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Adsorption of Hydrophobically Modified Poly(acrylamide)-co-(acrylic acid) on an Amino-Functionalized Surface and Its Response to the External Solvent Environment Xiaoyan Song,† Meiwen Cao,† Yuchun Han,† Yilin Wang,*,† and Jan C. T. Kwak‡ Key Laboratory of Colloid and Interface Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, PR China, and Department of Chemistry, Dalhousie UniVersity, Halifax, NoVa Scotia, Canada B3H 4J3 ReceiVed October 8, 2006. In Final Form: January 12, 2007 The adsorption of hydrophobically modified poly(acrylamide)-co-(acrylic acid), designated as PAM-C14-AA (x%) (x ) 5, 10, 20, representing the mole percent of acrylic acid units), at an amino-functionalized silicon surface was studied. The effect of polymer charge density was determined by varying the acrylic acid content of the copolymer. Characteristics of the adsorbed layer were evaluated by atomic force microscopy, water contact angle measurements, and X-ray photoelectron spectroscopy. The results showed that the adsorption behavior of PAM-C14-AA (x%) is influenced by the balance among the electrostatic, hydrogen-bonding, and hydrophobic interactions. Adjusting the solution pH and polymer charge density significantly affects the morphology and thickness of the adsorbed film. Furthermore, it was found that the adsorbed PAM-C14-AA undergoes conformational rearrangements when the surface is wetted by selected organic solvents. The resultant morphology and wettability of the films indicated that the different affinities of the solvents for different segments of PAM-C14-AA (x%) can be considered to be the possible cause of the conformational rearrangements of adsorbed polymer.
Introduction Water-soluble hydrophobically modified polymers (HM polymers) are very important as thickening agents and stabilizers in dispersions and emulsions.1One interesting subclass of HM polymers is based on weak polyelectrolytes. The fact that their surface activity can be adjusted both chemically and by manipulating the solution pH makes them of interest for many applications. In most cases, the hydrophobic groups are alkyl chains grafted to the backbone of the polymer. The hydrophobic side groups tend to associate with one another in aqueous solution.2 This intermolecular hydrophobic association leads to the building of a transient network that results in unique rheological properties.3 In addition, at the solid/liquid interface, the alkyl groups can modify the adsorption properties of the polymers. The characterization of the adsorption behavior of these types of polymers on solid surfaces has been the subject of several recent studies.1,3,4 Argillier and co-workers3 explored the adsorption of hydrophobically modified polyacrylamides (HMPAM) at montmorillonite and siliceous minerals. Samoshina et al.4 studied the adsorption of HMPAM on a silica surface by varying the degree of hydrophobicity. Go¨bel et al.1 explored the adsorption of hydrophobically modified poly(acrylic acid) on a noncharged hydrophobic polystyrene surface. Cochin and Laschewsky5 investigated the layer-by-layer self-assembly of HM polymers. One of the main conclusions reached in these studies is that * To whom correspondence should be addressed. E-mail: yilinwang@ iccas.ac.cn. † Chinese Academy of Sciences. ‡ Dalhousie University. (1) Go¨bel, J. G.; Besseling, N. A. M.; Stuart, M. A. C.; Poncet, C. J. Colloid Interface Sci. 1999, 209, 129-135. (2) Wang, X.; Li, Y.; Li, J.; Wang, J.; Wang, Y.; Guo, Z.; Yan, H. J. Phys. Chem. B 2005, 109, 10807-10812. (3) Argillier, J. F.; Audibert, A.; Lecourtier, J.; Moan, M.; Rousseau, L. Colloids Surf., A 1996, 113, 247-257. (4) Samoshina, Y.; Diaz, A.; Becker, Y.; Nylander, T.; Lindman, B. Colloids Surf., A 2003, 231, 195-205. (5) Cochin, D.; Laschewsky, A. Macromol. Chem. Phys. 1999, 200, 609-615.
hydrophobic association results in an increased thickness of the adsorption layer. Stimuli-response polymer layers are capable of responding to very subtle changes in the surrounding environment including pH,6 surface pressure,7 temperature,8,9 light,10 and solvent quality.11 Surface composition and, hence, the surface energy, adhesion, friction, wettability, optical characteristics, and biocompatibility can be sensitive to the environment and can be tuned to a desired physical state. Such surfaces hold great promise in the design of biomaterials, drug delivery systems, reversible surface patterning, biosensors, and so forth.12-17 Stimuliresponsive polymer coatings can be designed by using a variety of approaches including the swelling/collapse of grafted polymers and phase separation in binary copolymers. Recently, there have been sophisticated molecular designs for block copolymers with adaptive properties such as star-shaped,18 tapered-shaped,19 combshaped,20 and Y-shaped copolymers.21 Homopolymers with (6) Harnish, B.; Robinson, J. T.; Pei, Z.; Ramstro¨m, O.; Yan, M. Chem. Mater. 2005, 17, 4092-4096. (7) Lee, M.; Kim, J.; Yoo. Y.; Peleshanko, S.; Larson, K.; Vaknin, D.; Markutsya, S.; Tsukruk, V. V. J. Am. Chem. Soc. 2002, 124, 9121-9128. (8) Jaber, J. A.; Schlenoff, J. B. Macromolecules 2005, 38, 1300-1306. (9) Sun, T.; Wang, G.; Feng, L.; Liu, B.; Ma, Y.; Jiang, L.; Zhu, D. Angew. Chem., Int. Ed. 2004, 43, 357-360. (10) Feng, X.; Feng, L.; Jin, M.; Zhai, J.; Jiang, L.; Zhu, D. J. Am. Chem. Soc. 2004, 126, 62-63. (11) Minko, S.; Mu¨ller, M.; Motornov, M.; Nitschke, M.; Grundke, K.; Stamm, M. J. Am. Chem. Soc. 2003, 125, 3896-3900. (12) Graighead, H. G. Science 2000, 290, 1532-1535. (13) Jacobs, H. O.; Tao, A. R.; Schwartz, A.; Gracias, D. H.; Whitesides, G. M. Science 2002, 296, 323-325. (14) Tsukruk, V. V. Prog. Polym. Sci. 1997, 22, 247-311. (15) Jones, D. M.; Smith, J. R.; Huck, W. T.; Alexander, C. AdV. Mater. 2002, 14, 1130-1134. (16) Nath, N.; Chilkoti, A. AdV. Mater. 2002, 14, 1243-1247. (17) Rohr, T.; Ogletree, D. F.; Svec, F.; Frechet, J. M. AdV. Funct. Mater. 2003, 13, 264-270. (18) Lupitskyy, R.; Roiter, Y.; Tsitsilianis, C.; Minko, S. Langmuir 2005, 21, 8591-8593. (19) Xu, C.; Wu. T.; Mei, Y.; Drain, C. M.; Batteas, J. D.; Beers, K. L. Langmuir 2005, 21, 11136-11140. (20) Adiga, S. P.; Brenner, D. W. Nano Lett. 2002, 2, 567-572.
10.1021/la062954u CCC: $37.00 © 2007 American Chemical Society Published on Web 03/17/2007
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Figure 1. Chemical structure and nomenclature of hydrophobically modified PAM-C14-AA (x%) polymers.
functional end groups have also been reported to be sensitive to external stimuli.22 These polymers are usually chemically grafted to solid substrates through one end of the polymer brushes18 or through surface-initiated polymerization.19 Compared with such chemical reaction methods, adsorption directly from solution through intermolecular interaction may be a simpler and more feasible method. Direct adsorption from solution may be an attractive and versatile technique for creating switchable surfaces reacting to changes in the external solvent environment and may lead to the development of tailored surface properties. In this work, we investigate the factors controlling the adsorption of hydrophobically modified poly(acrylamide)-co(acrylic acid) (PAM-C14-AA (x%)) on a positively charged surface and present a study on the response behavior of adsorbed PAM-C14-AA (x%) layers to selective solvents. To our knowledge, the adsorption of HM polymers on a positively charged surface has been scarcely reported, and there are no systematic reports on the solvent effect on the aggregation properties of polymers at surfaces. We use amino-functionalized silicon surfaces, which maintain a positive charge under proper pH conditions, providing linkage points for poly(acrylic acid) segments of the polymers through electrostatic interactions. The charge density of PAM-C14-AA (x%) was varied by changing the proportion of acrylic acid from 5 to 20%. To enhance the hydrophobic interaction among the polymer molecules, copolymers with hydrophobic alkyl side groups with 14 carbon atoms were used. Of particular interest is the growth of the polymer film thickness on the surface resulting from both hydrophobic interactions and hydrogen bonding. In addition, the characterization of the response for the HM polymers to different organic solvents is an important aspect of the characteristics of the adsorption-modified surface. The structural reorganization upon treatment with selective organic solvents was evaluated with atomic force microscopy (AFM) imaging. The differences in the water contact angle (CA) measurement further supported the morphology alterations under external solvent stimuli. Experimental Section Materials. 3-Aminopropyltrimethoxysilane (APTMS, H2N(CH2)3Si(OCH3)3) was obtained from TCI and stored in a desiccator. n-Hexane, acetone, dehydrated ethanol, and toluene were of analytical purity and were obtained from Sinopharm Chemical Reagent Co., Ltd (SCRC). n-Hexane and toluene were distilled over sodium before use. Acetone and dehydrated ethanol were used without further purification. The single-crystal silicon wafers (100) polished on one side were obtained from the General Research Institute for Nonferrous Metals (Beijing, PR China). Triply distilled water (with a specific conductivity of 1.5-2.5 µs cm-1 at 289.15 K) was used in the experiment. Polymer Synthesis. The HM polymers used in this investigation were random terpolymers (PAM-C14-AA (x%)) of acrylamide (AM, Aldrich), alkylacrylamide (AAM), and acrylic acid (AA, Aldrich), whose chemical structures are shown in Figure 1. By balancing the AA/AM ratio in the polymerization process, polyelectrolytes with different charge densities were synthesized, and x indicates the (21) Julthongpiput, D.; Lin, Y.; Teng, J.; Zubarev, E. R.; Tsukruk, V. V. J. Am. Chem. Soc. 2003, 125, 15912-15921. (22) Anastasiadis, S. H.; Retsos, H.; Pispas, S.; Hadjichristidis, N.; Neophytides, S. Macromolecules 2003, 36, 1994-1999.
Song et al. respective percentage of acrylic acid. The detailed synthesis method was described previously.23-26 The hydrophobic side groups are expected to be randomly distributed along the hydrophilic backbone. Polymer molecular weights can be estimated from the capillary viscometry of dilute polymer solutions in 0.1 M Na2SO4 at 289.15 K using the Mark-Houwink-Sakurada constants for poly(acrylamide)-co-(acrylic acid) as determined by Hunkeler et al.27 This method can be considered to be only approximate for determining the molecular weights of the polymers in this case because the effect of the added alkyl side groups on the Mark-Houwink-Sakurada constants has not been determined. For all of the polymers, the molecular weight estimated by this method is between 1.6 × 105 and 2.5 × 105 Da. These polymers can be assumed to have similar molecular masses and different charge densities. Preparation of Self-Assembled Films. Polished silicon wafers were ultrasonicated in detergent solution and acetone for 30 min. After the samples were rinsed with water, they were submerged in a freshly prepared mixture of H2SO4 (98%) and H2O2 (30%) at a volume ratio of 7:3. The solution was heated to 80 °C for 1 h to remove organic contaminants, and then the samples were rinsed thoroughly with copious amounts of triply distilled water and dried with a N2 gas stream. After such treatments, a thin oxide layer forms on the surface of silicon wafers. The substrates were placed in a 50 mL sealed vessel with a container filled with 0.7 mL of toluene and 0.1 mL of APTMS, avoiding direct contact between the liquid and the substrates. The vessel was put in an oven at 100 °C for 1 h. APTMS vaporized and reacted with the OH groups on the surface, resulting in the formation of an APTMS monolayer. Then the substrates were ultrasonicated for 5 min in toluene and ethanol and dried under a flow of N2. Each of the different PAM-C14-AA (x%) polymers was dissolved in triply distilled water to create a 1 mg/mL solution. We used pH values of 4.8 and 7.0 on the basis of a previous report28 and our pH titration curves. HCl and NaOH solutions of 0.1 M were added to the polyelectrolyte solutions to adjust the pH to the desired values. The adsorption experiments were performed at 20 °C. Samoshina et al.4 reported that solutions of high-molecular-weight polyacrylamide solution are subject to aging, which was attributed to the disentangling of polymer chains, long-term conformational changes, degradation, and hydrolysis. The polyacrylamide copolymers used in this study are of median molecular weight and may be expected to be less affected by such aging phenomena. Nevertheless, we used only freshly prepared solutions and kept the stock solutions at a temperature below 5 °C. The amino-functionalized substrates were submerged in a pH-adjusted water bath for 20 min, and then the samples were immersed in the polymer solutions for a certain period of time. This was followed by another 1 min dip in the pH-adjusted water. A 1 min rinse helped to remove the polymer molecules that were not truly bound to the surface. Finally, the treated substrates were dried under a flow of N2 and kept in a vacuum-drying container. Solvent Treatment. The substrates coated with PAM-C14-AA (20%) at pH 4.8 and 7.0 for 20 h were immersed in three different solventssn-hexane, acetone, and dehydrated ethanolsfor 24 h. The selective solvents have different affinities with the polymer backbones and side chains. Then, the substrates were withdrawn from the solvents and dried under a flow of N2. Finally, the substrates were kept in a vacuum-drying container overnight before the following surface characterization. The CA values have been compared between the vacuum-dried films after drying under a N2 flow and the films dried only under a N2 flow. The differences in their CA values are within experimental error, so there were no residual solvents on the surfaces after the above drying procedure was carried out. (23) Bai, G.; Wang, Y.; Yan, H.; Thomas, R. K.; Kwak, J. C. T. J. Phys. Chem. B 2002, 106, 2153-2159. (24) Effing, J. J.; McLennan, I. J.; Kwak, J. C. T. J. Phys. Chem. 1994, 98, 2499-2502. (25) Effing, J. J.; McLennan, I. J.; Van Os, N. M.; Kwak, J. C. T. J. Phys. Chem. 1994, 98, 12397-12402. (26) Brouwer White, B.; Kwak, J. C. T. Colloid Polym. Sci. 1999, 277, 785791. (27) Hunkeler, D.; Wu, X.; Hamielec, A. Polym. Prepr. 1993, 3, 1071-1072. (28) Clark, S. L.; Hammond, P. T. Langmuir 2000, 16, 10206-10214.
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Figure 2. APTMS self-assembled film on a smooth silicon surface through chemical vapor deposition for 1 h. (a) AFM morphology and the height profile along the white line in the image and (b) high-resolution XPS N 1s spectrum. Surface Characterization. Water contact angles (CA) were measured with a 2 µL water droplet at 20 °C with an optical contact angle meter (Dataphysics Inc., OCA20). The static CA values reported were the averages of five measurements made on different areas of the samples. All measurements for all of the samples were within (2.0° of the averages. The morphology of the sample surfaces was observed with atomic force microscopy (AFM, Digital Instruments, Nanoscope IIIa, tapping mode). Etched silicon probes attached to 125 µm cantilevers with a nominal spring constant of 40 N/m (Digital Instruments, model RTESPW) were operated at resonance frequencies of ∼300 kHz. The CA and AFM measurements were all carried out in air at a relative humidity of 40-50%. The chemical composition of the surfaces of the modified substrates was studied by X-ray photoelectron spectroscopy (XPS, VG Scientific ESCALab220iXL spectrometer) with a 225 W Al KR monochromatic X-ray source and a takeoff angle of 90°. The base pressure was about 3 × 10-9 mbar. The typical exposure time was ∼120 s for a survey scan and 40-160 s for a high-resolution elemental scan. The binding-energy scales were referenced to 284.8 eV as determined by the location of the maxima of peaks in the C 1s spectra of hydrocarbons (CHx) associated with adventitious contamination.
Results and Discussion Preparation of the Amino-Functionalized Surface. The amino-functionalized surface provides anchors to act as linkage points for the attachment of other molecules to the surface. To avoid self-polymerization in solution, the self-assembly reaction of 3-aminopropyltrimethoxysilane on the silicon surface was performed on the vapor/solid interface according to the procedure reported previously by our group.29,30 In this way, a reproducibly smooth amino-functionalized surface with a high content of primary amines was obtained, with carefully control on the silylation condition. The morphology and XPS N 1s spectrum of the amino-functionalized surface are shown in Figure 2. Clearly, the amino-functionalized surface appears to be very smooth and shows no existence of aggregate particles. The height profile along the white line in the height image indicates a uniform monolayer assembly. The water CA of the amino-functionalized surface reaches 54.5°, which is consistent with that of the entirely amino-terminated self-assembled films reported in previous studies.31 XPS can provide further information concerning the (29) Song, X.; Zhai, J.; Wang, Y.; Jiang, L. J. Colloid Interface Sci. 2006, 298, 267-273. (30) Song, X.; Zhai, J.; Wang, Y.; Jiang, L. J. Phys. Chem. B 2005, 109, 4048-4052. (31) Heise, A.; Menzel, H.; Yim, H.; Foster, M. D.; Wieringa, R. H.; Schouten, A. J.; Erb, V.; Stamm, M. Langmuir 1997, 13, 723-728.
Figure 3. Schematic illustration of the self-assembly of hydrophobically modified PAM-C14-AA (x%) polymers on an aminofunctionalized surface.
extent to which amino groups on the surface are reactive. Highresolution data for element N are collected to provide a quantitative understanding of the surface chemistry. In Figure 2b, the N 1s spectrum can be deconvoluted into two peaks centered at 398.7 and 400.7 eV. The free primary amines, -NH2, correspond to the peak component at 398.7 eV. Some amino groups may also undergo hydrogen bonding with each other or with substrate hydroxyls, as revealed by the presence of a shoulder at 400.7 eV.32 The free primary-amine content can reach 71.6%, which is established as the percentage of the free -NH2 peak component in the XPS N 1s spectrum. We conclude that the chemical vapor deposition process results in a reproducibly smooth amino-functionalized surface with high free primary-amine content available for further reaction. In all applications of amino-functionalized surfaces, the reactive primary-amine moieties exposed on the substrate surface serve as a platform to interact with other molecules. Adsorption of Polymers. Figure 3 shows a schematic illustration of the adsorption of PAM-C14-AA (x%) on the aminofunctionalized surface. Usually, the value of pKb for the primary amino group is around 9. However, when the amino group is fixed on the silica surface, its pKb may be changed as a result of the change in the molecular environment. The exact degree of ionization of the amino-functionalized surface is hard to obtain. Because pH 4.8 and 7.0 are much lower than pKb 9, the primary amino groups at the APTMS surface should be highly protonated, and the surface is more positively ionized28 at pH 4.8 than at 7.0. To determine the degrees of dissociation for all of the polymers, (32) Zhang, F.; Srinivasan, M. P. Langmuir 2004, 20, 2309-2314.
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Song et al. Table 1. XPS Relative Intensities of C, O, N (Normalized to the Si Signal) for Silica Substrates after APTMS and APTMS/ PAM-C14-AA Treatments APTMS/PAM-C14-AA (pH 7.0) C/Si O/Si N/Si
APTMS/PAM-C14-AA (pH 4.8)
APTMS
20 min
20 h
20 min
20 h
0.86 0.62 0.10
0.90 0.72 0.14
1.40 0.85 0.22
1.40 0.90 0.26
2.10 0.98 0.30
Table 2. Water Contact Angles of the Solid Surfaces with Adsorbed PAM-C14-AA (20%) at Different pH Values and after Different Adsorption Times pH 7.0 pH 4.8
Figure 4. AFM images of amino-functionalized surfaces in PAMC14-AA (20%) solution at pH 7.0 after (a) 20 min and (b) 5, (c) 10, and (d) 20 h.
Figure 5. AFM images of amino-functionalized surfaces in PAMC14-AA (20%) solution at pH 4.8 after (a) 20 min and (b) 5, (c) 10, and (d) 20 h.
pH titration experiments on the polymers were carried out separately. From the pH titration curves, the values of pKa for the acrylic acid groups of PAM-C14-AA (5%), PAM-C14-AA (10%), and PAM-C14-AA (20%) are 4.6, 4.6, and 4.2, respectively. The degrees of dissociation for the acrylic acid groups of these three polymers are 60, 50, and 30% at pH 4.8 and 100, 100, and 73% at pH 7.0. Considering the different contents of AA in these polymers, the ratios for the ionized AA groups for PAMC14-AA (5%), PAM-C14-AA (10%), and PAM-C14-AA (20%) are 3:5:6 at pH 4.8 and 5:10:14 at pH 7.0. That is to say, the polymers carry negative charges, the charge densities increase following the above polymer order, and the polymers all have larger charge densities at pH 7.0 than at pH 4.8. The adsorption of HM polymers on the functionalized surface will be affected by the competition among van der Waals interactions, electrostatic forces, hydrogen bonds, hydrophobic interactions, and so forth.3,4 Figures 4 and 5 present the AFM morphology of the substrates adsorbed in PAM-C14-AA (20%) solution at pH 7.0 and 4.8 for 20 min and 5, 10, and 20 h, respectively. Compared with the amino-functionalized surface shown in Figure 2a, the substrate presents many aggregates and pores after the adsorption of PAMC14-AA (20%). With the extended deposition time, the aggregates
20 min
5h
10 h
20 h
53.4° 47.8°
54.4° 57.5°
56.3° 58.0°
56.5° 61.3°
on the modified surface protrude more. For films adsorbed at pH 4.8, the aggregates protrude more than at pH 7.0 with longer adsorption times. Consequently, this suggests that the adsorbed amount of PAM-C14-AA (20%) increases continuously with increased adsorption times at both pH 4.8 and 7.0. The XPS relative intensities for the polymer-treated surfaces of C, O, and N, normalized to the Si signal, at different pH values and adsorption times are listed in Table 1. Meanwhile, the corresponding XPS relative intensities of C, O, and N, normalized to the Si signal, for the APTMS-treated silica surface in the absence of polymer are also presented in the Table. The intensity of the Si signal can be attributed to the bare silicon surface. The intensities of the C, O, and N signals can be ascribed to APTMS and the adsorbed polymer. In previous reports,33,34 the XPS method was employed to determine polyelectrolyte adsorption on a mica basal plane quantitatively, and the adsorbed amount was evaluated by comparing the signal intensity for K from the mica substrate with that for the nitrogen (N 1s) signal from the adsorbed polyelectrolyte. Here, the change in relative intensities normalized to the Si signal of C, O, and N can reflect the alteration of the amount of adsorbed polymer. From the data listed in Table 1, it can be found that all of the relative intensities of C, O, and N for the surface treated with both APTMS and the polymer are stronger than those for the surface treated with only APTMS. Also, these relative intensities for longer adsorption times are stronger than those for shorter adsorption times. This increase in relative intensity may reflect the increase in the amount of adsorbed polymer with adsorption time. Because the penetration depth of XPS is less than 10 nm, any changes in the Si 2p relative intensity would reflect changes in the overlayer thickness. Moreover, in the XPS measurements, any nonhomogeneity on the surface is averaged out. As we can see, the Si 2p relative intensity decreases with either increasing adsorption time or decreasing pH of the deposition solutions. The phenomenon indicates an increase in the thickness of the polymer films during the two processes. The increase in the XPS relative intensities observed coincides with the result from AFM images. The variation of water CAs agrees with the above conclusion as well. The values of water CAs of the substrates adsorbed with PAM-C14-AA (20%) at different pH values and adsorption times are shown in Table 2. Upon an increase in adsorption time from 20 min to 20 h, the water CA increases from 53.4 to 56.5° for the samples at pH 7.0 and from 47.8 to 61.3° for the samples at pH 4.8. The difference in CA values is not significant after (33) Rojas, O. J.; Ernstsson, M.; Neuman, R. D.; Claesson, P. M. Langmuir 2002, 18, 1604-1612. (34) Rojas, O. J.; Ernstsson, M.; Neuman, R. D.; Claesson, P. M. J. Phys. Chem. B 2000, 104, 10032-10042.
Adsorption of Hydrophobically Modified PAM-C14-AA
5 h. That is to say, the adsorption equilibrium has been reached within 5 h. However, the polymers used are weak polyelectrolytes and have long hydrophobic side chains. Besides the electrostatic interaction between the opposite charges of the polymers and the surface, the hydrogen-bonding and hydrophobic interactions among the polymer molecules also affect the adsorption behavior. After 5 h, the amount of adsorbed polymers may not change obviously any more, but the adsorbed polymers may have some slow conformational rearrangements through weak interactions including hydrogen-bonding and hydrophobic interactions, which probably need a longer time to complete. This may be the reason that the CA values increase slightly from 5 to 20 h. The differences in the adsorbed polymer aggregates on the surfaces from 5 to 20 h can also be seen in the AFM images in Figures 4 and 5. At pH 7.0, owing to the higher charge densities of the polymers, the adsorption is faster, as indicated by the CA of 53.4° for 20 min, which is 5.6° higher than that for adsorption at pH 4.8 for 20 min. At the solid/liquid interface, polyelectrolyte molecules attach to the oppositely charged surface because of electrostatic attraction. As described by Rojas et al.,35 in the case of lowcharge-density polyelectrolytes, the charged segments anchor the polyelectrolyte to the oppositely charged surface, and the uncharged segments extend out from the surface in a configuration of loops and tails. As to the polymer conformation of loops and tails, here we have only indirect evidence from the different roughnesses in the AFM images. The polymers that we used are low-charge-density polyelectrolytes. As shown in Figures 4-6, the aggregate size and height are significantly different with the variations in pH value, adsorption time, and AA group content in the polyelectrolytes. The resultant roughnesses are also obvious larger for the PAM-C14-AA/APTMS-treated surface than for the very smooth APTMS-treated surface. The presence of loops and tails on the PAM-C14-AA/APTMS-treated surface should be the reason for the obvious increase in surface roughness. Hydrogen-bonding and hydrophobic interactions also influence the adsorption of polyelectrolytes on solid surfaces. The increase in the amount of adsorbed polymer with time suggests that such further adsorption is the result of hydrophobic interactions between the polymer hydrophobes in the adsorbed polymer layer, although hydrogen bonding may also play a role. Eventually, the additional charges in the additional adsorbed polymers will cause a reversal of the surface charge, preventing further adsorption and leading to a state of adsorption saturation. Weak polyeletrolytes provide an opportunity to use pH as a tool to manipulate adsorption on solid surfaces by tuning interactions between the polyions and the surface functional groups. All of the C, H, and N XPS relative intensities at pH 4.8 in Table 1 are stronger than those at pH 7.0, indicating stronger adsorption at the lower pH. As the pH is varied for these polymers, the degree of ionization is altered, thus affecting the relative number of charges along the backbone as well as the number of sites available for hydrogen bonding and other secondary attractions. At pH 7, the surface amino groups are already protonated and charged, and the polymer carboxyl groups are already partially deprotonated. Therefore, PAM-C14-AA (x%) may adsorb on the surface in a manner similar to the case for strong polyelectrolytes. The adsorbed layer should be very thin, and the number of loops and tails would be reduced because of the extended configuration of ionized adsorbed chains, allowing for fewer hydrophobic contacts with polymers that are still in solution. At pH 4.8, the carboxylates are largely protonated and (35) Rojas, O. J.; Claesson, P. M.; Muller, D.; Neuman, R. D. J. Colloid Interface Sci. 1998, 205, 77-88.
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Figure 6. AFM images of amino-functionalized surfaces exposed to solutions of hydrophobically modified polymers at pH 4.8 after 20 h: (a) PAM-C14-AA (5%), (b) PAM-C14-AA (10%), and (c) PAM-C14-AA (20%).
neutral. This may lead to more polymer loops and tails extending from the surface, allowing for more contacts for the hydrophobes as well as stronger hydrogen-bonding interactions between adsorbed and solution polymers,28 leading to additional adsorption. Thus, both hydrophobic and hydrogen-bonding interactions may contribute to the increase in PAM-C14-AA adsorption at the amino-functionalized silica surface at pH 4.8. AFM images of the silicon surfaces exposed to solutions of PAM-C14-AA of different charge densities at pH 4.8 are shown in Figure 6. Compared with Figure 6b,c, the surface in Figure 6a is relatively smooth. Less heterogeneous adsorption can be observed, and the root-mean-square (rms) roughness value is 0.395 nm. Additional heterogeneous adsorption takes place in Figure 6b, and the rms roughness increases to 0.432 nm. In Figure 6c, the heterogeneous adsorption becomes much more obvious, and the number of aggregates appears to be growing. The rms roughness is increased to 0.932 nm. This is likely due to the effect of charge density on the aggregate morphology of PAM-C14-AA (x%) at the surface. The effect of polyelectrolyte charge density on the adsorption behavior at the solid/liquid interface has been extensively investigated.33-37 One of the conclusions reached is that a decrease in polyelectrolyte charge density will induce a coil-to-globule transition of adsorbed polyelectrolyte molecules as a result of the reduced electrostatic (36) Kirwan, L. J.; Papastavrou, G.; Borkovec, M.; Behrens, S. H. Nano Lett. 2004, 4, 149-152. (37) Steitz, R.; Jaeger, W.; Klitzing, R. V. Langmuir 2001, 17, 4471-4474.
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repulsion. This in turn will result in an increase in the adsorbed layer thickness and the adsorbed amount.34,37 In our present work, the weak polyelectrolyte PAM-C14-AA can be expected to form loops and tails away from the surface. Hydrophobic interactions among the hydrophobic side chains may lead to the formation of aggregates and accompanying heterogeneous adsorption in the adsorbed layer. For the lower-charge-density PAM-C14-AA with lower electrostatic repulsion, the molecules may be packed more closely, and the loops and tails may overlap each other, resulting in the formation of a smoother surface. For higher polyelectrolyte charge density, the effect of charge matching between the adsorbing polymers and the surface and electrostatic repulsion between the polyelectrolyte aggregates may lead to heterogeneous adsorption in the surface layer. Response to Solvent Environment. Because many segments of the polymer chains have to undergo conformational rearrangement38 upon exposure to the solvents, a long time is required for the polymers to reach their new equilibrium conformation upon changing the solvent environment. In this study, substrates coated with PAM-C14-AA (20%) at pH 4.8 and 7.0 for 20 h were immersed in three different organic solventssn-hexane, acetone, and dehydrated ethanolsfor 24 h. AFM in conjunction with water CA measurements was used to follow the changes in the adsorbed polymer layer. Figure 7 presents the AFM morphologies of the substrates coated with PAM-C14-AA (20%) at pH 4.8 and treated with the three different solvents. The AFM morphologies of the substrates coated with PAM-C14-AA (20%) at pH 7.0 are similar and are not shown here. At pH 4.8, the substrate shows only a very minor change upon treatment with dehydrated ethanol, as can be seen in Figure 7a. The rms roughness value for the surface treated with dehydrated ethanol is 0.40 nm. After treatment with acetone, the aggregates become more pronounced (Figure 7b), and the rms roughness increases to 0.71 nm. Larger aggregates with a height of 4.2 nm appear following treatment with n-hexane (Figure 7c), and the rms roughness is raised to 1.50 nm. As shown in Table 3, water CA values of the substrate with adsorbed PAM-C14-AA (20%) change greatly upon treatment with organic solvent. Treatment with ethanol leads to very low water contact angles at both pH 7.0 and 4.8, 41.8 and 46.1°, respectively, which correspond to a decrease of about 15° from the original CAs before immersion in the solvent. Immersion in acetone for 24 h results in only a minor change in the water CA. Immersion in n-hexane results in a major increase in the water CA to 83.7° at pH 7.0 and to 77.0° at pH 4.0. The changes in surface morphology and wettability reflect the conformational reorganization of adsorbed polymer upon exposure to the three solvents. Either low surface-energy composition or high surface roughness can raise CA values of surfaces. Here, organic solvents ethanol, acetone, and hexane lead to a conformational reorganization in the adsorbed polyelectrolytes, which changes the roughness values of the surfaces as well as the chemical composition of the outer adsorption layer at the surface. The conformational reorganization should be attributed to the affinity differences of the selective solvents with respect to the polymer backbone and side chains. n-Hexane is a good solvent for hydrocarbon moieties and a poor solvent for ionic and polar segments (AA and AM). The adsorbed polymer molecules may aggregate with each other and adjust the conformation to avoid unfavorable contacts with n-hexane, resulting in the hydrocarbon side chains located in the outer adsorption layer in contact with hexane. This would lead to the larger aggregates observed in (38) Yu, X.; Somasundaran, P. J. Colloid Interface Sci. 1996, 178, 770-774.
Song et al.
Figure 7. AFM images of amino-functionalized surfaces after adsorption of PAM-C14-AA (20%) at pH 4.8 for 20 h followed by 24 h of solvent immersion: (a) dehydrated ethanol, (b) acetone, and (c) n-hexane. Table 3. Water Contact Angles of Solid Surfaces Adsorbed with PAM-C14-AA (20%) Following Treatment with an Organic Solvent
pH 7.0 pH 4.8
before solvent immersion
n-hexane
acetone
ethanol
56.5° 61.3°
83.7° 77.0°
57.8° 57.1°
41.8° 46.1°
Figure 7c and the high water CA following exposure to n-hexane. Acetone has a weaker affinity than n-hexane for the hydrocarbon side chains of the polymers but also has a relatively weak affinity for the hydrophilic segments on the polymer backbone. Thus, the adsorbed molecules will contract and the aggregates will become smaller than in the case of n-hexane but more notably so than in the case of dehydrated ethanol. In this case, the chemical composition of the outer adsorption layer does not exhibit a notable change. Accordingly, we do not observe a change in the
Adsorption of Hydrophobically Modified PAM-C14-AA
water CA and wettability upon treatment with acetone. Dehydrated ethanol is a good solvent for both AM and AA but a poor solvent for the alkyl hydrophobes. This would be expected to lead to a relatively flat, uniform conformation for the adsorbed polymer as observed in Figure 6a. In this conformation, the hydrocarbon side chains of the polymer are protected from the solvent, and the polymer hydrophilic components exposed to the solvent, leading to the observed lower water CA values.
Conclusions In summary, a reproducibly smooth amino-functionalized surface with high free primary-amine content has been prepared. The adsorption of hydrophobically modified poly(acrylamide)co-(acrylic acid) (PAM-C14-AA (x%)) with different acrylic acid contents on amino-functionalized surfaces, studied by AFM and contact angle measurements, provides information about the conformation of the polymer in the adsorbed surface layer. Hydrophobic modification of the polymer and the resulting hydrophobic interactions among the side chains can promote inter- and intrapolymer aggregation. Tuning the solution pH and thereby changing degree of ionization of the weak polyelectrolytes allows us to change the surface characteristics of the substrate with the polymer adsorption layer. At lower pH, increased adsorption is observed, possibly as a result of the combined effects of hydrogen bonding and hydrophobic interactions, with lower electrostatic repulsion between the polymer segments. As the pH is increased from 4.8 to 7.0, the increased charge density
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of the polymer chain increases the electrostatic repulsion among the aggregates, resulting in heterogeneous adsorption along with the effect of charge matching between the adsorbing polymers and the surface. The response of the PAM-C14-AA (x%) adsorbed layer to exposure to selective organic solvents is explored by analyzing the alteration of film morphology and wettability. A good solvent for the polymer backbone leads to a flatter conformation for the adsorbed film, whereas a poor solvent induces the aggregation of the adsorbed polymer and leads to a more heterogeneous adsorption layer. Contact with a nonpolar solvent with a high affinity for the polymer’s alkyl side chains leads to rearrangements in the adsorption layer with the hydrophobic side chains located in the outer part of the adsorption layer exposed to the solvent, resulting in a significant increase in surface hydrophobicity when the surface is again brought into contact with water. However, exposure to polar solvents with low affinity for the hydrophobic side chains leads to more hydrophilic components in contact with the solution, resulting in low surface hydrophobicity. Acknowledgment. We appreciate Professor Lei Jiang for helpful discussions and for providing the instrument for CA measurements. We are grateful for financial support from the National Natural Science Foundation of China (grant nos. 20633010 and 20473101). J.C.T.K. acknowledges support from the Natural Sciences and Engineering Council of Canada. LA062954U
