Direct Surface Force and Contact Angle Measurements of an

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Langmuir 1998, 14, 2838-2845

Direct Surface Force and Contact Angle Measurements of an Adsorbed Polymer with a Lower Critical Solution Temperature F.-J. Schmitt,*,†,‡ C. Park,‡ J. Simon,§,| H. Ringsdorf,§ and J. Israelachvili‡ Department of Chemical Engineering, University of California, Santa Barbara, California 93106-5080, and Institute of Organic Chemistry, J. Gutenberg-University, D-55099 Mainz, Germany Received August 15, 1997. In Final Form: February 19, 1998 As an example of a polymer with a lower critical solution temperature, TLC, in water, we studied the phase behavior of a poly(N-isopropylacrylamide) copolymer. To adsorb it to a negatively charged surface, we introduced a positively charged comonomer. The combination of direct force measurements with the surface forces apparatus (SFA) and contact angle measurements reveal a lower TLC of the adsorbed layer than in bulk solution. At and above TLC the adsorbed polymer chains collapse and the adsorbed layers on the two opposing surfaces attract each other. The resulting adhesion is strongly dependent on the applied speed of separation, emphasizing the role of slow molecular relaxations and rearrangements above TLC. The results also point to the validity of the time-temperature superposition principle to this polymer system at T > TLC: at a low speed of separation the polymer layer is “liquid-like”, and at high speeds its behavior is more “solid-like”.

Introduction In recent years, water soluble polymers have regained a lot of interest both in research and in industrial applications, e.g., in fields such as water-based coatings and paints, additives to food and cosmetics, or medicine and biotechnology.1 Some of these polymers undergo a reversible phase separation on changing the temperature above a certain threshold. The temperature where such a transition from a one-phase (solution) to a two-phase region occurs is called the lower critical transition temperature, or TLC.2 Examples of such polymers are poly(ethylene oxide) (PEO),3,4 ethyl(hydroxyethyl)cellulose (EHEC),5 and poly(N-isopropylacrylamide) (PNIPAM).6-8 An understanding of the underlying cause for this temperature-induced phase separation would be invaluable for designing solute specific reactions or separation schemes that could be regulated via temperature control. With the current advances in polymer synthesis, novel polymers can be designed that exhibit the TLC property in different solvents. Further, polymers may be engineered to have an affinity for a particular reaction product where, for example, a small change in temperature creates * To whom correspondence should be addressed. Phone: 49 (0)351 4658-222. Fax: 49 (0)351 4658-284. E-mail: schmfj@ argos.ipfdd.de. † Now with the Institute of Polymer Research Dresden, P.O. Box 120411, D-01005 Dresden, Germany. | Now with Bayer AG, ZF-MFF, D-51368 Leverkusen, Germany. ‡ University of California. § J. Gutenberg-University. (1) Water-Soluble Polymers: Synthesis, Solution Properties, and Applications; American Chemical Society: Washington, DC, 1991. (2) Taylor, L. D.; Cerankowski, L. D. J. Polym. Sci. 1975, 13, 2551. (3) Bailey, F. E. J.; Koleske, J. V. Poly(ethylene oxide); Academic Press: New York, 1976. (4) Claesson, P. M.; Go¨lander, C.-G. J. Colloid Interface Sci. 1987, 117, 366. (5) Malmsten, M.; Claesson, P. M.; Perzon, E.; Perzon, I. Langmuir 1990, 6, 1572. (6) Heskins, M.; Guillet, J. E. J. Macromol. Sci., Chem. 1968, A2, 1441. (7) Hirokawa, Y.; Tanaka, T. J. Chem. Phys. 1984, 81, 6379. (8) Schild, H. G. Prog. Polym. Sci. 1992, 17, 163.

a phase separation that can be made selective for the reaction products or catalyst poisons. PNIPAM is a well investigated polymer material and has been recently reviewed by Schild.8 Most of the publications on PNIPAM are either concerned with elucidating the physical basis of the phase transition at TLC or with potential applications. The TLC is often measured by visual inspection of the cloud point or, more accurately, by absorption spectroscopy. At the temperature when the solution becomes cloudy, the scattering and thereby the measured light absorbance increases. The mechanism of the phase transition has been investigated by various methods, on both single molecules and gels (see, e.g., refs 9-12). Most likely, the phase transition consists of two steps: At TLC, individual polymer chains first collapse and then aggregate into particles. Lightscattering experiments on high molecular weight PNIPAM revealed that the molecules undergo a coil-to-globule transition on collapse.8,13-16 The relative importance of the two steps clearly depends on the concentration of PNIPAM. In a dilute solution, the first step of intramolecular collapse dominates and intermolecular aggregation is less likely. On the other hand, nonradiative energy transfer fluorescence studies showed that there is a gradual shrinking of the solvated polymer coils into the collapsed state, followed by intermolecular aggregation. The different conclusions reached on the abruptness of the phase transition could be caused by the different measurement techniques used. Nonradiative energy (9) Bae, Y. H.; Okano, T.; Kim, S. W. J. Polym. Sci., Part B: Polym. Phys. 1990, 28, 923. (10) Tokuhiro, T.; Amiya, T.; Mamada, A.; Tanaka, T. Macromolecules 1991, 24, 2936. (11) Tanaka, T.; Fillmore, D.; Sun, S.-T.; Nishio, I.; Swislow, G.; Shah, A. Phys. Rev. Lett. 1980, 45, 1636. (12) Beltran, S.; Hooper, H. H.; Blanch, H. W.; Prausnitz, J. M. J. Chem. Phys. 1990, 92, 2061. (13) Fujishige, S. Polym. J. (Tokyo) 1987, 19, 297. (14) Fujishige, S.; Kubota, K.; Ando, I. J. Phys. Chem. 1989, 93, 3311. (15) Kubota, K.; Fujishige, S.; Ando, I. Polym. J. (Tokyo) 1990, 22, 15. (16) Kubota, K.; Fujishige, S.; Ando, I. J. Phys. Chem. 1990, 94, 5154.

S0743-7463(97)00919-0 CCC: $15.00 © 1998 American Chemical Society Published on Web 04/21/1998

Force and Contact Angle Measurements of an LCST Polymer

transfer examines a smaller length scale than light scattering and is thus more sensitive to microscopic (molecular scale) changes.17,18 For a discussion on the order of the phase transition see, e.g., ref 7. The TLC of PNIPAM is around 32 °C, which is close to both room and body temperature. This makes PNIPAM convenient to work with and very interesting for biological and medical studies. Another advantage is the ability to alter the properties of the polymer and shift the TLC up or down by adding a comonomer to the NIPAM units, e.g., hydrophobic8,19 or ionic groups.11,12 The modification of PNIPAM with ionic comonomers raises TLC, whereas hydrophobic groups shift the phase transition to smaller temperatures. Besides investigations of the bulk behavior, there has been increasing interest during the last years in the properties of PNIPAM immobilized at an interface.20-24 One way to immobilize PNIPAM at a solid surface uses the electrostatic attraction between oppositely charged groups on the solid surface and the polymer. We introduced positively charged groups into the molecules by synthesizing a copolymer of NIPAM and an aminomodified acrylamide comonomer (see Materials and Methods). It has already been shown that a few percent of charged comonomers change the properties of the polymer by a slight but measurable amount.12 To preserve the PNIPAM properties as much as possible, we introduced only 5% charged monomers into the copolymer. By adsorbing this positively charged PNIPAM copolymer to a negatively charged solid support, we could drastically modify the properties of the solid interface: First, the charge of the solid interface was reversed, and second, we could control the interfacial properties by selecting an appropriate temperature. In this paper we investigated the interfacial properties of a thin layer of modified PNIPAM adsorbed onto a negatively charged mica surface. We chose mica as a model for a negatively charged solid body, since it is very well investigated and is the preferred material for studies with the surface forces apparatus (SFA). We measured and compared the interfacial properties of these coated surfaces as a function of temperature using two different methods: direct force measurements with the surface forces apparatus and contact angle measurements. The surface forces apparatus is ideally suited to investigate the interactions of modified solid surfaces and is complemented by the much easier to perform contact angle measurements. We hope to shed further insight into the unique behavior of TLC materials, which may lead to the “tailoring” of specialized materials and surfaces for future applications. Materials and Methods Materials. The material under investigation, shown in Figure 1a, is poly(N-isopropylacrylamide)x-co-(N-((dimethylamino)propyl)acrylamide)y, with a mole fraction x (17) Winnik, F. M. Polymer 1990, 31, 2125. (18) Ringsdorf, H.; Simon, J.; Winnik, F. M. Macromolecules 1992, 25, 5353. (19) Winnik, F. M.; Ringsdorf, H.; Venzmer, J. Macromolecules 1991, 24, 1678. (20) Okano, T.; Yamada, N.; Sakai, H.; Sakurai, Y. J. Biomed. Mater. Res. 1993, 27, 1243. (21) Zhu, P. W.; Napper, D. H. J. Colloid Interface Sci. 1994, 168, 380. (22) Tanahashi, T.; Kawagushi, M.; Honda, T.; Takahashi, A. Macromolecules 1994, 27, 606. (23) Takei, Y. G.; Aoki, T.; Sanui, K.; Ogata, N.; Sakurai, Y.; Okano, T. Macromolecules 1994, 27, 6163. (24) Zhang, J.; Pelton, R.; Deng, Y. Langmuir 1995, 11, 2301.

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Figure 1. (a) Molecular formula of the poly(N-isopropylacrylamide)x-co-(N-((dimethylamino)propyl)acrylamide)y, which is called the modified PNIPAM. The mole fraction of the amino modified comonomer is y ) 0.05, and the NIPAM mole fraction is x ) 0.95. (b) Schematic drawing of the new, small volume lower bath for the SFA3, which is especially suitable for use with absorbing or scattering solutions and with small quantities of liquid. Note that the lower window can be moved to any vertical position. For a detailed description of the standard SFA Mk III, see ref 26.

) (1 - y) ) 0.95. Since the copolymer consists mainly of N-isopropylacrylamide (NIPAM) units, it will be called the modified PNIPAM. For control experiments we use the corresponding PNIPAM homopolymer, with a mole fraction of x ) 1.00. The synthesis of this novel copolymer has been described previously.25 The electrolyte KNO3 was puriss grade and the water of Millipore quality. Characterization. The molecular weight of the modified PNIPAM was determined by viscometry and yielded Mn ) 200 000. The polydispersity, as determined by gel permeation chromatography (GPC) in tetrahydrofuran, is 1.9. Cloud point measurements were performed by heating the polymer solutions at a constant rate and observing the absorbance of the solution. The lower critical solution temperature (TLC) in pure water is 33.7 °C. The corresponding values for the NIPAM homopolymer are Mn ) 360 000, polydispersity ) 1.6, and TLC ) 31.8 °C. Force Measurements. We used a Mk III surface forces apparatus (SFA3)26,27 for the measurement of the force vs distance, F vs D, behavior between the surfaces and the thickness, T ) 1/2D, of the adsorbed polymer layer. The interaction forces between the polymer-coated surfaces were determined from the deflection of a double cantilever spring on which one of the surfaces was mounted. The deflection was measured by an interferometric technique using fringes of equal chromatic order (FECO). The adhesion between the polymer-coated surfaces was calculated from the force needed to separate the surfaces from contact, the pull-off force, Fpo. A novel lower bath was designed for the SFA3 used in these experiments (see Figure 1b). The new lower bath (25) Simon, J. Ph.D. Thesis, University Mainz, 1994. (26) Israelachvili, J. N.; McGuiggan, P. M. J. Mater. Res. 1990, 5, 2223. (27) Israelachvili, J. N. Intermolecular and surface forces; Academic Press: London, 1991.

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offers two major advantages: a small bath volume and the possibility to perform measurements in slightly turbid or absorbing (opaque) solutions. The total volume of the new lower bath is only 20 mL and thereby reduces the amount of solution and solute (e.g., polymer). This is especially valuable if custom-made and/or expensive materials are used. The small volume lower bath was also equipped with a movable optical tube, instead of a fixed window, located below the lower surface. This tube can be brought within a fraction of a millimeter of the lower surface, reducing the optical path length that the illuminating light must pass through in the (opaque) solution. In this way, light scattering and/or absorption are reduced and the intensity of the FECO fringes remains high. Refractive Index Determination. By analyzing the wavelength of the FECO fringes, one can determine the refractive index of the fluid or material between the two surfaces.28 The fringes of odd order depend only on the distance between the mica sheets. The even order fringes also depend on the refractive index. Thus, one can obtain the distance between the surfaces by analyzing the wavelength position of an odd fringe and then use this information to calculate the refractive index from an even fringe. The accuracy can be as good as (0.01 for a film 10 nm thick but decreases for thinner films.28 Contact Angle Measurements. All contact angle measurements were performed with a home-built apparatus consisting of a sealed contact angle cell, a microscope with an eyepiece goniometer, and a microliter syringe. The syringe could be manipulated from the outside, so that different positions on the surface or droplet could be tested. The temperature was controlled by putting the whole instrument in a room held at a constant temperature ((0.1 °C). The ambient humidity was measured with a hygrometer and varied from 58 to 65% RH in all experiments. Sample Preparation. To perform the force measurements, we either placed a macroscopic droplet of solution between the separated mica surfaces or we filled the whole lower chamber of the SFA. The latter was done when the new small volume lower bath was used (see Figure 1b). The reference state of the solution before adsorption, including the definition of D ) 0, was measured in pure water or 1 mM KNO3, and the distance between the mica sheets in adhesive contact was defined as D ) 0. The polymer was adsorbed from bulk solution either by replacing the droplet with a polymer solution or by injecting a concentrated polymer solution into the filled compartment. In the adsorption measurements we defined the time when the replacement took place as the adsorption time, t ) 0. All thickness measurements, refractive index determinations, and force runs were performed in the presence of the bulk polymer solution with a concentration of 1.00 mg/mL (0.1 wt %) in 1 mM KNO3 after an adsorption time of t ) 24 h. For the contact angle measurements, a sheet of mica was immersed in a solution of modified PNIPAM. The mica sheet was freshly cleaved and 30 mm × 30 mm in size. The solution contained 1.00 mg/mL modified PNIPAM in 1 mM KNO3. During the adsorption, the sample was kept in the experimental room at a temperature Tadsorb. After an adsorption time of 24 h, the sample was rinsed in a beaker of pure water for 1 min and then slowly withdrawn, while the dewetting behavior was observed. The mica sheet was then placed in the contact (28) Israelachvili, J. N. In CRC Handbook of Micro/Nanotribology; Bhushan, B. Ed.; CRC Press: Boca Raton, FL, 1995; Chapter 8, p 267.

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Figure 2. Force vs distance behavior of adsorbed layers of modified PNIPAM at two different temperatures. At T ) 27 °C (a) the behavior is repulsive at all distances, whereas at T ) 37 °C (b) an attraction occurs at small separations. Note that the inward (0) and the outward (9) movements are shown. The dashed line in (a) refers to the force vs distance function of the (unmodified) PNIPAM homopolymer, which does not adsorb to the mica.

angle cell and equilibrated at a temperature, Tmeasure. A droplet of pure water (Millipore quality) was placed on the surface, and the advancing, receding, and sessile contact angles were measured. Since the sessile contact angle was not constant, we noted its behavior with time. When we increased the volume of the droplet, the contact angle at first increased but the three-phase line remained fixed (pinned) in place until it suddenly began moving. We define the contact angle during motion as the advancing contact angle, because it best represents the properties of the unchanged surface. The receding contact angle was defined correspondingly as the contact angle during receding motion. This measurement procedure turned out to be the most reliable and reproducible for characterizing the polymer wetting behaviors under different conditions. Results Force vs Separation Behavior. Figure 2 shows the force vs distance functions between fully adsorbed layers of the modified PNIPAM on mica at two different temperatures, above and below the bulk TLC. The bulk solution consisted of 1.0 mg/mL modified PNIPAM in 1 mM KNO3, and the polymer was allowed to adsorb for t ) 24 h before the forces were measured. In Figure 2a we see that at T ) 27 °C (T < TLC) the force is purely repulsive.

Force and Contact Angle Measurements of an LCST Polymer

At large distances an electrostatic “double layer” repulsion is observed, which extends out to approximately D ) 50 nm and decays with a Debye length of κ27°C ) 10.4 nm. If we only consider the electrolyte ions (1 mM KNO3) and neglect the modified PNIPAM as a polyelectrolyte, one would expect a Debye length of 9.6 nm. If we treat the PNIPAM as charged, independent monomers, the resulting theoretical Debye length is 8.0 nm. At smaller distances the repulsion increases more steeply as the adsorbed polymer layers become sterically compressed. Figure 2b shows the corresponding forces, when the temperature is increased to T ) 37 °C (T > TLC). The electrostatic repulsion decreases in magnitude, but the Debye length (κ37°C ) 8.9 nm) is not very different from the value at 27 °C. With decreasing distance, an attraction becomes noticeable and the surfaces “jump” into contact from a distance of 17.4 nm (Jin in Figure 2b). On moving the surfaces further in, the adsorbed polymer layer is compressed to a “hard wall” at a distance of D ) 4.8 nm, where the surfaces are deformed and flattened. When the surfaces are separated again, they jump apart from a distance of D ) 7.8 nm (Jout in Figure 2b). The depth of the adhesive force minimum, as determined from the jump distance multiplied by the spring constant, is Fpo/R ) -2.8 mN/m. For the adhesion force values at different temperatures, see below. As a control experiment, unmodified PNIPAM homopolymer was introduced into a solution of 1 mM KNO3 at the same concentration as above (1 mg/mL) (see dashed line in Figure 2a). The force vs distance function measured at T ) 25 °C is monotonically repulsive and shows a pure electrostatic decay, which is the same as was measured in electrolyte solution without polymer. In contrast to the modified polymer experiments, there is no evidence of the polymer unless the surfaces are made to approach very quickly. When this is done, a small deviation from DLVO theory can be observed at a distance of D ) 4 nm, which is attributed to kinetically trapped PNIPAM molecules. When the temperature is slowly raised to T ) 29 °C, “bumps” in the fringes can be seen and the surfaces are prevented from coming into close contact. Presumably, this is caused by multimolecular aggregates that form bigger particles that adsorb weakly and become trapped between the surfaces. Polymer Adsorption Process. The above results are for polymer layers that had been allowed to adsorb for over 24 h and are therefore presumed to be at or near equilibrium coverage. To study the adsorption process and approach toward equilibrium, we used three different bulk concentrations of the modified PNIPAM in pure water, c ) 0.01, 0.10, and 1.00 mg/mL, and measured the adhesion or pull-off forces at different adsorption times, t. The pull-off force, Fpo, can be related to the energy of adhesion as

2 Fpo Eadh ) (3π) R using the theory of Johnson, Kendall, and Roberts.27,29 The temperature in these experiments was T ) 27 °C, i.e., T < TLC. The results are shown in Figure 3. In the presence of the polymer the adhesion between the (partially coated) polymer surfaces is different for each concentration and decreases overall with time. Thickness of the Adsorbed Layer. The thickness of the adsorbed polymer layer, as compared to the bare mica and corrected for the mica expansion with temperature, (29) Johnson, K. L.; Kendall, K.; Roberts, A. D. Proc. R. Soc. London A 1971, 324, 301.

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Figure 3. Pull-off force of two mica surfaces that are coated with different amounts of modified PNIPAM, as a function of adsorption time. The modified PNIPAM was adsorbed from three different bulk concentrations, as indicated. Only in the case of 1.00 mg/mL was complete coverage with the modified PNIPAM obtained.

Figure 4. Thickness of the adsorbed layer of modified PNIPAM on mica as a function of temperature. The measurements were taken at compressive loads of F/R ) 10 and 30 mN/m, at different spots and temperatures. The solid lines are guides to the eye. The arrows indicate the extent of the thickness change with load (see text for details).

was measured at different spots and temperatures. We report the measured thicknesses at two different points of the force curves, i.e., at F/R ) 10 and 30 mN/m (see force vs distance curves in Figure 2). At these high forces, the surfaces flatten elastically and it is possible to measure the thickness of the compressed layers accurately. The results are shown in Figure 4. The measured thicknesses of 2-3 nm at T < TLC are similar to PNIPAM layers, which were covalently attached to solid surfaces.30 We can also define the compressibility of the adsorbed layers as C ) ∆pressure/∆D. The compressibility is qualitatively proportional to the difference of the layer thickness, as measured at 30 and 10 mN/m, respectively. As can be seen by looking at the arrows in Figure 4, the compressibility decreases with increasing temperature. We can quantify the compressibility by calculating the inverse slope of the linear part of the force curve at small distances (30) Bohanon, T.; Elender, G.; Knoll, W.; Ko¨berle, P.; Lee, J.-S.; Offenha¨usser, A.; Ringsdorf, H.; Sackmann, E.; Simon, J.; Tovar, G.; Winnik, F. M. J. Biomater. Sci. Polym. Ed. 1996, 8, 19.

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Figure 5. Refractive index of the material between the mica surfaces as a function of distance. The measurements were taken at three different temperatures, one below the bulk TLC (27 °C), one approximately at TLC (31 °C), and one above TLC (37 °C). The solid lines are linear fits to the data at a given temperature. The horizontal dotted lines indicate the refractive index of the bulk solution and the calculated value for solid PNIPAM.

(large forces). The average values are C(27 °C) ) -25 nm/(mN/m), C(32 °C) ) -11 nm/(mN/m), and C(37 °C) ) -7 nm/(mN/m). Refractive Index Measurements. In these experiments we used a bulk concentration of 1 mg/mL modified PNIPAM and an adsorption time of 24 h to ensure complete coverage on the mica surfaces (see above). Figure 5 shows the refractive index of the trapped material between the surfaces as a function of distance (gap thickness) for three different temperatures: one below TLC (27 °C), one approximately at TLC (31 °C), and one above TLC (37 °C). The lines are linear fits to the data points at each temperature. The refractive index is constant at large distances and equal to that of the bulk solution, viz. pure water (n ) 1.33), since we have a dilute polymer solution. At a distance of around 20 nm the refractive index starts to increase and, as can be seen from the linear fits to the data points in Figure 5, the increase is qualitatively similar at all the temperatures studied. But the absolute values of the refractive index show a strong temperature dependence: For a given film thickness, the refractive indices are higher at higher temperatures. Effect of Speed on the Pull-Off Force. Figure 6 shows the pull-off forces, Fpo, at different contact positions and temperatures. At T ) 27 °C, i.e., below the bulk TLC, the measured pull-off forces are zero and there is no adhesion. At about 28 °C we find the crossover point, beyond which the surfaces become adhesive. The pull-off force increases with temperature: At a temperature around 32 °C the average pull-off force is Fpo/R ) -0.9 mN/m and at T ) 37 °C it has increased to Fpo/R ) -3.0 mN/m. These pull-off forces were determined while the surfaces were separated out of contact during a force run. The average speed of a force run was 1 nm/min. Doing these separations, we found a strong dependence of the pull-off force on the speed of separation. If the surfaces were separated fast, the measured pull-off force was bigger (more negative) than at lower speeds. If, on the other hand, the surfaces were separated very slowly, i.e., slower than at a force run, the pull-off force decreased. For example at T ) 32 °C and using very low speeds, the pull-off force was reduced to about 0 mN/m and no adhesion could be measured. Also, at a fast speed of separation, the distance where the jump-off occurred is close to the

Schmitt et al.

Figure 6. Pull-off forces of two surfaces with adsorbed layers of modified PNIPAM as a function of temperature. For a given measurement speed of 1 nm/min the crossover temperature, where the surfaces become adhesive, is T ) 28 °C. Using a lower speed at T ) 32 °C, the pull-off force was reduced to about 0 mN/m and no adhesion could be measured. In this case the curve and the crossover point most likely will be shifted to higher temperatures, as indicated by the dashed line. The vertical arrow indicates TLC of the bulk solution.

Figure 7. Advancing and receding contact angles at different temperatures, as well as the resulting contact angle hysteresis, ∆Θ, shown for a sample that was adsorbed at Tadsorb ) 30 °C. Note that at T ) 28 °C the behavior changes abruptly.

hard wall, whereas at low speeds the surfaces stick together even at larger distances and the jump-off takes place at larger surface separations. Contact Angle Measurements. We observed a strong difference between the advancing and receding contact angles under all experimental conditions, so we define a contact angle hysteresis as ∆Θ ) Θadv - Θrec. The results of a sample that was adsorbed at a temperature of Tadsorb ) 30 °C and measured at various temperatures Tmeasure are shown in Figure 7. We see that the advancing contact angle is about constant and has an average value of Θadv ) 76 ( 2°. The receding contact angle is much smaller and falls in the range Θrec ) 18°-33°. Most importantly, the variation in the receding contact angle shows a pronounced step at T ) 28 °C. This effect is also reflected by the contact angle hysteresis (cf. Figure 7). A different set of data, with samples that were adsorbed and measured at the same temperatures (Tadsorb ) Tmeasure), showed the same behavior (data not shown). When a drop of water was placed on the polymer-coated surface (sessile drop), its contact angle immediately started

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to decrease. We measured the sessile contact angle as a function of time and fitted the data to an exponential decay. The decay constant was temperature dependent and showed two distinct sets of values: At temperatures below TLC in the range T ) 20-28 °C, the average decay time was τ ) 0.5 min-1, whereas at temperatures above TLC in the range T ) 30-35 °C, it was τ ) 1.4 min-1; i.e., the decay of the sessile contact angle was slower at higher temperatures (data not shown). Discussion Adsorption and Interactions at T < TLC. The control experiments with the PNIPAM homopolymer showed that it does not adsorb to mica but that we can modify the surfaces with modified PNIPAM in a controlled way. Because of its positive charge, the modified PNIPAM has a strong tendency to adsorb at the negatively charged mica surface. This adsorption layer can reverse the effective surface charge, and the resulting electrostatic, van der Waals, and other, polymer-associated, forces can be overall attractive or repulsive depending on the coverage, temperature, and other experimental conditions. As can be seen in Figure 3, the adhesion of the modified PNIPAM covered surfaces decreases with concentration and time. Generally, an attractive interaction between polymer-covered surfaces can result from intersegment, depletion, or bridging attraction (see, e.g., ref 27, Chapter 14). At temperatures below TLC there is no intersegment attraction, but bridging attraction can occur if the polymer-surface interaction is attractive and there are free binding sites on the opposing surfaces. These conditions were met in the case of c ) 0.01 and 0.10 mg/ mL concentrations, especially at short adsorption times. In contrast, at 1.00 mg/mL the modified PNIPAM occupies all the available binding sites on both surfaces, so that little or no bridging occurs, even at small adsorption times. Depletion attraction may occur but has a small effect, because diluted solutions are used in this study. Densification with Increasing Temperature. The refractive index of the medium between the mica surfaces starts to increase above the bulk value at around 20 nm (see Figure 5). This is as expected because the polymer has a higher refractive index than that of water (1.53 compared to 1.33). As the two adsorbed layers are compressed, the effective polymer concentration increases in the gap between the two surfaces, which increases the optical density and refractive index. For a given distance, the refractive index increment shows a strong temperature dependence. At 27 °C the value is smallest, increases at 31 °C, and remains constant to 37 °C. This implies a sharp densification around the bulk TLC, which subsequently plateaus at higher temperatures. This densification can be attributed to the adsorption of additional molecules of modified PNIPAM at TLC. Before proceeding further, we would like to comment on the absolute magnitude of the refractive index. Using Vogel’s well-known method for calculating the refractive index of a molecule as the sum of the specific reflectivities of its molecular groups, we calculated the refractive index of this molecule in the solid, amorphous state.31 Assuming as a first approximation that the optical properties of the modified PNIPAM is dominated by the 95 mol % NIPAM fraction, we used Vogel’s formalism and calculated a refractive index of n ) 1.53. Since this value is for the solid material, we should compare it to the refractive index (31) van Krevelen, D. W.; Hoftyzer, P. J. Optical Properties. Properties of Polymers. Correlations with Chemical Structure; Elsevier: Amsterdam, 1972; Chapter 11.

Figure 8. Schematic model of the molecular conformation of the adsorbed modified PNIPAM below (a) and above (b) TLC. (c) Different conformations of the adsorbed PNIPAM layer in contact with air (advancing mode) and with water (receding mode). The gray arrow indicates a reorientation of the charged groups away from the mica and toward the water. The black arrow symbolizes the polymer segment-segment attraction, which suppresses the polymer mobility at temperatures above the TLC.

measured at small distances in Figure 5, where the adsorbed layers have the highest density. We see that the calculated maximum value of 1.53 is right in the middle of the low- and high-temperature fits, confirming the validity of the data, irrespective of the relatively large experimental error. In Figure 4 we see that the thickness of the adsorbed layer is very small at T ) 27 °C and high loads, indicating that the layer can be easily compressed. This, together with the low refractive index of the layer, means that the adsorbed layer at 27 °C has an open structuresmost likely only one monomolecular layer thick and having a conformation with many loops and tails. This is depicted schematically in Figure 8a. With increasing temperature, the layer thickness increases and, independently, the compressibility decreases, and at 37 °C the polymer chains completely collapse under a compressive load (see Figure 8b). Both curves in Figure 4 show a maximum in the adsorbed thickness at around T ) 31 °C. The two processes mentioned above, additional adsorption of material and

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chain collapse, contribute differently to the adsorbed layer thickness. It seems that at T ) 31 °C additional modified PNIPAM has adsorbed, yielding the highest layer thickness, while at T ) 37 °C there is no more adsorption but the polymer chains have further collapsed, thereby reducing the thickness again. Alternatively, because the relaxation time is longer at higher T, the adsorption time is also longer, and it takes much longer to reach the equilibrium thickness. Light-scattering data of a recent investigation by Gao et al. support our findings.32 Temperature and Rate (Time) Effects. The force curve at T ) 27 °C is purely repulsive and shows an exponential decay at large distances consistent with an electrostatic double-layer force Figure 2a). Since the adsorbed PNIPAM layer most likely has a conformation with loops and tails (see above), we can conclude that there are excess positive charges at the interface compared to the bare mica charge. This could lead to charge reversal and the observed repulsion. In the calculation of the theoretical Debye length, only ions from the bulk concentration of 1 mM KNO3 were considered. Nevertheless, the measured Debye length was larger than the theoretical one by about 8%, so that the effective ion concentration must be smaller than 1 mM. This can be explained by the adsorption and binding of some of the NO3- counterions to the positively charged groups of the modified PNIPAM, and thereby, leading to an effective reduction of free ions in the solution. At the higher temperature (see Figure 2b), the magnitude of the double-layer force decreases slightly, which can be due to two different effects. First, the protruding chains or tails have undergone a coil to globule transition, so that their extension into the bulk solution decreases (see, e.g., ref 8). This would be in agreement with the predicted chain collapse at temperatures above the TLC and has been previously observed for PNIPAM molecules attached to an interface.33 Second, the surface potential, Ψ, has decreased from about 75 to 50 mV, because of charge neutralization/ion condensation on collapse. Also, the measured Debye length has decreased to 8.9 nm, indicative of an increased free ion concentration. This value is now between the theoretical Debye lengths for a 1 mM KNO3 solution with and without 1 mg/mL of dissociated modified PNIPAM (see Results). It appears that at T ) 37 °C NO3counterions no longer bind to the modified PNIPAM and that the modified PNIPAM itself also contributes to the total concentration of charges. Looking at Figures 2 and 6, we see at T ) 27 °C a purely repulsive behavior and at T ) 37 °C an adhesive minimum on surface contact. The crossover point between repulsive and attractive behavior is at T ) 28 °C. One model of the phase transition at the TLC assumes that with increasing temperature, the interaction between polymer segments and water becomes less favorable, i.e., that the segments lose their water of hydration. This decreases the excluded volume and hence the osmotic repulsion between the segments and allows the van der Waals forces to act down to contact separations. A similar mechanism operates in poly(ethylene glycol) solutions.34 This concept of solvent quality and temperature is often used in polymer science (see, e.g., ref 8 for further literature). It also means that above TLC the attraction between PNIPAM segments within one surface is enhanced, but so is the attraction between segments on opposite surfaces. Combined with (32) Gao, J.; Wu, C. Macromolecules 1997, 30, 6873. (33) Zhu, P. W.; Napper, D. H. J. Colloid Interface Sci. 1994, 164, 489. (34) Kuhl, T.; Guo, Y.; Alderfer, J. L.; Berman, A. D.; Leckband, D.; Israelachvili, J.; Hui, S. W. Langmuir 1996, 12, 3003.

Schmitt et al.

the likelihood that fewer charged groups are facing the water semispace (see above), this leads to an increased attraction between the two coated surfaces. The introduction of charged comonomers into PNIPAM led to an increased TLC in bulk solution (see Materials and Methods). This is as expected, because the charged groups repel each other. If the modified PNIPAM is adsorbed to a negatively charged surface such as mica, some of the charges are balanced by the negative groups on the mica surface, so that the electrostatic repulsion between segments will be reduced. Additionally, the immobilization of the PNIPAM chains to the interface restricts the available conformations and forms a denser layer than in solution and thus reduces the temperature where chain collapse occurs. This explains why TLC is shifted to lower temperatures for absorbed PNIPAM molecules, as compared to the behavior in a bulk solution. A reduction of the LCST of adsorbed PNIPAM as compared to the bulk phase has recently also been observed by Gao et al.32 The dependence of the pull-off force and the distance where the jump-off occurs shows the important relation between the time of interaction and the time of measurement. The relative magnitude of these two parameters is often the determining factor of the performance of a material and may be quantified by the so-called Deborah number.28 The experimental data at high temperatures show that two adsorbed PNIPAM layers in contact have different adhesion forces when they are separated at different speeds. At fast speeds, the polymer layers behave like solids: The adhering layers separate before they have time to relax and a large number of attractive “bonds” are broken at the same time, which leads to a large pull-off force. If the surfaces are separated slowly, the polymer behaves liquid-like: The layers on the opposing surfaces have time to relax and deform as they are pulled apart. During the deformation only a few attractive “bonds” are broken at a time, leading to a small but measurable adhesion force. Therefore, adhesion measurements should have a third axis, indicating the time of the measurement. Only measurements performed with the same speed of separation can be compared, and a decreased speed will presumably shift the crossover point to attractive behavior in Figure 6 toward higher temperatures. This is another example where the time-temperature superposition principle35 can be applied to the results of direct force measurements:28,36 At the one extreme case of small speeds, the pull-off force is zero, or close to zero. At intermediate speeds, as in the force runs, the pull-off force has a finite value. At the other extreme, if the surfaces are approached and separated infinitely fast (zero time in contact), the pull-off force is again expected to be zero since there is now no time for any bonds to form in the first place. It follows that the pull-off force in any adhesion cycle goes through a maximum at a given temperature or rate when the measurement and interaction rates are equal or, in other words, when the Deborah number is equal to unity. Molecular Reorientations. In the Results we showed that the conformational changes that occur at the TLC can also be characterized by contact angle measurements. Whereas the (dynamic) advancing contact angle remains constant over the whole temperature range, the (dynamic) receding contact angle shows a clear transition at T ) 28 °C. These effects can best be seen when looking at the (35) Ferry, J. D. Viscoelastic Properties of Polymers; John Wiley & Sons: New York, 1980. (36) Schmitt, F.-J.; Yoshizawa, H.; Schmidt, A.; Duda, G.; Knoll, W.; Wegner, G.; Israelachvili, J. Macromolecules 1995, 28, 3401.

Force and Contact Angle Measurements of an LCST Polymer

contact angle hysteresis, which is very sensitive to changes on the molecular scale.37 We would like to propose the following model, which is illustrated in Figure 8c: The adsorbed PNIPAM adopts different conformations when exposed to air and water, respectively. At the air semispace, the positively charged molecular groups are oriented toward the negatively charged mica surface and the nonpolar parts are facing the air, producing a surface with an average contact angle of 76°. When the adsorbed PNIPAM layer is exposed to water, molecular reorientations of some of the charged groups occur, leading to a smaller (receding) contact angle. This concept of segment reorientations to produce a minimum interfacial energy is well-known in the surfactant and polymer literature.5,38,39 Using the concept of molecular reorientations to explain the observed contact angle hysteresis, the explanation of the temperature dependence also becomes straightforward. At temperatures below TLC, these molecular motions can occur and are fast. Their velocity is linked to the experimentally measured time constant of the decreasing sessile contact angle, as mentioned in the Results. Above TLC, the molecular motions are hindered and become slower; e.g., the above-mentioned time constant increases by a factor of about 3. Using the concept of solvent quality as discussed, we can conclude that above TLC the polymer segments increasingly attract themselves and are less hydrated by water, so that the PNIPAMwater interface is more hydrophobic. Because of the increased polymer-polymer interaction, the motions of polymer segments become more difficult, so that molecular reorientations occur to a lesser extent and the contact angle is therefore higher than at lower temperatures. This corresponds to a smaller contact angle hysteresis and is in accordance with the experimental data. Parallel to our studies, other reports of contact angle measurements on PNIPAM were published.23,24 Takei et al. used PNIPAM copolymers with 3% charged groups and also obtained a large contact angle hysteresis. But in their study the advancing contact angle showed a strong temperature dependence while the receding angle was about constant. They used the immersion technique for contact angle measurement and found different behavior for different immersion cycles. On first immersion the advancing contact angles were high, as in our experiments, but on further immersions they were much lower. This may indicate that during the first immersion cycle some changes in the film had occurred and that the PNIPAM never regained its original, hydrophobic state when reexposed to air and during subsequent advancing immersions. Zhang et al. obtained similar results as did Takei et al., but did not supply details of the experimental procedure. Contact angle data are known to be very dependent on (37) Chen, Y.-L.; Helm, C. A.; Israelachvili, J. N. J. Phys. Chem. 1991, 95, 10736. (38) Yasuda, H.; Sharma, A. K.; Yasuda, T. J. Polym. Sci., Polym. Phys. Ed. 1981, 19, 1285. (39) Grainger, D.; Okano, T.; Kim, S. W. In Advances in Biomedical Polymers; Gebelein, C. G., Ed.; Plenum Press: New York, 1987; p 229.

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the precise experimental and preparation procedures and are thus often thought to be unreliable. Therefore, it is important to be consistent and to use more than one experimental technique. In our case the contact angle data are supported by the results of the force measurements, and the three separate data sets of the contact angle measurements (Tadsorb * Tmeasure, Tadsorb ) Tmeasure, decay time constants) are in themselves consistent. Summary We have investigated the temperature dependent properties of modified PNIPAM layers adsorbed to a solid surface from aqueous solution. By varying the bulk concentration of the modified PNIPAM solution and the adsorption time, we determined the necessary condition for complete coverage of the surface. At low temperatures the coverage is low, the adsorbed layer has an “open structure” with a high content of loops and tails, and the layer is highly compressible. With increasing temperature, the solvent quality worsens and the polymer segments start to attract each other increasingly. This leads to the adsorption of additional molecules, a chain collapse on the surface and to an attraction between the two opposing surfaces. Direct force and refractive index measurements coupled to contact angle measurements are suitable and complementary techniques to study the equilibrium and nonequilibrium phase behavior of adsorbed polymers and showed excellent agreement in their results. In both studies the TLC of the adsorbed layer of the modified PNIPAM exhibited the phase transition at T ) 28 °C, which is slightly lower than that in bulk solution where TLC ) 33.7 °C. The contact angle data suggest that the modified PNIPAM adopts different conformations when exposed to air and to water. To reduce the respective interfacial energy, different molecular groups are oriented toward the medium. During the passage of a water phase across the surface, these reorientations occur on a time scale of minutes and are slower at temperatures above TLC because of the increased polymer-polymer attraction. The measured adhesion of two contacting surfaces is strongly dependent on the speed of separation. At high speeds the polymer behaves like a solid: exhibiting sudden fracture, whereas at low speeds the polymer layers dissipate energy quickly by deforming like a liquid. The overall behavior can be classified by the relative magnitude of the time of interaction and the time of measurement. The adhesion should be a maximum when the two time scales are approximately equal. This study shows again the importance of time as an experimental parameter in adhesion measurements and gives an example of the validity of the time-temperature superposition principle for adsorbed polymer layers. Acknowledgment. We would like thank R. Hill for his expert technical skill in making the small volume lower bath. This work was supported by the MRL Program of the National Science Foundation under Award No. DMR-9123048. LA9709195