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Acrylamide Polymerized in the Presence of Surfactants: Surface Analysis Using Atomic Force Microscopy Mukundan Chakrapani,† David H. Van Winkle,*,† Brian C. Patterson,‡ and Randolph L. Rill‡ Department of Physics and Center for Materials Research and Technology, and Department of Chemistry and Biochemistry, The Florida State University, Tallahassee, Florida 32306-4250 Received January 31, 2002. In Final Form: May 28, 2002
Introduction Polymer gel research has advanced rapidly in the past 20 years.1 The scope of application of these materials is extensive and encompasses diverse areas from medicine to personal hygiene, civil engineering to biomedical engineering, and food preservation to environmental protection. Polyacrylamide gels are well-known and have many realized and potential applications. These include drug delivery systems, biosensors, adhesion materials for proteins and cells, biomimetic actuators, chemomechanical devices, and matrixes for biochemical separations. Some recent research has focused on characterizing gels polymerized in the presence of other included particles or materials (typically nonreactive solutes or colloids). Several groups have used dynamic light scattering (DLS), small-angle X-ray scattering (SAXS), or small-angle neutron scattering (SANS) to investigate their properties.2-7 Gels have also been formed in microemulsions and in the presence of surfactants.8-17 These novel synthesis techniques have been used to produce materials that have * Address correspondence to this author. Telephone: (850) 6446019. Fax: (850) 644-6504. E-mail:
[email protected]. † Department of Physics and Center for Materials Research and Technology. ‡ Department of Chemistry and Biochemistry. (1) Gels Handbook; Osada, Y., Kajiwara, K., Eds.; Academic Press: San Diego, 2001. (2) Shibayama, M.; Isaka, Y.; Shiwa, Y. Macromolecules 1999, 32, 7086-7092. (3) Krall, A. H.; Weitz, D. A. Phys. Rev. Lett. 1998, 80, 778-781. (4) Krall, A. H.; Huang, Z.; Weitz, D. A. Physica A 1997, 235, 19-33. (5) Joonsten, G. H.; Gelade, T. F.; Pusey, P. N. Phys. Rev. A 1990, 42, 2161-2175. (6) Xue, J.-Z.; Pine, D. J.; Milner, S. T.; Wu, X.-l.; Chaikin, P. M. Phys. Rev. A 1992, 46, 6550-6563. (7) Shibayama M. Macromol. Chem. Phys. 1998, 199, 1-30. (8) Goltner, C. In Reactions and Synthesis in Surfactant Systems; Texter, J., Ed.; Marcel Dekker: New York, 2001. (9) Rill, R. L.; Locke, B. R.; Liu. Y.; Dharia, J.; Van Winkle, D. H. Electrophoresis 1996, 17, 1304-1310. (10) Anderson, D. M.; Strom, P. In Polymer Association Structures: Microemulsions and Liquid Crystals; El-Nokaly, M. A., Ed.; ACS Symposium Series 384; American Chemical Society: Washington, DC, 1989; pp 204-224. (11) Laversanne, R. Macromolecules 1992, 25, 489-491. (12) Burban, J. H.; He, M.; Cussler, E. L. AIChE J. 1995, 41 (4), 907-914. (13) Antonietti, M.; Goltner, C.; Hentze, H.-P. Langmuir 1998, 14, 2670-2676. (14) Antonietti, M.; Caruso, R. A.; Goltner, C. G.; Weissenberger, M. C. Macromolecules 1999, 32, 1383-1389. (15) Antonietti, M.; Caruso, R. A.; Hentze, H.-P.; Goltner, C. Macromol. Symp. 2000, 152, 163-172. (16) Hentze, H.-P.; Antonietti, M. Curr. Opin. Solid State Mater.Sci. 2001, 5, 343-353. (17) Paul, E. J.; Prud’homme, R. K. In Reactions and Synthesis in Surfactant Systems; Texter, J., Ed.; Surfactant Science Series 100; Marcel Dekker: New York, 2001; pp 525-535.
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geometries or functionalities that cannot be achieved using traditional methods. We report here an analysis of the surface of polyacrylamide gels formed in the presence of surfactants. Polyacrylamide gels were formed in the presence of different concentrations of tetradecyltrimethylammonium bromide (TTAB) surfactant. The presence of significant amounts of surfactant during the polymerization dramatically altered the structure of polyacrylamide gels. Atomic force microscopy (AFM) has been previously used to investigate the surface structure of hydrogels.18,19 AFM of the gels synthesized revealed that the surface roughness increased strongly with increasing surfactant concentration. We document the surface features seen in AFM, relate these features to the optical transparency of the gels, and quantify the roughness of the gel surfaces as a function of the initial surfactant concentration. Experimental Section Polyacrylamide gels were formed at room temperature by freeradical polymerization with chemical initiators between two glass plates separated by 1.5 mm spacers. Acrylamide (Electrophoresis grade, Fisher Scientific) solutions were polymerized in the presence of a TBE buffer (45 mM trisborate, 1 mM EDTA) at pH 8.3. The chemical initiators were ammonium persulfate and N,N,N′,N′-tetramethylethylenediamine (TEMED,Fisher Scientific). The initial concentration of acrylamide plus cross-linker (N,N′-methylenebisacrylamide) in the stock solution was 40 wt %. The concentration of the cross-linker with respect to acrylamide was 7 wt %. The acrylamide concentration in the stock solution was calculated as a mass percentage, assuming a density of 1.00 g/cm3 for water and buffer solution added; that is, %T ) [Ma/(Ma + Ml)] × 100, where Ma is the mass of acrylamide (including bisacrylamide) and Ml is the mass of solution (water plus buffer concentrate). Various amounts of the surfactant, TTAB (Sigma-Aldrich), were added to form the gels. All the gels were made after dissolving the surfactant in the stock solution. The surfactant concentration was varied from S ) 0% to 40 wt % of the stock solution in steps of 10%. The surfactant concentration was calculated as %S ) [Ms/(Ms + Mp)] × 100, where Ms is the mass of surfactant and Mp is the mass of the stock solution. A set of “pre-gel” solutions and gels containing surfactant were imaged in a polarizing microscope (Nikon Optiphot-Pol). No birefringence was observed in any samples at any surfactant concentration. When a piece of gel with surfactant was left on the slide for a few minutes, birefringent crystallites formed on the gel surfaces. These were apparently surfactant crystallites forming on the surface as water evaporated and surfactant diffused out from the gel. Another set of gels was then polymerized for studies of the changes upon removal of surfactant. After polymerization between glass plates in sealed, watertight, plastic bags, gels were soaked in water to remove the surfactants by diffusion. More than 98% of the surfactant diffused out of the gel upon successive soaking in water. Several methods including mass balance and Raman scattering have been used previously to analyze the removal of surfactants by free diffusion into water.20 The gel samples were secured in a sample holder (Figure 1) and imaged by a Digital Instruments (D3000) atomic force microscope in contact mode under water. This procedure ensured that the tip-sample interaction was minimal and the images were highly reproducible. All scans were done at room temper(18) Suzuki, A.; Yamazaki, M.; Kobiki, Y. J. Phys. Chem. 1996, 104, 1751. (19) Suzuki, A.; Yamazaki, M.; Kobiki, Y.; Suzuki, S. Macromolecules 1997, 30, 2350. (20) Patterson, B. P. Surfactant micelles as templates in hydrogels. Ph.D. Dissertation, The Florida State University, Tallahassee, 2000.
10.1021/la020107v CCC: $22.00 © 2002 American Chemical Society Published on Web 07/12/2002
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Notes Table 1. Image Roughness [nm] and Surface Area [µm2] Data for Various Scan Sizes as a Function of Surfactant Concentration square scan size of L [µm] % surfactant 0 10 20 30 40 0 10 20 30 40
Figure 1. Schematic representation of the sample holder designed for imaging hydrogels using an AFM. ature. Any background slope in the images was removed using the flatten function available with the D3000 software.21
Results and Discussion Obvious macroscopic changes resulted from forming polyacrylamide gels in solutions containing up to 40% TTAB surfactant. The rate of polymerization decreased with increasing concentration of surfactants. Polymerization was essentially complete after 30 min for pure polyacrylamide gels and only after several hours with 40% TTAB added. Prior to polymerization, all solutions were clear and homogeneous liquids with viscosities dependent on the amount of surfactant. All resulting gels were still optically transparent after polymerization, but before surfactant removal. Gels formed in the presence of 30% or more surfactant became uniformly white as the surfactants were removed. Those formed with 0-20% surfactant remained transparent. The observation of the change in clarity upon removal of the surfactant was puzzling. Optical clarity is an indication that the index of refraction does not vary on optical length scales. White materials either reflect all colors without absorption or, more commonly, scatter all colors well. When broken up, the pieces of white gel remained white. Thus, the whiteness was not because the surface became highly reflective. The transformation of a gel from clear to white thus indicates that there was a significant change in light scattering as the surfactant diffused out. Apparently, as the TTAB diffused out, a structure that was homogeneous on optical length scales became inhomogeneous. In a series of papers, Antonietti et al. discuss formation of a variety of polymer gels such as polyacrylamide in the presence of lyotropic surfactant mesophases.13-15 The surfactant concentrations were such that the surfactants were in a hexagonal columnar lyotropic liquid crystalline phase, identified by the characteristic birefringence observed in polarized optical microscopy. In their observations, “prior to polymerization all mixtures are transparent, and become opaque or turbid-white shortly after the start of the reaction.”15 This led them to propose that prior to and during polymerization a binary phase separation occurred, with water present in both phases, one of which was rich in surfactant and one rich in (21) D3000 Command Reference Manual; Digital Instruments, Santa Barbara, CA, 1996.
1.25
2.50
5.00
10.00
(a) Image Roughness [nm] 0.249 0.309 0.490 0.607 0.921 2.153 3.426 4.236 3.836 7.080 7.121 8.469 6.163 11.578 19.93 25.676 10.473 33.253 38.253 55.730 (b) Surface Area [µm2] 1.563 6.251 25.009 1.564 6.257 25.031 1.570 6.281 25.104 1.585 6.384 25.640 1.689 7.021 26.744
100.01 100.10 100.38 102.81 105.54
20.00 0.772 5.005 12.059 32.560 62.421 400.04 400.24 402.12 414.13 427.08
monomer. They proposed that the polymer gel is formed by cross-linking the phase low in surfactant. This phenomenon did not occur in the polymerization process described here, because translucent gels were formed and they became white only as the surfactant was soaked out. Atomic force microscopy was used to study the surfaces of the gels after removal of surfactant. The presence of TTAB during polymerization markedly affected the morphology of the surfaces, as was evident from the variations in the surface topographies of the AFM images. AFM images of 20 µm × 20 µm regions of the gel surfaces are shown in Figure 2. Conventional gels formed without any surfactants were very flat and exhibited a variation in height on the order of only a few nanometers in a several square micrometer scan area. White gels formed in the presence of high concentration of surfactants (S g 30%), on the other hand, exhibited 300 nm high peaks and valleys in 20 µm × 20 µm scan areas. Gels formed in the presence of 30% or 40% surfactant had features that were comparable in both height and separation to the wavelength of visible light. Thus, the translucency of the gels can be related to the surface features observed. The surface properties of samples imaged by AFM are usually quantified by the roughness and surface area measurements. Roughness is defined as the rootmean-square variation of heights (rms roughness
x∑(Zi-Zave)2/N, where Zi is the height of the ith pixel, Zave is the average height of the of all pixels in an image, and N is the number of pixels).21 The data for various images of different gel surfaces obtained using the D3000 software21 are summarized in Table 1. The roughness of the gel surface increased dramatically with surfactant concentration. The roughness variations for different scan sizes are plotted on a log-log scale against the surfactant concentration in Figure 3. The surface roughness increased from less than 1 nm in the conventional gel (S ) 0%) to 62 nm in the gel formed in the presence of 40% TTAB (20 × 20 µm2 scan size data). Surface area is the three-dimensional area of the given image. This value is the sum of the areas of all the nonoverlapping triangles formed by each point in the image and its two nearest neighbors in both the negative and positive x and y directions. The change in surface morphology can also be quantified in terms of changes in the topographic surface area. The difference in the twodimensional scan area from the topographic surface area, termed as the surface area difference, is plotted as a function of the surfactant concentration in Figure 4. The difference increased by almost 3 orders of magnitude as surfactant concentration increased from 0 to 40%, regardless of the scan sizes.
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Figure 2. AFM images of polyacrylamide gels. (a) No surfactant, (b) 10% surfactant, (c) 20% surfactant, (d) 30% surfactant, and (e) 40% surfactant. The x and y range is 20 µm while the z range is 300 nm.
These AFM observations demonstrate that there is a dramatically different surface morphology for the gels that turned white compared with those that remained clear. Prior work on synthesis of polymer gels in the presence of lyotropic surfactant phases provides some insight into our results.8-17 Antonietti et al.13-15 reported scanning electron microscopic images of replica structures of polymer gels that had been formed in the presence of various surfactants. Many of those images seem to show large irregular pores on many length scales, including hundreds of nanometers. Our results are in agreement with these previously reported observations in the sense of finding length-scale variations on the order of hundreds of nanometers.
However, our observations differ from those reported in the literature in the following important aspects. All the gels synthesized in the presence of up to 20% TTAB form and stay clear. Gels made in the presence of 30% and 40% TTAB were translucent but turned white upon soaking in water. It is known that TTAB exists as spherical micelles up to 35% (by weight) at room temperature.22 Antonietti et al. reported that gels synthesized in the presence of spherical micelles end up with strongly birefringent regions, indicating an increase of the surfactant concentration in a surfactant-rich phase during polymerization.14 We do not observe any birefringence at (22) Warheim, T.; Jonsson, A. J. Colloid Interface Sci.1988, 125, 627.
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Notes
surfactant-rich and a polymer-rich subphases during polymerization, because they do not turn white until the surfactant is removed. This observation indicates that the surfactants are homogeneously distributed in the gel matrix. A restructuring of the gel architecture must occur while the surfactant diffuses out to make these gels scatter light strongly.
Figure 3. Mean image roughness as a function of surfactant concentration. The five data sets correspond to the different scan sizes indicated in the inset (in µm2).
Figure 4. Surface area difference as a function of surfactant concentration. The five data sets correspond to the different scan sizes indicated in the inset (in µm2).
all either in the “pre-gel” solution or after the gel has been formed. The transformation of gels made in the presence of 30% and 40% TTAB into homogeneously white material cannot be explained by separation of the solution into
Conclusions Polyacrylamide gels have been formed in the presence of surfactants and analyzed. Atomic force microscopy has revealed dramatic changes in the surface topography of gels as a function of the surfactant concentration. Polymerization and the subsequent removal of the surfactants cause a change in the internal structure of the gel matrix. Translucent gels are transformed into homogeneously white gels with features as large as several hundred nanometers that scatter light strongly. AFM has shown an almost exponential increase in the roughness and surface area of these gels. The surface areas and dimensions of features increase systematically with the concentration of surfactant present during polymerization. Thus, by choosing appropriate conditions, one can synthesize gels exhibiting greatly modified structure and surface topographies. This controlled modification of gel surfaces and internal structure may provide new opportunities in technology. Possible areas of application may include enzyme fixation for building biosensors and controlled-release systems where the increase in the surface area of hydrogels formed in the presence of surfactants can be exploited. The process of gel formation in the presence of unreactive surfactants can also be extended to alter the structure of other polymer materials. Acknowledgment. We thank B. R. Locke, P. A. Rikvold, S. J. Mitchell, A. Beheshti, and E. Lochner of The Florida State University for fruitful discussions and assisting in the design of the AFM sample holder. This work was made possible in part by support from NSF Grant BES-951381 and the FSU Center for Materials Research and Technology (MARTECH). LA020107V
