Preparation of Tunable Wettability Polymer (Ionic Liquid) Brushes at Rough Substrate using Surface Initiated Atom Transfer Radical Polymerization

Através da técnica de polimerização radicalar por transferência atômica (ATRP) foram preparadas novos polímeros tipo pente com molhabilidade ajustável (líquido iônico) de hexafluorfosfato poli[2-(1-butilimidazolino-3-ila)etila metacrilato (PBIMH-PF6 ), a partir de um substrato bruto modificado com uma camada iniciadora composta de brometo 2-bromoisobutirila. Várias técnicas de caracterização como elipsometria, medidas de ângulo de contato estático, espectroscopia fotoeletrônica de raios X (XPS) e microscopia de força atômica (AFM) foram utilizadas para caracterizar os filmes em cada etapa de modificação da superfície. Estudos cinéticos revelaram um aumento linear na espessura do filme polimérico em função do tempo de reação, indicando que o crescimento da cadeia a partir da superfície, é um processo cujo controle apresenta características de polimerização “viva”. As medidas de molhabilidade indicaram que as interações de troca iônica podem ser usadas para alterar as características da superfície de hidrofilicidade para hidrofobicidade de forma reversível. O substrato bruto obtido por ataque químico exibiu uma melhor hidrofobicidade quando comparado à superfície lisa do substrato plano modificado com as mesmas cadeias poliméricas tipo pente (líquido iônico).


Introduction
Wettability is an important property of a solid surface depending on the surface free energy and surface roughness, and it is governed by both chemical composition and surface structure. [1][2][3][4][5] Recently, control of surface wettability has attracted extensive interests 6,7 because of its wide variety of applications, such as discrete liquid droplet manipulators, 8 self-clean surfaces, 9 tunable optical lenses, 10 drug delivery and biomimetic materials, 11,12 microfabrication, 13,14 and bioanalytical 15 and microfluidic devices 16 with tunable properties.
Among the surface modification methods, grafting surfaces with polymer brushes provides a versatile way to modify surface properties, [17][18][19][20] because of increasing the grafting density and immobilized tethering of polymer brushes on a solid substrate by performing an organic reaction with high efficiency 21 including grafting a polymer chain through a monomer covalently linked to the surface 22 and from a surface modified with polymerization initiators. [23][24][25] Surfaceinitiated atom transfer radical polymerization (ATRP) is one Vol. 22, No. 3, 2011 of the most efficient grafting processes [26][27][28][29] because it can be used to polymerize a wide range of vinyl monomers, does not require stringent conditions, and is tolerant of various functional groups and impurities that are detrimental to living anionic, cationic and some transition-metal-mediated polymerizations. 30 Facile tunability of physicochemical properties of ionic liquids (ILs), particularly imidazolium ions 31,32 bearing simple alkyl appendages as cations, has led to intense interest in ILs as an alternative of conventional organic solvents in a range of synthesis, catalysis, and electrochemistry. Recent developments include their uses for biopolymers, 33 chemical sensors for specific ions, 34 determination of acidity scale, 35 formation of nanosized metal particles 36 and hollow metal oxide microspheres 37 and so on. However, there has been little study on poly (ionic liquid) based materials. We are currently seeking to develop novel poly (ionic liquid) brushes modified substrate surface. The availability of potentially hydrophobic and hydrophilic ILs attests to the ease in the manipulation of their physical properties. 38,39 This paper aims at developing a novel and simple way to achieve tunable wettability switching surface by a chemical etching of the surface of silicon substrate and following modification with polymer (ionic liquid) via surface initiated ATRP. Reversible switching between hydrophilicity and hydrophobicity can be achieved by simply exchanging their counteranions. Such switchable surfaces have potential applications in functional materials and intelligent microfluidic switching.

Characterizations
The static water contact angles were determined by a KRUSS DSA100 contact angle goniometer. The thickness of polymer layer was measured with a spectroscopic ellipsometer (Gaertner model L116C). Morphological images of the substrate surfaces were obtained with an atomic force microscope (AFM) (Seiko Instruments Inc.) in a tapping mode.
The X-ray photoelectron spectroscopy (XPS) analysis was carried out on a PHI-5702 multifunctional XPS, using AlK α radiation as the exciting source at a measurement angle of 54°. The binding energies of the target elements were determined at a pass energy of 29.35 eV and a resolution of ± 0.2 eV, using the binding energy of C 1s at 284.6 eV as the reference. Hydrogen nuclear magnetic resonance ( 1 H NMR) spectra were measured with an INOVA 400 MHz spectrometer with DMSO-d 6 or D 2 O as solvent.

Silicon rough surface construction, hydroxylation and immobilization of the ATRP initiator
The smooth flat silicon was immerged into freshly prepared mixed acid solution (2.4 mol L -1 HF + 15.0 mol L -1 HNO 3 ) at room temperature for 8 min, then rinsed with plenty of distilled water and followed by boiling water for 10 min, then washed using distilled water, and dried under a stream of nitrogen.
Then the rough substrate was immersed in (0.1 mol L -1 ) NaOH aqueous solution for 10 min and subsequently in (0.1 mol L -1 ) HNO 3 for 10 min to generate surface hydroxyl groups. The rough substrate had been washed with an excess of distilled water and dried under a flow of nitrogen.
The hydroxylation rough substrate was put in anhydrous methanol solution which contained (100:5, v/v) APS for at least 10 h to obtain chemically bonded -NH 2 groups on the surface. The surface was rinsed with dichloromethane, dried under a flow of nitrogen gas. It was them immersed in dry dichloromethane contained pyridine (100:5, v/v), the polymerization initiator 2-bromoisobutyryl bromide was added dropwise at 0 °C, and the mixture was maintained for 1 h at this temperature with electromagnetic stirring, then kept for 12 h at room temperature. The initiator modified silicon substrate was cleaned with dry dichloromethane, and dried under a nitrogen flow. This procedure was carried out according to a modification method of Sun et al. 40 Above procedure is shown in Scheme 1.

Synthesis of the ionic liquid monomer
The synthesis of the ionic liquid monomer 2-(1-butylimidazolium-3-yl)-ethyl methacrylate hexafluorophosphate (BIMH)PF 6 was performed according With cuprous chloride as catalyst, the reaction temperature was maintained at the boiling point of the system for 70 min at a feed ratio of 2:9 (the ratio of methacrylic acid to phosphorus trichloride). Then obtained methacryloyl chloride and 2-chloroethanol reacts to achieve 2-chloroethyl methacrylate. The mixture of 2-chloroethyl methacrylate, 1-butyl imidazole and a little amount of 2,6-di-tert-butyl-4 methylphenol was stirred at 40 °C for 48 h to yield a very viscous liquid, 2-(1-butylimidazolium 3-yl)-ethyl methacrylate chloride. Then a suspension of NH 4 PF 6 in anhydrous acetonitrile was added and a white precipitate appeared. After the mixture was stirred for 48 h at room temperature, the sodium chloride precipitate was removed by filtration, and the filtrate was concentrated. The concentrated filtrate was diluted with methylene chloride (250 mL) and filtered through a silica gel column. By reduced pressure distillation methylene chloride was removed and the ionic liquid monomer 2-(1-butylimidazolium 3-yl)-ethyl methacrylate hexafluorophosphate was obtained as a viscous oil. 1

Characterization of functionalized surfaces
Solution etching method was used to construct appropriate surface roughness on silicon substrate.
Surface morphology of rough silicon substrate is shown in Figure 1. Figure 1 shows that some holes of 1-3 mm have been etched in the substrate surface.
In order to carry out surface-initiated ATRP, initiator molecules must be at first introduced onto the substrate surface. Figure 2 shows the typical XPS survey spectrum and high-resolution elemental scan of Br 3d and N 1s , which were recorded from the bromosilane initiator modified silicon surface. The C 1s core-level spectrum of the initiator modified surface showed a peak at BEs of 284.4 eV. The N 1s peak at the BEs of 398.8 eV was attributed to the O=C-N species and Br 3d peak at the BEs of 70.8 eV resulted from Br-C. These results proved that bromosilane initiator has been successfully anchored on the silicon surface.

Preparation and characterization of PBIMH-PF 6 brushes using a surface-initiated ATRP and tunning of the wettability by exchanging their counteranions
Copper chloride purified (0.092 mmol), 2,2-bipyridine purified (0.184 mmol), and BIMH-PF 6 -(9.2 mmol) were charged into a Bunsen flask with branch pipe, which was then tightly sealed and degassed. Degassed acetonitrile (1.6 mL) was added with a degassed syringe. The mixture was bubbled with nitrogen for 15 min, and then the beforehand prepared rough substrate modified with the surface initiator was added. The flask was immersed in an oil bath at 60 °C. At regular intervals, sample brushes were taken from the flask via a nitrogen-purged, and then were put into DMF to wash-up to remove physically absorbed polymer chains. After the polymer brushes were placed into a Soxhlet extractor to extract with DMF for 16 h, they were dried in a stream of nitrogen and kept in a refrigerator.
The tuning of wettability of poly (ionic liquid) brushes was carried out by exchanging their counteranions and Scheme 3 showed a typical procedure for counteranion exchanging.  Figure 4, whereas the peak of F 1s disappeared in survey spectrum, which showed hexafluorophosphate had completely been exchanged with chloride ion. The [Cl]:[N] ratio was about 0.9:2.0, which was consistent with theoretical values. Figure 5 and 6 are respectively AFM topographies of the rough silicon substrate surface and the smooth flat silicon substrate modified with PBIMH-PF 6 polymer brushes. Figure 5 showed that the polymer brushes had homogeneously been grafted on silicon substrate surface and a micro/nanometric composite structure had been constructed. Grafted layer thickness was about 200 nm which was nearly consistent with ellipsometric result (180 nm). Figure 6 showed that the film thickness was also about 200 nm and the polymer brushes arrayed densely and homogeneously on the flat substrate. Figure 7 shows typical water droplet shapes and corresponding contact angles (CA) on different substrates.   It was found that through counteranion exchange the surface wettability had successfully been reversed and the rough PBIMH-PF 6 brushes surface (R-Si-PBIMH-PF 6 ) had higher hydrophobicity than flat one (Si-PBIMH-PF 6 ). Figure 8 is the relationship of static water contact angle, reaction time and film thickness (nm). From Figure  8 it was obviously seen that CA of PBIMH-PF 6 brushes increased with the reaction time but when it exceeded 24 h CA maintained a constant on the whole. However, CA of PBIMH-Clbrushes did not changed nearly. It was also found that the surface wettability could be completely tuned reversibly only by exchanging counteranions. Figure 8 also showed that the thickness of the polymer (ionic liquid) brushes linearly increased with the reaction time, indicating that the polymerization reaction was a "controlled/active" procedure.

Conclusion
Well-defined poly (ionic liquid) brushes with tunable wettability were successfully produced on the surface of rough silicon wafer by surface initiated ATRP polymerization. The tunable wettability of poly (ionic liquid) brushes can be achieved by exchanging their counteranions. Moreover, if the same two counteranions are sequentially replaced, the surface with tunable wettability, reversible switching between hydrophilicity and hydrophobicity can be easily obtained.