Reversible dry micro-fibrillar adhesives with thermally controllable adhesion

Transcription

1 PAPER Soft Matter Reversible dry micro-fibrillar adhesives with thermally controllable adhesion Seok Kim, a Metin Sitti,* a Tao Xie* b and Xingcheng Xiao b Received 19th May 2009, Accepted 2nd July 2009 First published as an Advance Article on the web 3rd August 2009 DOI: /b909885b This work reports thin-film terminated micro-fibrillar adhesives made of adhesive polymers and shape memory polymers as reversible dry adhesives with thermally controllable adhesion. Structurally different adhesives were fabricated by coating a continuous thin layer of an elastomeric adhesive polymer onto either a flat or a fibrillar shape memory polymer surface. Experimental results exhibited that pull-off forces of the adhesives can be up to four times different depending on thermal conditions. These differences originate from the temperature dependence of either the intrinsic adhesion properties of the adhesive polymer and/or the stiffness of the sub-surface shape memory polymer. Introduction Many researchers have been inspired by the unique reversible adhesion ability of gecko feet on almost any surface. 1 6 The feet of large lizards such as the Tokay gecko have a complicated hierarchical structure with microscale diameter base fibers which branch down to sub-micron diameter terminal fibers. Gecko toes have been shown to adhere with high interfacial shear strength to smooth surfaces. 1 These animals have adhesive footpads with multi-level hierarchy of compliance including their toes, foot tissue, lamellae, setae, and spatulae. This hierarchical hairy structure conforms to the surface roughness with various frequency and wavelength scales. The fibers are angled with respect to the animals toes, and the branched tips are also oriented with respect to the base of the fiber. 3 The result is that the gecko footpad exhibits a high level of directional dependence, high adhesion while dragging the toe inwards, and almost no adhesion in the opposite direction. 4 This directionality is sometimes referred to as frictional anisotropy or, more appropriately, directional adhesion. Studies of gecko footpads have revealed that due to their asymmetric angled structure, they are nonadhesive in their resting state, dragging motion is required to induce adhesive behavior. Reversing the direction of this dragging motion removes the fibers from the surface with very little force. Inspired by the gecko s hairy footpads, synthetic fibrillar structured surfaces have been fabricated for various applications in recent years As an attempt to mimic the gecko s in-situ adhesion reversal mechanism, Xie and Xiao have developed selfpeeling reversible dry adhesive systems (SPRA) 22 consisting of AP a thermoset elastomeric adhesive polymer (AP) (T g ¼ 3.0 C) SMP and a thermoset shape memory polymer (SMP) (T g ¼ 39.9 C) in a layered configuration. The SPRA possesses a high adhesion value of 60 N cm 2 and an in-situ peeling adhesion reversal mechanism similar to the mechanical actions of gecko toes. It is worth noting that both the adhesive strength and the surface mechanical properties of the original SPRA is dominated by the a Department of Mechanical Engineering, Carnegie Mellon University, Pittsburgh, PA, 15213, USA. sitti@cmu.edu b Materials & Processes Laboratory, General Motors Research & Development Center, Warren, MI, 48090, USA. tao.xie@gm.com adhesive polymer layer due to its thickness of 0.7 mm, and the shape memory polymer backing layer plays a role only in the adhesive detachment. In addition to their work, strategies to switch adhesion of dry adhesives have been demonstrated through ex-situ means 20,21 and in-situ means with mechanically tunable adhesion. 23 Intrigued by the self-peeling mechanism of the SPRA and the adhesion tunability of other previous works, we set to fabricate a double layer adhesive (DLA) by coating a thin AP layer (1 mm) on a flat shape memory polymer layer (65 mm). Such an adhesive is expected to maintain the intrinsically high adhesive strength of the adhesive polymer yet its bulk mechanical properties should be dominated by the shape memory polymer (such as thermally controllable stiffness). Furthermore, these properties of the DLA can be combined with fibrillar structuring to enhance adhesion properties. Accordingly, a second dry adhesive (herein called an AP-SMP fiber array) was fabricated with a shape memory polymer fiber array terminated with a continuous thin adhesive polymer layer (1 mm). We demonstrate that the adhesion of both adhesives can be thermally controlled. Materials and methods We should note that both the adhesive polymer and shape memory polymer are thermoset epoxy polymers. The chemical structures of their precursors as well as the characterization of the fully cured adhesive polymer and shape memory polymer have been reported in a previous publication dedicated to epoxy shape memory polymers. 24 Specifically, the adhesive polymer and shape memory polymer in this paper correspond to samples NGDE4 and NGDE2 in ref. 24, respectively. The AP-SMP fiber array was fabricated according to a modified procedure from our previous work. 14,17 First, a negative silicon fiber array template was created using the notching effect during deep reactive ion etching. 14 Next, the liquid precursor mixture of shape memory polymer 22 was poured into the template. After vacuum degassing, the shape memory polymer precursor was cured at 100 C for 1.5 hours. Upon completion of the curing, the silicon template was removed by XeF 2 dry etching, releasing the shape memory polymer fiber array. 14 To coat a thin adhesive polymer layer onto the fiber array, the liquid precursor for the adhesive This journal is ª The Royal Society of Chemistry 2009 Soft Matter, 2009, 5,

2 polymer 22 was first precured (60 C for 1.5 hours) to obtain optimal viscosity. The precured liquid adhesive polymer was then spun onto a Mylar (Dupont) film (2000 rpm for 30 seconds) to form a uniform thin layer, as illustrated in Fig. 1(a). The shape memory polymer fiber array was immediately placed in contact with the precured spun liquid adhesive polymer on the Mylar film and then retracted with some of the liquid adhesive polymer retained on the tips of the fibers (Fig. 1(b)). The fiber array was placed onto another clean Mylar film and cured (100 C for 1.5 hours and 130 C for another hour) (Fig. 1(c)). Finally, the AP-SMP fiber array was released from the Mylar film by gentle mechanical peeling (Fig. 1(d)). The obtained AP-SMP fiber array consists of shape memory polymer fiber arrays terminated with a continuous thin adhesive polymer layer shown in Fig. 2. Since the backing layer thickness of soft fibrillar adhesives would affect the adhesion significantly, 25,26 the AP-SMP fiber array was fabricated with small thickness of 65 mm shape memory polymer backing layer and approximately 1 mm thick adhesive polymer thin layer. The non-structured DLA sample was fabricated according to the same procedure except that a flat silicon Fig. 1 Schematic process flow steps for the fabrication of AP (adhesive polymer)-smp (shape memory polymer) microfiber arrays: (a) the shape memory polymer fiber array was placed in contact with the precured liquid adhesive polymer; (b) the dipped shape memory polymer fiber array was retracted with some of the liquid adhesive polymer; (c) the fiber array was placed onto a Mylar substrate and cured; (d) AP-SMP fiber array was released from the substrate. template was used instead of the negative silicon fiber array template. The DLA was fabricated with thicknesses of the shape memory polymer backing layer (65 mm) and the adhesive polymer layer (1 mm) identical to those in the AP-SMP fiber array to ensure a fair comparison between them. In addition, a flat adhesive polymer film (115 mm, without the shape memory polymer layer) on a glass substrate was produced to investigate temperature dependence of the adhesive strength of adhesive polymer without any influence of the shape memory polymer. It should be noted that, for both the non-structured DLA and the AP-SMP fiber array, the adhesive polymer started as liquid epoxy precursors and cured onto the shape memory polymer. The interfacial adhesion strength between the two polymers is thus very strong, as would be expected for most traditional wet adhesives (at least several MPa for epoxy adhesives). Thus, we did not observe any adhesion failure between the adhesive polymer and the shape memory polymer in any of our experiments described below. To characterize the adhesion performance of the samples, a custom tensile macroscale adhesion measurement setup was built (Fig. 3). We used a glass hemisphere instead of a flat glass surface as the test contact surface to minimize alignment errors during the measurements. In addition, the glass hemisphere stimulated a rough surface with a well-defined height distribution and a wavelength larger than the size of the AP-SMP fiber. 28 A 6 mm diameter smooth glass hemisphere (ISP Optics, QU-HS-6) attached to a load cell (Transducer Techniques, GSO-25) was moved vertically by a motorized stage (Newport, MFA-CC) with 100 nm resolution. The hemisphere was pushed to the sample surface with a preload force and retracted at a slow speed of 5 mm s 1. During the measurement, the contact area between the hemisphere and the sample was monitored using a camera (Dage-MTI, DC330) attached to an inverted optical microscope (Nikon, Eclipse TE200). With the custom setup, the pull-off forces of the DLA and the AP-SMP fiber array were measured in three different conditions. Under the Hot-Hot (HH) condition, the adhesion tests were conducted after the samples were heated to 80 C with focused infrared light (Infrared Heat Lamp, Exo-terra). In this case, the shape memory polymer was at its rubbery state (above its T g ) with a low modulus (E ¼10 MPa) 24 during both loading (preloading) and unloading (adhesion measurement). In the Cool-Cool (CC) case, the same tests were conducted on the samples at room temperature at which the Fig. 2 SEM image of AP-SMP fiber array of 1 2 mm thick adhesive polymer layer and shape memory polymer fibers with 5 mm stem diameter and 20 mm length (scale bar: 10 mm). Fig. 3 A custom macroscale adhesion measurement setup using a 6 mm diameter glass hemisphere and focused infrared light Soft Matter, 2009, 5, This journal is ª The Royal Society of Chemistry 2009

3 shape memory polymer was at its high modulus (E ¼3 GPa) 24 state throughout the adhesion measurement. In the Hot-Cool (HC) case, the sample was heated up to 80 C before loading, cooled down to room temperature for 100 seconds while maintaining the contact area generated during loading, and the glass hemisphere was retracted from the sample. In this HC case, the shape memory polymer was at its low and high modulus states during loading and during unloading, respectively. Pull-off force measurements were conducted for 10 preloads up to 20 mn as a maximum instead of repeated measurements for a lower number of preloads to show more obvious pull-off force trends. Results and discussion Before adhesion tests of the DLA and the AP-SMP fiber array, contact area and pull-off force measurements for the 115 mm thick adhesive polymer film were conducted to inspect thermally dependent material properties of the adhesive polymer film. The contact areas (Fig. 4(a)) for the adhesive polymer film in the HH and CC cases are nearly identical at the same preloads. Typical maximum contact areas and force distance curves from the adhesion tests with the adhesive polymer film in the HH and CC cases are demonstrated in Fig. 5. This observation is consistent with the expectation that contact area is determined by the material modulus, which is not expected to change for a thermoset elastomer in its rubbery state. The pull-off force of the adhesive polymer film measured under the CC condition, however, is approximately 3 to 5 times higher than that obtained under the HH condition, depending on the preload (Fig. 4(b)). Using the saturated approximate pull-off force values, the overall work of adhesion values between the glass hemisphere and the HH and CC state AP films can be computed as around 0.6 J m 2 and 3.1 J m 2, respectively using the Johnson Kendall Roberts theory. 27 This large difference in adhesion strength at two different temperatures can only be attributed to the Fig. 4 Contact area and pull-off force of 115 mm thick adhesive polymer film (a, b), and macroscale pull-off forces of 65 mm thick DLA and AP-SMP fiber array (c, d) on a 6 mm diameter glass hemisphere for varying preloads. Fig. 5 Optical microscope images of maximum contact areas and force distance curves from the adhesion experiments on 6 mm diameter glass hemisphere for 20 mn preload with 115 mm thick adhesive polymer film in the Cool-Cool (a, c) and Hot-Hot (b, d) cases, respectively (scale bars: 200 mm). temperature dependence of the noncovalent interactions responsible for the adhesion, not the bulk material mechanical property. While important, a detailed definitive explanation of such a phenomenon demands significant efforts that are beyond the scope of this particular paper. The 65 mm thick DLA exhibits 1.3 to 2.5 times higher pull-off force under the CC condition than that under the HH condition, depending on the preload (Fig. 4(c)). Typical maximum contact areas and force distance curves from the adhesion tests with the DLA in the HH and CC cases are demonstrated in Fig. 6. This difference in adhesion is smaller than that observed for the adhesive polymer film under the HH and CC conditions. Clearly, the intrinsic higher pull-off strength for the adhesive polymer layer in the DLA under the CC condition is partially offset by the smaller contact area due to the high modulus of the shape memory polymer layer under CC condition. Under the HC condition, the loading event for the DLA is expected to be identical to that under the HH condition, with larger contact area compared to that for loading at room temperature. As for the unloading behavior, the HC condition takes advantages of the higher intrinsic pull-off strength of adhesive polymer at room temperature. The above benefits of the HC conditions led to higher pull-off forces for DLA than those measured under the HH and CC conditions (Fig. 4(c,d)). Compared to the DLA, the 65 mm thick AP-SMP fiber array exhibits higher pull-off forces under the HH condition at preloads above 10 mn (Fig. 4(c)). Typical maximum contact areas and force distance curves from the adhesion tests with the AP-SMP fiber array in the HH and CC cases are demonstrated in Fig. 7. Since higher preloading generates larger contact area and larger difference in penetration depth between the edge and the center regions of the contact area, the experiments under higher preloads provide insight into the adhesion performance against a rough surface. 28 Given these considerations, the AP-SMP fiber array appears to be more adaptive to rough surfaces compared to This journal is ª The Royal Society of Chemistry 2009 Soft Matter, 2009, 5,

4 Fig. 6 Optical microscope images of maximum contact areas and force distance curves from the adhesion experiments on 6 mm diameter glass hemisphere for 20 mn preload with 65 mm thick double layer adhesive in the Cool-Cool (a, c) and Hot-Hot (b, d) cases, respectively (scale bars: 200 mm). Fig. 7 Optical microscope images of maximum contact areas with image subtraction methods and force distance curves from the adhesion experiments on 6 mm diameter glass hemisphere for 20 mn preload with 65 mm thick AP-SMP fiber array in the Cool-Cool (a, c) and Hot-Hot (b, d) cases, respectively (scale bars: 200 mm). Since contact areas from adhesion experiments with the AP-SMP fiber array are not quite visible on the corresponding videos, we used the image subtraction methods to identify maximum contact areas in these cases. the DLA. Additional factors contributing to the higher adhesion of the AP-SMP fiber array may also include higher elongation and crack trapping during unloading, both of which are expected for fibrillar dry adhesives. 14,29 However, these benefits associated with the fibrillar structure of the AP-SMP fiber array were not observed under the CC condition. In fact, under such a condition, the adhesion of the AP-SMP fiber array was lower than that of its non-fibrillar counterpart (i.e. DLA) at any given preload (Fig. 4(c)). This observation suggests that the advantages of a fibrillar surface may have been compromised due to the high stiffness of the shape memory polymer fibers under such a condition. In addition, the flexible adhesive polymer thin layer between two neighboring shape memory polymer fibers may be still in contact with the glass hemisphere when the tensile force reached its peak value during the unloading. As a result, the thin adhesive polymer layer between fibers might not have contributed to the maximum pull-off force as much as the adhesive polymer layer on top of the fibers. The adhesion of the AP-SMP fiber array was maximized in the HC case (Fig. 4(d)) in the same way as the DLA; that is, larger contact area or more roughness adaptation during loading and higher intrinsic adhesion of the adhesive polymer layer during unloading. Fig. 4(d) also shows that the pull-off force exhibited by the AP-SMP fiber array is about 10% higher than the DLA for preloads higher than a preload of 10 mn. The indentation difference between the center and the edge of the contact area generated due to the glass hemisphere curvature is larger for higher preloads. Thus, the AP-SMP fiber array is interpreted to be more efficiently adapting to the curvature of the glass hemisphere during loading than the DLA. This interpretation is also supported by the fact that the fibrillar structuring is potentially more advantageous in terms of its ability to adapt to a surface roughness. 11 Under the HH state, the AP-SMP fiber array also showed higher pull-off forces than the DLA for only higher preloads in the same way as HC state case. Conclusion In summary, the AP-SMP fibrillar adhesive exhibits four attractive properties: (1) bulk material properties, such as the stiffness of shape memory polymer, are thermally controllable; (2) surface material properties, such as the intrinsic adhesive strength of the adhesive polymer layer, are also thermally controllable; (3) the shape memory polymer can be deformed and the deformed shape memory polymer can recover its permanent shape thermally; (4) topographically, its fibrillar structure enhances its adhesion performance against rough surfaces. Under the Hot-Cool condition, the combined benefit of (1), (2), and (4) is maximized in the adhesion performance of the AP-SMP fiber array towards rough surfaces. The deformed AP-SMP fiber array after pressing and detaching can return to the original shape because of (3), as observed in our experiment. DLA possesses similar properties except (4). These properties can be useful for developing more versatile dry adhesives with on/off adhesion switching and self-peeling References 1 K. Autumn, Y. Liang, T. Hsieh, W. Zesch, W.-P. Chan, T. Kenny, R. Fearing and R. J. Full, Nature, 2000, 405, K. Autumn, M. Sitti, Y. A. Liang, A. M. Peattie, W. R. Hansen, S. Sponberg, T. Kenny, R. Fearing, J. N. Israelachvili and R. J. Full, Proc. Natl. Acad. Sci. U. S. A., 2002, 99(19), K. Autumn and A. M. Peattie, Integr. Comp. Biol., 2002, 42, K. Autumn, A. Dittmore, D. Santos, M. Spenko and M. Cutkosky, J. Exp. Biol., 2006, 209, W. R. Hansen and K. Autumn, Proc. Natl. Acad. Sci. U. S. A., 2005, 102(2), G. Huber, H. Mantz, R. Spolenak, K. Mecke, K. Jacobs, S. N. Gorb and E. Arzt, Proc. Natl. Acad. Sci. U. S. A., 2005, 102(45), Soft Matter, 2009, 5, This journal is ª The Royal Society of Chemistry 2009

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