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Temperature- and/or pH- responsive surfaces with controllable wettability: From parahydrophobicity to superhydrophilicity Melani A. Frysali, and Spiros H Anastasiadis Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b02098 • Publication Date (Web): 09 Aug 2017 Downloaded from http://pubs.acs.org on August 10, 2017

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Temperature- and/or pH- responsive surfaces with controllable wettability: From parahydrophobicity to superhydrophilicity

Melani A. Frysali†,‡ and Spiros H. Anastasiadis *,†,‡ †

Institute of Electronic Structure and Laser, Foundation for Research and Technology - Hellas, P.O. Box 1385, 711 10 Heraklion Crete, Greece

‡

Department of Chemistry, University of Crete, P. O. Box 2208, 710 03 Heraklion Crete, Greece

*

Corresponding author. E-mail: [email protected]

ACS Paragon Plus Environment

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ABSTRACT Multifunctional surfaces with reversible wetting characteristics are fabricated utilizing end-anchored polymer chains onto hierarchically roughened surfaces. Temperatureand/or pH-responsive surfaces are developed that exhibit reversible and controllable wettability, from the “parahydrophobic” behavior of natural plant leaves all the way to superhydrophilic properties in response to the external stimuli. For this purpose, dual scale micro/nano-roughened surfaces were prepared by laser irradiation of inorganic surfaces (Si wafers) utilizing ultrafast (femtosecond) laser pulses under a reactive gas atmosphere. End-functionalized polymer chains were anchored onto those surfaces utilizing the “grafting to” method; poly(N-isopropyl acrylamide), PNIPAM, and poly(2-vinyl pyridine), P2VP, were used for the formation of monofunctional as well as mixed brushes. The surfaces exhibit “parahydrophobic” behavior in the hydrophobic state (high temperature and/or high pH), with high static contact angles (~120°) and high water adhesion (~30° contact angle hysteresis), whereas they show superhydrophilic behavior in the hydrophilic state (low temperature and/or low pH). The surfaces were tested for their wettability under repetitive cycles and found to be stable and reproducible.

KEYWORDS: Responsive surfaces, hysteresis, hydrophobicity, hydrophilicity, grafting to, P2VP, PNIPAM, fs laser.


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INTRODUCTION The investigation of functional and superhydrophobic surfaces has raised the interest of the research community1 due to their multiple applications, including controllable drug delivery,2 microfluidic devices,3,4 sensing,5 antifogging materials6 and self-cleaning surfaces.7,8 Mimicking nature has been a central strategy in this field, since biological species have many extraordinary wetting properties. Insects, like Cicada orni and Rhinotermitidae, and plants, such as Nelumbo nucifera (the sacred Lotus), exhibit remarkable wetting characteristics.7 Barthlott and Neinhuis were the first to report the properties of the Lotus leaf, the symbol of purity, because of its selfcleaning nature.1,7,9,10 The water repellency and the self-cleaning properties of these biological species have been attributed to both their chemical composition and their surface topology.1 Research efforts have focused over the years on fabricating surfaces, which can replicate the behavior of the Lotus leaf.1,11,12 For example, Zorba, et al. produced dual-scale roughened surfaces, by ultrafast femtosecond laser micro/nanostructuring, which exhibited superhydrophobic and water repellent properties.7,13,14 Nowadays the main focus of the research community is on the fabrication of responsive surfaces with controllable wettability. Such surfaces can alter their wetting properties in response to external stimuli, like pH, light, solvent quality, electric field, heating, magnetic field, etc.15–26 A number of smart material surfaces have been reported, which can switch between (super)hydrophobicity and (super)hydrophilicity in response to an appropriate external stimulus, with promising applications as switchable valves,27 biosensors,19,28 selective separation,29,30 tunable microlenses31 and “lab-on-a chip” devices.32,33 Among them, the wetting properties of responsive polymers have been widely studied.34,35 Polymer-functionalized pH-responsive surfaces based on poly(2-(diisopropyl-amino)ethyl methacrylate), PDPAEMA, brushes


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4 synthesized onto hierarchically roughened inorganic surfaces have been previously developed, which can reversibly switch between superhydrophilicity at low pH and superhydrophobicity and water repellency at pH>8.32 A dual-responsive material with tunable wettability and ability to respond to both temperature and pH changes, based on poly(N-isopropyl acrylamide)-co-poly(acrylic acid), P(NIPAAm-co-AAc), films, was investigated by Xia, et al.15 Despite the wide range of investigations reported, the fabrication of stimuli-responsive surfaces is still a challenging issue; the magnitude of the wettability changes, the relevant time scales and, most importantly, the stability and reproducibility of the switching behavior for extended cycles of the external stimuli are still under scrutiny in the scientific literature. The investigation of the wettability of plant surfaces revealed another interesting phenomenon, the so-called rose-petal effect.12 Unlike the Lotus leaf, where superhydrophobicity is accompanied by water repellency, i.e., by very low water adhesion (expressed as very low contact angle hysteresis), certain rose petals, scallions and garlic exhibit superhydrophobic behavior with high water adhesion (expressed as high contact angle hysteresis).36,37 Although the origin of high values of contact angle hysteresis has not been fully understood, it appears that the main factors affecting the water surface adhesion are the height and density of the morphological protrusions of the biological surfaces as well as their chemical defects.36–38 There is a number of investigations in the literature that attempt to imitate the “adhesive” superhydrophobic behavior of the latter species.39–42 Besides the “sticky” superhydrophobic species, another remarkable category of plant surfaces are the so-called “parahydrophobic” plants, which exhibit high contact angles, but much less than 150°, with high water adhesion. An important family among this kind of species are most of the thermogenic plants, which can raise their


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5 temperature above the ambient,36 thus, attracting insect pollinators to them. Skunk cabbage is one of the most studied thermogenic plant, known for its ability to melt its way through frozen ground, that flowers while there is still snow and ice on the ground. The abaxial surface of the leaf has hierarchical roughness with an advancing angle of ~110° and a contact angle hysteresis of ~27°. The high adhesion in this case is attributed to hydrophobic defects, due to diallyl disulfide, on the leaf surfaces.36 Plants with “parahydrophobicity” or, else, “adhesive hydrophobicity”, usually exist in regions with dry conditions as a way to minimize the water losses.43 Other plants that belong to this category are the banana leaf, the red-veined prayer plant44, the oak leaves45, the ceratonia siliqua43 and many others.46,47 The term “parahydrophobicity” has been recently proposed by Marmur48 to describe this particular behavior and unify the various terms used before. Various attempts have been made to design and develop such parahydrophobic surfaces with contact angles ~120°-150° and high water adhesion by utilizing electropolymerization of appropriately modified monomers to form nanofibers,49,50 by electrodepositing appropriately modified conducting polymers by a cyclic voltammetry process51 or by taking advantage of the surface roughness induced by spontaneous phase separation during photopolymerization;52 the behavior of naturally occurring parahydrophobic surfaces and the design and properties of such synthetic surfaces have been recently reviewed.53 Adhesive hydrophobic materials can find applications as barrier materials, micromanipulators and oil-water separators.54 In this work, we aim at developing surfaces inspired by nature with reversible behavior all the way from “parahydrophobic” to superhydrophilic. For this purpose, poly(2-vinyl pyridine), P2VP, and poly(N-isopropyl acrylamide), PNIPAm, were selected for the development of responsive surfaces. P2VP is a pH responsive polymer with an effective pKa of the protonated P2VP ~5,55 while PNIPAm is a


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6 thermoresponsive one that exhibits a lower critical solution temperature (LCST) of ~32°C.56,57 Silicon wafers were initially treated with femtosecond (fs) laser pulses in order to produce the hierarchically micro/nano-structured surfaces, the so-called Si spikes.7,14,32 Organic coatings, P2VP or PNIPAm, were subsequently introduced by anchoring end-functionalized polymer chains utilizing the “grafting to” method. The main drawback of this method is that the anchoring density of the polymer chains is determined by the polymer molecular weight and the strength of the end-group – surface interactions; thus, the anchoring density cannot be independently varied and is usually not very high as the attached polymer chains prevent the anchoring of even more chains.58 However, in this work, this apparent disadvantage turns into an advantage as the low density of anchored chains may allow interaction of water with a not perfectly homogenous surface that can essentially act as chemical nano-defects, which resulted to high water contact angle hysteresis and, thus, high water adhesion. Moreover, we combined the responsive behavior of the two polymers, P2VP and PNIPAm, in one surface. So we achieved to develop a dual pH- and thermo-responsive surface, with high water adhesion in the hydrophobic state and superhydrophilic behavior in the hydrophilic state, utilizing a mixed P2VP/PNIPAm brush. To the best of our knowledge this is the first work referring to pH/thermo-responsive surfaces with “parahydrophobic” behavior with high water adhesion on hierarchically roughened surfaces.

EXPERIMENTAL SECTION Materials. Carboxy-terminated poly(2-vinyl pyridine), P2VP (Mn = 10,000 g⋅mol-1, Mw/Mn = 1.08; Mn = 53,000 g⋅mol-1, Mw/Mn = 1.06), hydroxy-terminated P2VP (Mn =


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7 80,500 g⋅mol-1, Mw/Mn = 1.17; 172,000 g⋅mol-1, Mw/Mn = 2.3) and carboxy-terminated poly(N-isopropyl acrylamide), PNIPAm (Mn = 30,000, Mw/Mn = 1.25; Mn = 43,000, Mw/Mn = 1.25; Mn = 80,000, Mw/Mn = 1.2; Mn = 130,000 g⋅mol-1, Mw/Mn = 1.3), were purchased from Polymer Source, Inc. Poly(glycidyl methacrylate) (PGMA) with Mn = 20,000 g⋅mol-1 was purchased from Sigma-Aldrich. Chloroform, hydrogen peroxide, sulfuric acid, hydrochloric acid, sodium hydroxide as well as highly polished silicon wafers were purchased from Sigma-Aldrich. Surface Modification with fs laser. Single crystal n-type Si(100) wafers with a resistivity of ρ = 2-8 Ω·cm were utilized as substrates for the fabrication of silicon spikes. Micro/nano-structuring of Si surfaces was performed by femtosecond (fs) laser irradiation under a reactive gas (SF6) atmosphere at a pressure 500 Torr. The irradiation source (Yb:KGW) with wavelength of 1026 nm and pulse duration of 170 fs was adjusted at a repetition rate of 1 kHz. The laser pulse fluences tested were 0.53, 1.40 and 2.10 J⋅cm-2 whereas the fluence of 1.40 J⋅cm-2 was selected as the optimum for this work. The samples were kept in a chamber, mounted on a high precision X-Y translational stage, perpendicular to the incident laser beam.13,32 End-Anchored PNIPAm and P2VP Thin Films. The laser irradiated micro/nanostructured silicon substrates (as well as the flat ones) were first cleaned with chloroform in an ultrasonic bath for 30 min and then placed in a piranha solution (sulfuric acid and hydrogen peroxide, 3:1) for 1h. The samples were, then, rinsed with MilliQ water and dried under a nitrogen flow.21,59,60 Subsequently, the samples were heated at 1000°C for 1h at a heating rate of 25°C/min in order to achieve the formation of a native oxide (SiO2) onto their surfaces. After cooling, a 0.002% w/v solution of PGMA in chloroform was spin-coated onto the substrates and the samples were heated for 10 min at 120°C. The thin film of PGMA was utilized to enable the formation of robust


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8 chemical bonds between the oxidized silicon substrate and the carboxy- or hydroxyterminated polymer chains, due to the very active epoxy groups of PGMA.61–63 Carboxy-terminated P2VP and PNIPAm, dissolved in chloroform (2% w/v), were spincoated onto the PGMA-covered substrates and the coated surfaces were annealed under vacuum at 150°C for 24 h in order to form the layer of end-grafted polymer chains. To remove the ungrafted polymer chains, the samples were ultrasonicated in chloroform for 30 min and thoroughly rinsed in chloroform. The same procedure was repeated for hydroxy-terminated polymers of different molecular weights. Furthermore, flat silicon wafers were also coated with polymers utilizing exactly the same procedure to be used as reference materials. In order to combine the pH- and thermo-responsiveness of P2VP and PNIPAm, respectively, onto one surface, a 50/50 solution of carboxy-terminated P2VP and carboxy-terminated PNIPAM in chloroform was prepared. The solution of mixed polymers was subsequently spin-coated onto the laser micro/nano-structured silicon substrates, cleaned and functionalized with PGMA. The same procedure was repeated with 30/70 and 70/30 solutions of carboxy-terminated P2VP/PNIPAm for comparison. It should be noted that utilizing a solution with a particular composition of chains (e.g., 50/50) does not necessarily provide a 50/50 composition of grafted chains on the surface. Surface characterization. Silicon substrates, irradiated at 1.40 J⋅cm-2 laser fluence, were characterized by field emission scanning electron microscope (FE-SEM, JEOL JSM-7000F) before and after the polymer anchoring. The thickness, h, of the polymer films prepared on flat silicon substrates was estimated by variable angle spectral ellipsometry (VASE);64,65 The reflectance data for various wavelengths and angles are analyzed to extract the film thicknesses and the refractive indices of the layers; the


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9 refractive indices are extracted in the form of the coefficients of the Cauchy relationship for various wavelengths, n = A + (B / λ2) + (C / λ4), where A, B and C are the Cauchy coefficients and λ the wavelength. The layer thickness was utilized to determine the anchoring density of the chains, σ, using the equation h = σ N υ, where h the thickness, N the number of segments of the chain and υ the segmental volume; this leads to the equation h = σ Mn / (ρ NAVOG), where Mn is the chain molecular weight, ρ the polymer density and NAVOG the Avogadro’s number. It is noted that this is only a rough estimate of the anchoring density on the micro/nano-structured surfaces, which cannot be measured by ellipsometry due to the high roughness. Attenuated total reflection Fourier transform infrared (ATR-FTIR) spectra were recorded to investigate the anchoring of the polymer chains onto the flat silicon surfaces; spectra were measured on flat silicon wafers because the high roughness of the micro/nano-structured silicon surfaces does not allow a valid measurement. A Bruker Vertex 70v FTIR spectrometer equipped with an A225/Q Platinum attenuated total reflection (ATR) integrating sphere with single reflection diamond crystal was employed. Wettability. Wettability tests were performed using a surface tensiometer (OCA-40, Dataphysics) utilizing the sessile drop method.66 Static equilibrium contact angle measurements of P2VP-coated samples were performed after immersion of the surfaces in solutions of different pH and PNIPAm-coated samples after heating at different temperatures. The solutions of different pH were prepared by dissolving HCl acid or NaOH base in distilled water and the pH was measured by a pH meter (Crison, GLP21). Water droplets of 2 μL were deposited onto the samples in every case and digital images of the water droplets were taken. The drop profiles were analyzed to determine the equilibrium contact angles; equilibration is considered when the obtained contact angles do not change any more with time. Contact angle titrations were repeated five


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10 times for each sample. The contact angle hysteresis was defined as the difference between the advancing and receding contact angles for a contact line moving to an opposite direction measured with the same tensiometer in the dynamic mode.67,68

RESULTS AND DISCUSSION Surface characterization. Figure 1 depicts the SEM micrographs of the Si substrates, irradiated at a laser fluence of 1.40 J⋅cm-2, before and after the coating with the endanchored carboxy-terminated P2VP (Mn = 53,000 g⋅mol-1) chains. A two-scale hierarchical morphology is clearly evident consisting of micron-scale features, consisting of conical or pyramidal asperities with average sizes of ~5-10 µm and aspect ratio of ~4, decorated with nano-scale protrusions on the scale of a few hundred nanometers.13,14,69 Comparison of the two samples shows that the procedure for the formation of the polymer brushes did not affect the hierarchical micro- and nanostructure of the artificial surface. Ellipsometry showed that the thickness of the respective P2VP film onto a flat Si substrate was 8±2 nm, which allowed the estimation of the anchoring density as σ = 0.10 chains⋅nm-2. It should be noted that the thickness of the polymer brush on the flat surfaces is very thin and, thus, the brush does not mask the micro/nano-structuring of the surfaces. Similar results are obtained for the surfaces following the anchoring of carboxy-terminated PNIPAm (Mn = 43,000 g⋅mol-1) chains, for which thickness of the PNIPAm film was 7.7±2 nm and the anchoring density was found to be σ = 0.12 chains⋅nm-2. For comparison, the surface characterization procedure was repeated for samples irradiated at fluences of 0.53 and 2.10 J⋅cm-2 (Supporting Information, Figure S1).


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Figure 1. SEM micrographs of micro/nano-structured Si substrate by fs-laser irradiation at a fluence of 1.40 J⋅cm-2 under reactive SF6 atmosphere before (left) and after (right) coating with end-anchored carboxy-terminated P2VP chains (Mn = 53,000 g⋅mol-1).

Figure 2 shows the FTIR-ATR spectra of the respective flat silicon substrates coated with end-anchored carboxy-terminated P2VP (Mn = 53,000 g⋅mol-1) and carboxy-terminated PNIPAM (Mn = 43,000 g⋅mol-1) chains as well as with the nominally 50/50 mixed layer of carboxy-terminated P2VP and PNIPAm chains. The characteristic absorption bands of P2VP due to pyridine group are evident at 1475, 1570 and 1590 cm-1 in the spectra for the P2VP brush, while the amide bands of PNIPAm at 1530 and 1645 cm-1 are evident in the spectra for the PNIPAm brush. In the case of the mixed brush, the characteristic peaks of both the amide and pyridine groups were identified,70 which proves the successful anchoring of both polymers. Comparing the spectra of the mono and mixed brushes, a shift of the amide band was observed from 1530 to 1550 cm-1, probably due to interactions between the two polymers.70 It should be noted that quantitative analysis of the presented FTIR-ATR data for the mixed brush system in order to extract the exact composition of chains that were actually grafted on the micro/nano-structured silicon wafer is not attempted because of the uncertainty and arbitrarity of the FTIR-ATR measurements especially for so thin films on silicon


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12 wafers. The FTIR-ATR spectra of Fig. 2 indeed provide evidence on the formation of the monofunctional and mixed polymer brushes on the silicon surface; this is in agreement with the traditional practice of earlier investigations in the literature.71,72 Thus, one is led to use the nominal compositions used in the preparation of the grafted layer to indicate the particular brush. XPS measurements are planned for the future studies in order to measure the exact composition on the surface.

Absorbance (arbitrary units)

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1700

50/50 P2VP/PNIPAm P2VP PNIPAm

P2VP

P2VP/PNIPAm

PNIPAm

1650

1600

1550

1500

1450

1400

-1

Wavenumber (cm ) Figure 2. ATR-FTIR spectra of the end-grafted polymers coated onto flat Si surfaces. The spectra are shown for the coatings of carboxy-terminated P2VP (Mn = 53,000 g⋅mol-1), carboxy-terminated PNIPAm (Mn = 43,000 g⋅mol-1) and the nominally 50/50 mixed layer of carboxy-terminated P2VP/PNIPAm chains.

Wettability of the P2VP-coated surfaces. P2VP brushes exhibit switching properties upon exposure to solutions of different pH. For low pH, the chains exhibit a hydrophilic behavior due to protonation of the pyridine nitrogen, while, for high pH, the chains turn back to hydrophobic. Carboxy-terminated P2VP (Mn = 53,000 g⋅mol-1) coated samples


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13 were immersed for 20 min in aqueous solution of pH 2 and pH 9 and the contact angles were measured. The results are illustrated in Figure 3.

Figure 3. Photographs of representative water droplets and the respective equilibrium static contact angles on (a) flat silicon and (b) laser micro/nanostructured silicon substrates (at 1.40 J⋅cm-2 fluence) coated with carboxy-terminated P2VP chains (Mn = 53,000 g⋅mol-1), for σ = 0.10 chains⋅nm-2, following immersion in solutions of high pH (pH 9, left column) and low pH (pH 2, right column), respectively.

The equilibrium static contact angle of water on the coated flat silicon substrate was measured at ~83° after immersion in a pH 9 solution and ~51° following immersion in a pH 2 solution. In the case of the micro/nano-structured surfaces, the difference between the contact angle values following immersion in pH 2 and pH 9 solutions was significantly higher (equilibrium static contact angles of 11° and 121°, respectively) due to the effects of the hierarchical roughness of the surfaces. The irradiated silicon sample coated with P2VP was found to be almost superhydrophilic at low pH and hydrophobic at pH 9 with a contact angle hysteresis of ~27° (138° advancing and 107° receding contact angles), reproducing the behavior of parahydrophobic surfaces like that of the skunk cabbage leaf.36 It is worth mentioning that silicon substrates, irradiated


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14 under identical conditions and coated with dichlorodimethylsilane (a hydrophobic silane), were found to be superhydrophobic, with contact angles of ~160°, with a very low contact angle hysteresis (Supporting Information, Figure S2). Once more, this proves that the hierarchical roughness requires the appropriate chemistry in order to lead to superhyphobicity or/and superhydrophilicity. The reproducibility and stability of the systems were investigated by subjecting the samples to cycles of successive immersion in solutions of high and low pH and investigation of their wettability behavior (Figure 4). Both systems, the flat silicon and the hierarchical Si surface, both coated with P2VP end-anchored chains, proved to be reproducibly and stably responsive to the changes of the pH of the environment, with the effects being consistently more amplified for the P2VP brushes onto micro/nanostructured substrates.

P2VP on spikes P2VP on flat

140 120

Contact angle (°)

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100 80 60 40 20 0

2

9

2

9

2

9

2

pH variation

9

2

9

Figure 4. Equilibrium static contact angle values of water droplets onto flat Si substrate (circles) and micro/nano-structured artificial Si surface (squares), irradiated at 1.40 J⋅cm-2 fluence, coated with carboxy-terminated P2VP chains (Mn = 53,000 g⋅mol-1), for 5 cycles of pH environment changes.


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Wettability tests were also performed on P2VP-coated substrates, which were micro/nano-structured under different fluences of the fs laser irradiation. For high fluence (2.10 J⋅cm-2), there was no difference in the results to those reported above, while, for a lower fluence (0.53 J⋅cm-2), the results were reproducible but with a much lower contact angle value variation (Supporting Information, Figure S3). The same procedure was repeated for P2VP brushes formed by utilizing P2VP chains of different molecular weights. In the case of carboxy-terminated P2VP with Mn = 10,000 g⋅mol-1, there was no responsive behavior observed. This may be related to a not-sufficient charging of the P2VP chains at low pH, especially since it has been reported that the molecular weight affects the charge density per molecule of a polyelectrolyte with a polymer with lower molecular weight showing a lower charge density.73 In contrast, responsive behavior was detected for the laser irradiated silicon substrates functionalized with hydroxy-terminated P2VP of Mn = 80,500 and 172,000 g⋅mol-1. In both cases, the equilibrium static contact angle values were fluctuating between ~110° (with a hysteresis of ~28°) following immersion in a pH 9 solution to ~20° following immersion in a pH 2 one, (Supporting Information, Figures S4-S5). The small difference of ~10°, compared to the lower molecular weight, is probably attributed to the lower mobility of the higher molecular weight P2VP chains.74 Wettability of the PNIPAm-coated surfaces. At temperatures below the lower critical solution temperature, LCST, of the PNIPAm solutions, the PNIPAm brushes are arranged into a swollen and hydrated conformation, while at temperatures above 32°C, the chains are hydrophobic and, thus, collapsed.75,76 Carboxy-terminated PNIPAm chains (Mn = 43,000 g⋅mol-1) were end-grafted onto the silicon surfaces according to


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16 the procedure described in the experimental part. The wettability of the PNIPAm coated samples was investigated at ambient temperature (25°C) and at 40°C. The heating of the samples was performed with a heating rate of 0.2°C/min under nitrogen flow and the samples remained at 40°C for 4h before the first measurement. It has been found that humidity affects the characteristic properties of the polymer and its water absorption dynamics;57 therefore, nitrogen flow was utilized, which played an important role for the reproducibility of the experimental conditions during the contact angle measurements and, thus, of the results. Note that exposure to different humidity conditions has been utilized before as a stimulus used to modify the surface behavior of polymer films, as well.77 Figure 5 shows images of water droplets on flat and laser micro/nano-structured silicon substrates, coated with end-anchored carboxy-terminated PNIPAm chains (Mn = 43,000 g⋅mol-1), following temperature treatments at 25°C and 40°C.

(a)

68o

51o

T ~ 25oC

(b)

T ~ 40oC 123o

16o

Figure 5. Photographs of representative water droplets and the respective equilibrium static contact angles on (a) flat silicon and (b) laser micro/nanostructured silicon substrates (at 1.40 J⋅cm-2 fluence) coated with carboxy-terminated PNIPAm chains (Mn = 43,000 g⋅mol-1), for σ = 0.10 chains⋅nm-2, following temperature treatments at 40°C (left column) and 25°C (right column), respectively.


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In the case of the laser irradiated silicon substrates, the equilibrium static contact angles were measured as 16° (at 25°C) and 123° (at 40°C), behavior analogous to that of the pH-responsive surfaces described above. A significant contact angle hysteresis of approximately 29° was measured for the hydrophobic state (135° advancing and 106° receding contact angles), reproducing again the behavior of parahydrophobic plant surfaces. Flat silicon substrates, similarly coated with the same PNIPAm chains, were also measured as reference and the contact angles were measured to change from ~51° (at 25°C) to ~68° (at 40°C); these contact angles on flat surfaces are in agreement with those reported in the literature.78,79 Comparison of the flat and the laser irradiated surfaces illustrates the influence of the laser micro/nano-structuring of the substrates on the amplification of the effects of surface chemistry. The stability of the systems was investigated through ten repeated cycles of contact angle measurements after heating and cooling of the specimens; Figure 6 shows the results for 5 cycles. In parallel, surfaces were prepared with end-anchoring of PNIPAm chains of different molecular weights. According to the literature, low-molecular weight PNIPAm does not collapse above the LCST.76,78 Indeed, when the same procedure was repeated for carboxy-terminated PNIPAm chains of lower molecular weight (Mn = 30,000 g⋅mol-1) end-anchored onto irradiated silicon substrates at a fluence of 1.40 J⋅cm-2, the results were slightly different, with the contact angle of only ~85° at 40°C. In contrast, when PNIPAm chains with higher molecular weight of Mn = 80,000 g⋅mol-1 and 130,000 g⋅mol-1 were end-anchored onto the substrates, the results were almost the same with the data in Figures 5-6. The systems were stable and


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18 reproducible with the static contact angles fluctuating from ~10° (at 25°C) to ~120° (at 40°C) (Supporting Information, Figures S6-S7).

140

PNIPAM on spikes PNIPAM on flat

120

Contact angle (°)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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100 80 60 40 20 0

40 25 40 25 40 25 40 25 40 25

Temperature Variation (°C) Figure 6. Equilibrium static contact angle values of water droplets onto flat Si substrates (circles) and micro/nano-structured artificial Si surface (squares) (irradiated at a fluence of 1.40 J⋅cm-2) coated with carboxy-terminated PNIPAm chains (Mn = 43,000 g⋅mol-1), for 5 cycles of temperature changes.

Wettability of the surfaces coated with mixed P2VP/PNIPAm brushes. In order to achieve a multi-responsive system and to combine the pH- and thermo-responsiveness on one surface, samples were fabricated with mixed brushes of carboxy-terminated P2VP (Mn= 53,000 g⋅mol-1) and PNIPAm (Mn = 43,000 g⋅mol-1) chains. For this purpose, chloroform solutions of the two polymers with composition ratios 30/70, 50/50 and 70/30 were prepared. These solutions were spin-coated onto the laser irradiated micro/nano-structured silicon substrates, which were modified by the PGMA layer, according to the procedure described in the Experimental section. The samples were tested for their wettability in response to pH and temperature variations.


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19 Table 1 shows the results from the contact angle measurements after immersion in solutions at pH 2 and pH 9 for temperatures 25°C and 40°C for the three different mixed brushes. At 25°C and pH 2, the PNIPAm chains are hydrophilic and the P2VP ones are positively charged; the surfaces are, thus, superhydrophilic independently of the polymer composition. In the opposite scenario, at 40°C and pH 9, when both polymers are in their hydrophobic state, high hydrophobicity is expected and observed in all cases. For 25°C and pH 9, the PNIPAm chains are hydrophilic whereas the P2VP ones are hydrophobic. So, the contact angle values are expected to increase gradually with increasing the ratio of the P2VP chains in the brush layer; indeed, the contact angle values increase from 64°, when the P2VP/PNIPAm ratio is 30/70, to 95°, when the ratio is 50/50, and to 110°, when the P2VP/PNIPAm ratio is 70/30. The opposite behavior is observed for temperature 40°C and pH 2, where the P2VP chains are charged and, thus, hydrophilic, whereas the PNIPAm chains are hydrophobic, with the contact angles being higher for high ratios of PNIPAm brushes (82°, when the P2VP/PNIPAm ratio is 30/70, and 115-112°, when the ratio is 50/50 and 70/30).

Table 1. Average contact angle values of water droplets onto micro/nano-structured silicon substrates coated with the mixed P2VP/PNIPAm brushes at three different P2VP/PNIPAm ratios: 30/70, 50/50 and 70/30. Contact angle values (°) Temperature (°C)

25

40

pH

P2VP/PNIPAm:

P2VP/PNIPAm:

P2VP/PNIPAm:

30/70

50/50

70/30

2

17

20

14

9

64

95

110

2

82

115

112

9

127

135

125


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20 It should be mentioned here that when one compares the results from the samples coated with mono-functional brushes with the ones coated with the mixed ones, one notices that the surfaces coated with mixed brush are more hydrophobic, probably due to interactions between the two kinds of chains.61 On contrast, no significant difference was observed in the estimation of the hysteresis values. Figure 7 illustrates the effects of pH and temperature on the wettability of the dually responsive surfaces, where photographs of representative water droplets are shown during the measurements of the contact angles after immersion in solutions of for varying pH and heating at varying temperatures for the sample with a mixed 50/50 P2VP/PNIPAm brush.

Figure 7. Photographs of representative water droplets and respective contact angle values for a hierarchically roughened silicon substrate coated with a mixed 50/50 P2VP/PNIPAm brush exposed to immersion to solutions of different pH and heating to different temperatures.


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21 At temperatures below 30°C, the contact angle values increase from ~5° to ~90° by increasing the solution pH from 2 to 10. However, for the case of a constant pH 2 and by increasing temperature from 20°C to 50°C, the contact angle value increase from ~5° to ~123°. Apparently, the hydrophobicity of the PNIPAm chains at higher temperatures dominate in the competition between the two polymers and, as a result, the temperature effect is more pronounced. At 30°C, which is close to the LCST of PNIPAm, the contact angle values measured by increasing the solution pH changed from ~20° to ~125°, with a rapid increase at pH 4 (~95°), which is close to the pKa of P2VP (in bulk solutions). At temperatures above the LCST of PNIPAm, there is no obvious change in the wettability results, which also supports the hypothesis that the influence of the PNIPAm chains is stronger in comparison to that of the P2VP ones.

140

Contact angle (°)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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50/50 P2VP/PNIPAM brush on spikes

120 100 80 60 40 20 0

a

b

a

b

a

b

a

b

a

b

cycles Figure 8. Equilibrium static contact angle values of water droplets onto micro/nanostructured artificial Si surfaces (irradiated at a fluence of 1.40 J⋅cm-2) coated with a mixed 50/50 P2VP/PNIPAm brush, for 5 cycles of pH / temperature variations. State a: immersion in a solution of pH 2 and exposure to 25°C. State b: immersion in a solution of pH 10 and exposure to 50°C.


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22 The dually responsive surfaces were subjected to pH/thermal cycles in order to test their stability. Figure 8 shows the results following 5 cycles of immersion in a pH 2 solution and exposure to 25 °C vs. immersion in a pH 10 solution and exposure to 50°C. The samples with the P2VP/PNIPAm mixed brushes proved to be stable with controllable and reproducible wettability changes. The average contact angle value were measured as ~15°, when the samples were immersed in solutions with pH 2 at 25°C, i.e., in the hydrophilic state, and ~120° when the samples were immersed in solutions with pH 10 at 50°C, i.e., when both polymers are hydrophobic and collapsed.

CONCLUSIONS In summary, hierarchically roughened surfaces micro/nano-structured utilizing fs laser irradiation of silicon substrates were fabricated and were functionalized using endanchoring of end-functionalized P2VP or PNIPAm chains utilizing the “grafting to” method. The surfaces exhibited reversible and controllable wetting characteristics: “parahydrophobic” behavior, when the polymer chains are in their hydrophobic state with the surfaces exhibiting contact angles ~120-130°C with high contact angle hysteresis of ~30°C imitating the “parahydrophobicity” of certain plant leaves, and superhydrophilic behavior when the chains are in their hydrophobic state. Such values of contact angle hysteresis correspond to highly water-adhesive surfaces. The effect of molecular weight of the polymers on the wettability was investigated as did the type of the end group (carboxy- vs. hydroxy-termination). Furthermore, multi-responsive surfaces were developed utilizing a mixture of P2VP/PNIPAm chains “grafted to” the hierarchically roughened surfaces. This kind of surfaces can be successfully applied in


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23 industry as switchable valves, biosensors, tunable microlenses and for selective liquid separation.

ACKNOWLEDGEMENTS This research was partially supported by the European Union (European Social Fund, ESF) and Greek national funds through the "ARISTEIA II" Action (SMART_SURF) of the Operational Programme “Education and Lifelong Learning”, NSRF 2007-2013, via the General Secretariat for Research & Technology, Ministry of Education and Religious Affairs, Greece. This work was also supported by the European Research Infrastructure LASERLAB EUROPE (Grant Agreement No. 228334) and was partially funded in the framework of the Hellenic Republic – Siemens settlement agreement. We would like to acknowledge the interaction with Dr. Emmanuel Stratakis on the surface micro/nano-structuring. We want to thank Mrs. Aleka Manousaki for her assistance with the SEM measurements and Dr. George Kenanakis for the ATR-FTIR measurements.

SUPPORTING INFORMATION SEM images of micro/nano-structured surfaces, images of water droplets and water contact angles for different molecular weights cycles of environmental changes.


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Temperature- and/or pH- responsive surfaces with controllable wettability: From parahydrophobicity to superhydrophilicity

Melani A. Frysali and Spiros H. Anastasiadis

parahydrophobic to superhydrophilic surfaces


or pH-Responsive Surfaces with ... - ACS Publications

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