Chapter 3 Modification of Polymer Substrates with Low Surface Free Energy
3.4 Conclusions
The free radical initiator AIBN, induced polymerization of the N-allyl group and produced phenol-containing oligomers. These oligomers were able to catalyze the ring opening of the oxazine ring at a relatively lower curing temperature (120 °C) to produce polybenzoxazine with stronger intramolecular hydrogen bonding but lower surface energy. B-ala and B-ala/AIBN PBZ thin films both possessed low surface free energy because the strong intramolecular hydrogen bonds were formed during the curing process. B-ala/AIBN PBZ system had a relatively lower surface free energy than the pure B-ala system because of the higher extent of the ring-opening of oxazine.
Moveover, it can modify many polymer substrates that are thermally stable at or above 120 °C.
References
[1] (a) Li, H.; Wang, X.; Song, Y.; Liu, Y.; Li, Q.; Jiang, L.; Zhu, D. Angew. Chem.
2001, 113, 1793. (b) Sun, T.; Wang, G.; Liu, H.; Feng, L.; Jiang, L.;Zhu, D. J. Am.
Chem. Soc. 2003, 125, 14996 (c) Russell, T. P. Science 2002, 297,964 (d) Aussillous, P.; Qumrm, D. Nature 2001, 411, 924.
[2] (a) Ryu, D. Y.; Shin, K.; Drockenmuller, E.; Hawker, C. J.; Russell, T. P. Science F. C. Macromol. Rapid Commun. 2006, 27, 333.
[4] Agag, T.; Takeichi, T. Macromolecules, 2003, 36, 6010.
[5] Tung, P. H.; Kuo, S. W.; Jeong, K. U.; Cheng, S. Z. D.; Huang, C. F.; Chang, F. C.
Macromol. Rapid Commun. 2007, 28, 271.
[6] Ishida, H.; Ohba, S. Polymer, 46, 2005, 5588.
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Polymer Science: Part A: Polymer Chemistry 1999, 37, 1913.
[8] Severini, F.; Gallo, R. J. Thermal Analysis, 1984, 29, 561.
[9] (a) van Oss, C. J.; Ju, L.; Chaudhury, M. K.; Good, R. J. J. Colloid Interface Sci.
1989, 128, 313. (b) van Oss, C. J.; Chaudhury, M. K.; Good, R. J. Chem. Rev.
1988, 88, 927.
[10] Fowkes, F. W. “Dispersion Force Contributions to Surface and Interfacial Tensions” Gould, R. F., Ed.; American Chemical Society: Washington, DC, 1964.
Table 3-1. Advancing contact angles for water, ethylene glycol (EG), and diiodomethane (DIM) and their corresponding surface free energy of B-ala and B-ala/AIBN polybenzoxazine films.
* Molar ratio of B-ala monomer/AIBN = 5/1
* γs (HD)：Test liquids are deionized water and diiodomethane
* γs (ED)：Test liquids are ethyleneglycol and diiodomethane
Table 3-2. Fraction of hydrogen bonding of B-ala/AIBN=5:1 PBZ film cured at 120
°C for 2, 4, 8, and 24 h.
O----H+N Intramolecular hydrogen bonding
(%)
OH---N Intramolecular hydrogen bonding
(%)
OH---O Intermolecular hydrogen bonding
(%)
B-ala-AIBN-5-1-2h 63.64 31.66 4.69
B-ala-AIBN-5-1-4h 59.84 36.06 4.10
B-ala-AIBN-5-1-8h 56.86 39.52 3.62
B-ala-AIBN-5-1-24h 56.65 40.65 2.70
Table 3-3. The advancing contact angle for water, ethylene glycol, and diiodomethane of poly(4-vinyl phenol), poly(4-vinyl pyridine) and polycarbonate substrates before and after modification with B-ala/AIBN=5/1 PBZ thin film cured 8 h at 120 °C.
Contact angle(°) Surface energy Polymer substrates
Water DIM E.G. γs (mJ/m2) Before modification
Poly(4-vinyl phenol) 72.1 44.4 45.6 40.5
Poly(4-vinyl pyridine) 62.2 0 48.0 57.4
Polycarbonate 89.5 35.0 56.0 42.3
After modification
Poly(4-vinyl phenol) 107.0 80.6 87.3 17.6
Poly(4-vinyl pyridine) 108.7 83.2 86.3 16.6
Polycarbonate 107.3 80.3 86.2 17.9
8 7 6 5 4 3 2 1 0
Figure 3-1. The 1H NMR spectrum of B-ala monomer.
180 160 140 120 100 80 60 40 20
Figure 3-2. The 13C NMR spectrum of B-ala monomer.
0 50 100 150 200 250 300 350 400 450 500
Figure 3-3. The Mass spectrum of B-ala monomer.
50 100 150 200 250 300
Heat flow(mW)
Temperature(oC) (a)B-ala
(b)B-ala-AIBN-5/1 Exo
a b
Figure 3-4. DSC diagram of B-ala and B-ala/AIBN molar ratio = 5:1.
100 150 200 250
b
Heat flow(mW)
Temperature
(
oC)
(a) B-ala
(b) B-ala-phenol-5/1 exo
a
Figure 3-5. DSC diagram of B-ala and B-ala/phenol molar ratio = 5:1.
4000 3600 3200 2800 2400 2000 1600 1200 800 400
Figure 3-6. FTIR spectra of B-ala and B-ala/AIBN PBZ film. (a) B-ala, (b)B-ala and B-ala/AIBN = 5:1 cured at 120 °C, (c) B-ala and B-ala/AIBN = 5:1 cured at 120 °C, and (d)B-ala and different molar ratio of B-ala/AIBN cured for 8h at 120 °C.
4000 3500 3000 2500 2000
a
4000 3500 3000 2500 2000
b
4000 3500 3000 2500 2000
Absorbance(a.u.)
c
4000 3500 3000 2500 2000
Wavenumber
(
cm-1)
d
Figure 3-7. Curve fitting for the FTIR spectra of B-ala/AIBN=5:1 PBZ film cured at 120 °C for (a) 2, (b) 4, (c) 8, and (d) 24 h.
0 5 10 15 20 25 15
20 25 30 35 40 45 50
Surface energy(mjm-2 )
Curing time(hour) B-ala
B-ala/AIBN=20:1 B-ala/AIBN=10:1 B-ala/AIBN=7:1 B-ala/AIBN=5:1
Figure 3-8. Surface free energy of B-ala and B-ala with different molar ratio of AIBN cured at 120 °C.
Figure 3-9. The advancing contact angle for water, ethylene glycol, and diiodomethane of (a)poly(4-vinyl pyridine) thin film (b)modified with B-ala/AIBN=5 PBZ thin film.
Chapter 4
Tuning the Surface Free Energy of Polybenzoxazine Thin Films
Abstract
A novel approach to manipulate the surface free energy and wettability on polybenzoxazine thin films can be achieved simply by varying time of thermal treatment or UV exposure. Fraction of the intramolecular hydrogen bonding of the as cured sample will convert into intermolecular hydrogen bonding upon thermal treatment or UV exposure and thus results in increase of hydrophilicity and wettability.
This UV approach provides a simple method to generate wettability patterns or wettability gradients on the surface of polybenzoxazine film. In addition, we have applied this technique to the preparation of a large-area periodic array of CdTe colloidal nanocrystals on polybenzoxazine thin films.
4.1 Introucrtion
The surface and interfacial properties of materials are directly related to their surface energies. Current approaches toward engineer tunable surfaces include light irradiation [1,2]and UV [3] thermal treatment, [4, 5] acidification, [6-8] and applying electrical potentials, [9,10] among others. [11] Intra- and intermolecular interactions play important roles in determining the surface properties of polymers. For example, Jiang et al. [4] found that at temperatures above its lower critical solution temperature (LCST), the compact, collapsed conformation of poly(N-isopropylacrylamide) (PNIPAAm), induced by intramolecular hydrogen bonding between the C=O and N-H groups of the main chains results in a low surface free energy and a high contact angle for water. When the temperature is below the LCST, however, intermolecular hydrogen bonding between the PNIPAAm main chains and water molecules predominates leading to a higher surface free energy and a lower water contact angle.
Similarly, Chung et al. [12] reported that the presence of amide groups in a fluorinated-main-chain liquid-crystalline polymer system induces strong intermolecular hydrogen bonding resulting in higher surface free energies and higher degrees of hydrophilicity.
Structured surfaces that exhibit lateral patterns of varying wettability have received extensive attention because it can apply as preparing fluid microchips [13]
and the periodical arrangements of metallic nanoparticles [14,15] or nanowires [16]
and self-assembly of block copolymer [17] or carbon nanotubes [18]. Besides, a gradient surface displays a gradual change in the chemical and physical properties along its length.has a wide range of applications in material science. [19-22] A gradient in a surface can induce the net mass transport of liquids, which affords a driving force for operation of microfluidic devices and for biological cell motility in
vitro. Therefore, a simple method to create wettabilty pattern or wettability gradient on a polymer thin film was needed to be developed.
Polybenzoxazines (PBZs), feature strong intramolecular hydrogen bonds that result in extremely low surface free energies, even lower than that of pure Teflon. [23]
In PBZ systems, strong intramolecular hydrogen bonding between the hydroxyl groups and the amino groups in the Mannich bridges tends to decrease the surface free energy, whereas intermolecular hydrogen bonding between hydroxyl groups results in higher surface free energies. We are unaware, however, of any previously available methods that allow precise control over the surface free energies of PBZ films. In this paper, we present a simple strategy for obtaining wettability patterns and wettability gradients on PBZ thin films by using UV irradiation to modify the extent of intra- and intermolecular hydrogen bonding. We have applied this technique to the preparation of a large-area periodic array of CdTe colloidal nanocrystals on PBZ thin films.
4.2 Experiment Section 4.2.1 Materials
All chemicals were used as received. Bisphenol A and paraformaldehyde (95%) were supplied by the Showa Chemical Company (Japan). The synthesis of bis(3-allyl-3,4-dihydro-2H-1,3- benzoxazinyl)isopropane (B-ala) was based on the reaction of bisphenol A, allylamine, and paraformaldehyde (Scheme 4-1). Column chromatography (eluent: ethyl acetate/hexane, 2:1) was used to separate the impurities, which were identified as unreacted phenols, amines, and benzoxazine oligomers.
4.2.2 Contact Angle Measurement
The surface free energy of the polymer sample was determined through contact angle goniometry of a liquid drop (5 μL) at 25 °C using a Krüss GH-100 goniometer interfaced with image-capture software. Deionized water, ethylene glycol (≥99%;
Aldrich), and diiodomethane (99%; Aldrich) were used as standards for measuring the surface free energies.
4.2.3 Fourier Transform Infrared (FTIR) Spectroscopy
All infrared spectra were recorded using a Nicolet Avatar 320 FTIR spectrophotometer; 32 scans were collected at a spectral resolution of 1 cm–1. FTIR spectra of the polymer films were determined using the conventional potassium bromide (KBr) plate method. Each sample was prepared by casting a THF solution directly onto a KBr plate and then curing under conditions similar to those used for the bulk preparation. All films were sufficiently thin to exist within the absorbance range in which the Beer–Lambert law is obeyed.
4.2.4 Ultraviolet Irradiation Exposure
Five 6W, low-pressure mercury lamps (λ = 265 nm) were used as the UV irradiation source. Samples were placed at a distance of 25 cm from the source to receive 30 W/m2 of radiation.
4.2.5 Electron Spectroscopy for Chemical Analysis (ESCA)
The chemical composition of the substrate surface was analyzed using a Thermo VG Scientific ESCALAB 250 spectrometer equipped with a monochromatic Al Ka X-ray source (1486.6 eV photons); the vacuum within the analysis chamber was maintained at or below ca. 10–8 mbar.
4.2.6 Thin-Film Formation and Polymerization
A solution of the B-ala monomer (0.5 g) in THF (10 mL) was filtered through a 0.2-μm syringe filter before spin-coating onto a glass slide (100 × 100 × 1 mm3) and the sample was then cured in an oven at 210 °C.
4.2.7 Periodic Arrangement of Arrays of CdTe Colloidal Nanocrystals
Aqueous colloidal CdTe solutions were prepared by adding a freshly prepared NaHTe solution to 1.25 × 10–3 N N2-saturated CdCl2 solutions at pH 9.0 in the presence of mercaptocarboxylic acid (stabilizing agent). The molar ratio of Cd2+, stabilizer, and HTe– was fixed at 1:2.4:0.5. The resulting mixture was then heated under reflux to control the growth of the CdTe nanocrystals. The B-ala polybenzoxazine (PBZ) thin films were exposed through a mask to 265-nm UV radiation at a distance of 1 cm for ca. 20 min to produce a hydrophilic pattern. The periodic arrays of CdTe colloidal nanocrystals were prepared through direct
evaporation of a drop of an aqueous colloidal CdTe solution on PBZ thin films exhibiting patterned lyophobicity. Fluorescent images were acquired with a Leica DMI 6000B CS laser scanning confocal microscope equipped with diode, argon blue, green DPSS, and helium-neon lasers for excitation.
4.3 Results and discussion
After optimizing its thermal curing conditions, B-ala PBZ contains predominantly intramolecular hydrogen bonds and, therefore, possesses an extremely low surface free energy. Figure 4-1 displays the advancing contact angles and surface free energies (γs) of three test liquids on B-ala PBZ films after various curing times at 210 °C. From Table 4-1, the lowest surface free energy we obtained for a B-ala film was 14.4 mJ m–2, calculated using van Oss and Good’s three-liquid method, [24]
which is substantially lower than that of pure Teflon (21 mJ m–2). The surface free energy in this B-ala PBZ system decreased initially and then increased steadily upon increasing the curing time. This phenomenon can be explained in terms of changes in the ratio of intra- and intermolecular hydrogen bonds (Figure 4-2). [23] Therefore, the surface free energy of this PBZ is tunable—from 14.4 to 46.3 mJ m–2—merely by controlling the length of time that it is subjected to thermal curing. Nevertheless, this technique would be very difficult to use to fabricate a gradient in the surface free energy or provide a wettability pattern on PBZ.
Ishida et al. [25, 26] determined that C=O-containing species are formed when a bisphenol-A-based PBZ resin is exposed to UV radiation under ambient conditions.
The isopropylidene linkages of PBZ are the reactive sites where oxidation and cleavage occur upon UV exposure, forming 2, 6-disubstituted benzoquinone units.
The presence of these benzoquinone moieties decreases the extent of intramolecular hydrogen bonding while increasing the extent of intermolecular hydrogen bonding.
Because radical formation and oxidation reactions induced by UV radiation are usually concentrated at polymer surfaces, we suspected that the surface properties of PBZ thin films would be greatly affected by their length of UV exposure and the corresponding photo-oxidation mechanism is shown in Scheme 4-2.
Figure 4-1 reveals that the advancing contact angles of the three test liquids decreased upon increasing the UV exposure time of B-ala PBZ films (cured 2 h at 210
°C). This behavior is consistent with a partial destruction of intramolecular hydrogen bonding and a corresponding increase in the extent of intermolecular hydrogen bonding after UV exposure, resulting in higher surface free energies and higher degrees of hydrophilicity. From Table 4-2, we find that the advancing contact angles of the polar test liquids (water, ethylene glycol) decrease substantially after UV exposure; the decrease in the advancing contact angle of the nonpolar liquid (diiodomethane) is less pronounced. To determine the interactions occurring between these liquids and the PBZ thin film, we used the two-liquid geometric method [27] to determine the corresponding values of γs, γd, and γp (Table 4-3). The values of γd and γp can be calculated from the measured contact angles; the superscript “d” refers to the London dispersion forces, whereas the superscript “p” refers to polar forces, including all of the interactions established between the solid and liquid, such as Keesom dipole–dipole, Debye dipole-induced dipole, and hydrogen bonding interactions. The value of γp increased rapidly upon increasing the UV exposure time, but the change in γd was relatively insignificant; these phenomena imply that the polar forces between the PBZ thin film and the testing liquids increased substantially after UV exposure. The presence of new polar quinone C=O functional groups on the irradiated surface led to stronger polar forces between the PBZ film and the testing liquid, resulting in lower advancing contact angles for both water and ethylene glycol.
The ESCA results in Table 4-3 reveal that the atomic fraction of oxygen, an indication of the degree of photo-oxidation of the surface, increased dramatically after UV exposure.
The relationship between the surface free energy of B-ala PBZ thin films and the UV exposure time suggested that we could manipulate the surface free energy at selected regions merely by varying the UV exposure time to create wettability patterns or wettability gradients. Scheme 4-3 provides an illustration of the procedures used to control the surface free energy of PBZ thin films. In the procedure, we controlled the surface free energy of the B-ala PBZ thin films through thermal curing and then created hydrophilic regions on them through UV exposure. Figures 4-3 (a) and 4-3 (b) present photographic images of a wettabilty pattern and a wettablity gradient formed upon two PBZ films after UV exposure. Furthermore, we also deposited CdTe colloidal nanocrystals through direct evaporation of a nanocrystal solution [28] in periodic arrangements on PBZ thin films exhibiting patterned lyophobicity (Figure 4-4).
4.4 Conclusion
In conclusion, the surface free energy and hydrophilicity of PBZ films can be controlled through a combination of thermal treatment and UV exposure to change the ratios of intra- to intermolecular hydrogen bonds. This simple method allows wettability patterns and wettability gradients to be produced on the surfaces of PBZ films; in addition, we used this technique to pattern periodic arrangements of CdTe colloidal nanocrystals on PBZ thin films.
References
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Langmuir 2002, 18, 8062-8069
[2] Feng, X.; Feng, L.; Jin, M. ; Zhai, J.; Jiang, L.; Zhu, D. B. J. Am. Chem. Soc.
2004, 126, 62-63.
[3] Lahann, J.; Mitragotri, S.; Tran, T.; Kaido, H.; Sundaram, J.; Choi, I. S.; Hoffer, S.; Somorjai, G. A.; Langer, R. Science 2003, 299, 371-374.
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Fulghum, J. E.; López, G. P. J. Am. Chem. Soc. 2004, 126, 8904-8905.
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Langmuir 2004, 20, 9916-9919.
[7] Yu, X.; Wang, Z.; Jiang, Y.; Shi, F.; Zhang, X. Adv. Mater. 2005, 17, 1289-1293 [8] Choi, Se-Jin; Suh, K. Y.; Lee, H. H. J. Am. Chem. Soc. 2008, 130, 6312-6313.
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Science 2005, 308, 236-239.
[12] Ma, K. X.; Chung, T. S. J. Phys. Chem. B 2001, 105, 4145-4150
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Table 4-1. Advancing contact angles for water, ethylene glycol (EG), and diiodomethane (DIM) and corresponding surface free energies of B-ala PBZ films after thermal curing
contact angle(°) surface free energy (mJ/m2) curing
time (h)
roughness (nm)
water DIM E.G. γs(HD) γs(ED) γs
B-ala-210°C
0.5 1.3 98.2 69.4 75.4 23.3 30.2 24.1
1 1.2 112.0 83.2 84.4 15.9 22.7 16.1
2 1.8 112.9 87.2 88.2 14.0 20.1 14.4
4 1.4 104.6 76.7 76.8 19.2 28.4 20.1
8 1.3 91.0 68.6 52.6 24.7 49.0 26.8
24 1.2 19.8 48.9 10.2 68.6 75.0 46.3
* Curing temperature = 210 °C
Table 4-2. Advancing contact angles for water, ethylene glycol (EG), and diiodomethane (DIM) and corresponding surface free Energies of B-ala PBZ films cured for 2 h at 210 °C and then Subjected to UV exposure.
Contact angle(°) Surface free energy (mJ/m2) UV exposure
time (min) Roughness (nm)
Water DIM E.G. γs
B-ala-210°C-2h
0 1.8 112.9 87.2 88.2 14.4
10 1.6 106.0 85 81.5 16.4
20 1.4 84.6 79.6 75.9 19.9
40 1.9 65.6 74.8 60.8 26.5
60 1.4 53.4 69.1 37.8 38.4
80 1.7 34.7 64.9 25.3 43.4
100 1.7 21.3 60.8 18.3 44.8
120 1.6 6.2 54.6 3.8 46.6
* UV wavelength = 265nm
* distance between UV source and sample: 25cm
Table 4-3. Surface free energy and ESCA analysis of B-ala PBZ films cured for 2 h at 210 °C and then subjected to UV exposure
UV exposure
CH3 H3C
OH
OH
H2N
O N
O N
+ 4CH2O+ 2
reflux
Scheme 4-1. Preparation of the allylamine-based benzoxazine monomer
Scheme 4-2. Mechanism of the B-ala photo-oxidation
Scheme 4-3. Fabrication of wettability gradients and wettability patterns on B-ala PBZ films
0 5 10 15 20 25
Figure 4-1. Advancing contact angles of (●)water, (▲)diiodomethane, and (▼)ethylene glycol and the respective surface free energies (γs) (■) of B-ala PBZ films
4000 3500 3000 2500 2000
Intensity (a.u.)
Wavenumber(cm-1)
(a)
4000 3500 3000 2500 2000
(b)
Intensity (a.u.)
Wavenumber((cm-1)
4000 3500 3000 2500 2000
(c)
Intensity (a.u.)
Wavenumber(cm-1)
4000 3500 3000 2500 2000
(d)
Intensity (a.u.)
Wavenumber((cm-1)
Figure 4-2. Curve fitting of the FTIR spectra of B-ala PBZ films cured at 210 °C for (a) 1, (b) 2, (c) 4, and (d) 8 h
Figure 4-3. Water drops on (a) wettabilty pattern, (b) wettablity gradient B-ala PBZ films.
Figure 4-4. Fluorescence microscope images of periodic arrangement of arrays of CdTe colloidal nanocrystals on line patterned PBZ thin film in a magnification of (a)200 (b)400 (c)600 and square patterned PBZ thin film in a magnification of (a)200 (b)400 (c)600
Chapter 5
Fabrication of patterned superhydrophobic Polybenzoxazine-hybrid surfaces
Abstract
The hydrophilicity of B-ala PBZ film and superhydrophobic polybenzoxazine-hybrid surface can be controlled through UV exposure to change ratio of intra- to intermolecular hydrogen bonds. Fraction of the intramolecular hydrogen bonding of the as cured sample will convert into intermolecular hydrogen bonding upon UV exposure and thus results in increase of hydrophilicity. This simple method allows for manipulating the hydrophilicity at selected regions on superhydrophobic polybenzoxazine-hybrid surface to create patterned surface with superhydrophobic and superhydrophilic regions. Besides, we have found that the superhydrophobic polybenzoxazine-silica hybrid surface exhibits good adhesion of water droplets after UV exposure which can be served as a “mechanical hand” to transfer water droplets from a superhydrophobic surface to a hydrophilic one.
5.1 Introucrtion
A solid surface’s water repellency is one of the most important characteristics in both theoretical research and industrial applications. The wettability of solid surfaces can be controlled by surface topography and/or surface chemistry. With this controllability, many useful methods [1] have been developed to produce numerous superhydrophobic surfaces. Among these, an approach for artificial superhydrophobic surfaces [2, 3] such as mimic lotus leaves has drawn great interest where the surface is covered by branch-like nanostructure on top of micropapillae. The multiscale hierarchical structure observed in nature could be effective in manipulating important surface properties such as wettability, friction, and adhesion for electronic, optical, and biological applications. [4] The wettability of the multiscale hierarchical structure can also be alternated between superhydrophobicity and superhydrophilicity by a change in surface chemistry. [5] Wettability-switching surfaces have also been realized by applying an external stimulus, such as light irradiation, [6] electrical potential, [7] temperature, [8] and solvent. [9] While the reversibility is desirable in certain applications, however, this approach cannot be used for selective transformation of the wettability. [7-9]
Patterned surfaces with dissimilar wetting properties have been achieved using techniques such as microcontact printing, [10, 11] chemical vapor deposition, [12]
and photolithography. [13-15] However, these approaches often involve complicated procedures to introduce functional groups to the patterned areas. Therefore, a simple and more effective route to generate arrays of patterns with different wetting properties and chemical functionalities is highly desirable. Furthermore, surfaces with extreme wetting properties such as superhydrophilic patterns on a superhydrophobic surface offer new possibilities in the fabrication of novel devices such as planar
microcanals (open-air microfluidic channels). [16] Open-air microfluidic channels offer advantages such as the facile handling of small amount of liquids, the possibility of massive parallel processing, direct accessibility, and ease of cleaning. [16-18] The availability of patterned surfaces with superhydrophobic and superhydrophilic regions can greatly enhance the utility and function of such devices and move us beyond nature’s impressive accomplishment with the Namib beetle.
In our previous study, we have discovered that polybenzoxazine (PBZ) is a new class of nonfluorine, non-silicon low surface free energy polymeric material and the superhydrophobic polybenzoxazine hybrid surface with excellent environmental
In our previous study, we have discovered that polybenzoxazine (PBZ) is a new class of nonfluorine, non-silicon low surface free energy polymeric material and the superhydrophobic polybenzoxazine hybrid surface with excellent environmental