Langmuir 1999, 15, 8691-8694
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Effect of Water Traces upon Adsorption of PMMA on Flat Substrates of Glass and Silica Marc Balastre and Jean-Marc Berquier* Laboratoire CNRSsSaint-Gobain, “Surface du Verre et Interfaces”, 39 Quai Lucien Lefranc, F-93303 Aubervilliers Cedex, France Received March 30, 1999. In Final Form: July 15, 1999 Using IR spectroscopy and AFM, we investigated the effect of traces of water on the adsorbed amount and the morphology of the adsorbed layer in the adsorption of PMMA on flat silica and flat glass. To obtain very anhydrous solutions, we used a technique that involves the addition of molecular sieves to the solution. We showed that this method allows direct comparison of PMMA adsorption between dispersed and flat substrates of silica; in the absence of water in the solution, as is obtained with this technique, the amount of polymer adsorbed on flat silica is similar to amounts reported in the literature for dispersed silicas. We also showed that the adsorption of PMMA occurs in a similar way, with the same adsorbed amount and same layer morphology, onto glass and silica only when the adsorption takes place in an extremely anhydrous solution. On the contrary, when water molecules are present, the adsorbed amount of polymer increases on both substrates, with a higher amplitude on the glass substrates, and the structures of the polymer adsorbed layers are different on the two substrates.
Introduction Knowledge of the chemical interactions at the interface between polymer molecules and glass and of the conformation of polymer chains in contact with glass surfaces is important for the understanding of the fundamentals of polymer/glass adhesion as well as how they contribute to the control and the improvement of this adhesion. Because the same type of interactions are also expected to occur in the adsorption process, the investigation of the adsorption of the polymer on glass appears as an interesting approach to these interactions. The adsorption of polymer, especially PMMA, on dispersed silica1-9 or on silicium wafers3,10-13 has been the subject of numerous articles. The adhesion of different polymers on flat glass and silica samples has been compared by different authors.14-17 But, to our knowledge, the adsorption of polymer on flat glass has not been investigated. We have compared the adsorption of a model polymer, poly(methyl methacrylate) (PMMA), on flat samples of * Corresponding author. Fax: 33 1 48 34 74 16. Tel.: 33 1 48 39 58 22. E-mail:
[email protected]. (1) Fontana, B. J.; Thomas, J. R. J. Phys. Chem. 1961, 65, 480. (2) Yamagiwa, S.; Kawaguchi, M.; Kato, T.; Takahashi, A. Macromolecules 1989, 22, 2199. (3) van der Beek, G. P.; Cohen Stuart, M. A.; Fleer, G. J. Macromolecules 1991, 24, 3553. van der Beek, G. P. Thesis, Landbouwuniversiteit, Wageningen, The Netherlands, 1991. (4) Kobayashi, K.; Araki, K.; Imamura, Y. Bull. Chem. Soc. Jpn. 1989, 62, 3421. (5) Kawaguchi, M.; Yamagiwa, S.; Takahashi, A.; Kato, T. J. Chem. Soc., Faraday Trans. 1990, 86, 1383. (6) Thies, C. J. Polymer Sci. C 1971, 34, 201. (7) Fowkes, F. M.; Mostafa, M. A. IEC Prod. R&D 1978, 17, 3. (8) Sakai, H.; Imamura, Y. Bull. Chem. Soc. Jpn. 1987, 60, 1261. (9) Kobayashi, K.; Sugimoto, S.; Yajima, H.; Araki, K.; Imamura, Y.; Endo, R. Bull. Chem. Soc. Jpn. 1990, 63, 2018. (10) Johnson, H. E.; Granick, S. Macromolecules 1990, 23, 3367. (11) Kuzmenka, D. J.; Granick, S. Colloids Surf. 1988, 31, 105. (12) Van Alsten, J. G. Macromolecules 1992, 25, 3007. (13) Zazzera, L. A.; Tirrell, M.; Evans, J. F. Mater. Res. Soc. Symp. Proc. 1993, 304, 125. Zazzera, L. A.; Tirrell, M.; Evans, J. F. J. Vac. Sci. Technol. 1993, 11, 2239. (14) Fowkes, F. M. J. Adhes. Sci. Technol. 1987, 1, 7. (15) Fowkes, F. M.; Dwight, D. W.; Cole, D. A.; Huang, T. C. J. NonCrystalline Solids 1990, 120, 47. (16) Creuzet, F.; Ryschenkow, G.; Arribart, H. J. Adhes. 1992, 40, 15. (17) Buffeteau, T.; Desbat, B.; Arribart, H.; Chartier, P. Proceedings of the XVth Adhesion Society Meeting, Hilton Head Island, 1992.
silica and glass. We used for this purpose FT-IR external reflection spectroscopy and reported that larger quantities of PMMA are adsorbed on glass than on silica.18 This result was in contradiction with what one may expect from adhesion experiments. Fowkes,14 indeed, showed that the difference in poly(methyl methacrylate) (PMMA) adhesion between flat silica and flat soda-lime glass is due to different contributions of the acid-base interaction in the adhesion energy. PMMA, which contains Lewis basic groups, has a higher affinity for silica, which has acid sites, than for glass, which the presence of alkali and alkaline earths makes basic. Considering only the acidbase properties of both substrates, a higher, or in any case similar, adsorbed amount of PMMA on flat silica was expected. This discrepancy is not the only singularity of our results: we have also undertaken an atomic force microscopy (AFM) investigation of the silica and glass substrates after adsorption and reported19 that the morphology of the adsorbed polymer layers on these two substrates are very different. We made the hypothesis that these unexpected results concerning the adsorbed amount and morphology are due to the presence of water traces in the solvent. In most of the studies about polymer adsorption onto flat surfaces, the possible presence of water traces, a fourth partner in addition to the surface, the polymer and the solvent, is neglected. However, to achieve a strictly anhydrous environment around a flat surface in a solution is difficult, and the possibility of such a water presence can hardly be ruled out. On the contrary, in the domain of polymer adhesion, the presence of water at the polymer/silica interface has been the subject of several studies.20-23 Water (18) Berquier, J.-M. In Organic Coatings, Lacaze, P. C., Ed.; AIP Conference 354: New York, 1995; p 424. (19) Berquier, J.-M.; Creuzet, F.; Grimal, J.-M. Langmuir 1996, 12, 597. (20) Nguyen, T.; Bentz, D.; Byrd, E.; J. Coatings Technology 1994, 66, 39. (21) Nguyen, T.; Byrd, E.; Bentz, D. J. Adhes. 1995, 48, 169. (22) Nguyen, T.; Byrd, E.; Bentz, D.; Lin, C. Prog. Org. Coatings 1996, 27, 181. (23) McKnight, S. H.; Gillespie, J. W., Jr. J. Appl. Polym. Sci. 1997, 64, 1971.
10.1021/la990373h CCC: $18.00 © 1999 American Chemical Society Published on Web 10/27/1999
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at the organic coating/substrate interface is indeed often the main cause of degradation of the adhesion of the system.24,25 In this paper, we investigate the effect of the presence of water in the solution on the characteristics of the adsorbed polymer layer on the flat glass and silica substrates. We report a simple technique allowing us to study the adsorption on flat samples in extremely anhydrous conditions. With these conditions, we find the same adsorbed amount of PMMA on flat silica as on dispersed silicas. We show that the behaviors of the two surfaces of glass and silica in relation to PMMA adsorption are similar in extremely anhydrous conditions and differ dramatically when traces of water are present. Experimental Section Flat substrates of soda-lime silicate glass (Saint-Gobain Vitrage, France) and amorphous silica (WQS) were cleaned in a detergent solution, rinsed several times with deionized water, and finally irradiated with UV radiation under an O2 flow for 1 h (UV-O3 treatment). This last treatment has proved to be essential for reproductibility of the experiment. After this cleaning procedure, the substrates were rapidly immersed in the CCl4 solutions of PMMA. When the adsorption was complete, the substrates were rinsed with solvent. The solvent was then allowed to evaporate at room temperature for 1 day. Three types of experimental conditions have been used in order to obtain different levels of water concentration in the solution. The “equilibrated” solution was obtained simply by using the solvent as received, which is assumed to be in equilibrium with the atmospheric humidity. The substrates were transferred in the atmosphere of the laboratory. The “anhydrous” solution used solvent that had been dried over molecular sieves for several days. The “extremely anhydrous” solution used the same dried solvent, but molecular sieves were also added to the solution. In these last two cases, the substrates were transferred under the anhydrous atmosphere of nitrogen. Karl Fischer measurements for water concentrations in the equilibrated solution and the anhydrous solution were around 3 × 10-3 and 3 × 10-4 M, respectively. It was expected that the water concentration in the “extremely anhydrous” solution was lower than 10-4 M and so is under the detection limit of the Karl Fischer technique. The adsorbed amount of PMMA was estimated by infrared external reflection at a 60° incident angle. To convert band absorbance into the adsorbed amount, it was necessary to simulate the reflection spectra from the complex indices of the different products. The spectra of thin PMMA layers on glass and silica were simulated for different layer thicknesses, and the height of the ν(CO) band was measured. For these very low thicknesses, the band height increased linearly with thickness. The slopes were different for glass and silica: 1.4 × 103 and 0.8 × 103 mg/m2/absorbance unit, respectively. This difference can be ascribed to the difference in indices between the two substrates. These theoretically obtained slope values were used for estimation of the adsorbed amount from the experimental value of the band height. X-ray reflectometry has occasionally been used to measure the thickness of the PMMA layers on several samples. This technique confirmed the results obtained with the IR external reflection. The surfaces were examined in air by AFM using a commercial instrument from Park Scientific Instrument with a scanning frequency equal to 2 Hz. The operation mode was the repulsive one. In this mode, the AFM tip is always in contact with the surface.
Results and Discussion We present in Figures 1 and 2 some typical FTIR external reflection spectra of PMMA layers on glass and (24) Wu, S. Polymer Interface and Adhesion; Marcel Dekker: New York 1982; p 589. (25) Kinloch, A. J., Ed.; Durability of Structural Adhesives; Applied Sci.: New York, 1983.
Balastre and Berquier
Figure 1. External reflection IR spectra obtained on two samples of float glass that were immersed simultaneously for 1 day in the same anhydrous solution of PMMA. Please note the good reproductibility of the experiment. The downwarddirected band at ca. 1730 cm-1 is due to the carbonyl groups of the adsorbed polymer chains.
Figure 2. External reflection IR spectra obtained on two samples of flat silica that were immersed simultaneously for 1 day in the same anhydrous solution of PMMA. Contrary to Figure 1, one can also note here the presence of a shoulder at ca. 1715 cm-1 which could be ascribed to the carbonyl groups interacting with the surface silanols.
silica, respectively. The bands due to the polymer are directed downward. This is a characteristic point of the external reflection on glass and silica.26-28 The band due to the stretching vibration of the carbonyl group, νCO, appears in the wavenumber range presented in these figures. It is the only band which will be used. Our experience is that this band can be detected for PMMA layers with thicknesses down to 0.2 nm. In each figure two spectra are presented, which correspond to two samples in which polymer adsorption has occurred simultaneously and in the same solution. The very good agreement between the two spectra demonstrates the reproductibility of the adsorption experiment. On the contrary, we observed slight differences between samples obtained from different solutions or from the same solution but at different times, and we felt that these differences may be due to minute modifications in the water concentration. So, we became interested in studying in more detail the effect of the “water concentration” parameter, which indeed appears to be crucial, even at very low concentrations. Accordingly, we searched for a simple method with extremely anhydrous experimental conditions and especially very anhydrous solutions. (26) Udagawa, A.; Matsui, T.; Tanaka, S. Appl. Spectrosc. 1986, 40, 794. (27) Berquier, J.-M.; Fernandes, A.-C.; Chartier, P.; Arribart, H. In Chemically Modified Oxide Surfaces; Leyden, D. E., Collins, W. T., Eds.; Gordon and Breach Science: Langhorne, PA, 1989; p 245. (28) Blaudez, D.; Buffeteau, T.; Desbat, B.; Fournier, P.; Ritcey, A.M.; Pe´zolet, M. J. Phys. Chem. B 1998, 102, 99.
Effect of Water Traces on the Adsorption of PMMA
Langmuir, Vol. 15, No. 25, 1999 8693 Table 1. Effect of Water Concentration on Adsorption
Figure 3. Adsorbed amounts (mg/m2) of PMMA on glass and silica from extremely anhydrous, anhydrous, and equilibrated solutions.
Care in obtaining very anhydrous solutions is indeed all the more required in this case because the adsorption is investigated on flat substrates. If one compares the ratios (substrate surface area)/(solvent volume) for two typical adsorption experiments involving substrates of the same nature but with different geometries, flat or dispersed, it readily appears that these ratios have a huge effect on the amount of water which is eventually adsorbed at the surface of these substrates. In the case of the flat substrate, one liter of solvent is in contact with less than 1 m2 of substrate surface area. This is much lower than the ratio of 10 000 m2 per solvent liter, which is currently obtained experiments with dispersed silica. We now consider the example of dry substrates immersed in a solvent whose water concentration is equal to 2 × 10-4 M. Such a concentration is typically measured by the Karl Fischer technique for solvents that have been dried over molecular sieves. In such a solvent, this water concentration in the case of the flat samples is so high that the adsorption of a water layer of ca. 5 H2O molecules/nm2 on the sample surface only slightly modifies the water concentration in the solvent. On the contrary, in the case of the dispersed silica, the same water concentration in the solution appears to be so low that it does not even allow the adsorption of 1/100 of such an adsorbed layer of water. Because some water adsorption eventually occurs, it is then expected that, in an experiment involving dispersed silica, the presence of this dry dispersed silica will even decrease the water concentration in the solvent. It is clear from this simple calculation that only the achievement of very low water concentrations in the solutions will permit a quantitative comparison of the adsorption onto substrates of different geometries. This conclusion leads us to introduce the molecular sieves in the solution and to let them in as the flat samples are immersed. The ratio (substrate surface area)/(solvent volume) then becomes similar to the case of the dispersed silica, and the adsorption of water molecules on the flat substrates is limited. It is just as if the solution were dried in situ. In the following discussion we call this solution an extremely anhydrous solution. Concerning the polymer, it should be noted that its concentration in the solution remains constant, as the very small diameter of the pores in the molecular sieves prevents any polymer adsorption in the pores. This point has been checked by transmission IR spectroscopy. Dispersed silica could also have been added to the solution instead of the molecular sieves, and it is expected that such an addition would have the same effect. However, it would also have the disadvantage of decreasing the concentration of PMMA, owing to polymer adsorption on the dispersed silica. Figure 3 presents the values of the adsorbed amounts on glass and silica for the three solutions prepared in
ext. anhydrous
anhydrous
equilibrated
glass
21 °C 35 °C
0.8 0.9
2.3 2.2
5.4 3.2
silica
21 °C 35 °C
0.8 0.8
1.5 1.8
2.9 2.1
different conditionssthe “extremely anhydrous”, the “anhydrous” and the equilibrated solutionsscorresponding to three different water concentrations. Typically, the water concentration in an equilibrated solution is around 3 × 10-3 M, and in an anhydrous solution it is around 3 × 10-4 M.29 Our experiments were conducted mainly at room temperature. To check for the possible effect of temperature, we also conducted some experiments at 35 °C. This temperature was chosen because it is higher than the theta temperature of PMMA in CCl4, which is reported to be equal to 27 °C.30 Similar results were obtained for both temperatures. They are reported in Table 1. For the “extremely anhydrous” solution, a value of ca. 1 mg/m2 was found for both substrates. This value is typically found in the case of PMMA adsorption on dispersed silicas.3-9 This similarity was understood as a direct consequence of our method for preparation of the solutions. As expected, the very low water concentration obtained in the solution permits a direct comparison between polymer adsorption onto high-specific area and low-specific area silicas. For the “anhydrous” solution, higher values were measured, between 1.5 and 2 mg/m2. In this case, the mean square deviation is larger. This may possibly be ascribed to a larger deviation in the water concentration. These values of the adsorbed amount are similar to the sparse quantitative data obtained by internal multireflection IR spectroscopy,10 where care is taken to minimize the presence of water but without molecular sieves added to the solution. For the equilibrated solution, values larger than 3 mg/m2 were obtained. The adsorbed amount increases with the water concentration, and in these two cases with higher water concentrations, the adsorbed amount on glass is larger than on silica. At this point, one may wonder about the possible roles of water in the adsorption process. If one considers water as a “cosolvent” in a binary mixture, even if this cosolvent is present in a very low concentration, two roles appear possible for the water molecules: displacer or nonsolvent. The first involves interaction of the water molecules with the surface, where the water can act as a displacer3,31,32 for the adsorbed polymer segments. Such an effect was reported by Marra and Christenson,33 who observe that water acts in cyclohexane as a displacer of polystyrene (PS) from the mica surface; 0.5% water is enough to completely inhibit the adsorption of PS. It can be reasonably expected that a higher water concentration would be needed to displace the polar PMMA from the polar silica surface than the nonpolar PS from the polar mica surface. This effect would result in a decrease of the adsorbed amount with an increase in the water concentration. In the domain of polymer/glass adhesion science, this effect (29) In ref 19, the water concentration is erroneous: it should read 3 × 10-4 instead of 3 × 10-5 M. (30) Brandrup, J.; Immergut, E. H. Polymer handbook, 2nd ed.; Wiley: Ithaca, NY, 1975. (31) Fleer, G. J.; Cohen Stuart M. A.; Scheutjens, J. M. H. M.; Cosgrove, T.; Vincent, B. Polymers at Interfaces; Chapman and Hall: London, 1993. (32) Cohen Stuart, M. A.; Fleer, G. J.; Bijsterbosch, B. H. J. Colloid Interface Sci. 1982, 90, 321. (33) Marra, J.; Christenson, H. K. J. Phys. Chem. 1989, 93, 7180.
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of water molecules at the surface is well-known14,24,25 and contributes to a process responsible for degradation of the adhesion. The second possible role of water concerns the solvent quality of the mixture. As water is a nonsolvent of PMMA, the addition of water to tetrachlorocarbone tends to decrease the quality of the mixture. From the molecular point of view, it is expected that the water molecules can interact through hydrogen bonds with the carbonyl groups of the polymer. For high concentrations of water and polymer, one can also envisage the presence of water molecules which interact through two hydrogen bonds with two carbonyl groups of the same polymer chain or of different chains. A decrease of the solvent quality leads to an increase of the adsorbed amount before the phase separation. For example, in the case of the adsorption of polystyrene from a cyclopentane/n-pentane mixture, Granick and collaborators34 found that the adsorbed quantity increases with higher volume fraction of the poorer solvent, amounting to a factor of two before the onset of precipitation. The observed increase of the quantity of adsorbed PMMA with the water concentration of the solution indicates that the second effect is predominant in the case of glass and silica. One can be surprised by the occurrence of such an effect for such a low water concentration, here typically in the 10-3-10-4 M range. A possible explanation should be found in the fact that, due to the adsorption of water at the surfaces of silica and glass, the water concentration is significantly higher near the surfaces than in the bulk solution.35 Because water has an effect on solvent quality, the higher water concentration near the surface could induce a lower solvent quality near the surface than the one which is experienced in the bulk solution. Such an explanation also allows us to understand the difference in the adsorbed amounts on glass and silica. Indeed, it has been known for many years36 that, under the same conditions, the concentration of adsorbed water molecules at the glass surface is higher than at the silica surface. For the same water concentration in the bulk solution, the polymer chains will indeed experience different water concentrations near the surfaces of glass and silica and, accordingly, different solvent qualities. We also investigated the effect of successive immersions of a sample in two solutions with different water concentrations. The resulting adsorbed amounts in the cases of the anhydrous and equilibrated solutions are shown in Table 2. It appears that the final adsorbed amount depends only on immersion in the more hydrated solution. Immersion in the anhydrous solution either before or after immersion in the equilibrated solution has at best only a minute effect on the adsorbed amount. We recently presented the images obtained by AFM of (34) Johnson, H. E.; Hu, H.-W.; Granick, S. Macromolecules 1991, 24, 1859. (35) Trens, P. Ph.D. Thesis, Marseille, 1994. (36) Holland, L. The Properties of Glass Surfaces; Chapman and Hall: London, 1964; Chapter 4.
Balastre and Berquier Table 2. Effect of Water Traces on Adsorption solution 1
solution 2
adsorbed amount (mg/m2)
67 000
anhydrous anhydrous equilibrated equilibrated -
equilibrated equilibrated anhydrous anhydrous
2.2 3.8 4.0 4.3 4.1 2.2
218 000
anhydrous anhydrous equilibrated equilibrated -
equilibrated equilibrated anhydrous anhydrous
2.2 4.3 4.3 4.9 4.6 2.2
the PMMA layers adsorbed on flat substrates of glass and silica19 from “anhydrous solutions” (as named in this paper); in these conditions, in which water traces are present, the PMMA adsorbed layer covers the silica surface in a homogeneous way, but the adsorbed layer on glass is constituted of islands. Since then, we obtained AFM images (not presented here) of adsorbed PMMA layers in “extremely anhydrous” conditions; in these conditions, the adsorbed layers are homogeneous on both substrates. This proves that the presence of water in the solution, and accordingly near the surfaces, is needed for the formation of the island structure of the adsorbed layer on glass. Summary We have shown in this work that the use of molecular sieves added directly to the polymer solution allows a direct comparison of PMMA adsorption between dispersed and flat substrates of silica; in the case of a very low concentration of water in the solution, as is obtained with this technique, the amount of polymer adsorbed on flat silica is similar to the values reported in the literature for dispersed silicas. We have shown that the adsorption of PMMA occurs in a similar way, with the same adsorbed amount and same layer morphology, onto glass and silica only when the adsorption takes place in an extremely anhydrous solution, as was obtained in this work. On the contrary, for higher concentrations of water, the adsorbed amount of polymer increases on both substrates, with a higher amplitude on the glass substrates than on the silica substrates, and the structures of the polymer-adsorbed layers are different on the two substrates: island structure on glass versus continuous film on silica. Two effects of the presence of water molecules in the solution are assumed to explain these results. On one hand, the decrease of the solution quality due to the water presence promotes the adsorption of PMMA. On the other hand, the displacement of polymer groups from the glass surface sites by water molecules permits formation of the island structure. LA990373H
