Reversible Adhesion Switching of Porous Fibrillar ... - ACS Publications

Letter pubs.acs.org/NanoLett

Reversible Adhesion Switching of Porous Fibrillar Adhesive Pads by Humidity Longjian Xue,*,†,‡ Alexander Kovalev,‡ Kirstin Dening,‡ Anna Eichler-Volf,† Henning Eickmeier,† Markus Haase,† Dirk Enke,§ Martin Steinhart,*,† and Stanislav N. Gorb*,‡ †

Institut für Chemie neuer Materialien, Universität Osnabrück, Barbarastrasse 7, 49069 Osnabrück, Germany Functional Morphology and Biomechanics, Zoological Institute, Kiel University, Am Botanischen Garten 1-9, 24098 Kiel, Germany § Institut für Technische Chemie, Universität Leipzig, Linnéstraße 3, 04103 Leipzig, Germany ‡

S Supporting Information *

ABSTRACT: We report reversible adhesion switching on porous fibrillar polystyrene-block-poly(2-vinyl pyridine) (PS-b-P2VP) adhesive pads by humidity changes. Adhesion at a relative humidity of 90% was more than nine times higher than at a relative humidity of 2%. On nonporous fibrillar adhesive pads of the same material, adhesion increased only by a factor of ∼3.3. The switching performance remained unchanged in at least 10 successive high/low humidity cycles. Main origin of enhanced adhesion at high humidity is the humidity-induced decrease in the elastic modulus of the polar component P2VP rather than capillary force. The presence of spongelike continuous internal pore systems with walls consisting of P2VP significantly leveraged this effect. Fibrillar adhesive pads on which adhesion is switchable by humidity changes may be used for preconcentration of airborne particulates, pollutants, and germs combined with triggered surface cleaning. KEYWORDS: Block copolymers, nanorods, adhesion, biomimetics, switching, humidity Here we show that adhesion on porous fibrillar adhesive pads consisting of the block copolymer polystyrene-b-poly(2-vinyl pyridine) (PS-b-P2VP) can be reversibly switched by humidity changes. For example, airborne particulates and germs might be preconcentrated on the fibrillar adhesive pads under humid conditions (high adhesion state) and released under dry conditions (low adhesion state). Thus, fibrillar adhesive pads with adhesion switchable by humidity changes could be used as self-cleaning components in filters and separation systems. As discussed below, the presence of spongelike, continuous pore systems resulted in significant reduction in the elastic modulus of porous fibrillar adhesive pads as compared to solid fibrillar adhesive pads and leveraged increases in adhesion associated with increases in humidity. To prepare the fibrillar adhesive pads, we infiltrated cylinderforming PS-b-P2VP (Mn(PS) = 101 000 g/mol; Mn(P2VP) = 29 000 g/mol; Mw/Mn = 1.60) with a P2VP volume fraction of 21% and a bulk period of ∼51 nm (see Supporting Information Figure S1) by capillary wetting19 at a temperature above the glass transition temperatures of both components into selfordered anodic aluminum oxide (AAO).20 The self-ordered AAO contained arrays of aligned cylindrical pores with narrow size distribution (lattice period ≈ 500 nm, pore diameter ≈ 300

T

he design of surfaces with switchable adhesive properties has emerged as a new challenge in materials engineering.1 Up to now, adhesion switching of nonfibrillar polymeric surfaces has commonly been accomplished by changing the pH values of supernatant solutions2 or by exposure to different solvents.3,4 The combination of adhesion switching and the functional advantages of fibrillar adhesive pads making use of the contact splitting principle5−10 is particularly attractive but has remained challenging. The contact splitting principle involves contact formation between the discrete fibrillar contact elements of fibrillar adhesive pads and counterpart surfaces at numerous discrete contact points. Functional advantages of fibrillar adhesive pads include high adaptability to rough or curved counterpart surfaces, constrained crack propagation, pronounced adhesion reversibility and durability. Systems that combine all these features with adhesion switching are attractive as self-cleaning components for advanced separation and filtration processes. A limited number of approaches have been evaluated for the realization of fibrillar adhesive systems with adhesive properties responsive to external stimuli. Adhesion of continuous films supported by micropillar arrays to counterpart surfaces could be modulated by temperature changes11 or by pressure changes.12 Moreover, the adhesive properties of fibrillar adhesive pads could be modified by magnetic fields,13 strain,14 pressure-induced tilt of contact interfaces15 and temperature-change-induced phase transitions.16−18 © 2013 American Chemical Society

Received: August 22, 2013 Revised: October 23, 2013 Published: October 30, 2013 5541

dx.doi.org/10.1021/nl403144w | Nano Lett. 2013, 13, 5541−5548

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nm, pore depth ≈ 1.5 μm). Since the polar component P2VP segregates to oxidic surfaces such as the AAO pore walls, the surface of the PS-b-P2VP nanorods formed in the AAO pores consisted of P2VP.21,22 Wet-chemical etching of the AAO yielded nonporous fibrillar adhesive pads consisting of arrays of PS-b-P2VP nanorods serving as fibrillar contact elements with convex-hemispherical tips (negative replicas of the concavehemispherical AAO pore bottoms). The PS-b-P2VP nanorods were connected to PS-b-P2VP substrates (initially the PS-bP2VP surface films on top of the AAO). Below, the obtained fibrillar adhesive pads with an overall thickness of ∼110 μm are referred to as solid-hemispherical adhesive pads. Subjecting solid-hemispherical adhesive pads to swelling-induced morphology reconstruction21−24 in ethanol at 60 °C for 4 h yielded fibrillar adhesive pads containing spongelike, continuous pore systems, thereafter referred to as porous-hemispherical adhesive pads (Figure 1). Characterization of porous-hemispherical

terminal contact shapes oriented along the shear direction were obtained, which were treated with ethanol at 60 °C for 4 h while their stems were still located in the AAO nanopores. Only the footlike terminal contact shapes, which were directly exposed to ethanol, had pores open to the environment, while the stems of the contact elements had a closed outer P2VP skin (Figure 2a). The interior of fibrillar contact elements swollen while located inside the AAO pores was porous. To verify the porous nature of porous-footlike adhesive pads, PS-b-P2VP located in AAO pores with closed pore bottoms was treated with ethanol for 4 h at 60 °C. Wet-chemical etching of the AAO yielded an array of released PS-b-P2VP nanorods with closed outer P2VP skin and convex-hemispherical tips that was connected to an underlying porous PS-b-P2VP substrate (Figure 2b). Transmission electron microscopy (Figure 2c) as well as scanning electron microscopy after additional treatment with oxygen plasma (100 W; 4 min; Figure 2d) revealed the presence of pores even at the convex-hemispherical tips of the PS-b-P2VP nanorods. Adhesion measurements were performed by approaching and retracting a sapphire sphere mounted on a metal spring to and from the tested fibrillar adhesive pad at constant speed (2 μm/ s). The spring deflection was converted into force. Further displacement of the sapphire sphere after contact formation to the tested fibrillar adhesive pad resulted in the detectable loading force Fl. The pull-off force Fad required to separate the spherical probe from the tested fibrillar adhesive pad represented adhesion (Figure 3a). Reversible adhesion switching of the fibrillar adhesive pads induced by humidity changes was tested in a humidity chamber (Supporting Information Figure S3) equipped with a humidity control system (Supporting Information Figure S4). A high/low humidity cycle comprised a first adhesion test carried out in N2 atmosphere with a relative humidity of 90% and a second adhesion test carried out in N2 atmosphere with a relative humidity of 2%. Before each adhesion test, N2 with the desired relative humidity was piped into the humidity chamber for 15 min. Even after 10 successive high/low humidity cycles carried out on a porous-footlike adhesive pad using a loading force of ∼300 μN no deterioration in switching performance was apparent; the Fad/Fl values (adhesion normalized to the loading force) of the low-adhesion and high-adhesion states remained constant (Figure 3b). Thus, humidity-induced switching of adhesion on porous-footlike adhesive pads is highly reversible. If adhesion at a relative humidity of 90% is considered as “on”state and adhesion at a relative humidity of 2% as “off”-state, the mean on/off ratio (Fad/Fl value of “on”-state divided by Fad/ Fl value of “off”-state) of porous footlike adhesive pads amounted to ∼9.3. The mean on/off ratio of a solidhemispherical adhesive pad tested under the same conditions amounted to ∼3.3 (Supporting Information Figure S5). To investigate the dependence of adhesion on humidity in more detail, we measured the dependence of Fad on Fl at different humidities (Figure 4a). The outer surfaces of all tested fibrillar adhesive pads and a ∼110 μm thick smooth porous PSb-P2VP film prepared by swelling with ethanol at 60 °C for 4 h consisted of P2VP blocks with similar mobility. At a given relative humidity, attractive interactions between adhesive pads and counterpart surfaces such as that of the spherical probes used in the adhesion tests should be similar. At low relative humidities of 2 and 24%, porous-footlike adhesive pads showed only weak adhesion even for high Fl values. At a relative humidity of 24%, Fad reached only ∼18 μN at Fl ∼ 300 μN

Figure 1. Electron microscopy images of rodlike PS-b-P2VP contact elements of porous-hemispherical adhesive pads obtained by swelling at 60 °C in ethanol for 4 h after removal of AAO. (a) Scanning electron microscopy (SEM) image; (b) transmission electron microscopy (TEM) image.

adhesive pads by mercury intrusion confirmed the presence of continuous pore systems with an average pore diameter of ∼98 nm and a cumulative pore volume of ∼1.7 cm3/g (Supporting Information Figure S2). Footlike terminal contact shapes of fibrillar contact elements mimic spatula-like contact elements in the adhesive systems of various animals.25 To prepare fibrillar adhesive pads with contact elements having footlike terminal contact shapes (porous-footlike adhesive pads), AAO was infiltrated with PSb-P2VP. Then, the pore bottoms of the AAO membranes were etched and the exposed convex-hemispherical tips of the PS-bP2VP nanorods were rubbed with a polymer rod adapting a shearing procedure reported previously.10 Hence, footlike 5542


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Figure 2. Electron microscopy images of rodlike PS-b-P2VP contact elements swollen in ethanol at 60 °C for 4 h while located in AAO pores. (a) SEM image of a porous-footlike adhesive pad. (b) SEM image, (c) TEM image and (d) SEM image taken after additional treatment with oxygen plasma (100 W; 4 min) of PS-b-P2VP nanorods after swelling while located in AAO.

Figure 3. (a) Schematic representation of a single force−displacement curve. The insets illustrate the position of the sapphire sphere relative to the surface of the tested adhesive pad. Loading force Fl, adhesion force Fad and loading distance Dl are indicated. (b) Reversibility of switching between “on” state (high adhesion) and “off” state (low adhesion) on porous footlike adhesive pads (normalized adhesion Fad/Fl vs run number at Fl ∼ 300 μN). Measurements were carried out on the same sample spot in an alternating manner at 90% humidity (adhesion “on”) and 2% humidity (adhesion “off”). Before each measurement, humidity was equilibrated for 15 min.

μN (Fad/Fl ∼ 0.23). Similar humidity dependence was found on porous-hemispherical adhesive pads and on smooth porous PSb-P2VP films (Supporting Information Figure S6). Figure 4b

(Fad/Fl ∼ 0.06). However, at a relative humidity of 50%, Fad increased to ∼59 μN for Fl ∼ 310 μN (Fad/Fl ∼ 0.19); at a relative humidity of 90% Fad amounted to ∼62 μN at Fl ∼ 275 5543


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Information Figure S7). A representative force−distance curve measured on a porous-footlike adhesive pad at a relative humidity of 90% is shown in Figure 5a. Only weak hysteresis

Figure 4. (a) Plot of Fad versus Fl on a porous-footlike adhesive pad at different humidities. (b) Normalized adhesion Fad/Fl (Fl ∼ 300 μN) on porous smooth PS-b-P2VP films (gray) as well as on solidhemispherical (dark yellow), porous-hemispherical (blue) and porousfootlike (magenta) adhesive pads at different relative humidities. Each data point in panel (a) or bar in panel (b) represents at least five measurements at different positions on two samples. The error bars indicate standard deviations.

Figure 5. (a) Typical force-displacement curve of a porous-footlike adhesive pad taken at a humidity of 90%. (b) Effective elastic moduli of solid-hemispherical adhesive pads (squares, dashed line) and porous-hemispherical adhesive pads (circles, solid line) at different relative humidities. (c) Indentation depths for solid-hemispherical adhesive pads (squares, dashed line) and porous-hemispherical adhesive pads (circles, solid line) at Fl ∼ 300 μN and different relative humidities. Each data point in (b,c) represents the mean value of at least five measurements. Standard deviations are indicated. The lines in (b,c) are guides to the eyes.

shows Fad/Fl values for Fl ∼ 300 μN of porous smooth PS-bP2VP films as well as of solid-hemispherical, porous-hemispherical and porous-footlike fibrillar adhesive pads at different humidities. Apart from the expected outcome that fibrillar adhesive pads showed significant higher adhesion than the corresponding smooth films, the following results were apparent. (1) Increases in relative humidity from 25 to 90% were accompanied by significant increases in adhesion for all tested samples. Fad/Fl increased from 0 to ∼0.09 for porous smooth PS-b-P2VP films, from ∼0.03 to ∼0.12 for solidhemispherical fibrillar adhesive pads, from ∼0.06 to ∼0.20 for porous-hemispherical fibrillar adhesive pads, and from ∼0.06 to ∼0.23 for porous-footlike fibrillar adhesive pads. (2) Porous samples had significantly higher adhesion than the corresponding nonporous samples, especially at an intermediate relative humidity of 50%. We attribute the increase in adhesion caused by increased humidity predominantly to humidity-induced softening of P2VP, while structural integrity is conserved by the hydrophobic and glassy component PS. P2VP is slightly hydrophilic and has a water contact angle of 65.8 ± 1.2° (Supporting

was apparent in the load-retraction cycles. This outcome indicates that even at high humidity the porous fibrillar adhesive pads were deformed elastically rather than inelastically. The shape of the force−distance curve, especially the absence of a jump-in event and the rapid jump-off, indicated that capillary forces were not the main origin of the observed adhesion enhancement at 90% relative humidity. Enhanced adhesion has been related to a reduction in the elastic modulus.26 We calculated effective elastic moduli of the fibrillar adhesive pads based on the modified Schargott− Popov−Gorb (SPG) model10 from force-displacement curves. For a loading force Fl ∼ 300 μN at a relative humidity of 50%, the effective elastic moduli of solid-hemispherical and poroushemispherical adhesive pads were calculated to be 41.2 ± 4.0 and 6.0 ± 0.4 MPa, respectively (Figure 5b). The presence of internal pore systems is accompanied by significant reduction in 5544


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solid PS-b-P2VP contact elements. However, the mechanical properties of one-dimensional block copolymer nanostructures are difficult to predict as they depend on complex, confinement-induced nanoscopic domain structures. We found that reducing the diameter of solid fibrillar PS-b-P2VP contact elements resulted in poorer mechanical stability; only fibrillar contact elements with diameters of 300 nm and above were stable enough after release from AAO. We assume there are two reasons for this outcome. (1) The mechanical stability of solid PS-b-P2VP nanorods depends on the number of intermolecular entanglements between different blocks of the majority component PS with a volume fraction of ∼79%. Assuming a bulklike mean molecular weight Me of PS segments between two entanglements (Me ∼13 000 g/mol),28 one PS block forms on average ∼7 entanglements. In nanoscale confinement, the proportion of intramolecular self-entanglements increases at the expense of intermolecular entanglements, resulting in a much less stable network of PS chains.29−31 Hence, PS nanostructures show poorer mechanical properties than bulk PS.32 In the strong cylindrical confinement imposed by AAO pores having diameters only a few times larger than the bulk period of an infiltrated block copolymer, the nanoscopic domain structure of the latter is strongly disturbed.33 It is reasonable to assume that strong confinement suppresses the formation of intermolecular entanglements between PS blocks that would stabilize fibrillar PS-b-P2VP contact elements. Solid PS-b-P2VP nanorods with a diameter of ∼300 nm show a complex arrangement of cylindrical P2VP domains in a PS matrix. Curved P2VP domains partially winding around the long axes of the PS-b-P2VP nanorods which do not form fully developed helices, and cylindrical P2VP domains aligned with the PS-b-P2VP nanorod axes formed. The P2VP cylinders are surrounded by continuous PS domains (Figure 6). Intermolecular entanglements between PS blocks may form especially in the triangular volumes between three neighboring P2VP cylinders. Therefore, geometric constraints leading to the replacement of intermolecular entanglements between PS blocks by intramolecular entanglements should be much less pronounced than in solid PS-b-P2VP nanorods having smaller diameters with strongly perturbed domain structures. (2) Junctions between the PS and P2VP blocks, which show strong mutual repulsion, are concentrated in narrow interfacial regions.34 PS and P2VP blocks near the domain interface are stretched in the direction normal to the domain interface.35 In narrow AAO pores, most of the junctions between the PS and P2VP blocks are located at highly curved domain interfaces between P2VP shells and PS cores, which are concave for the PS blocks. Conformational perturbation caused by mutual repulsion between neighboring blocks, which results in reduced conformational entropy, is stronger at concave domain interfaces than at flat or convex domain interfaces. Mutual repulsion at concave domain interfaces affects the longer PS majority blocks much more than the shorter P2VP minority blocks. The conformational perturbation of the PS blocks at the concave core/shell domain interface increases the free energy of the nanoscopic domain structure. Increased free energies of block copolymer domain structures related to perturbed conformations of block chains near domain interfaces are known to reduce the stability of nanoscopic domain structures.36 In the solid PS-b-P2VP nanorods with a diameter of ∼300 nm, the proportion of longer PS majority blocks located at concave domain interfaces to P2VP adsorbed at the

the effective elastic modulus and in turn by significant increase in adhesion under otherwise comparable conditions. Load applied to the adhesive pads in the course of adhesion tests distorts the domain structure of PS-b-P2VP. Inside porous fibrillar adhesive pads, the internal P2VP chains are located at the walls of the internal pores so that they form a soft interface to the free volume of the internal pores. It is reasonable to assume that solid PS-b-P2VP adhesive pads, in which P2VP blocks are confined to solid P2VP domains, show much smaller elastic deformation at a given loading force. This is obvious from a comparison of the indentation depths Δl (see Figure 8 in ref 10) determined from force-displacement curves using the modified SPG model. Indentation depths Δl specify to what extent the spherical probes emboss the adhesive pads at the predefined loading force Fl during adhesion measurements and can be considered as qualitative measure of actual contact area. For a loading force Fl ∼ 300 μN at a relative humidity of 50%, Δl of solid-hemispherical and porous-hemispherical adhesive pads was calculated to be ∼140 and ∼400 nm, respectively (Figure 5c). The comparison of the mean on/off ratios of porous-footlike and solid-hemispherical adhesive pads reveals that humidityinduced changes in adhesion are much more pronounced in the presence of pores. P2VP blocks located at internal pore walls of porous fibrillar adhesive pads can much easier interact with water than internal P2VP blocks of solid adhesive pads. Apart from the fixed junctions of the P2VP blocks to the glassy PS domains, no geometric constraint hampers humidity-induced partial swelling of the internal P2VP blocks at the pore walls of porous adhesive pads. In solid PS-b-P2VP, partial swelling of internal P2VP blocks is impeded by geometric confinement imposed by rigid, glassy PS matrices that can only be overcome if strong osmotic pressure drives swelling agent into the solid P2VP domains. As a result, interactions between water and internal P2VP blocks changed the effective elastic modulus of porous-hemispherical adhesive pads much more than that of solid-hemispherical adhesive pads (Figure 5b). The effective elastic modulus of porous-hemispherical adhesive pads at Fl ∼ 300 μN decreased by a factor of ∼4.3 when relative humidity increased from 25 to 50%, whereas the effective elastic modulus of solid-hemispherical adhesive pads decreased only by a factor of ∼2.8. Under otherwise identical conditions (Fl ∼ 300 μN), the Δl values of porous-hemispherical adhesive pads were more than twice as large as those of solid-hemispherical adhesive pads (Figure 5c). The Δl value of porous-hemispherical adhesive pads increased by ∼210 nm when relative humidity increased from 25 to 50%, whereas Δl of solid-hemispherical adhesive pads increased by only ∼50 nm. We attribute both the higher overall adhesion as well as the better switching ratio of porous adhesive pads as compared to solid adhesive pads to the higher actual contact area with counterpart surfaces. In the strong cylindrical confinement of narrow AAO pores PS-b-P2VP forms core/shell structures with P2VP shells at the AAO pore walls and PS cores (possibly containing embedded internal P2VP domains, which are commonly strongly disturbed).27 As a high proportion of the P2VP blocks will be located at the outer surface of the resulting solid fibrillar PS-bP2VP contact elements, one might assume that the effective elastic modulus of the corresponding solid fibrillar adhesive pads decreases along with the diameter of their contact elements. The question arises whether adhesion performance and humidity-induced adhesion switching of solid fibrillar adhesive pads could be optimized by reducing the diameter of 5545


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preserves structural integrity. Reduced stiffness of the polar component P2VP results in reduced effective elastic moduli of the porous fibrillar adhesive pads. Smaller effective elastic moduli lead in turn to larger adhesion. Humidity-induced increase in adhesion is significantly enhanced by the internal porous structure of the porous fibrillar adhesive pads. Because in porous fibrillar adhesive pads internal P2VP blocks are located at internal pore walls, partial swelling of the P2VP chains with water at high humidity is not impeded by rigid confinement. However, in the absence of pores the rigid PS scaffold hampers humidity-induced softening of the P2VP domains. Thus, the adhesion difference between the highadhesion state at high humidity and the low-adhesion state at low humidity of porous adhesive pads is nearly three times larger than that of nonporous solid fibrillar adhesive pads. Fibrillar adhesive pads showing humidity-responsive adhesion combine switchability with functional advantages related to the contact splitting principle, such as adhesion to rough and curved surfaces, constrained crack propagation and pronounced adhesion reversibility and durability. Potential applications may include preconcentration of particulates, pollutants, and germs at high humidity (high-adhesion state) combined with triggered surface cleaning at low humidity (low-adhesion state).

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EXPERIMENTAL DETAILS Asymmetric PS-b-P2VP (Mn(PS) = 101 000 g/mol; Mn(P2VP) = 29 000 g/mol; Mw/Mn = 1.60; volume fraction P2VP = 21%, bulk period ∼51 nm) was obtained from Polymer Source Inc., Canada. Self-ordered nanoporous AAO with a pore diameter of 300 nm, a lattice period of 500 nm and a pore depth of 1.5 μm was prepared by anodizing aluminum chips with a diameter of 2 cm (Goodfellow, purity >99.99%) following procedures described elsewhere.20 PS-b-P2VP was infiltrated into AAO at 220 °C for 48 h under vacuum while applying a load of ∼160 mbar. Then, the samples consisting of an AAO layer infiltrated with PS-b-P2VP, a PS-b-P2VP surface film on top of the AAO and an Al substrate to which the AAO layer was connected were cooled to room temperature at a rate of −1 K/min. The PS-b-P2VP-infiltrated AAO templates were embedded into epoxy resin (ATACS5104/4103 supplied by Aldrich) in such a way that the underside of the Al substrate remained uncovered while the interface between the PS-b-P2VP film and the AAO was sealed. After curing the epoxy resin at room temperature for 24 h, the Al substrates were selectively etched with a solution of 100 mL 37% HCl and 3.4 g CuCl2·2H2O in 100 mL deionized water at 0 °C. Thus, the hemispherical pore bottoms of the AAO layer consisting of barrier oxide were uncovered. To obtain solid-hemispherical fibrillar adhesive pads, the AAO was etched with 10% aqueous HCl solution at 60 °C. To convert solid-hemispherical into porous-hemispherical adhesive pads, the former were treated with ethanol heated to 60 °C for 4 h. To prepare porous-footlike adhesive pads, the tips of the PS-b-P2VP contact elements (negative replicas of the AAO pore bottoms) were uncovered by selectively opening the AAO pore bottoms by etching with 10% phosphoric acid at 60 °C for 10.5 min. Then, a PS cylinder with a diameter of 2 mm tilted at a small angle was rubbed on the arrays of PS-b-P2VP contact elements protruding from the AAO underside at room temperature. The samples treated in this way were then subjected to ethanol at 60 °C for 4h. Finally, the AAO membrane was etched with 10% aqueous HCl solution at 60 °C.

Figure 6. (a,b) TEM images of solid PS-b-P2VP nanorods. P2VP domains were stained with iodine and appear dark.

AAO pore walls is smaller than in narrower pores. PS blocks are predominantly located at convex domain interfaces to the cylindrical P2VP domains. Locating the long PS majority blocks at convex domain interfaces is entropically much more favorable than their predominant location at concave interfaces. On the other hand, locating the shorter P2VP minority blocks at concave interfaces results in a much smaller entropic penalty than locating the longer PS majority blocks at concave surfaces. Moreover, if the diameter of solid fibrillar contact elements is reduced, their aspect ratios (ratio length/diameter) need to be reduced also. Adhesion performances of the corresponding adhesive pads would then predominantly be determined by the contributions of the underlying solid PS-b-P2VP substrates. Owing to the rigidity of the underlying solid PS-b-P2VP substrates, adhesion performance and humidity-induced adhesion switching performance would be poor. In conclusion, we demonstrated that adhesion on arrays of porous fibrillar adhesive pads consisting of the amphiphilic block copolymer PS-b-P2VP can be reversibly switched between low-adhesion states at low humidity and high-adhesion states at high humidity. Adhesion in the high-adhesion state was more than nine times larger than adhesion in the low-adhesion state. High adhesion under humid conditions is related to reduced stiffness of the polar component P2VP related to P2VP/water interactions, while the glassy component PS 5546


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Mercury intrusion measurements were performed with Pascal 140 and 440 devices from Porotec (Thermo Finnigan). The samples were dried at 45 °C for 8 h and degassed for 20 min before the measurements. SEM investigations were carried out on a Zeiss Auriga microscope operated at 3 kV. The samples were sputter-coated with a 5 nm thick iridium layer. For TEM investigations, solid and porous PS-b-P2VP nanorods were prepared as described above, the only difference being that for the preparation of solid PS-b-P2VP nanorods AAO with a pore depth of 100 μm was used. Prior to the etching of the AAO, any PS-b-P2VP was mechanically removed from the AAO surface. We first etched the aluminum substrate to which the AAO was connected with a mixture of 3.4 g CuCl2·2 H2O, 100 mL H2O, and 100 mL concentrated HCl(aq) at 0 °C and then the AAO with 10% phosphoric acid at 60 °C. The suspensions of the solid or porous PS-b-P2VP nanorods were neutralized by several centrifugation/washing steps and then dropped on copper grids coated with carbon films. P2VP domains in solid PS-b-P2VP nanorods were stained with iodine by heating the PS-b-P2VP nanorods deposited on TEM grids in a sealed container for 30 min to 60 °C in the presence of iodine. TEM measurements were conducted on a JEOL JEM2100 microscope operated at 200 kV. Adhesion was tested by recording force−displacement curves on sample pieces with areas of about 5 × 5 mm2 using a homebuilt force tester Basalt-02. A sapphire sphere with a diameter of 3 mm glued to the free end of a metal spring was used as probe that could be vertically moved with a piezo drive. The sapphire sphere was cleaned with acetone prior to measurements on new samples. The samples were placed on a hexapod nanopositioning stage (Physik Instrumente, Karlsruhe, Germany). The approach and retraction speeds were 2 μm s−1. The retraction started immediately after the predefined Fl value was reached. The deflection of the metal spring on which the spherical sapphire probe was mounted was monitored with a laser interferometer during loading and unloading. Since the spring constant Cspring = 69.8 N/m was known, the deflection Fl/Cspring could be converted into force. For data collection, a custom-made Labview software package was used. All measurements were carried out in a homemade humidity chamber (Supporting Information Figure S3) at room temperature. The humidity was controlled by mixing dry N2 and saturated water vapor at different ratios (for details see Supporting Information Figure S4). Effective elastic moduli E were obtained from forcedisplacement curves using the modified SPG model. According to the modified SPG model,10 all fibrillar contact elements are modeled as elastic springs with uniform length l0 and a spring constant E/(Al0), where A is the contact area per single contact element. The overall apparent contact area corresponds to the projection of the curved surface portion of the spherical sapphire probe touching the fibrillar adhesive pad into the plane normal to the long axes of upstanding fibrillar contact elements. The probability p of contact formation between the spherical sapphire probe and fibrillar contact elements within the apparent contact area is linearly proportional to a spring compression (corresponding to an indentation depth) Δl (cf. Figure 8 in reference 10) until p equals one. Then, all fibrillar contact elements within the apparent contact area are in contact with the sapphire probe. The spring compression Δl corresponds to the loading distance (piezo displacement) Dl minus the deflection Fl/Cspring of the metal spring on which the spherical sapphire probe is mounted

Δl = D l −

Fl Cspring

E can be calculated from Δl and the applied loading force Fl, which is directly apparent from the force-displacement curves

E=

F ll 0 παR Δl 2

R = 1.5 mm is the radius of the sapphire probe; α is ratio of the actual contact area between contact elements and sapphire sphere to the apparent contact area. On the basis of the diameter of the fibrillar contact elements and the lattice constant of the arrays they formed, α was estimated to 0.1.

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ASSOCIATED CONTENT

S Supporting Information *

Additional information on the setups used for adhesion tests, the evaluation of the bulk period of the PS-b-P2VP, porosity measurements, the contact angle of P2VP, and results of control experiments. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: (L.X.) [email protected]. *E-mail: (M.S.) [email protected]. *E-mail: (S.N.G.) [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank C. Hess and H. Tobergte for the preparation of AAO membranes and for SEM investigations as well as R. Hillebrand (Max Planck Institute of Microstructure Physics, Halle) for providing a program used for the calculation of pair distribution functions. Support by the German Research Foundation (DFG Priority Program 1420 and project No C10 within SFB 677) is gratefully acknowledged. L.X. thanks the Alexander-von-Humboldt Foundation for a fellowship.

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REFERENCES

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