Photocured Zwitterionic Coatings Containing POSS for Antifogging Applications

Abstract

The conventional fabrication of antifogging polymer coatings such as zwitterionic or amphiphilic copolymers typically require multiple processes. In this work, a simple photocuring method was used to create a series of zwitterionic coatings containing polyhedral oligomeric silsesquioxane (POSS) without the need to prepare copolymer. Surface analysis demonstrated that the coating thickness was typically about 6 μm, and the surface POSS content showed a tendency of increasing with POSS. A wettability analysis demonstrated that zwitterionic coating with high POSS content held better water absorbing capability than that with low POSS content and without POSS. Furthermore, it was found that a high proportion of POSS contributed towards the enhancement of transmittance. The excellent antifogging properties of coatings with a high mass fraction of POSS can be ascribed to the aforementioned good wettability and transmittance. It is expected that zwitterionic coating via the simple incorporation of POSS can be utilized for practical application.

1. Introduction

The widely existing fogging phenomenon in nature is due to the exposure of a cold surface to warm and wet water vapor, which greatly reduces the light transmission of transparent materials such as windshields, eyeglasses, goggles, and display devices in the medical field [1,2,3]. To eliminate or alleviate the fogging-caused threat, two main strategies have been proposed thus far, including physical and chemical antifogging. Physical antifogging is carried out mainly by tuning the surface temperature of the material via electroheating [4] or photoheating [5,6]. Chemical antifogging involves a large series of surface modification materials and technologies.

A hydrophobic, especially superhydrophobic, surface with a water contact angle greater than 150° and a sliding angle smaller than 10° is deemed as a desirable surface, which is capable of effectively hindering water droplets adhesion on transparent material surfaces. The fabrication of superhydrophobic surfaces tends to be involved in the construction of micro/nanostructures and the application of low-surface-energy chemicals. However, the practical application of the surface is limited by imperfect properties such as fragile microstructure and poor scratch resistance and durability, directly leading to the disappearance of superhydrophobicity [7]. To overcome the intrinsic defect of the surface, a strategy called “enough-air-trapped” was presented to design antifogging structures, and showed a long-lasting robust superhydrophobic and antifogging performance in a harsh environment [8]. For the improvement of the mechanical strength of the superhydrophobic surface, a hierarchical structure combining an array of microscale inverted-pyramidal cavities with highly water-repellent nanostructures were created and showed superior superhydrophobic robustness [9]. During the pursuit of antifogging for practical application, the design ideas of delicate surface structures benefit from a colorful nature. More recently, a large number of biomimetic superhydrophobic materials have been investigated on antifogging performance such as lotus leaf [10] and green bottle fly compound eyes [11]. Unfortunately, it is difficult to prepare a hierarchical micro/nanostructure on a solid surface on a large scale, although the human imitation of living things is getting closer to reality.

Hydrophilic/superhydrophilic materials featuring a spread mechanism are another greatly important class of antifoggant. According to materials chemistry, they generally fall into four categories, including surfactant [12], inorganic nanomaterials [13], organic polymer materials [14], and organic/inorganic hybrid composites [15]. The materials generally contain hydrophilic groups such as one or the combination of carboxyl (COOH), amino (NH₂), sulfonic acid (SO₃H), phosphoric acid (PO₄H₂), and hydroxyl groups (OH). These chemical groups are responsible for the formation of condensed water droplets into a continuous water membrane, and, thus, function as antifogging. However, hydrophilic coatings based on the above materials have experienced some problems. For example, superhydrophilic inorganic materials such as TiO₂ will lose their antifogging effect in the absence of UV illumination [16], whereas hydrophilic polymer coatings are prone to solubility in water [17,18] and show weak mechanical properties [19]. To this end, two surface chemistry strategies were introduced to fabricate superhydrophilic coatings, including EDC/NHS coupling [20] and surface silanization [21,22]. The incorporation of comonomers such as a silane coupling agent or dicarboxylate compound improved not only the antifogging properties, but also the stability and durability of coatings.

Different from superhydrophobic and superhydrophilic antifogging, a new kind of coating containing both hydrophilic and hydrophobic functional components, commonly termed as “Zwitter-Wettable surfaces”, has been created to combat the fogging problem [23]. The amphiphilic surface has been extensively investigated due to its highly controllable hydrophilic and hydrophobic parts [24]. Recently, a dual-thermosensitive block copolymer was prepared to build amphiphilic semi-interpenetrating polymer network (SIPN) coatings, showing excellent antifogging properties [25]. Implementation of the coating antifogging depends on coating hydrophilic groups with absorption of the cond ensation droplets, whereas the hydrophobic part of the coating surface regulates the dispersion of water molecules within the coating and contributes to the stability of the coating. However, the antifogging coatings were involved in the synthesis of linear homopolymer or copolymer. More recently, a water-absorbing anionic monomer has been exploited to fabricate a simple and efficient antifogging coating by a two-step process [26]. Different from the previous crosslinking strategy, a hydrogen bond crosslinking strategy was also exploited to construct an antifogging coating, which can be applied over a wide range of temperatures.

Zwitterionic polymer, as an important antifouling material, has attracted great attention and research interest, including poly(phosphorylcholine) (PPC), poly(carboxybetaine) (PCB), and poly(sulfobetaine) (PSB). The materials have a pair of oppositely charged groups in a repeating unit, which appears electrically neutral as a whole, exhibiting strong hydration through the ionic solvation effect. Recently, zwitterionic polymers were viewed as candidate antifogging materials due to their excellent ability to form water thin films. However, the fabrication of these coatings was performed by crosslinking polymers, which were obtained via additional polymerization and purification processes. In this study, we fabricated a simple antifogging coating via photocuring. These coatings contained a zwitterionic component and a different content of POSS on glass substrate pretreated with a silane coupling agent. We hypothesized that the incorporated POSS contributed to the improvement of the coating on some properties, such as transmittance and antifogging. The as-prepared coating was characterized by Fourier transform infrared (FTIR), water contact angles (WCAs), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and visible spectrophotometry. The antifogging properties and transmittance of the as-prepared coating were studied via the hot vapor and cold–warm method.

2. Materials and Methods

2.1. Materials

Acryloyl POSS (POSS), γ-methacryloxypropyl trimethoxy silane (KH-570), and n-hexane were purchased from Meryer (Shanghai, China). Poly(ethylene glycol) dimethacrylate 600 (PEGDMA), 2-hydroxy-4′-(2-hydroxyethoxy)-2-methylpropiophenone (HHMP), 2,2,2-trifluoroethanol (TFE), and (N-3-Sulfopropyl-N,N-dimethyl ammonium)ethyl methacrylate (SBMA) were purchased from Heowns (Tianjin, China). Sulfuric acid (98.3%) and hydrogen peroxide (30%) were purchased from Jinshan Chemical Test (Chengdu, China). Ultrapure water was obtained from a Millipore Mill-Q purification system (Millipore, Bedford, MA, USA).

2.2. Glass Slide Pretreatment

Clean glass slides (Golo, Luoyang, China) were treated in a piranha solution with H₂O₂ and H₂SO₄ in a ratio of 3:7 for 1 h. After that, the slides were removed from the piranha solution and washed with pure water until they reached neutrality. After drying, the hydroxylated glass (OH-glass) was obtained. Then, the OH-glass was immersed in a mixture of KH570 and n-hexane with a volume ratio of 1:100 for 1 h in a covered container at room temperature. After rinsing with n-hexane, the silanized glass (KH-570-glass) with KH-570 grafted on the surface was prepared and immediately used as the substrate for polymerization.

2.3. Zwitterionic Coating Fabrication

A schematic diagram of the glass coated with zwitterionic coatings containing POSS is presented in Figure 1. An organic polymerization solution of SBMA was prepared, with 50 wt% SBMA (monomer), 1 wt% HHMP (photoinitiator), and 10 wt% PEGDMA in TFE with 0, 2%, 5%, and 10% POSS relative to the SBMA monomer, respectively. The untreated glass was named Pristine in order to distinguish it from the OH-glass and KH-570-glass samples. A volume of 20 μL of the polymerization solution was pipetted onto KH-570-glass (15 mm × 15 mm) and a coverslip was applied to the sample surface to spread the liquid. The samples were illuminated using a LED UV lamp (Yanxizao, Zhongshan, China) at a light density of 100 mW/cm² of 365 nm for 20 min. After polymerization, the coverslips were removed, and the samples were allowed to dry at room temperature. The coatings applied to glass are abbreviated as POSS0, POSS2, POSS5, and POSS10 according to the weight percentages of POSS to SBMA.

2.4. Characterization

Infrared spectra were recorded by a single-reflection diamond attenuated total reflection (ATR) on the FTIR spectrometer (Nicolet iS5, Thermo Scientific, Waltham, MA, USA) equipped with a deuterated L-alanine triglycine sulfate deuterated triglycine sulfate (DLATGS) detector. WCAs were measured at ambient temperature on a contact angle goniometer (JY-PHa, Chengde Yote, Chengde, China) according to the sessile drop method. A volume of 5 µL of water droplets were dropped carefully onto the coating surface, and the measurement was carried out on at least three different areas of the coated surface. The surface morphology and cross-section were observed on an SEM (FEI Quanta FEG250, Thermo Scientific, Hillsboro, OR, USA) operated at 20 kV. The samples were coated with a layer of platinum with a thickness of 2 nm by ion sputtering before observations. The chemical composition of the coating surface was probed by XPS (Thermo Scientific ESCALAB Xi+, Waltham, MA, USA) using monochromatic Al Kα radiation at 1486.6 eV. Transmission spectra were recorded using a Shimadzu UV-3600i Plus spectrophotometer (Kyoto, Japan) equipped with a high-performance double raster monochromator using the 400–800 nm wavelength range.

2.5. Antifogging Test

The antifogging performance was tested with the hot vapor. Briefly, one side of the coated glass was placed over hot water (about 80 °C, 100% relative humidity) about 5 cm above it. The beaker with hot water was placed on the top of a sheet of A4 printing paper, which was printed with Chinese and English characters of Tongren University. The photo of samples was taken immediately after exposure to hot vapor for 30 s.

3. Results and Discussion

3.1. Coatings Preparation and Characterization

In order to improve the adhesion of the coating on the glass substrate, the clean glass was treated with the piranha solution to obtain a hydroxylated surface. After that, the treated surface was immersed in KH-570 solution of n-hexane to produce a transparent silanized glass, onto which the polymerization solution was dropped and illuminated to create a zwitterionic antifogging coating. So far, various strategies have been exploited to fabricate antifogging coatings. Using commercial or synthetic water-absorbing polymers is a highly simple and effective strategy. A kind of SIPN coating via a photo-crosslinking technique has previously been reported, which employed amphiphilic linear copolymer synthesized by free radical polymerization [17,27], atom transfer radical polymerization (ATRP) [25], or reversible addition-fragmentation chain transfer polymerization (RAFT) [28,29] to facilitate the absorption of water into the coatings. In spite of excellent coating properties, however, the preparation of coatings involves the additional synthesis of copolymer and encounters possible polymer dissolution problems. In contrast to that aforementioned, an alternative important class of coatings was prepared by spin coating [30], dip coating [31], and casting [2], as well as thermal crosslinking procedures. The coating preparation strategy ingeniously exploited hydrogen-bonding crosslinking with water-absorbing polyvinyl alcohol (PVA) as hydrogen bond donor, salicylic acid (SA), polyacrylic acid (PAA), or Nafion as corresponding hydrogen bond receptors. Additionally, polymer brush by surface-initiated ATRP (Si-ATRP) [19], layer-by-layer [23], coordination-driven grafting [32], and photothermal [33] techniques were also utilized to fabricate such coatings. POSS is short for polyhedral oligosiloxane, which contains a caged inorganic core composed of an alternately connected silicon–oxygen skeleton. Acryloisobutyl-POSS is a kind of POSS with eight acryloisobutyl groups connected to eight silicon atoms in the inorganic core. In our case, a zwitterionic coating was handily fabricated by a one-step photocrosslinking zwitterionic monomer and POSS with PEG dimethacrylate crosslinker without the need to synthesize copolymers for coatings. POSS as an additive can be readily incorporated into the coatings for the improvement of their properties compared to the reported work [25].

In order to investigate the chemical composition of the coatings, the coatings applied to the glass were carefully scraped off with a utility knife and characterized using FTIR spectra. As shown in Figure 2, the C-H stretching vibration peaks in SBMA appeared at 3040 cm⁻¹, whereas the counterparts were not observed in POSS0, POSS2, POSS5, and POSS10. The C=O stretching vibration peaks were observed at 1722 cm⁻¹ in all of the coating films and the SBMA monomer. The C=C stretching vibration peaks and the C-H bending vibration in -CH₂=C of SBMA were observed at 1635 cm⁻¹ and 1300 cm⁻¹, respectively, but disappeared in all of the coating films. The disappearance of characteristic absorption peaks in the films indicated that the main component SBMA had been successfully crosslinked using the photocuring method. The asymmetrical and symmetrical stretching vibration peaks of S=O were observed at 1169 cm⁻¹ and 1034 cm⁻¹, respectively. These results suggest the successful preparation of the coatings. Herein, we found that the characteristic peaks of S=O were similar to that reported in the literature [29]. However, the characteristic C=O peaks of POSS were not identified at 1722 cm⁻¹ due to its overlap with that of SBMA. Although it was reported that characteristic peaks of Si-O-Si were observed at 1119 cm⁻¹ [34], the characteristic band falls into the asymmetrical characteristic peak of S=O, resulting in an inability to distinguish the respective characteristic peaks.

Because we know that the surface morphology of the coating greatly affects the transmittance, the coating surface was characterized using SEM. The coating surface appeared to be smooth such as Pristine and did not change greatly as a whole (Figure 3a). The cross-sections of POSS0 and POSS10 coating were typically about 6 μm thick by SEM. Previous reports showed that the thickness of semi-interpenetrating polymer network (SIPN) coatings were typically about 7~10 μm [30], and some hydrophilic coatings were even up to 13~15 μm [28,31]. A bigger coating thickness generally contributes to a longer durability to some extent, however, which may cause a decrease in optical transmittance. Therefore, the fabrication of antifogging coating demands a delicate balance between the thickness of the coating and the transmission of light. In contrast to thicker coatings of a few microns, an approximately 120 nm polymer coating has been developed, exhibiting excellent antifogging and frost-resistant properties [30].

Apart from coating surface morphology and thickness, the distribution of POSS in the coating played an important part in the performance of the coating [25]. It was, therefore, imperative to investigate the surface chemical composition by XPS. To determine the elemental composition of the coating surface, the XPS wide-scan and high-resolution spectra were measured (Figure 3b–e). The corresponding binding energies of O 1s, N 1s, C 1s, S 2p, Si 2s, and Si 2p peaks were observed at 531.6, 401.9, 284.2, 166.8, 150.6, and 100.5 eV, respectively. The high content of nitrogen, oxygen, and carbon of the coating surface showed a consistent pattern with the change in POSS proportion. The oxygen content of the coating surface of POSS0, POSS2, POSS5, and POSS10 appreciably increased with POSS due to its high oxygen content, whereas the carbon content on the surface discernably decreased with POSS due to its low carbon content (

Table 1. Surface chemical composition (at. %) of glass coated by POSS0, POSS2, POSS5, and POSS10 calculated from XPS Spectra.

Samples	O 1s	N 1s	C 1s	S 2p	Si 2p
POSS0	15.9	2.2	76.3	3.8	1.8
POSS2	21.1	2.8	65.3	1.9	8.9
POSS5	24.3	2.5	60.8	2.1	10.3
POSS10	26.7	2.3	54.2	2.4	14.4

). In addition, the silicon content on the coating surface appeared to significantly increase with the introduction of POSS, which contains a high percentage of silicon. In the high-resolution XPS spectra, the C 1s spectra of POSS0 were deconvoluted into four Gaussian peaks, whereas that of POSS2, POSS5, and POSS10 were deconvoluted into five Gaussian peaks. The peaks corresponding to the C-Si bond implied that POSS was successfully incorporated into the coating surface of POSS2, POSS5, and POSS10.

3.2. Coating Surface Wettability

Surface wettability tends to dominate coating antifogging performance. To differentiate the antifogging property of the as-prepared coating, the variation of the WCA of the coatings over time was recorded, as shown in Figure 4. The WCA of Pristine was originally about 65° and decreased to 62° after 60 s, whereas that of POSS0 remained within the range of 25°~24° during the same interval. When a small mass fraction of POSS was incorporated into the coating, it had negligible influence on the WCAs. However, upon the incorporation of a higher proportion of POSS into the coating, the WCA on the POSS5 surface made an appreciable change from about 26° to 20°. When the POSS content came up to 10% relative to SBMA, the WCA on coating surface declined rapidly from about 25° to 16°. These results suggest that an optimal percentage of POSS aids in the rapid spreading of water droplets. It was extensively reported that the incorporation of POSS in the polymer coating was conducive to strengthening the coating stability and mechanical properties [35]. Recently, research showed that POSS functionalized polymer coating exhibits excellent antifogging properties [36], in which a typical water-absorbed coating has WCAs of 40–110°. Zwitterionic polymers are a highly desirable class of candidate materials for antifogging due to their strong hydration capacity. However, the rapid absorption of water could lead to a decrease in the coating stability [28]. Achieving a delicate balance between hydrophilicity and hydrophobicity has, therefore, been deemed the key to coatings with excellent performance. Several coatings consisting of bulk amphiphilic polymers have been fabricated with superior antifogging properties [25,28,29]. From the perspective of changes in WCAs, this showed that precise control of the proportion of the hydrophobic components delays the rapid absorption of water over a hundred seconds. Accordingly, in this case, the proportion of POSS incorporated into the coatings also increased as expected to adjust the ratio of hydrophilicity and hydrophobicity. However, the initial WCAs and its changes on the coatings demonstrated that POSS failed to effectively enhance the coating surface hydrophobicity and, to some extent, increased the hydrophilicity of the coating. It was shown that the distribution pattern of POSS on the coating surface could be different from that reported in Ref. [25].

3.3. Coating Transmittance and Antifogging Properties

In order to examine the transmittance of the as-prepared coating, we measured the visible light in the range of 400~800 nm (Figure 5a). It was evidently shown that coating transmittance had a gradual increase from 86% relative to Pristine for POSS0 without POSS, to 89% for POSS2, 93% for POSS5, and finally to about 99% for POSS10. Our results were in line with the reported research, which showed that the incorporation of POSS rendered the enhancement of light transmission of optical materials despite the different ways of incorporation [37]. Although in most of the literature, POSS as a physical enhancer has been introduced into polymer materials in the form of pendant moieties, it was capable of good maintenance of the transmittance of materials.

To test the as-prepared coating’s antifogging characteristic, a hot vapor method was implemented. It was quite clear that the coating surface showed varied degrees of an antifogging effect (Figure 5b). Obviously, glass covered by coating with zwitterionic materials and the POSS modified version presented clear print characters, whereas the bare surface was blurry. Especially, POSS10 held a more distinct picture than the other POSS-containing samples and the sample without POSS. The good antifogging properties can be ascribed to the excellent transmittance and water-absorbing capability of the zwitterionic coating with a high proportion of POSS. Recently, a study showed that POSS in blending polymer coating had a trivial effect on antifogging performance [36]. However, an alternative investigation demonstrated that aggregated POSS was beneficial to the absorption of water molecules into the coatings to strengthen the antifogging effect [25]. So far, the simplicity, stability, and durability of coatings are still problems to be solved, although antifogging coatings have made great progress with respect to antifogging properties. The simple fabrication of coating is always desirable for the practice application. Silanized surface chemistry is the promising candidate for antifogging coating. The introduction of the silane coupling agent into coating has been performed to enable the simplicity, stability, and durability of coating fabrication. Additionally, it is noteworthy that it is of great importance to achieve additional properties such as self-cleaning [20,22] and self-healing [20] for antifogging coating in long-term applications. The long-lasting exposure of coatings to environmental pollutants such as dust and grease eventually leads to the failure of antifogging. The effective removal of stains by self-cleaning function is obviously beneficial to prolong the life of antifogging coatings.

4. Conclusions

In this work, we fabricated zwitterionic antifogging coatings with varied content of POSS by a simple and effective photocuring method with monomers. The coatings contained hydrophobic and zwitterionic components for regulating the balance between hydrophobicity and hydrophilicity. The incorporation of POSS strengthened the coating hydrophilicity to a certain extent over time, resulting in the enhancement of coating antifogging. This can be attributed to the water-absorbing zwitterionic component and hydrophobic POSS. The transmittance of coating, in turn, was improved by POSS under control of the thickness of the coating. The simple and effective fabrication of antifogging coating is desirable for potential applications.