2.3 Wettability Pattern and Wettability gradient
2.3.1 Methods to Control the Wettability of Surface
Wettability is a very important property governed by both the chemical composition and the geometrical structure of solid surfaces. Super-hydrophobic surface (with water contact angle (CA) larger than 150°) and super-hydrophilic surfaces (CA close to 0°) have been extensively investigated due to their importance for industrial applications. Recently, smart surfaces with tunable wettability have aroused great interest because of their myriad applications as biosensors, microfluidic
devices, intelligent membranes, and so on. [90, 91] Current approaches toward engineer tunable surfaces include UV, [92, 93] thermal treatment, [94, 95]
acidification, [96-98] and applying electrical potentials, [99, 100] among others. [101, 102]
Among these method, UV approach to control the wettability of surface is the most way which was used. Jiang et al. reported the wettability of aligned ZnO nanorod films could be controlled by UV illumination. [92] From Figure 2-27, reversible super-hydrophobicity to super-hydrophilicity transition on aligned ZnO nanorod films was observed and intelligently controlled by alternation of UV illumination and dark storage. Besides, Cho et al. reported a facile method for the fabrication of a wetting surface that is photoswitchable from superhydrophobicity to superhydrophilicity, which combines layer-by-layer assembly and the introduction of photoresponsive moieties onto the top surface (Figure 2-28). [103] This strategy can be extended to other stimuli-responsive surfaces with similar nanostructure and higher stability, which is certainly significant for future industrial applications.
There are a number of thermal approaches have been employed to control the wettability of surface. For example, Jiang et al. [94] reported on the use of poly(N-isopropylacrylamide) (PNIPAAm) grafted to texture surfaces formed by microlithography to generate the surfaces with reversibly transition behavior between the temperature above the lower critical solution temperature (LCST) and the temperature below the LCST. The rough surface structures enhanced thermally responsive wettability of a PNIPAAm-modified surface. Reversible switching between superhydrophilicity and superhydrophobicity can be achieved in a narrow temperature range of about 10°C (Figure 2-29), which is considered to result from the combined effect of the chemical variation of the surface, and surface roughness. Such
switchable surfaces may have wide applications in functional textiles, intelligent microfluidic switching, controllable drug release, and thermally responsive filters.
Besides, Jiang et al. have also reported that wettability of surface could be control by pH value. [104] Stable superhydrophobic or superhydrophilic colloidal crystal films have been successfully fabricated under ambient conditions from an amphiphilic material of poly-(St-MMA-AA) in the presence of hydrogen bonding or not (Figure 2-30).The consistent hydrogen-bonding network in the films contributes to the stable superhydrophobicity, while the absence of the hydrogen bonding leads to superhydrophilicity.
Choi et al. have reported that the design of surfaces that exhibit dynamic changes in interfacial properties, such as wettability, in response to an electrical potential (Figure 2-31). The change in wetting behavior was caused by surface-confined, single-layered molecules undergoing conformational transitions between a hydrophilic and a moderately hydrophobic state. [99]
Except UV, thermal treatment, pH value and electrical potential, solvent treatment is another approach to control the wettability of surface. Minko et al. have reported a route to fabricate two-level structured self-adaptive surfaces (SAS) of polymer materials as shown in Figure 2-32. [105] The first level of structure is built by a rough polymer film that consists of needlelike structures of micrometer size. The second level of structure is formed by the nanoscopic self-assembled domains of a demixed polymer brush irreversibly grafted onto the needles. By exposing the surface to solvents that are selective to one of the components of the brush, we reversibly tune the surface properties. The large scale surface structure amplifies the response and enables us to control wettability, adhesion, and chemical composition of the surface over a wide range.
Figure 2-27. (a, b) FE-SEM top-images of the as-prepared ZnO nanorod films at low and high magnifications, respectively. (c) Cross-sectional view of the aligned ZnO nanorods. (d) XRD pattern of the as-synthesized nanorod films. (e) Reversible super-hydrophobic-super-hydrophilic transition of the as-prepared films under the alternation of UV irradiation and dark storage. (f) Photographs of water droplet shape on the aligned ZnO nanorod films before (left) and after (right) UV illumination. [92]
Figure 2-28. (a) Fabrication and (b) Reversible Photoisomerization of a Roughness-Enhanced Photoswitchable Surface (c) photographs of substrates with patterned extreme wetting properties; angled views of water droplet profiles on the patterned substrate as a result of selective UV irradiation. (d) The relationships between the number of deposition cycles and the water contact angles: water droplet profiles on the smooth substrate (dotted arrows) and on the (PAH/SiO2) 9 multilayer film (solid arrows) after UV/visible irradiation. (e) Reversible wettability transitions of a smooth substrate (□) and a (PAH/SiO2) 9 multilayer film (■).
Figure 2-29. Surface-roughness-enhanced wettability of a PNIPAAm-modified surface. (a) The relationships between groove spacing (D) of rough surfaces and the water CAs at low temperature (triangles, 25 ℃) and at high temperature (squares, 40
℃). The groove spacing of ∞ represents flat substrate. (b) Water drop profile for thermally responsive switching between superhydrophilicity and superhydrophobicity of a PNIPAAm-modified rough surface with groove spacing of about 6 μm, at 25 ℃ and 40 ℃. The water CAs are about 0° and 149.3±2.5°, respectively. (c) Temperature (T) dependences of water CAs for PNIPAAm thin films on a rough substrate with groove spacing of about 6μm (triangles) and on flat substrate (squares). (d) Water CA in at two different temperatures for a PNIPAAm-modified rough substrate with groove spacing of 6μm. [94]
Figure 2-30. Typical comparison of SEM images of the colloidal crystal film assembly at different pH values. (a, b) Top view and side view of the films assembled at pH = 6.0. (c, d) Top view and side view of the film assembled at pH = 12.0. (Inset:
typical TEM image of core-shell spheres of poly-(St-MMA-AA); the bar is 100 nm. (e) Photographs of water droplet shape on the films assembled from suspensions with pH of 6.0 and 12 and illustrations of the structure of the latex sphere in the films. [104]
Figure 2-31. Idealized representation of the transition between straight (hydrophilic) and bent (hydrophobic) molecular conformations (ions and solvent molecules are not shown). The precursor molecule MHAE, characterized by a bulky end group and a thiol head group, was synthesized from MHA by introducing the (2-chlorophenyl)diphenylmethyl ester group. [99]
Figure 2-32. Two-level structure of self-adaptive surfaces (SAS): Schematic representation of needlelike surface morphology of the PTFE surface (first level) (a) and SEM image of the PTFE film after 600 s of plasma etching (b). Each needle is covered by a covalently grafted mixed brush that consists of hydrophobic and hydrophilic polymers (second level) depicted schematically in panels c-e. Its morphology results from interplay between lateral and vertical phase segregation of the polymers, which switches the morphology and surface properties upon exposure to different solvents. In selective solvents the preferred polymers preferentially occupies the top of the surface (c and e), while in nonselective solvents, both polymers are present in the top layer (d). The lower panels (f and g) show AFM images (model smooth substrate) of the different morphologies after exposure to different solvents. [105]
2.3.2 Fabrication of wettability pattern and periodic array of colloidal nanocrystals
The development of highly parallelizable means of creating ordered assemblies of colloidal particles of micrometer to nanometer length scales is a recent focus of research. Electrostatically guided deposition of particles on patterned substrates is one means of creating ordered structures. In this technique, patterned surfaces containing charged and uncharged regions are created by soft lithography to pattern alkanethiols on gold, [106] by photolithography to pattern siloxane layers on glass, [107, 108] by layer by layer adsorption of polyelectrolytes, [109] or by creating holes in an insulating substrate formed using a focused ion beam. [110] Colloidal particles bearing charge of the opposite sign of the patterned patches are then exposed to those regions. The particles adsorb to the charged regions via columbic interactions and pack to form arrays of single particles or multiple particles, depending upon the relative size of the particles to the charged regions. In the case of multiple particles adsorbing on a site, the particles, which in general are well-wet by the suspending fluids, are pulled into ordered structures in the late stages of fluid evaporation by the contraction of capillary bridges connecting them. Evaporation also provides a means of collecting particles near three-phase contact lines and so has been exploited as a means of particle self-assembly. Stebe et al. reported that ordered arrays of particles were created spontaneously by evaporative deposition of colloidal suspensions on surfaces of patterned wetting from parent drops with diameters large compared to the length scale of the underlying pattern (Figure 2-33). [111] Jonas et al. also reported 2D structured colloidal crystals can be obtained on chemically patterned surfaces by evaporative deposition of colloidal suspensions (Figure 2-34). [112] Furthermore, Zhang et al. demonstrated the fabrication of metallic photonic crystals, in the form of
a periodic array of gold nanowires on a waveguide, by spin coating a colloidal gold suspension onto a photoresist mask and subsequent annealing (Figure 2-35). This alternative method for fabricating metallic photonic crystals possessed advantages of simplicity, high speed, and low cost. [113] From recent report, the selective placement and alignment of individual SWCNTs could be also achieved on UV patterning surface (Figure 2-36). [114]
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). [115] 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. [115-117] 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. Lee et al. introduced a direct ultraviolet (UV)-assisted replica molding method for creating a biomimetic hierarchical structure and its use for selectively transforming the superhydrophobic surface to a superhydrophilically patterned surface on large area, regardless of the type of substrate (Figure 2-37). [118]
Figure 2-33. Colloidal particles assembled on 50 μm carboxylic acid terminated square patterned surfaces on a continuous methylterminated surface at pH=2, 24.5°C, 21% humidity. (a) An optical micrograph of 0.8 μm amidine functionalized microspheres deposited at 0.1% volume fraction. (b) SEM image. (c) An SEM image of 0.8 μm microspheres assembled on a surface patterned with alternating 5 μm carboxylic acid terminated stripes and 5 μm methyl terminated stripes at pH=2, 24.5
°C, 21% humidity, and 0.01% volume fraction. [111]
Figure 2-34. (a), (b) LVSEM image at higher magnification showing the stepwise increase of the thickness from the edge to the center of the stripe. (c) LVSEM image of the topmost layer showing the excellent crystal quality. (d) LVSEM image of the cross-section through a colloidal crystal stripe. [112]
Figure 2-35. (a) SEM Images of the gold photonic crystal structures, (b) AFM height image of the gold photonic crystal structures, (c) AFM phase image of the gold photonic crystal structures, (d) Measurements of contact angles of water on the ITO and on the PR surfaces: θ= 44-45°, θ= 73-75°. (e) Mechanisms for the confinement of the gold nanoparticles into the grating grooves when the PR channel is small enough.
(f) Two gaps will form for structures with large PR channel. [113]
Figure 2-36. (a) Illustration of creating the photopatterned monolayer, (b) illustration of the UV-induced reaction of SAM 6 and (c) AFM image of the photopatterned monolayer surface. (d) AFM image of the UV-patterned HD-UV-PA-modified surface after drop-casting SWCNTs form a H2O/methanol solution (3:1 volume). The higher resolution images on the left and right illustrate the high selectivity of the deposition. [114]
Figure 2-37. (a) Fabrication of a Biomimetic Dual-Scale Hierarchical Structure by Direct UV-Replica Molding with the Template and (b) Fabrication of Selectively Wetting Surface (c) SEM and AFM image of the template with dual-scale roughness (d) Selective wetting of water on the DUV-modified surface obtained with a SUS mask. [118]
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Chapter 3
Modification of Polymer Substrates with Low Surface Free Energy Material by Low-Temperature Curing
Polybenzoxazine
Abstract
The B-ala/AIBN PBZ system has a higher extent of the ring-opening of oxazine because phenol-containing oligomers are formed at the early stage of the curing process. As a result, the B-ala/AIBN PBZ system possesses a relatively stronger intramolecular hydrogen bonding and lower surface energy than the pure B-ala system at low temperature curing. In this context, Poly(4-vinyl pyridine), Poly(4-vinyl phenol) thin film and polycarbonate substrates which lack liquid resistance possess low surface free energy after modification with B-ala/AIBN = 5/1 PBZ.
3.1 Introduction
Low surface energy polymeric materials with good film-forming characteristics have attracted great interest because of their practical applications. [1] Consequently great attention has been placed on precise strategies modifying these solid surfaces. [2]
Most of the low surface energy polymeric materials that have been developed were based on flurorine- or silicon-containing polymers. Polybenzoxazine (PBZ), a new class of low surface energy material, has recently been developed displaying a strong intramolecular hydrogen bonding but extremely low surface free energy, even lower than pure Teflon. [3] However, the requirement of high-temperature curing (ca. 180 ~ 210°C) by PBZ limits its broader applications, especially for most polymer substrates.
A method of lower temperature curing for benzoxazine is thus urgently needed to broaden PBZ applications in temperature-sensitive substrates such as most polymeric materials. In this context, we discovered that Bis(3-allyl-3,4-dihydro-2H-1,3- benzoxazinyl)-isopropane (B-ala) can be cured at a relatively lower temperature (120
°C) with the aid of 2,2’-azobisisobutyronitrile (AIBN), resulting in even lower surface energy than that from the conventional method. Many polymer substrates such as polycarbonate, poly(4-vinyl pyridine), poly(4-vinyl phenol) and the like others can be coated by the modified polybenzoxazine in order to possess low surface energy.
3.2 Experiment Section 3.2.1 Materials
All the chemicals were used as received. Bisphenol A, paraformaldehyde (95%) and AIBN were supplied by the Showa Chemical Company of Japan.
2,2’-Bis(3-methyl-3,4-dihydro-2H-1,3- benzoxazinyl)propane (BA-m) was supplied by Shikoku Corporation. Finally, poly(4-vinylpyridine) (Mw = 60,000) was obtained from Aldrich of USA.The synthesis of B-ala was based on the reaction of bisphenol A with allylamine and paraformaldehyde according to the previously reported procedure. [4] Likewise poly(4-vinyl phenol) (Mw = 10,000; PDI = 1.4) was synthesized according to the previously reported method. [5]
3.2.2 Contact Angle Measurement
The surface free energy of the polymer sample was determined by contact angle goniometry at 25 °C using a Krüss GH-100 goniometer interfaced with image-capture software by injecting a 5 μL liquid drop. Deionized water, ethylene glycol (≥99%;
Aldrich), and diiodomethane (99%; Aldrich) were used as standards for measuring the surface free energies.
3.2.3 Thin-Film Formation and Polymerization
One-half grams of the B-ala monomer was pre-mixed with a certain mole ratio of AIBN in 10 mL tetrahydrofuran (THF) at room temperature. The solution was then filtered through a 0.2 μm syringe filter before spin coating onto a glass slide (100×100×1 mm3).
3.2.4 Polymer Thin Film Formation
Polymer solution was prepared by dissolving the polymer in ethanol at a concentration of 10 wt ％ . One mL of the appropriate polymer solution was spin-coated onto a glass slide using a photoresistant spinner operating at 1500 rpm for 45 s. The sample was then left to dry at 60 °C for 1 h to remove residual solvent. The 0.5 g B-ala monomer was pre-mixed with AIBN to produce a mole ratio of 5 in toluene(10ml) B-ala/AIBN at room temperature. Finally, the mixed solution was filtered through a 0.2 μm syringe filter before spin-coating onto the polymer thin film surface.
3.3 Results and Discussion
The 1H NMR spectrum of B-ala as shown in Figure 3-1 established that the structure of B-ala was recorded in deuterated chloroform (CDCl3) solution at 25° C by using a Varian UNITY INOVA-400 NMR spectrometer. The two multiples at 5.25 and 5.95 ppm were typical for the protons of ＝CH2 and ＝CH- in the allyl group, respectively. The protons of -CH2- of the allyl group showed a doublet at 3.36 ppm.
The characteristic protons of oxazine ring appeared at 3.92 and 4.82 ppm as assigned to -Ar-CH2-N- and -OCH2-N-, respectively, while the aromatic protons appeared as a multiplet at 6.77-7.0 ppm. Besides, 13C NMR spectrum and Mass spectrum of B-ala are shown in Figure 3-2 and 3-3 to provide evidences for the synthesis of B-ala monomer is successful.
The B-ala monomer contained the N-allyl group that can polymerize through free radical polymerization with the aid of a free radical initiator. The AIBN is known as
The B-ala monomer contained the N-allyl group that can polymerize through free radical polymerization with the aid of a free radical initiator. The AIBN is known as