250
Langmuir 2007, 23, 250-257
Switchable Friction of Stimulus-Responsive Hydrogels† Debby P. Chang,‡,§ John E. Dolbow,*,§,| and Stefan Zauscher*,‡,§ Department of Mechanical Engineering & Materials Science, Center for Biologically Inspired Materials and Material Systems, and Department of CiVil & EnVironmental Engineering, Duke UniVersity, Durham, North Carolina ReceiVed June 13, 2006. In Final Form: August 23, 2006 Poly(N-isopropylacrylamide) (pNIPAAm) gels are stimulus-responsive hydrogels that exhibit large reversible changes in their volume and surface physicochemical properties near the lower critical solution temperature (LCST) in response to external stimuli, such as a change in temperature or solvent composition. Here we report how different phase states, induced isothermally by changes in the solvent composition, affect the tribological properties of pNIPAAm hydrogels. Our measurements indicate that gels in a collapsed conformation (above the LCST) exhibit significantly more friction than swollen gels (below the LCST) at low shear rates. These differences arise from changes in the surface roughness, adhesive interactions, and chain entanglements of the gel surfaces associated with the phase transition. Importantly, we show that the changes in friction, triggered by an external stimulus, are reversible. These reversible and possibly tunable changes in friction may have a significant impact on the design of coatings for biosensors and for actuation devices based on stimulus-responsive hydrogels.
Introduction Stimulus-responsive hydrogels (SRHs) are cross-linked polymer gels that exhibit reversible changes in volume and surface physicochemical properties in response to a variety of environmental stimuli, such as temperature, pH, ionic strength, and solvent composition.1-4 Stimulus-responsive properties are of particular interest because they can be utilized for actuation and sensing in devices on the nano- and microscales. Although the successful integration of SRHs into molecular recognition sensor surfaces,5 cell culture substrates,6 optical switches,7 drug carriers,8 and microfluidic devices9 has been demonstrated, the rational design of SRHs and the optimization of hydrogel-based actuators requires a better understanding of hydrogel mechanical and tribological properties. Despite the rapid conceptual development of devices, little has been reported on the fundamental tribological properties of SRHs, and a more in-depth analysis of the surface energetic contribution to friction is needed. The objective of this work is to investigate the tribological properties of stimulus-responsive poly(N-isopropylacrylamide) (pNIPAAm) hydrogels as a function of the gel phase state. † Part of the Stimuli-Responsive Materials: Polymers, Colloids, and Multicomponent Systems special issue. * To whom correspondence should be addressed. E-mail: zauscher@ duke.edu,
[email protected]. Tel: (919) 660-5360. Fax: (919) 660-8963. ‡ Department of Mechanical Engineering & Materials Science. § Center for Biologically Inspired Materials and Material Systems. | Department of Civil & Environmental Engineering.
(1) Hirokawa, Y.; Tanaka, T. J. Chem. Phys. 1984, 81, 6379-6380. (2) Eichenbaum, G. M.; Kiser, P. F.; Simon, S. A.; Needham, D. Macromolecules 1998, 31, 5084-5093. (3) Winnik, F. M.; Ottaviani, M. F.; Bossmann, S. H.; Garciagaribay, M.; Turro, N. J. Macromolecules 1992, 25, 6007-6017. (4) Winnik, F. M.; Ringsdorf, H.; Venzmer, J. Macromolecules 1990, 23, 2415-2416. (5) Tanaka, T.; Wang, C. N.; Pande, V.; Grosberg, A. Y.; English, A.; Masamune, S.; Gold, H.; Levy, R.; King, K. Faraday Discuss. 1995, 201-206. (6) Yamato, M.; Konno, C.; Utsumi, M.; Kikuchi, A.; Okano, T. Biomaterials 2002, 23, 561-567. (7) Pardo-Yissar, V.; Gabai, R.; Shipway, A. N.; Bourenko, T.; Willner, I. AdV. Mater. 2001, 13, 1320-1323. (8) Kiser, P. F.; Wilson, G.; Needham, D. Nature 1998, 394, 459-462. (9) Beebe, D. J.; Moore, J. S.; Bauer, J. M.; Yu, Q.; Liu, R. H.; Devadoss, C.; Jo, B. H. Nature 2000, 404, 588-590.
pNIPAAm gels in water undergo a reversible hydrophilichydrophobic phase transition at the lower critical solution temperature (LCST) of approximately 32 °C.10 Below the LCST, pNIPAAm gels are in a swollen, hydrophilic state, and above the LCST, they exist in a collapsed, hydrophobic state. This conformational response essentially results from a balance between osmotic repulsive forces and hydrophobic attractive interactions of the pNIPAAm polymer chain segments in the gel. Below the LCST, the osmotic swelling pressure of the solvent associated with the polymer chains in the gel is large enough to outweigh hydrophobic attractions between the polymer chains; above the LCST, however, hydrophobic attractions dominate, and the gel adopts a hydrophobically collapsed conformation.11 In addition to temperature, the addition of co-nonsolvents can also induce a phase transition in pNIPAAm at room temperature.3,4 For example, the change of solvent from pure water to 50% methanol by volume lowers the LCST from 32 °C to below 0 °C. Therefore, at room temperature (∼25 °C), pNIPAAm gels are swollen in water and collapsed in a water/methanol (1:1 v/v) mixture.11 Here we make use of the methanol/water solvent system to induce the gel phase transition isothermally at room temperature. In general, friction between polymer surfaces has been related to, among other factors, changes in polymer chain conformation and entanglements on the surface and adhesion hysteresis at the interface.12-15 A series of previous studies investigated polymer gel friction against a variety of substrates, including glass and other polymer gels.16-24 An adsorption-repulsion model, (10) Zhang, J.; Pelton, R.; Deng, Y. Langmuir 1995, 11, 2301-2302. (11) Schild, H. G.; Muthukumar, M.; Tirrell, D. A. Macromolecules 1991, 24, 948-952. (12) Chen, N. H.; Maeda, N.; Tirrell, M.; Israelachvili, J. Macromolecules 2005, 38, 3491-3503. (13) Chen, Y. L.; Helm, A.; Israelachvili, J. N. J. Phys. Chem. 1991, 95, 10736-10747. (14) Maeda, N.; Chen, N. H.; Tirrell, M.; Israelachvili, J. N. Science 2002, 297, 379-382. (15) Maeda, N.; Chen, N. H.; Tirrell, M. V.; Israelachvili, J. 2003, 225, U624U624. (16) Baumberger, T.; Caroli, C.; Ronsin, O. Phys. ReV. Lett. 2002, 88, 075509. (17) Baumberger, T.; Caroli, C.; Ronsin, O. Eur. Phys. J. E 2003, 11, 85-93. (18) Gong, J.; Iwasaki, Y.; Osada, Y. J. Phys. Chem. B 2000, 104, 34233428. (19) Gong, J.; Kagata, G.; Osada, Y. J. Phys. Chem. B 1999, 103, 6007-6014.
10.1021/la0617006 CCC: $37.00 © 2007 American Chemical Society Published on Web 11/07/2006
Switchable Friction of Stimulus-ResponsiVe Hydrogels
Langmuir, Vol. 23, No. 1, 2007 251
Figure 1. Schematic representation of the rheometer setup for (a) gel-gel friction measurements and (b) gel-gel adhesion measurements.
proposed by Gong and Osada,20 explains friction in terms of the adhesive interaction of the polymer gel surfaces. According to this model, friction is thought to arise from the interaction of polymer chains on the surfaces of the opposing gel substrates. In the case of repulsive interactions, friction is believed to arise from hydrodynamic lubrication, which is governed by the sliding speed and viscosity of the solvent. For attractive interactions, below a critical sliding velocity, friction arises from the stretching of polymer chains adsorbed to the opposing surface and increases with increasing velocity up to a critical velocity. Above this critical velocity, friction decreases with increasing velocity because polymer chains do not have sufficient time to adsorb onto the opposing surface, and friction is then dominated by the viscous drag of the lubricating layer between the two surfaces. Whereas the normal load dependence and the velocity dependence of friction have been studied, the effect of the gel phase state on gel friction has not been investigated. In this article, we examine the effect of load and sliding velocity on friction as a function of phase state for pNIPAAm gels. We observed that friction depends strongly on the gel phase state and that friction is reversibly triggerable through an environmental stimulus. The contributions of adhesion, surface roughness, and mechanical material properties to this interesting behavior are discussed. (20) Gong, J.; Osada, Y. J. Chem. Phys. 1998, 109, 8062-8068. (21) Gong, J. P.; Higa, M.; Iwasaki, Y.; Katsuyama, Y.; Osada, Y. J. Phys. Chem. B 1997, 101, 5487-5489. (22) Gong, J. P.; Iwasaki, Y.; Osada, Y.; Kurihara, K.; Hamai, Y. J. Phys. Chem. B 1999, 103, 6001-6006. (23) Gong, J. P.; Osada, Y. Prog. Polym. Sci. 2002, 27, 3-38. (24) Kagata, G.; Gong, J. P.; Osada, Y. J. Phys. Chem. B 2002, 106), 45964601.
Experimental Section Gel Synthesis. Cross-linked poly(N-isopropylacrylamide) homopolymer hydrogels were synthesized by free radical polymerization of N-isopropylacrylamide (NIPAAM, Aldrich) with N,N′-methylenebis-acrylamide (MBAAm, Aldrich) as the cross linker, ammonium persulfate (AP, Acros) as the redox initiator, and N,N,N′Ntetramethylethylenediamine (TEMED, Acros) as a co-initiator.25 Deionized water (Myron L Company, 18 MΩ/cm) was used as the polymerization solvent. A 100:1 mixture molar ratio of monomer to cross linker was prepared by combining 1.5 g of NIPAAM, 0.0204 g of MBAAm, 30 µL of TEMED, and 18 mL of deionized water. The solution was bubbled with dry nitrogen gas in an ice water bath until complete dissolution occurred. The addition of 0.03 g of AP initiated the polymerization. The pregel solution was then quickly injected between two glass plates separated by a 3-mm-thick spacer. Polymerization was carried out at room temperature for about 6 h. After polymerization, the gel specimens were dialyzed against deionized water over a period of 2 days to remove free monomers before experimental testing. Friction Measurements. Friction measurements on pNIPAAm gel surfaces were performed with an ARES rotational rheometer (Advanced Rheometric Expansion System, Rheometrics Scientific Inc). All experiments were carried out in aqueous solution with parallel-disk plates, where the top plate was kept stationary and the bottom plate rotated at a specified rotation rate. A disk-shaped gel specimen with a 20 mm diameter was cut and glued onto the top plate of the rheometer with cyanoacrylate (CA) adhesive. A second gel disk, 25 mm in diameter, was glued to the bottom plate with CA adhesive; the larger area for the bottom gel was chosen to ensure that at any rotational position complete contact is made with the top (25) Makino, K. ThermosensitiVe Gels; Marcel Dekker: New York, 2000; Vol. 88.
252 Langmuir, Vol. 23, No. 1, 2007
Chang et al.
gel (Figure 1a). CA adhesive was chosen because it is a good adhesive for soft-wet materials.26 Friction measurements were performed under constant compressive strain conditions where the gap height between the two parallel disk plates was adjusted to set a desired normal load. The gap height was held constant at each normal load for the duration of an experiment. Because of the viscoelastic properties of the hydrogel, stress relaxation occurs after a constant strain is applied. For our gels, we observed that the normal stress typically reached a quasi-equilibrium value about 60 min after the initial normal load was applied. To minimize stress relaxation effects, friction measurements were performed 60 min after applying a change in normal load. The lower plate was then rotated against the top plate at a constant shear rate, ω, while recording torque, T, and normal force, N, as a function of time. The friction force, F, is determined from the measured torque, T, using eq 119 F)
4T 3R
(1)
where R is the radius of the contact area. The equation is derived by assuming a linear dependence of shear stress on sliding velocity. (A detailed derivation is presented in the Supporting Information). To measure friction forces for a range of applied normal loads, the two opposing pNIPAAm gel disks were compressed against each other until an initial normal load of 0.2, 0.4, 0.6, or 0.8 N was reached. The corresponding compressive strains were calculated from the gap spacing at contact (lower normal force threshold) and the compressive displacement necessary to set a desired normal load. At each normal load and after 60 min of stress relaxation, the gels were then sheared against each other at four different shear rates, 0.01, 0.1, 1, and 10 rad/s, maintaining a fixed gap distance between the rheometer disk geometries. Gel surfaces were sheared against each other for 30 min to allow the normal and friction forces to stabilize at each shear rate. Then the shear rate was increased incrementally to the next level. We define an instantaneous coefficient of friction (COF), µ, by dividing the time-averaged friction force, Fˆ , by the time-averaged normal force, N ˆ , at each shear rate, µ)
Fˆ N ˆ
(2)
Friction measurements in different gel phase states were carried out isothermally at room temperature (T ≈ 25 °C). Different gel phase states were obtained isothermally by changing the solution from water (swollen gel) to a water/methanol mixture (1:1 v/v) (collapsed gel) in the unloaded state. Prior to each measurement, the gel disks were compressed against each other until the desired normal load was reached. Viscometric measurements established that there is not any significant contribution to torque due to solvent viscosity when the two gel surfaces are sheared in close proximity to each other without being loaded (zero normal force). The viscosity of water at 25 °C is about 0.9 mPa‚s, and that of methanol is about 1.6 mPa‚s. To investigate whether the friction response is reversible, we cycled the pNIPAAm gels from the collapsed to the swollen state by changing the solvent from water/MeOH (1:1 v/v) to water and shearing at 1 rad/s at an initial applied normal load of 0.5 N. Gels were immersed in each solvent for over 15 min prior to measurements to allow sufficient time for the bulk phase transformation to occur. Adhesion Measurements. Bulk adhesion measurements were performed to determine the adhesive properties between pNIPAAm gel surfaces as a function of phase state. These experiments were performed in the same experimental setup described above for friction measurements (Figure 1b). The top plate, with an attached gel disk, was lowered at a constant speed of 0.008 mm/s toward the opposing substrate until a normal force of 0.5 N was registered. After the two gel surfaces were held in contact for approximately 100 s, the two plates were retracted at a constant speed (0.008, 0.08, or 5 mm/s) until contact was lost. Throughout this loading regime, the normal (26) Singer, A. J.; Thode, H. C. Am. J. Surg. 2004, 187, 238-248.
force was recorded as a function of time. The approach speed and gel contact time were kept constant such that the effect of retraction speed on adhesion could be observed. The adhesive force, defined as the maximum force reached upon unloading just before a “jump” off the surface occurs, was determined from the retraction curves. Microscopic adhesion measurements between gel substrates and pNIPAAm brush-modified AFM cantilevers were performed with atomic force microscopy (AFM) in force spectrometry mode (MultiMode AFM with Nanoscope IIIa controller, Digital Instruments). The pNIPAAm-decorated cantilever tips were prepared from V-shaped Si3N4 cantilevers (Nanoprobe, Veeco) fitted with spherical borosilicate glass beads (series 9005, 4.9 ( 0.5 µm radius, Duke Scientific) using an XYZ manipulator (Signatone, S-926) and a two-part, heat-hardening epoxy (Epo-Tek no. 377).27,28 The modified cantilever was then coated with 5 nm chromium followed by 20 nm of gold using e-beam evaporation (CHA Industries Solution E-Beam Evaporator). PNIPAAm brushes (about 50 nm thick) were grown by surface-initiated atom-transfer radical polymerization (ATRP) on the gold probe, as described in detail elsewhere.29 The gel substrate for the AFM experiment was synthesized with the pre-gel solution injected between two glass slides with a 0.1 mm spacer as described above. The lower glass slide was chemically functionalized with Bind Silane30 (3-methacryloxy-propyltrimethoxysilane, Alfa Aesar) to allow chemical attachment of the gel. The spring constants of the Si3N4 microcantilevers were determined before each experiment from the power spectral density of the thermal noise fluctuations in solution.31 The sensitivity of the photodetector was determined from the constant compliance regime after engaging the cantilever on the substrate surface. Adhesion measurements were carried out at a constant approach/retraction speed of 1.3 µm/s. Mechanical Testing. The mechanical properties of the pNIPAAm gels at different gel phase states were determined with the same rheometer used for adhesion and friction measurements. To avoid slip at the gel-plate interfaces, a disk-shaped gel specimen with 20 mm diameter was glued onto the top and bottom plates of the rheometer using cyaonoacrylate adhesive. The shear modulus, G, of the gel was obtained from the slope of the shear stress versus strain curve. The dynamic modulus was determined by smallamplitude oscillatory shear measurements in which the lower disk was driven in an oscillatory torsional motion with predetermined amplitude (controlled strain) and frequency. The in-phase and outof-phase components of the dynamic modulus at the fundamental (applied) frequency and in the limit of small strain amplitudes are reported as the storage and loss moduli, G′ and G′′, respectively. Dynamic modulus tests were performed at an oscillation frequency of 1.6 Hz, in the linear deformation regime, with a 0.2 N compressive load applied. Atomic Force Microscopy. PNIPAAm gel surfaces were imaged in solution by tapping mode AFM (NanoScope IIIa, Veeco) with a V-shaped silicon nitride cantilever (Nanoprobe, Veeco; spring constant 0.12 N/m). Cantilevers were oscillated at a frequency of 7.65 kHz with an amplitude set point of around 0.6 V. We adjusted the amplitude of the tapping oscillation at a given set point such that the surface could be imaged with no loss of surface engagement. These imaging conditions were found by gradually increasing the oscillation amplitude to just the point at which a good image could be obtained. Thus, the imaging force was “minimized” to a level that allowed consistent imaging. Clearly, there is some tip penetration into the soft gel surface, and the surface topography is a complicated convolution of the tip-surface interaction. To avoid slip, the gel specimen was chemically adhered onto a glass slide with Bind Silane.30 AFM images of the gel surfaces were obtained with tapping mode AFM, where the cantilever oscillation amplitude is held constant by a feedback loop. The force on the sample was set and maintained (27) Zauscher, S.; Klingenberg, D. J. Colloids Surf., A 2001, 178, 213-229. (28) Garoff, N.; Zauscher, S. 2002, 18, 6921-6927. (29) Kaholek, M.; Lee, W. K.; Ahn, S. J.; Ma, H. W.; Caster, K. C.; LaMattina, B.; Zauscher, S. Chem. Mater. 2004, 16, 3688-3696. (30) Suzuki, A.; Yamazaki, M.; Kobiki, Y. J. Chem. Phys. 1996, 104, 17511757. (31) Hutter, J. L.; Bechhoefer, J. ReV. Sci. Instrum. 1993, 64, 1868-1873.
Switchable Friction of Stimulus-ResponsiVe Hydrogels
Langmuir, Vol. 23, No. 1, 2007 253
Figure 2. (a) Normal force and (b) torque (friction force) plotted as a function of time for two pNIPAAm gel surfaces sliding against each other at four shear rates (0.01, 0.1, 1, and 10 rad/s) in water at room temperature. at a level at which imaging was possible, but compression and damage to the gel surfaces were minimized. Imaging soft surfaces in tapping mode essentially eliminates lateral forces exerted by the scanning tip and thus reduces unnecessary damage to the gel surface. The root-mean-square (rms) roughness of a scanned gel area (15 µm × 15 µm) was obtained with AFM image analysis software. The rms roughness of the scanned area was calculated with eq 3
x
∑(Z - Z˙ )
rms(roughness) )
2
i
i
N
(3)
where Zi is the height at position i, Z˙ is the average topographical height, and N is the number of data points within the given area.
Results Effect of Normal Force and Shear Rate on Friction. To understand the effect of the phase state on friction, pNIPAAm gels were sheared against each other at different normal loads and shear rates. Normal force and torque are plotted as a function of time at four different shear rates in Figure 2a,b, respectively. Because changes in the shear rate affect both normal and friction forces, measurements were time averaged over a 15 min interval that immediately followed an initial 15 min interval to allow a transient response to subside. Different normal force loading regimes were obtained prior to each experiment by setting the gap between the measurement geometries until the desired normal load was attained. Figure 3 shows that the coefficient of friction (COF) depends strongly on (i) the shear rate, (ii) the normal force, and (iii) the phase state of the gel (i.e., swollen or collapsed). Figure 3 also shows that at low shear rates (0.01-0.1 rad/s) the COF is small (on the order of 0.1) for the swollen gel and large (0.6-1.2) for the collapsed gel. Whereas the COF for swollen gels increases with increasing shear rate, the COF for collapsed gels decreases with increasing shear rate at shear rates greater
Figure 3. Coefficient of friction plotted as a function of shear rate at different applied normal loads for pNIPAAm gels in (a) swollen and (b) collapsed states.
than 0.1 rad/s. One notable exception occurs for the swollen gel at low shear rate and at the smallest normal force. In this case, the COF is initially large (comparable to that of the collapsed gel) and at first decreases with increasing shear rate (Figure 3a, filled circles). Interestingly, at 10 rad/s, the highest shear rate tested, the friction of the swollen gel surpasses that of the gel in the collapsed state. Localized shear heating effects, inducing a thermal phase transition in the water-swollen gels, likely cannot explain the friction behavior shown in Figure 3. First, all measurements were carried out in a temperature-controlled liquid bath that provides a significant heatsink. Second, a temperature rise of more than 5 °C over a large portion of the shearing interface would be necessary to induce a phase transition in the gel; considering the presence of a large thermal bath and only moderate shear rates, this seems unlikely to occur. The observed friction behavior is complex and likely depends on the surface roughness, surface energy (adhesion and adhesion hysteresis), surface wear, and mechanical properties (static and dynamic shear moduli) of the gels, all of which are dependent on the gel phase state. Surface Roughness. AFM tapping mode images (15 µm × 15 µm) show that there are significant differences in pNIPAAm gel surface morphology in the two phase states (Figure 4). The pNIPAAm gel in the swollen state (imaged in water) appears relatively smooth and diffuse, with a root-mean-square (rms) roughness of 5 nm compared to the same gel in the collapsed state (imaged in water/MeOH (1:1, v/v)) which displays large globular domains and has a rms roughness of 21 nm. The gel surfaces show significant topographical variation over the scan area, with a height range of approximately 130 nm for the
254 Langmuir, Vol. 23, No. 1, 2007
Chang et al.
Figure 5. Shear modulus, storage, and loss modulus for a typical pNIPAAm gel in swollen and collapsed states as measured with a rheometer. The shear modulus was obtained from a shear stress vs strain curve. The dynamic results were measured at an angular frequency of 1.6 Hz with a shear strain of 10%.
collapsed gel surface and 50 nm for the swollen gel surface. AFM images obtained over a 0.5 µm × 0.5 µm scan area reveal roughness and surface morphology that agree well with previously reported observations for pNIPAAm gels (Supporting Information).30,32 It is clear that, particularly in the collapsed state, the gel surfaces are not smooth and thus contact between these surfaces likely occurs at a multitude of contacting surface asperities. During sliding, interfacial forces responsible for friction are generated at these contact points. Friction thus depends in a complex way on the local topography, sliding speed, viscoelastic properties of the contact, and surface energy (adhesion and adhesion hysteresis). Mechanical Properties. The shear modulus, G, and the dynamic shear moduli, G′ (storage modulus) and G′′ (loss modulus), of a typical pNIPAAm gel specimen in its swollen and collapsed state are plotted in Figure 5. Figure 5 shows that the magnitudes of the shear modulus and the storage modulus are significantly larger for a pNIPAAm gel in the collapsed conformational state than for the same gel in the swollen state. This behavior is expected and reflects the profound structural change in gel structure that is associated with the hydrophobic collapse.
Adhesion. The adhesive properties of the contacting pNIPAAm gel surfaces likely contribute to friction. We argue that adhesion and adhesion hysteresis may arise from two fundamentally different sources: (I) the surface energy of the gel surfaces (i.e., surface polarity) and (II) the state of entanglement between polymer loops and chain ends emanating from the opposing gel surfaces.14 To directly probe the effect of the phase transition on surface energy and polymer conformation on the gel surface, we performed microscopic adhesion measurements below and above the LCST on gel surfaces by AFM surface force measurements, using colloidal probe cantilevers decorated with a pNIPAAm brush layer of high grafting density. The measurement configuration and the solvent-dependent pNIPAAM conformations are illustrated schematically in the insets to Figure 6a,b. Figure 6a shows that the interaction upon approach and retraction was monotonically repulsive in water and that the onset of repulsive force interactions occurred at about 200 nm. When two polymer-bearing surfaces are brought into increasingly compressive contact in a good solvent, repulsive steric forces arise from the restriction of conformational degrees of freedom of the thermally mobile polymer chains emanating from the polymer-bearing surfaces. We note, however, that part of the observed interaction also likely reflects the “indentation” of the colloidal probe into the soft and diffuse gel surface layer. Figure 6b shows that upon approach in the water/MeOH mixture the repulsive interaction distance is significantly reduced, when compared to the interaction in water, and that upon retraction a large nonspecific adhesion force occurs. These observations suggest that in the water/MeOH mixture the pNIPAAm brush and the gel surface are in a hydrophobically collapsed state. In contrast to the measurement in water, however, there is now a large adhesive component associated with the retraction of the probe from the gel surface. This adhesion likely arises from van der Waals forces that can effectively act between the hydrophobically collapsed polymer chain segments on the gel and colloidal probe.29 This is also consistent with theory that predicts that the force between two hydrophobic surfaces, interacting in a polar solvent (water), is attractive.33 We also performed macroscopic adhesion measurements by measuring the force required to separate two gel surfaces in a good and a poor solvent over a range of retraction speeds (Figure 7). In contrast to the microscopic adhesion measurements, our
(32) Suzuki, A.; Yamazaki, M.; Kobiki, Y.; Suzuki, H. Macromolecules 1997, 30, 2350-2354.
(33) Israelachvili, J. N. Intermolecular and Surface Forces; 2nd ed.; Academic Press: London, 1991.
Figure 4. Three-dimensional tapping mode AFM images of pNIPAAm gel in (a) swollen and (b) collapsed states. The rms roughness in a 15 µm × 15 µm surface scan area is 5 nm for the swollen gel and 21 nm for the collapsed gel.
Switchable Friction of Stimulus-ResponsiVe Hydrogels
Langmuir, Vol. 23, No. 1, 2007 255
Figure 6. Force plotted as a function of separation upon approach and retraction of a colloidal probe, decorated with a pNIPAAM brush layer, from a pNIPAAm gel surface under (a) good solvent conditions (swollen) and (b) poor solvent conditions (collapsed).
results here show that adhesion is significantly larger at all retraction velocities for the swollen gel (in water) than for the collapsed gel (in water/MeOH) and increases with increasing retraction speed (Figure 8). The larger adhesive force in the swollen state may be explained by three potential mechanisms: (i) disentanglement and stretching of polymer chain ends and loops in the diffuse interface between the contacting gel surfaces, (ii) hydrodynamic effects associated with solvent flowing into the gap between the separating disk plates, and (iii) a larger effective contact area. Chain entanglements should occur more readily when the contacting gel surfaces are in the swollen state, presenting a diffuse, hydrated interface. An apparent adhesion force due to the disentanglement and stretching of polymer chain ends and loops thus arises upon separating the surfaces. This mechanism also agrees with the observed rate dependence of the adhesion force. At low retraction speed, the two gel surfaces loose contact gradually, which is indicated by the gradual rise to zero force in the adhesion force curve (Figure 7a) as if the gel surfaces were slowly peeling apart. This behavior was not observed when separating the gel surfaces in a poor solvent (Figure 7b). At a large retraction speed, there is insufficient time for extensive chain disentanglement to occur, which causes a more elastic response upon surface separation, followed by abrupt rupture. We did not observe chain entanglement effects in our microscopic adhesion measurements in good solvent, and we believe that this is due to the use of polymer brushes, tethered to the colloidal probe at high grafting density, as the counter surface. The interpenetration of chain segments emanating from the gel surface into the brush is entropically disfavored because of the large osmotic pressure leading to chain stretching in the densely grafted polymer brush.
Figure 7. Normal force plotted as a function of time for a typical bulk adhesion measurement for gels in (a) swollen and (b) collapsed states.
Figure 8. Adhesion force (maximum pull-off force required to separate the gel surfaces) between two pNIPAAm gel surfaces plotted as a function of retraction speed for two gel phase states.
When separating the gel surfaces, solvent needs to enter the increasing space formed between the separating gel surfaces. The viscous, radial inflow of the solvent gives rise to an apparent adhesion force. This force can be calculated by the Stefan equation,34 which predicts that the force necessary to separate two disk plates in a liquid depends, among other things, linearly on the viscosity of the liquid and the rate of separation and (34) Bird, R. B.; Armstrong, R. C.; Hassager, O. Dynamics of Polymeric Liquids, 2nd ed.; Wiley: New York, 1987; Vol. 1.
256 Langmuir, Vol. 23, No. 1, 2007
inversely on the disk separation to the third power. The force contribution predicted by the Stefan equation for our measurement geometry and experimental protocol is small and cannot explain the observed adhesion behavior shown in Figure 7. This was further confirmed by control measurements, where we separated disk plates without attached hydrogels in water and a water/ MeOH mixture at the separation rates used in the adhesion experiments. In collapsed gels, polymer chain ends and loops are in a collapsed conformational state and likely remain tightly bound to one another and to polymer chain segments on the gel surface through hydrophobic interactions. This behavior is consistent with (i) the increase in the elastic shear modulus for gels in the collapsed state (Figure 5), (ii) the decrease in steric interaction distance observed in microscopic adhesion measurements (Figure 6), and (iii) the increase in surface roughness. These observations support the notion that contact for gels in the collapsed state is largely confined to contacting asperities. This is in contrast to pNIPAAm gels in the swollen state, where surfaces, although not molecularly smooth, likely are in significant conformal contact. This also suggests that the effective contact area for pNIPAAm gel surfaces contacting each other in the hydrophobically collapsed state is substantially smaller than that in the swollen gel state. Although attractive forces at the contacting asperities likely arise from hydrophobic interactions (Figure 6b), our macroscopic adhesion measurements do not show any adhesion force (Figure 7b) at the lowest separation rate for gels in the collapsed state and only significantly smaller forces than for the swollen gels at larger separation rates (Figure 8). Reversible Friction and Wear. The tribological properties of stimulus-responsive pNIPAAm gels depend strongly on the gel phase state. Here we examine whether this phase-dependent property is reversible by performing friction measurements between pNIPAAm gel surfaces after repeated cyclic exposure to water and water/methanol mixtures. At a shear rate of 1 rad/s, when the two opposing gel specimen were cycled repeatedly from the collapsed to the swollen state, the COF also switched from high (collapsed state) to low (swollen state) values (Figure 9a). The same experiment was repeated at a shear rate of 10 rad/s (Figure 9b). Here the friction behavior was different than that at 1 rad/s. In the first cycle, the COF for the collapsed gel was significantly larger than that for the swollen gel. As the two gel surfaces continued to shear against each other in the second and third cycles, the COF for the gel in the collapsed state dropped again below that of the swollen gel, as expected (Figure 3). This observation suggests that in addition to reversible interactions between the two sliding gel surfaces, irreversible interactions (i.e., wear) occurs and influences the frictional response for subsequent cycles. A macroscopic inspection of the gel surfaces before and after sliding at large shear rates revealed signs of wear (i.e., the surfaces scattered light differently and appeared roughened). It is reasonable that wear is most pronounced at a high shear rate and is associated with the gel being in a hydrophobically collapsed state, in which the compliance is large and surface contact occurs at asperities.
Discussion We found that the dependence of the COF on shear rate and normal force is a strong function of the phase state of the pNIPAAm gel. Furthermore, we showed that the roughness of the gel surfaces and their mechanical and adhesion properties also depend on the gel phase state. Friction between two sliding pNIPAAm gel surfaces thus likely arises from a combination of several different dissipative mechanisms. These include topo-
Chang et al.
Figure 9. Coefficient of friction of pNIPAAm gels plotted for three consecutive cycles from the collapsed to swollen gel state. (a) Shear rate, 1 rad/s; normal load, 0.35 ( 0.04 N and (b) shear rate, 10 rad/s; normal load, 0.26 ( 0.03 N.
graphical and chemical heterogeneities on the gel surfaces, timeand rate-dependent structural rearrangements of molecules at the sliding interface, and viscoelastic effects associated with the bulk deformations of adhering surfaces.12 It was previously shown that all of these factors can contribute to adhesion hysteresis and can thus provide an important dissipative mechanism for friction between sliding polymer surfaces.12,14 We argue that friction for pNIPAAm gels in the swollen state arises from viscous interactions between the gel surfaces, sliding past each other in conformal contact. Moreover, our macroscopic adhesion measurements suggest that shear-rate-dependent stretching and disentanglement of polymer chain ends and loops in the “diffuse” contact zone between the two sliding surfaces likely contribute significantly to friction. At low shear rates (∼0.1 rad/s), there is sufficient time for polymer chains to respond to an applied stress and to disentangle, and thus the COF is generally low (∼0.1). With increasing shear rate, however, the COF increases significantly, which can be explained by the increase in shear force associated with the stretching and rupture of polymer chain ends and loops tethering the opposing gel surfaces.35 In addition, the decrease in the friction force with increasing normal force observed for pNIPAAm gels in the swollen state at the lowest shear rate can be explained by the flattening of compliant asperities, bringing the contacting surfaces into better conformal contact and thus leading to “smooth” sliding.27 The effective contact area for pNIPAAm gel surfaces contacting each other in the hydrophobically collapsed state is likely (35) Cherry, B. W. Polymer Surfaces; Cambridge University Press: Cambridge, U.K., 1981.
Switchable Friction of Stimulus-ResponsiVe Hydrogels
substantially smaller than that in the swollen gel state. When pNIPAAm gel surfaces are in contact with each other in a collapsed state, then hydrophobic interactions should contribute to increased adhesion between contacting asperities and thus possibly also increase the friction. Macroscopic adhesion measurements, however, showed that the adhesion force is small. We thus argue that an additional and likely more important contribution to friction for pNIPAAm gels in the collapsed state is associated with the interlocking of surface asperities during sliding (i.e., when the surfaces of two pNIPAAm gels in the collapsed state slide past each other, friction arises from the shear forces associated with the deformation of contacting asperities, thus overcoming their mechanical interlocking).35 Our mechanical measurements of gel elasticity showed that the elastic shear modulus increases 3-fold when the gel undergoes a hydrophobic collapse. This would reasonably also imply an increase in the shear force necessary to slide asperities past each other. This friction mechanism is further supported by our observation that the COF decreases at the highest shear rate (10 rad/s) (Figure 3) (i.e., the friction response transitions from a stick-slip behavior to more of a slip behavior with increasing shear rate, leading to apparently “smoother” sliding and less friction). Finally, our data on the reversibility of friction showed that there is significant wear-in associated with the sliding of the gel surfaces at large shear rates and in the hydrophobically collapsed state. Once wear tracks have been established between the sliding surfaces, it is reasonable that the contribution of stickslip friction (arising from rough surfaces) to overall friction is greatly reduced.
Conclusions The tribological properties of stimulus-responsive pNIPAAm gels were investigated by shearing two pNIPAAm gels against
Langmuir, Vol. 23, No. 1, 2007 257
each other in a parallel disk rheometer. The friction response and normal load for two gel surfaces sliding past each other were obtained in the collapsed and swollen gel states after accounting for normal stress relaxation. We found that the COF for pNIPAAm gels depends not only on the shear rate and normal force but also strongly on the phase state of the gel (swollen or collapsed). The COF of the collapsed gel is generally larger than that of the swollen gel and is mainly due to significant shearing between surface asperities. Both surface roughness and the elastic shear modulus increase substantially for gels in the hydrophobically collapsed state. Friction for gels in the swollen state largely arises from chain entanglements between the two gel surfaces sliding past each other in conformal contact. Significantly, the friction response is reversible and can be manipulated by inducing a phase change in the stimulus-responsive pNIPAAm gel. The reversibility of the friction properties may have a significant impact on the design of coatings for biosensors and for actuation devices that exploit the unique mechanochemical properties of stimulus-responsive hydrogels. Acknowledgment. We thank Dr. Lori Setton at Duke University for generous access to the ARES rheometer. This research was supported by the National Science Foundation through grant CM5-0324459 (J.E.D. and S.Z.) and through grant NSF DMR-0239769 Career (S.Z.). D.P.C. gratefully acknowledges the financial support from the Center for Biologically Inspired Materials & Material Systems (CBIMMS). Supporting Information Available: Derivation of the friction/ torque equation. Three-dimensional tapping mode AFM images of the pNIPAA gel in the swollen and collapsed states. This material is available free of charge via the Internet at http://pubs.acs.org. LA0617006
