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Pulsed Plasma Polymerized Maleic Anhydride Films in Humid Air and in Aqueous Solutions Studied with Optical Waveguide Spectroscopy Li-Qiang Chu,†,‡ Renate Fo¨rch,† and Wolfgang Knoll*,†,‡ Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany, and Department of Material Science and Engineering, and Department of Chemistry, National UniVersity of Singapore, 117543, Singapore ReceiVed NoVember 10, 2005. In Final Form: January 13, 2006 Optical waveguide spectroscopy (OWS) was employed to monitor the swelling behavior of pulsed plasma polymerized maleic anhydride (PPPMA) films in humid air and in aqueous solutions by measuring the film thicknesses and refractive indices. With the relative humidity of air increasing, both the thickness and the refractive index of the PPPMA films increased, indicating water penetration into and uptake by the films. The swelling of the hydrated PPPMA films in humid air is reversible. In aqueous media, the thickness and the refractive index of the washed PPPMA film increased with an increase of pH and ionic strength, respectively. On the basis of the present data, a hypothesis concerning the structure of the PPPMA film is proposed. Our model suggests that the unique structure of the PPPMA films originates from the cyclic structure of maleic anhydride and depends on parameters of the plasma deposition process, and the interaction between H2O and the carboxylic groups.
Introduction It has been demonstrated that optical waveguide spectroscopy (OWS) is a suitable tool to characterize polymer thin films ranging from several hundred nanometers to a few micrometers in thickness.1,2 For example, the behavior of PMMA films at various temperature and pressure was studied using this optical technique.3-5 It was also applied successfully to monitor the swelling of polymer brushes.6,7 As compared to SPR, OWS is suitable for the characterization of thick films (d > 200 nm). Moreover, SPR provides the information only about the optical thickness (n × d) of a thin film, whereas OWS can determine the (isotropic) refractive index (n) and the thickness (d) simultaneously, provided at least two modes can be guided in the thin film structure. Plasma polymerization is an effective and economical surface modification technique for polymers and for many other materials.8-11 Traditionally, plasma polymerized films are often highly cross-linked and chemically inert because of the high fragmentation of the monomers during the plasma polymerization process.12 However, it was found that films deposited by plasma polymerization of acrylic acid and other monomers can be * To whom correspondence may be addressed. Telephone: (+49) 6131 379161. Fax: (+49) 6131 379360. E-mail:
[email protected]. † Max Planck Institute for Polymer Research. ‡ National University of Singapore. (1) Knoll, W. In Handbook of Optical Properties: Optics of Small Particles, Interfaces, and Surfaces; Hummel, R. E., Wissmann, P., Eds.; CRC: Boca Raton, FL, 1997; p 373. (2) Knoll, W. Annu. ReV. Phys. Chem. 1998, 49, 569-638. (3) Prucker, O.; Christian, S.; Bock, H.; Ru¨he, J.; Frank, C. W.; Knoll, W. Macromol. Chem. Phys. 1998, 199, 1435-1444. (4) Kleideiter, G.; Lechner, M. D.; Knoll, W. Macromol. Chem. Phys. 1999, 200, 1028-1033. (5) Fehrenbacher, U.; Jakob, T.; Berger, T.; Knoll, W.; Ballauff, M. Fluid Phase Equilib. 2002, 200, 147-160. (6) Biesalski, M.; Ru¨he, J. Langmuir 2000, 16, 1943-1950. (7) Biesalski, M.; Johannsmann, D.; Ru¨he, J. J. Chem. Phys. 2002, 117, 49881994. (8) Ratner, B. D. Biosens. Bioelectron. 1995, 10, 797-804. (9) Chan, C. M. Polymer Surface Modification and Characterization; Hanser: Munich, 1994. (10) Barton, D.; Bradley, J. W.; Steele, D. A.; Short, R. D. J. Phys. Chem. B 1999, 103, 4423-4430. (11) Strobel, M.; Lyons, C. S.; Mittal, K. L. Plasma Surface Modification of Polymers: ReleVance to Adhesion; VSP: Utrecht, The Netherlands, 1994.
structurally and chemically very similar to their conventional counterparts.13-15 These results indicate that the structure of a plasma polymerized film can vary dramatically with the variation of the chemical nature of the monomer and the deposition parameters employed during plasma polymerization. Swelling is a common phenomenon in polymers due to the penetration of small solvent molecules into the polymer (network), which may change the structure and the properties of polymers dramatically. Hence, the swelling behavior of many polymers has been investigated extensively.16-18 In most practical applications, plasma polymers will come into contact with air or aqueous solution. Therefore, it is of great importance to examine the swelling behavior of plasma polymerized films in humid air and in aqueous solution. The use of maleic anhydride as a monomer in pulsed plasma polymerization was first reported in 1996.19 Since then, this system has received much attention as an approach to achieve carboxylic functionalities, and the properties of the deposited films were investigated extensively using many techniques. Jacobsen et al.20 examined the behavior of PPPMA films in contact with moisture with a waveguide mode spectrometer (WaMS). They found that both the thickness d and the refractive index n increased if the plasma polymers were exposed to humidity. The change was not reversible because the water vapor reacted chemically with the anhydride groups. The increase of d was explained by two factors: (a) the swelling of the polymer network, and (b) the reaction of the anhydride groups leading to carboxylic acid groups. For the increase of n, the authors suggested that there were some (12) Inagaki, N. Plasma Surface Modification and Plasma Polymerization; Technomic: Lancaster, PA, 1996. (13) O’Toole, L.; Beck, A. J.; Short, R. D. Macromolecules 1996, 29, 51725177. (14) Alexander, M. R.; Duc, T. M. J. Mater. Chem. 1998, 8, 937-943. (15) Fraser, S.; Short, R. D.; Barton, D.; Bradley, J. W. J. Phys. Chem. B 2002, 106, 5596-5603. (16) Wind, J. D.; Sirard, S. M.; Paul, D. R.; Green, P. F.; Johnston, K. P.; Koros, W. J. Macromolecules 2003, 36, 6433-6441. (17) Jabbari, E.; Nozari, S. Eur. Polym. J. 2000, 36, 2685-2692. (18) Chen, W. L.; Shull, K. R.; Papatheodorou, T.; Styrkas, D. A.; Keddie, J. L. Macromolecules 1999, 32, 136-144. (19) Ryan, M. E.; Hynes, A. M.; Badyal, J. P. S. Chem. Mater. 1996, 8, 37-42. (20) Jacobsen, V.; Menges, B.; Fo¨rch, R.; Mittler, S.; Knoll, W. Thin Solid Films 2002, 409, 185-193.
10.1021/la0530283 CCC: $33.50 © 2006 American Chemical Society Published on Web 02/09/2006
Pulsed Plasma Polymerized Maleic Anhydride Films
structural changes when the anhydride group dissociated in water. In addition, surface plasmon resonance (SPR) spectroscopy was used to examine the swelling behavior of PPPMA films.21 However, the swelling behavior of maleic anhydride plasma films was very different from that of allylamine plasma polymers and di(ethylene glycol) mono vinyl ether plasma polymers. This was also attributed to the reactivity of the anhydride groups in aqueous solution. Both reports dealt with freshly deposited PPPMA films and indicated that the reaction between anhydride groups and H2O plays a vital part in their final structure. Moreover, Schiller et al.22 have investigated the film changes occurring in aqueous media using impedance spectroscopy. They stated that their PPPMA films showed some characteristic features of polyelectrolytes in aqueous solution because the film could swell considerably. To distinguish the factors influencing the behavior of PPPMA films, we prepared in this study three kinds of PPPMA films: (1) fresh PPPMA, which contains anhydride groups; (2) hydrated PPPMA, which was exposed to humid air for sufficient time allowing for the completion of the reaction of the anhydride groups in the fresh PPPMA; and (3) washed PPPMA, which was extracted in H2O overnight and hence has no reactive anhydride groups and is stable because the soluble material in the hydrated PPPMA had been washed out. The PPPMA films are deposited to a thickness of 500-700 nm aiming at exciting at least two waveguide modes in an OWS spectrum, which allows for the simultaneous determination of the thickness and the refractive index of the thin film. OWS is also employed to monitor in real time the swelling behavior of PPPMA films in moisture and in water of varying pH and ionic strength. Experimental Section Materials and Substrates. All chemicals were purchased from Sigma-Aldrich and used without further purification. Milli-Q water was used throughout the experiments. LaSFN9 glass slides (n ) 1.844 @ λ ) 633 nm, Hellma Optik, Jena, Germany) were used in the SPR and OWS measurements. The LaSFN9 slides were cleaned with 2% Hellmanex detergent, washed with water extensively, and dried with pure nitrogen gas. Next, 2 nm chromium and 50 nm gold were evaporated onto the glass slide. Here, the chromium layer is used to enhance the adhesion of gold on the LaSFN9 glass. A blank measurement by SPR with this substrate gave information about the bare Au and Cr layers. To improve the adhesion of the plasma polymer on gold, a monolayer of 1-octadecanethiol (5 mM in ethanol, 10 min immersion) was self-assembled on gold. The SAM/Au/Cr/ LaSFN9 substrates were dried after the SAM formation and placed into the plasma chamber. Aqueous solutions with various pH and salt concentrations were prepared with NaOH and NaCl. Preparation of PPPMA Films. The pulsed plasma polymerization was carried out in a home-built plasma reactor, as described before.23 The substrates were located halfway between two electrodes. Plasma polymerization was carried out at a pressure of 630 Pa (0.063 mbar), peak power 37 W, and duty cycle 1/41 ms (duty cycle ) Ton/(Ton + Toff)). These conditions correspond to an equilibrium power of 1.11 W (Pequil ) Ppeak * DC). The deposition time was varied from 60 to 90 min to obtain films of sufficient thickness for the excitation of at least two waveguide modes in the OWS spectrum. Two SAM/ Au/Cr/LaSFN9 slides are used for the PPPMA deposition at the same time to obtain two virtually identical samples. After deposition, one sample is characterized by OWS immediately, which is referred to as “fresh PPPMA”. After the OWS measurements, the anhydride (21) Zhang, Z.; Chen, Q.; Knoll, W.; Foerch, R. Surf. Coat. Technol. 2003, 174-175, 588-590. (22) Schiller, S.; Hu, J.; Jenkins, A. T. A.; Timmons, R. B.; Sanchez-Estrada, F. S.; Knoll, W.; Fo¨rch, R. Chem. Mater. 2002, 14, 235-242. (23) van Os, M. T.; Menges, B.; Foerch, R.; Vanco, G. J.; Knoll, W. Chem. Mater. 1999, 11, 3252-3257.
Langmuir, Vol. 22, No. 6, 2006 2823 Table 1. Relative Humidity over the Saturated Salt Solution at 25 °C salt solution relative humidity (%) salt solution relative humidity (%) LiBr CaCl2 K2CO3 NaBr
6 29 43 58
NaCl KCl ZnSO4 KNO3
75 84 90 98
group in the fresh PPPMA will react with H2O in humid air. Therefore, no anhydride groups exist any more. In this case, the sample is referred to as “hydrated PPPMA”. The other sample was washed with water at room temperature for 24 h, allowing for a complete hydrolysis reaction and the removal of low MW non-cross-linked material. The sample is referred to as “washed PPPMA” in this work. Fourier Transform Infrared Spectroscopy (FT-IR). FT-IR was carried out using a BIO-RAD FTS3000 spectrometer. Typically, 64 scans were acquired at a resolution of 4 cm-1. The FT-IR spectrum of the Si wafer was recorded first, which was used as the background for the sample measurement because the plasma polymers were deposited on the Si wafer in the FT-IR measurements. Optical Waveguide Spectroscopy (OWS). OWS measurements were carried out with a home-built setup, which is technically identical to an SPR setup and has been described before.6 The samples were mounted into a Teflon cell, which can be kept at a constant relative humidity. The humidity inside the Teflon cell was controlled by using a small vessel filled with a saturated aqueous salt solution in contact with excess salt. The high salt content of the solution reduces the water vapor pressure to a distinct value. The chosen salt solutions and the corresponding relative humidity inside the cell are shown in Table 1. 0% humidity was obtained by drying the samples over potassium hydroxide pellets. 100% humidity was obtained through pure H2O. Each sample was first dried with solid potassium hydroxide before introducing the salt solution. During the moisture exposure, the change in thickness was monitored by measuring the shift of the minimum dip of a waveguide mode as a function of time. The time resolution is better than 1.5 s. Once the equilibrium of the swelling process was reached, an OWS angular scan was carried out to determine the thickness and the refractive index of the swollen polymer. For the measurements in aqueous solutions, another Teflon flow cell was employed and thus the plasma polymer could be brought in direct contact with the liquid solution. The results were verified by measuring the OWS spectra using both p-polarized and s-polarized light.
Results and Discussion FT-IR Analysis. FT-IR allows for a unique identification of anhydride group by two strong bands at 1780 and 1860 cm-1, respectively. Conversely, carboxylic acid groups show strong adsorption bands at 1730 and 1630 cm-1, respectively.24 By a careful comparison of the relative intensities of the various absorption bands, some conclusions about the abundance of the functional groups in the films can be made. Figure 1 shows two typical FT-IR spectra of a fresh PPPMA and a hydrated PPPMA film, respectively. The FT-IR spectrum of the fresh PPPMA film shows a distinct band at 1781 cm-1, which indicates that the film contains a relatively high amount of anhydride groups. The anhydride groups in the fresh PPPMA films react readily with water and result in the formation of carboxylic acid groups. This can be observed by FT-IR via a relative intensity increase of the carboxylic acid band at 1716 cm-1, as shown in Figure 1. In Humid Air. Figure 2 shows OWS spectra of fresh PPPMA at 0% and 100% relative humidity, respectively. Generally, the higher order of the waveguide mode is located at low angles, which are more sensitive to thickness changes. The low-order (24) Jenkins, A. T. A.; Hu, J.; Wang, Y. Z.; Schiller, S.; Foerch, R.; Knoll, W. Langmuir 2000, 16, 6381-6384.
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Figure 1. FT-IR spectra of fresh PPPMA and hydrated PPPMA film. Figure 3. Swelling kinetics of hydrated PPPMA film at various relative humidity.
Figure 2. p-Polarized OWS spectra of fresh PPPMA film at 0% and 100% relative humidity (RH).
waveguide modes are located at high angles and are sensitive to refractive index changes. After introducing the humid air, all of the waveguide modes in the OWS spectrum shift to higher angles, indicating a large increase of both the thickness and the refractive index. For OWS spectra with more than two waveguide modes, the thickness and the refractive index can be determined independently by using Fresnel’s equations. It was found that the thickness of the fresh PPPMA layer is d0 ) 906.0 nm (at n ) 1.626) in the dry state, and d ) 936.5 nm (at n ) 1.632) at 100% relative humidity, respectively. Therefore, both d and n of a fresh PPPMA increase with increasing humidity. This result is in good agreement with the literature.20 OWS can be carried out with either a p-polarized or an s-polarized laser. The OWS spectrum measured with s-polarized light also confirms this conclusion (data not shown here). If the plasma polymerized maleic anhydride films are exposed to humid air, two processes are expected to occur: (a) hydrolysis of anhydride groups, which leads to acid groups; and (b) swelling of the polymer films because H2O molecules penetrate into the film. To minimize the effect of hydrolysis reaction, hydrated PPPMA films were investigated. The swelling kinetics of the hydrated PPPMA in contact with humid air can be monitored with OWS by measuring the angular position of one waveguide mode as a function of time. Figure 3 shows the change of the fourth-order waveguide mode (in s-polarized light OWS, data are not shown here) as a function of time. The hydrated PPPMA was attached to a special cell in which KOH pellets guaranteed
Figure 4. p-Polarized OWS spectra of washed PPPMA film at 0% and 100% relative humidity.
Figure 5. The behavior of washed PPPMA film with varying pH.
0% humidity. Upon changing KOH with various saturated salt solutions (i.e., various relative humidity), the waveguide mode shifts to higher angles very quickly within the first several minutes and reaches equilibrium within about 10 min. The only exception is the case at 100% relative humidity, which shows that the process is initially very fast, while after ca. 1 min a secondary much slower process takes over, and equilibrium is reached only after about 10 h. The fast process may represent diffusion of water molecules into the upper layers of the hydrated PPPMA followed by a significantly slower diffusion of H2O into the
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Figure 7. s-Polarized OWS spectra of the hydrated PPPMA film at pH 9.3 with various salt concentrations.
Figure 6. Hypothesis about the structure of washed PPPMA film in aqueous solution: (a) compact state with association of COOH at low pH; (b) expansion state with dissociation of COOH at high pH; and (c) Na+ incorporation into polymer network.
lower layers of the film. After equilibrium is reached, the humidity inside the cell is changed back to 0%. The waveguide mode goes back to the starting point. This can be seen from Figure 3 in that the starting point of every curve is almost at the same level. This indicates that the swelling of the hydrated PPPMA film is fully reversible. From Figure 3, it can be seen that a stepwise increase of the relative humidity results in a stepwise increase of the angle
position of the waveguide mode. The data indicate that the thickness (d) and the refractive index (n) increase simultaneously as the humidity increases. For hydrated PPPMA, the hydrolysis reaction is (almost) completed prior to the measurements. Therefore, the increase in thickness should be caused by the swelling of the PPPMA film only (i.e., H2O penetration into the plasma polymer). For the increase of the refractive index, Jacobsen et al.20 suggested that there were some structural changes if maleic anhydride dissociated in water. However, the present sample is not expected to undergo a hydrolysis reaction. The PPPMA films are hydrophilic due to the presence of functional groups and hence have a strong interaction with H2O. In a humid environment, H2O penetrates easily into the matrix of the polymer. Thus, the overall density of the polymer matrix may increase due to H2O substitution of air, which may partially explain the increase of the refractive index of the plasma polymerized maleic anhydride film observed in this work and elsewhere. Generally, fresh PPPMA films contain a small part of low MW molecules, which will dissolve if the samples are washed in water. In other words, the composition of the PPPMA films will only be relatively stable after the washing step. The OWS angular scans were recorded for a washed PPPMA at 0% and 100% relative humidity, respectively. For the former experiment, the washed PPPMA was first dried with N2 gas, and then stored in a desiccator containing KOH pellets for more than 24 h. Figure 4 shows the OWS angular scans for the washed PPPMA at 0% humidity and 100% humidity. It can be seen that Figure 4 is in excellent agreement with Figure 2. As the humidity increases, the angular positions of the waveguide modes shift to higher angles, indicating that the washed PPPMA also shows the increase of both refractive index and thickness upon humidity increase. The thickness of the washed PPPMA at 0% humidity is calculated to be d0 ) 896.5 nm (at n ) 1.624), and at 100% humidity the thickness is d ) 925.5 nm (at n ) 1.632). The thickness of the washed PPPMA at 0% humidity is lower than that of the fresh PPPMA. This can be ascribed to the loss of some low MW components during washing. In Aqueous Solutions. The behavior of the PPPMA films in aqueous solutions was investigated with particular emphasis on the effect of pH and ionic strength. Considering the stability, washed PPPMA films were employed rather than the fresh or hydrated PPPMA films. Figure 5 shows the kinetic behavior of the PPPMA film upon changing the pH. The third-order waveguide mode (at low angle position) in s-polarized OWS was monitored in the present experiment. OWS spectra were scanned at pH 2 and pH 10. The fitted data show that the refractive
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index at pH 2 and pH 10 does not change (n ) 1.632), and only the thickness increases a little (3 nm). Therefore, we assume that the change in Figure 5 is related to only a thickness change. From Figure 5, it can be seen that the position of the waveguide mode remains almost constant from pH 2 to pH 4. If the pH is increased to 5, the waveguide mode shifts slightly to lower angles and remains stable until pH 6. The waveguide mode then shifts by a small increment to a higher angle position at pH 8 and remains stable continuously until pH 9. At pH 10, the waveguide mode position increases again to a very small extent. However, if the pH is increased to 11, a relatively large increase of the waveguide position is observed, indicating a large increase of the thickness of the washed PPPMA film. The basicity of the solution was increased to pH 12 only for a short period of time because the PPPMA films may delaminate from the substrates. After the pH returns to 10, a quick decrease can be observed. However, the waveguide mode does not return to the same position as before, indicating that the change of the PPPMA induced by a pH change was not reversible. The behavior of the PPPMA films at different pH can be attributed to the presence of COOH groups in the films, as a result of the hydrolysis of the anhydride groups. The retention of anhydride groups in fresh PPPMA films results in some carboxylic acid close to each other inside hydrated and washed PPPMA. In the compact state (Figure 6a), these carboxylic acid groups may dimerize due to hydrogen bonding. If the pH of the aqueous solution increases above a certain value (from Figure 5 at pH > 8), the COOH dissociates to COO-. The repulsion of the negative charges associated with COO- may lead to a thickness increase of the PPPMA film, as depicted in Figure 6b. As the COOH groups continue to dissociate with increasing pH, the thickness of the PPPMA film follows. However, it should be noted that the PPPMA film is partially cross-linked, leading to a considerable resistance to the expansion of the film. The fitted results of the OWS spectra (at pH 2 and 10) show that the total change of thickness is only 3 nm, which is relatively small as compared to the overall thickness of the PPPMA film (d ) 931.5 nm). Therefore, the PPPMA films show a highly crosslinked polyelectrolyte characteristic in aqueous solution. The effect of ionic strength on the washed PPPMA was investigated by using various concentrations of NaCl solutions, while the pH was kept at 9.3. Once the films reached equilibrium, an angular scan was recorded. Figure 7 shows the OWS spectra (for p-polarized light) of the washed PPPMA film at various concentrations of NaCl. As the ionic strength in the aqueous
Chu et al.
Figure 8. The thickness and the refractive index of the hydrated PPPMA film in different NaCl concentrations.
solution increases, both waveguide modes shift to higher angles. This indicates that both the refractive index and the thickness of the PPPMA film increase with increasing ionic strength. This may be explained by incorporation of Na+ into the PPPMA films. At pH 9.3, the carboxylic group in the PPPMA is dissociated as COO-, which is able to adsorb Na+ (as depicted in Figure 6c). With increased concentration of NaCl, more Na+ penetrates into the polymer network, resulting in the increase of both d and n. These OWS spectra were fitted again according to Fresnel’s equations. The results are shown in Figure 8. The cyclic structure of maleic anhydride hinders the tight packing during the plasma deposition process, and thus the large free space will exist in the deposited film. Consequently, the density of the polymer matrix increased as the free space was occupied by Na+, which may explain the observed increase of the refractive index.
Conclusion The swelling behavior of PPPMA films in contact with moisture and aqueous media was investigated using optical waveguide spectroscopy. The hydrated PPPMA films shows a reversible swelling behavior in humid air. With increased pH, the thickness of washed PPPMA films increases because of the dissociation of COOH to COO- groups. High salt concentrations in the aqueous media result in more Na+ incorporation into the PPPMA film as seen in an increase of both the thickness and the refractive index. LA0530283
