C. Subsequently, the slides were immersed in hydrochloric acid solution (1 : 6 HCl : H₂O) for 15 min at room temperature and washed thoroughly using ethanol and water before drying in a N₂ atmosphere. The slides were immersed in a solution of 40 mM of the initiator in THF for 24 h at room temperature. Finally, the slides were washed sequentially with THF, ethanol, and water and dried under nitrogen before polymerization.

SI-ATRP of SBMA and SBVI

For the synthesis of pSBMA brushes, 7.5 g of SBMA was dissolved in 50 mL H₂O and MeOH (1 : 3). The solution was bubbled with nitrogen for 3 h to remove oxygen, and then 332.5 mg of Bpy and 152.5 mg of CuBr were added in the solution to achieve [SBMA] : [CuBr] : [BPy] molar ratio of 25 : 1 : 2. This solution was further degassed using three freeze–pump–thaw cycles. The mixture was transferred to vessels containing the initiator-immobilized slides and sealed under nitrogen. The reaction was carried out in a water bath at 60 °C for 24 h. Following a similar procedure, SI-ATRP of SBVI was performed at [SBVI] : [CuBr] : [BPy] molar feed ratio of 30 : 1 : 2 in 1 : 2 H₂O/DMF mixture at 60 °C. The concentration of monomers in the reaction solvents was 15% (w/v). To ensure that both glass slides and silicon wafers were grafted under the same conditions, the polymerization was carried out in the same reaction vessel. The polymerization was halted after the specified reaction time and the slides were repeatedly washed with warm water and methanol, and dried under nitrogen.

Characterization

NMR spectra were recorded on a Bruker DRX-600 instrument. The water contact angles were measured by the sessile-drop method using an optical CA goniometry (Phoenix mini, Surface Electro Optics) at ambient temperature. The volume of the droplets from a microsyringe was 5 μL, and the measurements were performed in triplicate at random positions on the samples to characterize the average wetting properties of the coatings. The brush thickness was measured using ellipsometry (Angle Stokes Ellipsometer LSE, gaertner scientific) on silicon wafers at ambient lab conditions in air. A light source with a wavelength in 400–800 nm range at incidence angle of 70° was used to scan the sample surface. The refractive index for the polymer brushes (n = 1.45) was measured automatically by ellipsometry. The atomic force microscopy (AFM) images were recorded on a commercial instrument (JSPM-5200) in tapping mode under ambient conditions. The chemical composition was detected by X-ray photoelectron spectroscopy (XPS) using a VG Sigma Probe spectroscope using an Al Kα excitation source (25 W, 15 μm) in a vacuum of 2 × 10⁻⁸ Pa. XPS spectra were acquired with a pass energy of 58.8 eV. Scans were performed at a take-off angle of 90° and the binding energy (BE) was estimated to be accurate within 0.2 eV. The UV-vis transmission spectra were measured using a UV-vis spectrophotometer (V-630, Jasco).

Anti-fog and anti-frost tests

Both hot and cold fogging tests were used to quantify the anti-fog properties of the polymer brush coatings. For the hot fogging test, the coated glass slides were placed 5 cm above hot water (80 °C), and held for different time periods. Photographs were then taken immediately after the samples were removed from the water vapour and compared to bare glass as a control sample for each case. More aggressive conditions were used to assess the anti-fog and anti-frost capabilities of the brushes. In brief, the samples were stored in a freezer at −20 °C for 30 min and 24 h, and photographs were taken immediately after the samples were removed from the freezer to visually estimate the frost formation, which turns to fog after exposure to ambient conditions. The light transmittance in the range of 400–700 nm was also recorded on UV-vis spectrophotometer during the fogging and frosting experiments to quantitatively evaluate the anti-fog and anti-frost properties of polymer brushes.

Results and discussion

Synthesis and characterization of polymer brushes

The fabrication process of zwitterionic pSBMA and pSBVI is illustrated in Scheme 1. The glass or silicon substrates were firstly modified with ATRP initiator via self-assembly of the initiator in anhydrous THF for 24 h, and then the polymer brushes were grown from the grafted initiators under appropriate conditions. In this work, pSBMA and pSBVI brushes with similar dry thicknesses were used. In order to achieve this, different polymerization times were conducted at a monomer concentration of 0.5 M. The brush thicknesses (d) are shown in Table 1. The brush thickness of pSBMA was 44 nm after polymerization for 8 h, and 105 nm after 24 h. Similar results were obtained for pSBVI with d = 45 nm and 134 nm after polymerization for 12 h and 24 h, respectively.

Scheme 1 Synthesis procedure of pSBMA and pSBVI polymer brushes via SI-ATRP polymerization.

Table 1 Polymerization time (t) for SI-ATRP, dry film thickness (d), contact angle (CA) and surface roughness (R_a) of hydrophilic (H) and superhydrophilic (SH) pSBMA and pSBVI grafted on silicon wafer substrates

Brush type	t (h)	d^a (nm)	CA	Ra^b (nm)	N/S ratio (theoretical)	N/S ratio by XPS
a Measured by ellipsometry.b Measured by AFM.
pSBMA-SH	8	44	<5°	0.27	1	0.9
pSBMA-H	24	105	15 ± 3°	0.45	—	—
pSBVI-SH	12	45	<5°	0.66	2	2.3
pSBVI-H	24	134	15 ± 3°	1.40	—	—

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The wettability of polymer brushes was measured using a water CA goniometer and the results are summarized in Table 1 and ESI (Fig. S3†). The results showed a CA of 33° for the unmodified glass with a negligible change after immobilizing the initiator. However, the CA decreased sharply after grafting both pSBMA and pSBVI for respective polymerization times of 8 and 12 h and the surfaces became completely wet (CA < 5°). On the other hand, hydrophilic brushes (CA ∼15 ± 3°) were obtained by increasing the brush thicknesses via increasing the polymerization time.

Herein, superhydrophilicity was observed after 4 h for pSBMA with d = 23 nm. The polymerization time for pSBMA was, therefore, increased to 8 h to obtain a similar thickness to that of pSBVI brushes. The superhydrophilic pSBMA and pSBVI coatings are respectively named pSBMA-SH and pSBVI-SH. Upon increasing the polymerization time to 24 h, both pSBMA and pSBVI brush coatings became hydrophilic, pSBMA-H and pSBVI-H, owing to the increase in thickness of the brushes. It has been reported that zwitterionic polymer brushes with a thickness less than 50 nm possess very strong interaction with water molecules, and so they are very hydrophilic, while thicker brushes (thickness > 50 nm) lead to an increase in the hydrophobicity.^32,37,38 Azzaroni et al. studied the correlation between the wettability of sulfobetaines and brush thickness. It was found that the polyzwitterionic brush exhibits a transition in the wetting characteristics, from non-associated hydrophilic regime to self-associated hydrophobic regime, which produces a completely collapsed self-associated state due to reorganization processes within the macromolecular film.³⁷ In this study, we could find that thinner brushes are superhydrophilic, while the hydrophobicity increases with increasing the thickness due to the strong inter- and intrachain associations.³⁷

XPS broad survey spectra for the initiator-modified silicon wafers and both pSBVI and pSBMA polymer brush samples are presented (Fig. 1). The surface initiator was successfully immobilized on silicon wafers and glass substrates, as confirmed by the detection of the characteristic XPS signals for N_1s (BE = 402.0 eV) and Br_3d (BE = 70.0 eV), as shown in Fig. 1a. It is worth noting that the bromine signal detected on the initiator surface was weaker than that of the nitrogen signal (0.12% of Br vs. 2.41% of N) and this does not match their 1 : 1 stoichiometric ratio in the initiator. This discrepancy was also observed by Liu et al. and other researchers and was attributed to the instability of C–Br bonds under X-ray irradiation.^29,39 Anywise, high resolution XPS profile of Br_3d is shown in Fig. 1b, which demonstrates successful immobilization of the initiator on the substrates. XPS spectra of pSBMA and pSBVI show an increase in the intensities of S_2p and N_1s for both pSBMA and pSBVI compared to initiator-grafted substrates (Fig. 1a); however, a weak Si signal originating from the underlying initiator and/or substrates was observed. High-resolution scans for both N_1s and S_2p (BE = 167.0 eV) are also shown (Fig. 1c and d). The ratios between N and S were 0.9 and 2.3 for pSBMA and pSBVI, respectively, which are accordingly approximate to their stoichiometric ratios of 1 and 2 (Table 1). The surface morphology of the samples was measured by AFM (Fig. 2) and the corresponding roughness values are shown in Table 1. The surface morphology of silicon wafers was flat and smooth with a low roughness value of 0.23 nm (Fig. 2a). After the initiator was immobilized on the substrates, the roughness increased to 0.54 nm (Fig. 2b). The surface roughness values were 0.27 and 0.45 nm for pSBMA-SH and pSBMA-H (Fig. 2c and d), respectively, and the roughness values were 0.66 and 1.4 nm for pSBVI-SH and pSBVI-H (Fig. 2e and f), respectively. The data suggest that the surface roughness of the polymers grafted onto the substrate surfaces increases proportionally with the thickness.

Fig. 1 XPS spectra for: (a) initiator-modified silicon wafers, pSBMA and pSBVI polymer brushes, (b) high resolution scan of Br_3d, (c) high resolution scan of N_1s region, and (d) high resolution scan of S_2p region.

Fig. 2 Tapping-mode AFM images in air (0.5 μm × 0.5 μm): (a) bare silicon wafers (R_a = 0.23 nm), (b) initiator-modified silicon wafers (R_a = 0.54 nm), (c) pSBMA-SH (R_a = 0.27 nm), (d) pSBMA-H (R_a = 0.45 nm), (e) pSBVI-SH (R_a = 0.66 nm), and (f) pSBVI-H (R_a = 1.4 nm) polymer brushes grafted on silicon wafers.

Anti-fog performance

The primary factor to determine the ability of a surface to prevent fog formation is the equilibrium of a droplet in contact with a surface as described by the Young's equation.⁴⁰ Furthermore, it has been proved that the anti-fog property of a surface is governed by its hydrophilicity where very hydrophilic surfaces can maintain good optical transmission.⁴¹ In this regard, pSBMA and pSBVI grafted glass substrates were placed above hot water vapour for different time periods and digital images were taken immediately after transfer to ambient lab conditions. The unmodified glass was severely fogged after 15 s exposure to hot water vapour (Fig. 3a), so did both pSBMA-H and pSBVI-H grafted glasses (Fig. 3c and e). On the contrary, no fog formation was observed at all for pSBMA-SH and pSBVI-SH samples (Fig. 3b and d). Exposure to hot water vapour for longer periods (i.e. 30 s and 60 s) further exhibited similar performance, which indicated that pSBMA-SH and pSBVI-SH could maintain good anti-fog property over long-term test (Fig. S4 in ESI†). When the exposure time increased for pSBMA-H and pSBVI-H, the absorption capacity of water increased and consequently led to the formation of free water domains on the surfaces resulting in more fogging occurrence.

Fig. 3 Digital photos of different samples: (a) bare glass, (b) pSBMA-SH, (c) pSBMA-H, (d) pSBVI-SH, and (e) pSBVI-H after exposure to hot water vapour for 15 s under ambient lab conditions (temperature ∼21 °C, 88% relative humidity).

UV-vis spectrophotometer was used to quantitatively assess the anti-fog performance. According to the ASTM procedure F 659-06,¹⁹ a coating is defined to be anti-fog if it can maintain a light transmittance over 80% after exposure to hot water vapour for 30 s. The light transmittance values of bare glass and the grafted polymer brushes are comparable (∼91–92%, Fig. 4a) before the fogging and frosting tests. In a different manner, when the samples were exposed to hot water vapour for 15, 30, and 60 s (Fig. 4b–d), low light transmittance was observed for unmodified glass (<56%), but all of the grafted polymer brushes maintained very high transparency (>87%). The finding confirms that both hydrophilic and superhydrophilic brush coatings can retain an optically clear surfaces, even though the condensation of water virtually occurs during hot fogging test.

Fig. 4 Light transmission at the normal incident angle for various samples (a) before, (b) after 15 s, (c) after 30 s and (d) after 60 s exposure to hot water vapour (5 cm above 80 °C water bath) under ambient conditions (temperature ∼21 °C, 88% relative humidity).

Anti-frost performance

The frost forms when the temperature of a substrate falls below its dew point, where water vapour can evade the liquid phase and transform directly into ice.^5,10 In this work, the samples were stored in a freezer at −20 °C for 30 min and 24 h to study the frost-resistance. Digital images were taken immediately after the samples were removed from the freezer (Fig. 5). The bare glass lost its transparency completely (Fig. 5a), because of severe frosting formation, which turned into fog after exposure to ambient lab conditions due to the thermal gradient.⁴² Also, pSBMA-H and pSBVI-H brushes (Fig. 5c and e) showed similar behaviour to that of bare glass due to frost nucleation on the substrates. On the contrary, neither frost nor fog was formed on pSBMA-SH and pSBVI-SH brushes which attained very high transparency throughout the test even after 24 h storage at −20 °C (Fig. 5b and d). This confirms the excellent anti-fog and frost-resistance of the superhydrophilic brushes.

Fig. 5 Digital photos of different samples: (a) bare glass, (b) pSBMA-SH, (c) pSBMA-H, (d) pSBVI-SH, and (e) pSBVI-H, after storage in a freezer at −20 °C for 30 min (upper) and 24 h (bottom), taken immediately after transfer to ambient lab conditions (temperature ∼21 °C, 88% relative humidity).

Additionally, UV-vis spectroscopy was used to monitor the transmittance soon after removing the samples from the freezer after being stored at −20 °C for 30 min (Fig. 6a) and 24 h (Fig. 6b). It is obvious that the optical transmittance of bare glass was significantly reduced to less than 15% owing to severe frosting on surfaces. Furthermore, there was a decrease in the transmittance of hydrophilic brushes (<50% for pSBVI-H and 30% for pSBMA-H), due to the condensation of water on the brush surfaces. The lower transmittance of hydrophilic coatings compared to superhydrophilic coatings can be likely due to surface wetting property and surface roughness. The roughness values of pSBMA-H and pSBVI-H coatings are higher than that of pSBMA-SH and pSBVI-SH brushes; therefore, frost can nucleate on the former brush surfaces more easily. Jung et al. found that hydrophilic surfaces with a minimal roughness, 1.4–6.0 nm, have the longest freezing delay times when compared to rougher surfaces.⁴³

Fig. 6 Light transmission at the normal incident angle for various samples after: (a) 30 min, and (b) 24 h storage in a freezer at −20 °C, immediately after transfer to ambient lab conditions (temperature ∼21 °C, 88% relative humidity).

On the other hand, the transmittance values of superhydrophilic coatings were as high as that before fogging and frosting tests. These results can emphasize the role of the surface wetting properties on the coalescence of adsorbed water and ice nucleation. A comparison of the anti-fog and anti-frost results for both hydrophilic and superhydrophilic polymer brushes is summarized in Table 2 as compared with bare glass samples.

Table 2 Summary of anti-fog and anti-frost performance of various samples under different conditions

Sample	Anti-fog	Anti-frost
Bare glass	No	No
pSBMA-SH	Yes	Yes
pSBMA-H	No	No
pSBVI-SH	Yes	Yes
pSBVI-H	No	No

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Conclusions

The SI-ATRP method was used to prepare hydrophilic and superhydrophilic zwitterionic pSBMA and pSBVI brushes by precisely controlling the thicknesses of the polymer thin films. Only superhydrophilic polymer brushes could attain an excellent anti-fog and anti-frost properties even under an aggressive fogging and frosting conditions. However, hydrophilic brushes could not afford a good capability to resist the formation of fog and frost. Here we could find that the main factor that affects the ability of pSBMA and pSBVI to prevent the fog formation and frost nucleation is the wettability of the coatings rather than the chemical structures. Importantly, the relation between the structure and the property of sulfobetaine polymers was determined using water molecules at different phases and under various conditions. This will certainly drive further studies towards the fabrication of decorated surfaces with novel properties and applications.

Acknowledgements

The authors acknowledge the Ministry of Science and Technology (MOST 104-2221-E-008-108 and 104-2119-M-194-004) for financial support of this project.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Detailed procedure for the preparation of the initiator and SBVI monomer, ¹H-NMR spectra for the initiator and SBVI monomer, CA measurements, and digital images for hot fogging after exposure to hot water vapour for 30 s and 60 s. See DOI: 10.1039/c6ra12399f

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