Reversible Surface Properties of Polybenzoxazine / Silica Nanocomposites Thin Films

We report the reversible surface properties (hydrophilicity, hydrophobicity) of a polybenzoxazine (PBZ) thin film through simple application of alternating UV illumination and thermal treatment. The fraction of intermolecularly hydrogen bonded O–H⋅ ⋅ ⋅O=C units in the PBZ film increased after UV exposure, inducing a hydrophilic surface; the surface recovered its hydrophobicity after heating, due to greater O–H⋅ ⋅ ⋅N intramolecular hydrogen bonding. Taking advantage of these phenomena, we prepared a PBZ/silica nanocomposite coating through two simple steps; this material exhibited reversible transitions from superhydrophobicity to superhydrophilicity upon sequential UV irradiation and thermal treatment.

Macko and Ishida reported that carbonyl-containing species were formed when a PBZ based on bisphenol A was exposed to UV radiation under ambient conditions.The 2,6-disubstituted benzoquinone moieties released from the isopropylidene linkages of PBZ are the primary reactive sites where oxidation and cleavage occur upon UV exposure.These benzoquinone moieties decrease the fraction of intramolecular hydrogen bonding while increasing the fraction of intermolecular hydrogen bonding [42,43].Liao et al. reported that the surface properties of PBZ become hydrophilic upon decreasing the fraction of intramolecular hydrogen bonds after UV exposure.Our motivation for this study was to test whether we could obtain reversible switching of the surface properties-between superhydrophobicity and superhydrophilicity-of a PBZ thin film [40].
Reversible transitions from superhydrophobicity to superhydrophilicity have been observed previously for aligned ZnO nanorod films through intelligent control over alternating UV illumination and dark storage [12,13].In addition, a photo-switched azobenzene thin film displaying a superhydrophobicity-to-superhydrophilicity transition has been realized, controlled through UV and visible irradiation [24,25].Hou and Wang reported that a polystyrene/titania (TiO 2 ) hybrid could also be adjusted from superhydrophilicity to superhydrophobicity by alternating UV illumination and thermal treatment with different drying temperatures [44].Similar behavior has also been reported for a methylsilicone/phenolic resin/silica composite surface [45].
In this study, we observed reversible surface properties, controlled through alternating UV illumination and thermal treatment at different temperatures, of a pure PBZ thin film.Taking advantage of these phenomena, we also prepared a PBZ/silica nanocomposite coating through two simple steps, using a previously reported procedure [35]; this material also exhibited reversible transitions from superhydrophobicity to superhydrophilicity upon sequential UV irradiation and thermal treatment.

Thin-Film Formation and Polymerization
A low-surface-energy thin film of PBZ was prepared from a solution of the VB-a monomer (0.5 g) in THF (10 mL) that was filtered through a 0.2 m syringe filter and then spin-coated in a glass slide (100 × 100 × 1 mm), which was then cured in an oven at 200 ∘ C. A superhydrophobic coating comprising PBZ and silica nanoparticles on a glass slide was formed through a two-step process.First, VB-a benzoxazine (0.5 g) was mixed with the silica nanoparticles (0.75 g) in THF (10 mL).After ultrasonication in a bath for 2 h, the mixture was spin-coated (1500 rpm, 45 s) in a glass slide (100 × 100 × 1 mm) and then cured in an oven (200 ∘ C, 1 h).Subsequently, the PBZ-silica hybrid surface was modified with 0.1% (w/v) VB-a PBZ film.The VB-a benzoxazine solution was spincoated (1500 rpm, 45 s) in a rough surface and then cured (210 ∘ C, 1 h).To produce a hydrophilic surface, the VB-a PBZ thin film was exposed (ca.120 min) through a mask to UV radiation (265 nm) at a distance of 25 cm from the source, receiving 30 W/m 2 of radiation.The sample was then thermally cured at different temperatures in an oven to reform the superhydrophobic surface.To investigate the effects of thermal treatment, the temperature was set at 80, 100, 120, 140, 160, 180, or 200 ∘ C.

Characterization
A Data Physics OCA20 goniometer interfaced to imagecapture software was used to measure the advancing contact angles of the samples at 25 ∘ C; a liquid drop of deionized water was injected in the polymer surface.Surface roughness profiles of the film structures were acquired using a NT-MDT solver PRO-M scanning probe microscope operated in the tapping mode.Root-mean-square (rms) roughnesses were calculated over scan areas of 5 m × 5 m.The chemical composition of the as-prepared surface was investigated through X-ray photoelectron spectroscopy (XPS) using a PHI 5000 VersaProbe electron spectrometer and an AlKR line excitation source with the reference of C 1s at 285.00 eV.The microstructures of the polymer surface and the mold were characterized using a JEOL JSM-6380 scanning electron microscope.

Results and Discussion
For the VB-a monomer system that had been heated at various curing temperatures (Figure 1(a)), the WCA increased upon increasing the curing temperature, from 82 ∘ at 80 ∘ C to 103 ∘ at 200 ∘ C, similar to the behavior of a previously reported BA-a PBZ system [29].Figure 1(b) reveals that the WCA of the VB-a PBZ films decreased upon increasing the UV exposure time.After UV exposure for longer than 120 min, the WCA reached very close to 0 ∘ , corresponding to a superhydrophilic surface.This behavior arose due to the partial decrease in the degree of intramolecular hydrogen bonding and a corresponding increase in the degree of intermolecular hydrogen bonding, thereby resulting in a higher surface free energy and a greater hydrophilicity [40].Liao et al. reported a simple method using a photomask to produce wettability patterns and wettability gradients on the surfaces of PBZ films; they also suggested that the surface free energy and hydrophilicity of these PBZ films could be controlled through a combination of thermal treatment and UV exposure to change the ratio of intra-to intermolecular hydrogen bonds [40].
Figure 2 displays the effects on the UV-exposed PBZ films after thermal treatment at various temperatures.Surprisingly, the WCA increased upon increasing the thermal treatment temperature, from 1.5 ∘ at 80 ∘ C to 102.9 ∘ at 200 ∘ C, behavior that is very similar to that observed after the first thermal curing procedure at 200 ∘ C. The wettability of this PBZ film revealed a transition from hydrophilicity to hydrophobicity upon increasing the thermal treatment temperature.Again, we used WCAs to measure the reversible switching of the PBZ   film between its hydrophilic and hydrophobic states.Figure 3 displays a WCA of approximately 100 ∘ for the PBZ film after initial thermal treatment at 200 ∘ C.After UV irradiation for 2 h, the WCA decreased to 2 ± 2 ∘ , a clear transformation in the wettability of this PBZ film from hydrophobicity to hydrophilicity.After UV exposure, we heated this sample again at 200 ∘ C for 1 h, regaining the hydrophobicity of its surface.We could repeat this process several times, obtaining good reversibility of the surface wettability.To determine the factors responsible for these transitions, we used XPS to investigate the chemical structures of the surface components of the PBZ films.Figures 4 and 5 present XPS analyses of the compositions of a PBZ film after alternating UV illumination and thermal treatment.Table 1 lists the peak assignments of the C 1s and O 1s signals as well as the curve fitting data [48,49].Let us first consider the C=O peaks in both the C 1s (288.5 eV) and O 1s (531.4 eV) spectra in Figures 4 and 5.After the first thermal treatment procedure, the C=O peaks were not detectable in either the C 1s or O 1s spectrum; the area fractions of the C=O peaks increased, however, after UV irradiation.For example, the area fraction of the C=O peak at 288.5 eV in Figure 4(b) increased significantly (to 16.62%) after UV exposure, relative to that in Figure 4(a) without UV exposure.This result is consistent with Ishida's report: that carbonyl-containing species are formed when bisphenol A-based PBZ is exposed to UV radiation [17].These benzoquinone moieties decrease the fraction of intramolecular hydrogen bonds while increasing the fraction of intermolecular O-H⋅ ⋅ ⋅ O=C hydrogen bonds, thereby inducing a hydrophilic surface for the PBZ film.After we subjected the UV-exposed PBZ film to thermal treatment, the area fraction of the C=O peak at 288.5 eV decreased (to 9.18%; Figure 4(c)).Again, decreasing the fraction of intermolecular hydrogen bonding enhanced the hydrophobic surface properties.Finally, a further application of UV exposure significantly increased the area fraction of the C=O peak at 288.5 eV (to 19.14%; Figure 4(d)), again inducing hydrophilic surface properties.Figure 5 reveals similar trends for the C=O peaks in the O 1s spectra.
The physical interactions among the PBZ polymer chains, mainly through intramolecular hydrogen bonding of the OH groups with the nitrogen atoms, as depicted in Scheme 1, have a critical effect on the properties of these materials [40].Let us turn our attention to the fraction of intramolecularly hydrogen bonded O-H⋅ ⋅ ⋅ N units in the PBZ film, based on Figure 5.The signal at 536.5 eV for the intramolecularly hydrogen bonded O-H⋅ ⋅ ⋅ N units in the PBZ film disappeared after UV exposure (area fraction: 0%; Figure 5(b)), relative to its value of 5.63% in Figure 5(a) for the sample that had not been subjected to UV exposure.After thermal treatment of the UV-exposed PBZ film, the area fraction of the O-H⋅ ⋅ ⋅ N peak at 536.5 eV in Figure 5(c) increased again to 3.05%; increasing the fraction of intramolecular hydrogen bonds again enhanced the hydrophobic surface properties.Finally, further UV exposure resulted again in the disappearance of the O-H⋅ ⋅ ⋅ N peak in Figure 5(d) (area fraction: 0%), thereby inducing hydrophilic surface properties once again.Therefore, the reversible switching of the wetting behavior of the PBZ film arose from the reversible chemical structures formed (cf.Scheme 1) upon alternating UV illumination and thermal treatment.Decreasing the fraction of intramolecular hydrogen bonds while increasing the fraction of intermolecular hydrogen bonds through UV exposure, as a means of inducing hydrophilic surface properties for PBZ films, has been widely discussed previously [15].In this present case, our XPS analyses suggested that the reversible switching of the surface properties appeared to arise from a partial increase in the fraction of intramolecularly hydrogen bonded O-H⋅ ⋅ ⋅ N units after UV exposure and a partial decrease in the fraction of intermolecularly hydrogen bonded O-H⋅ ⋅ ⋅ O=C units after subsequent thermal treatment.
Table 2 lists the atomic percentages in the coatings after treatment under the various conditions.The ratio of N atoms was 1.5% after the first thermal treatment procedure; it increased to 4.9% after the first UV exposure.Because these N atoms each have a lone pair of electrons, they would undergo intermolecular interactions with water; therefore, we would expect a higher percentage of N atoms on the surface to induce hydrophilicity.After we had heated the UV-irradiated coating at 200 ∘ C for 1 h, the ratio of N atoms decreased, consistent with the hydrophilic surface becoming hydrophobic once again.After a second bout of UV exposure, the ratio of N atoms increased again, thereby again inducing the hydrophilic surface properties.
The ability to control the WCA of this PBZ surface through irradiation with UV light suggested the intriguing  possibility of using this system to prepare materials with properties varying from superhydrophobicity and superhydrophilicity.Toward this goal, we used a simple two-step method to prepare a PBZ/silica nanocomposite coating, based on a previously reported procedure [35].We obtained a superhydrophobic coating of this material on a glass slide after combining the VB-a benzoxazine monomer with silica nanoparticles in solution, spin-coating in a glass slide, and then curing.We then further modified the resulting rough surface of the PBZ-silica hybrid thin film with a thin film of the pure VB-a PBZ.
Figure 6(a) presents top-view scanning electron microscopy (SEM) images of the as-prepared super-hydrophobic surface on a glass slide.This superhydrophobic PBZ surface had a rough surface possessing both micro-and nanoscale binary structures.Each microisland (300-700 nm) on the PBZ surface was covered with nanospheres (20-60 nm); this structure dramatically increased the surface roughness and formed composite interfaces in which air could become trapped within the grooves beneath the liquid, thereby inducing superhydrophobicity.The inset to Figure 6   this surface.After UV exposure for longer than 60 min, this WCA reached very close to 0 ∘ , corresponding to a superhydrophilic surface (Figure 7).Again, we believe that a partial decrease in the degree of intramolecular hydrogen bonding and a corresponding partial increase in the fraction of intermolecular hydrogen bonding resulted in a higher surface free energy and, therefore, a higher degree of hydrophilicity in the PBZ/silica hybrid system.In addition, this superhydrophilic PBZ surface had the same rough surface, possessing both micro-and nanoscale binary structures, as that of the superhydrophobic material (cf.Figures 6(b) and 8); thus, the morphology did not change after UV exposure or thermal treatment, as confirmed through AFM analyses.
The WCA of the PBZ surface after thermal treatment was 153 ± 3.2 ∘ ; subsequent UV exposure decreased the WCA to 12 ± 2.3 ∘ , but it returned to the superhydrophobic state after thermal treatment once again.We could repeat the reversible switching of the surface wettability several times with good reversibility (Figure 9).In order to realize the wetting phenomena between water droplet and PBZ/silica nanocomposite coating, we measured the advancing and receding contact angles (ARCA) by means of Needle-Syringe Injection method.In Figure 10, the WCAs and the Base Diameters (BDs) are plotted against the droplet volume when the drop is inflated and then deflated.The sessile drops are inflated with water injection rate of 0.5 L/s  and so are the drops deflated with suction rate of 0.5 L/s.In Figure 9, most of the static CAs of water droplets with volume of 3 L on the PBZ/silica nanocomposite coating show superhydrophobicity (≧150 ∘ ), and it is quite similar to the WCA of lotus leaf.The high water repellency of lotus leaf should be attributed to the micro-and nanoscopic structure of the leaf surface.However, Figure 10 reveals a large WCA hysteresis in our case of PBZ/silica system.Although the advancing angle of water drops can be as high as 150 ∘ , the receding angle gradually decreases to about 60 ∘ as the drop volume is deflated to less than 0.5 L.The WCA hysteresis is very significant with Δ ≧ 90 ∘ in consequence of the BDs and also obviously exhibits pinning phenomena at the contact line during deflation.This result is similar to the case of scallion and garlic leaves [50], which exist rough surface of microscopic architecture and hydrophilic chemical defects.Likewise, in the PBZ/silica nanocomposite films, both low surface energy PBZ and silica caused roughness construct the superhydrophobicity, and the hydroxyl groups in PBZ become the hydrophilic defects which dominate the WCA hysteresis.

Conclusions
A PBZ/silica nanocomposite coating, prepared using a simple two-step method, exhibits reversible transitions from superhydrophobicity to superhydrophilicity and back again upon sequential UV irradiation and thermal treatment.We prepared a superhydrophobic PBZ film after introducing silica nanoparticles possessing micro-and nanoscale binary structure; it exhibited a large reversible contact angle of approximately 140 ∘ -seven times larger than that of the corresponding flat PBZ film-that could be switched through the application of sequential UV irradiation and thermal treatment.Based on XPS analyses, the fraction of intermolecularly hydrogen bonded O-H⋅ ⋅ ⋅ O=C units increased after UV exposure, resulting in a PBZ film with a hydrophilic surface.The hydrophobicity of the surface returned after heating this film at 200 ∘ C, due to an increase in the degree of intramolecular O-H⋅ ⋅ ⋅ N hydrogen bonding in the PBZ film.Furthermore, the ARCA experiments help us to figure out the wetting model in PBZ/silica system which is a superhydrophobic surface with serious hysteresis.We suspect that the materials exhibiting reversible WCAs reported herein might have potential new applications in lithographic patterning systems.

Figure 1 :
Figure 1: Variations in WCA after (a) the first thermal curing of the VB-a monomer at various temperatures and (b) the first UV exposure for various lengths of time.

Figure 2 :
Figure 2: Variation in WCA after performing the second thermal treatment at various temperatures.

Figure 3 :
Figure 3: (a) Photographs of a water droplet on the PBZ coating before (left) and after (right) UV illumination.(b) Reversible hydrophobic-hydrophilic transitions of the as-prepared coatings through sequential alternating of UV irradiation and thermal treatment.

Figure 4 :
Figure 4: XPS spectra (C 1s peaks) of the PBZ film after its (a) first thermal treatment at 200 ∘ C for 1 h, (b) first UV irradiation for 2 h, (c) second thermal treatment at 200 ∘ C for 1 h, and (d) second UV irradiation for 2 h.

Figure 5 :Scheme 1 :
Figure 5: XPS spectra (O 1s peaks) of the PBZ film after its (a) first thermal treatment at 200 ∘ C for 1 h, (b) first UV irradiation for 2 h, (c) second thermal treatment at 200 ∘ C for 1 h, and (d) second UV irradiation for 2 h.

Figure 6 :Figure 7 :
Figure 6: SEM images of the PBZ-silica hybrid surface modified with VB-a PBZ (a) before and (b) after UV exposure.Inset: photograph of a water droplet on the surface.

Figure 8 :
Figure 8: AFM images of the PBZ-silica hybrid surface modified with VB-a PBZ after its (a) first thermal treatment at 200 ∘ C for 1 h, (b) first UV irradiation for 1 h, (c) second thermal treatment at 200 ∘ C for 1 h, and (d) second UV irradiation for 1 h.

Figure 9 :Figure 10 :
Figure 9: (a) Photographs of a water droplet on the PBZ-silica hybrid surface modified with a VB-a PBZ coating before (left) and after (right) UV illumination.(b) Reversible superhydrophobic-superhydrophilic transitions of the as-prepared coating upon sequential alternating of UV irradiation and thermal treatment.
(a) displays a spherical water droplet having a WCA of 158 ∘ on

Table 1 :
Fitting data from the XPS analyses.

Table 2 :
Surface atomic abundances of the PBZ films.