Spontaneous Polymerization at the Air−Water Interface: A Brewster

Langmuir 2007, 23, 12243-12248

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Spontaneous Polymerization at the Air-Water Interface: A Brewster Angle Microscopy Study Sophie Cantin,* Odile Fichet, Franc¸ oise Perrot, and Dominique Teyssie´ Laboratoire de Physicochimie des Polyme` res et des Interfaces (LPPI, EA 2528), UniVersite´ de Cergy-Pontoise - 5, mail Gay-Lussac, NeuVille-sur-Oise, 95031 Cergy-Pontoise Cedex, France ReceiVed July 10, 2007. In Final Form: August 29, 2007 When a dioctadecyldimethylammonium bromide (DODA) monolayer is spread onto a styrene sulfonate (SSt) aqueous solution, this monomer undergoes a spontaneous polymerization process [Fichet, O; Teyssie´, D. Macromolecules 2002, 35, 5352]. However, the polymer synthesized in this monolayer cannot be investigated by classical characterization techniques. Brewster angle microscopy has thus been used as a complementary method in order to study this spontaneous polymerization. From these measurements, the threshold concentration above which the spontaneous polymerization occurs has been determined more precisely; the monomer adsorption under the DODA monolayer has been evidenced as being very fast, as supposed previously; moreover, sodium bicarbonate is confirmed as an inhibitor of the polymerization. Also, the replacement of SSt by toluene sulfonate (TSt) confirms the SSt spontaneous polymerization. Finally, the molecular weight and/or the structure of the polymer synthesized in the monolayer seems to be different from those synthesized in solution.

1. Introduction Polyelectrolyte-amphiphile complexes form spontaneously when an amphiphilic molecule solution is spread on a subphase containing the polyelectrolyte.1-3 Complex formation may induce stabilization of a monolayer as well as a change in its structure and morphology.4 Studies of the monolayer behavior lead to information about the interaction between a given polyelectrolyte and an amphiphilic molecule and about the new type of structure that results from complexation. In this nonconventional medium, unexpected reactions can occur. As reported previously,5 spontaneous polymerization has been studied at the air-water interface, in which the polymerizable double bond is located in neither the hydrophilic nor the hydrophobic part of an amphiphilic monomer, as in most Langmuir and Langmuir-Blodgett (LB) polymerization examples described in the literature. Instead, a monolayer of a positively charged lipid, dioctadecyldimethylammonium bromide (DODA) was chosen as a template for the adsorption of a wide water-soluble monomer, sodium 4-styrenesulfonate (SSt), that is, bearing a negatively charged group (sulfonate) in the para position with respect to the vinyl polymerizable function. Above a monomer threshold concentration in the subphase, spontaneous polymerization of SSt at the interface was evidenced by means of π-A isotherm measurements, as well as UV, IR, and 1H NMR spectra of the transferred layers. As far as we know, this is the first evidence of spontaneous polymerization of a water-soluble monomer upon adsorption under or inside a monolayer (the exact location cannot as yet be defined from our experimental data). This spontaneous polymerization was followed in real time by isobaric and isochoric curve recordings, showing that its occurrence depends upon the monomer concentration, the applied surface pressure π, the ionic strength, and the pH of the subphase.6 * Corresponding author. E-mail: [email protected]. (1) Erdelen, C.; Laschewsky, A.; Ringsdorf, H.; Schneider, J.; Schuster, A. Thin Solid Films 1989, 153, 180. (2) Shimomura, M.; Kunitake, T. Thin Solid Films 1985, 132, 243. (3) Vagharchakian, L.; Desbat, B.; He´non, S. Macromolecules 2004, 37, 8715. (4) Engelking, J.; Menzel, H. Thin Solid Films 1998, 327-329, 90. (5) Fichet, O.; Teyssie´, D. Macromolecules 2002, 35, 5352. (6) Fichet, O.; Plesse, C.; Teyssie´, D. Colloids Surf., A: Physicochem. Eng. Aspects 2004, 244, 121.

On the other hand, carbon dioxide, sodium bicarbonate, and LiClO4 inhibit this spontaneous polymerization. These results lead to the conclusion that the polymerization occurs at the interface, most probably according to an anionic mechanism. Knowing that a Langmuir monolayer only contains approximately 1016 molecules, no classical polymer characterization can be carried out with a high degree of accuracy. Indeed, the possible tacticity of the polymer because of the monomer preorganization under the monolayer has not been evidenced, and the polymerization degree cannot be estimated. Thus, because of the originality of the studied system, nonconventional characterization techniques must be used in order to indirectly analyze the synthesized polymer. Brewster angle microscopy (BAM) is a sensitive and nondestructive technique7,8 that provides information on the morphology of amphiphilic monolayers,9,10 including the inner structure of condensed domains, phase transitions in monolayers,11,12 and the orientational order of the molecules in condensed domains.13,14 BAM has already been used for studying the complex formation between an anionic surfactant and a cationic polyelectrolyte15,16 or the reverse combination.4,3,17,18 However, polyelectrolytes and amphiphilic molecules can also form complexes driven by nonelectrostatic interactions, such as hydrophobic or hydrogen bonding.19 BAM has also been used (7) He´non, S.; Meunier, J. ReV. Sci. Instrum. 1991, 62, 936. (8) Ho¨nig, D.; Mo¨bius, D. J. Phys. Chem. 1991, 95, 4590. (9) Rivie`re, S.; He´non, S.; Meunier, J.; Albrecht, G.; Boissonnade, M. M.; Baszkin, A. Phys. ReV. Lett. 1995, 75, 2506. (10) Werkman, P. J.; Schouten, A. J.; Noordegraaf, M. A.; Kimkes, P.; Sudholter, E. J. R. Langmuir 1998, 14, 157. (11) Overbeck, G. A.; Honig, D.; Mobius, D. Thin Solid Films 1994, 242, 231. (12) Rivie`re-Cantin, S.; He´non, S.; Meunier, J. Phys. ReV. E. 1996, 54, 1683. (13) Brezesinski, G.; Scalas, E.; Struth, B.; Mo¨hwald, H.; Bringezu, G.; Gehlert, Weidemann, U. G.; Vollhardt, D. J. Phys. Chem. 1995, 99, 8755. (14) Rivie`re, S.; Meunier, J. Phys. ReV. Lett. 1995, 74, 2495. (15) Franco, O.; Katholy, S.; Morawetz, K.; Reiche, J.; Freydank, A.; Brehme, L.; Antonio de Saja, J. Colloids Surf., A: Physicochem. Eng. Aspects 2002, 198200, 119. (16) Engelking, J.; Wittemann, M.; Rehahn, M.; Menzel, H. Langmuir 2000, 16, 3407. (17) Jain, N. J.; Albouy, P-A.; Langevin, D. Langmuir 2003, 19, 5680. (18) Jain, N. J.; Albouy, P-A.; Langevin, D. Langmuir 2003, 19, 8371. (19) Gole, A.; Phadtare, S.; Sastry, M.; Langevin, D. Langmuir 2003, 19, 9321.

10.1021/la7020534 CCC: $37.00 © 2007 American Chemical Society Published on Web 10/19/2007

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to study reactions occurring in Langmuir monolayers. For example, the synthesis of a thermosetting resin starting from different hexadecoxyphenols was investigated at the airformaldehyde interface.20 Noticeably different morphologies were observed at a surface pressure just lower than the collapse pressure. BAM has also allowed for following the formation of a dimyristoylphosphatidylcholine bilayer under a polymerized octadecyltrimethoxysilane Langmuir monolayer.21 Polymerizations occurring at the air-water interface are generally characterized by an area decrease, which corresponds to the usual three-dimensional shrinkage evidenced in classical bulk polymerizations.22 Thus, the spontaneous polymerization studied in this paper should induce a change in the DODA alkyl chain organization, which might be visualized by BAM. Morphological changes are thus expected to appear, going from below to above the SSt monomer concentration threshold (5 × 10-4 mol‚L-1) evidenced by isobaric and isochoric curve recording for spontaneous polymerization.6 In this paper, isotherms and BAM characterizations of the DODA monolayers are presented on an aqueous subphase at different SSt monomer concentrations. The monomer is either dissolved in the subphase beforehand or injected under the compressed DODA monolayer. In addition, the effect of sodium bicarbonate as a polymerization inhibitor is checked by means of BAM. For comparison, the DODA monolayer is also studied by means of isotherms and BAM on sodium p-toluene sulfonate (TSt) solutions, since TSt bears no polymerizable function. Finally, the DODA monolayer is spread on atactic poly(sodium 4-styrenesulfonate) (PSSt) polymer solutions. Indeed, when SSt spontaneously polymerizes at the air-solution interface, the monomer is probably preorganized under or inside the DODA monolayer. It is accordingly possible that the resulting polymer synthesized at the interface would show some different microstructure. 2. Experimental Section 2.1. Reagents and Materials. SSt, TSt, and PSSt (Mw ) 70 000 g‚mol-1) were purchased from Aldrich. DODA (99%, Sigma), potassium persulfate (Acros), and sodium hydrogen bicarbonate (99%, SDS) were used as received. Spreading solutions were prepared from Purex chloroform (HPLC grade, Carlo Erba). The subphase water was purified by a Millipore system producing water with a resistivity of 18 MΩ‚cm. Aqueous PSSt solutions were also synthesized as follows: 1 g of SSt (4.9 × 10-3 mol) and 0.026 g of potassium persulfate (9.6 × 10-5 mol, 2.5% by weight with respect to SSt) were dissolved in 10 mL of pure water. Then, the solution was heated at 60 °C for 3 h. Two other solutions were prepared with the same compositions, except the potassium persulfate weight was changed to 0.13 and 0.31 g, which corresponds to 12 and 30% by weight with respect to SSt, respectively. Thus, since the initiator proportion changed, the synthesized PSSt polymers had different molecular weights: the higher the initiator proportion, the smaller the polymer weight. 2.2. Instrumentation. All surface pressure isotherms (π-A curves) were recorded on a 600 cm2 Teflon Nima trough (611D system). The surface pressure measurements were carried out using the Wilhelmy plate technique. An appropriate amount of DODA chloroform solution (1-2 mg‚mL-1) is spread onto the water or solution subphase. The initial molecular area is set at 1.50 nm2, and the molecules are thus in an expanded state (gas/liquid-expanded (LE) phase transition). A 10 min waiting period prior to compression is necessary so that the (20) Shin, H.-K.; Kim, J. U.; Lee, B.-J.; Kwon, Y.-S. Thin Solid Films 2001, 393, 34. (21) Saccani, J.; Castano, S.; Desbat, B.; Blaudez, D. Biophys. J. 2003, 85, 3781. (22) Ringsdorf, H.; Schupp, H. J. Macromol. Sci. Chem. 1981, A15 (5), 1015.

Cantin et al. solvent evaporates. Then, Langmuir monolayers are compressed up to the desired surface pressure, using a continuous barrier speed of 4 × 10-2 nm2‚molecule-1‚min-1. BAM takes advantage of the reflectivity properties of an interface illuminated at the Brewster angle with light polarized in the incidence plane. This technique is sensitive to the density and the thickness of the film at the air-water interface. Without a monolayer at the air-water interface, no reflectivity signal is detected. When a monolayer is spread over pure water, a direct observation of the coexistence between an expanded phase (black on the image) and a condensed one (white on the image) is possible, meaning that first-order phase transitions can be visualized in this way. In the presence of monomer or polymer in the subphase, the organization of the monolayer phases may be modified by the adsorption of the species at the interface. The changes in the monolayer, which depend on the chemical species formed or present under the monolayer, can thus be evidenced. The temperature is systematically maintained at 20 ( 1 °C, and at least two measurements are carried out for each experimental condition.

3. Results and Discussion Knowing that a Langmuir monolayer contains about 10-9 mole, no classical characterization technique of polymer can be carried out in situ in order to analyze the polymer synthesized in the DODA monolayer. The aim of this work is to highlight the spontaneous polymerization of SSt monomer at the air-solution interface by BAM and to obtain information about the synthesized polymer. Indeed, the SSt polymerization under or inside the DODA monolayer could induce a morphological change in the monolayer, which should be evidenced by this technique and further correlated to previous results.5,6 The compression isotherms and simultaneously recorded BAM images for a DODA monolayer on different subphase natures are discussed. 3.1. DODA Monolayer on a Pure Water Subphase. Initially, the DODA monolayer is characterized on a pure water subphase in order to establish a reference under the same experimental conditions as those used for the spontaneous polymerization. The DODA monolayer has already been investigated at the airwater interface by ellipsometry,23 attenuated total reflection,24 reflection spectroscopy,25 grazing incidence X-ray diffraction,26,27 and BAM.28,29 The DODA isotherm on pure water is in good agreement with those reported in the literature,30,31 as well as the 0.50 nm2 molecular area that corresponds to a collapse pressure (πc) equal to 42 mN‚m-1 (Figure 1). The first increase in surface pressure upon compression occurs at a molecular area of approximately 1.25 nm2, indicating that the DODA monolayer is in an LE phase. This LE phase exists alone until 0.80 nm2‚molecule-1 corresponding to a 12 mN‚m-1 surface pressure. By BAM, the monolayer appears to be homogeneous, with an overall slight reflectivity in agreement with an LE phase (Figure 1). When the molecular area decreases from 0.80 to 0.60 nm2‚molecule-1, a pseudo surface pressure plateau leading to a liquid-condensed (23) Ruths, J.; Essler, F.; Decher, D.; Riegler, H. Langmuir 2000, 16, 8871. (24) Miyano, K.; Asano, K. Langmuir 1991, 7, 444. (25) Asano, K.; Miyano, K. U.; Shimomura, M.; Ohta, Y. Langmuir 1993, 9, 3587. (26) Goubard, F.; Fichet, O.; Teyssie´, D.; Fontaine, P.; Goldmann, M. J. Colloid Interface Sci. 2007, 306, 82. (27) Symietz, C.; Schneider, M.; Brezesinski, G.; Mo¨hwald, H. Macromolecules 2004, 37, 3865. (28) Ahuja, R. C.; Caruso, P. L.; Mo¨bius, D. Thin Solid Films 1994, 242, 195. (29) Cuvillier, N.; Bernon, R.; Doux, J.-C.; Merzeau, P.; Mingotaud, C.; Delhae`s, P. Langmuir 1998, 14, 5573. (30) Mingotaud, A. F.; Mingotaux, C.; Patterson, L. K. Handbook of Monolayers, 1st ed.; Academic Press: San Diego, CA, 1993. (31) Ahrens, H.; Baltes, H.; Schmitt, J.; Mo¨hwald, H.; Helm, C. A. Macromolecules 2001, 34, 4504.

Spontaneous Polymerization at Air-Water Interfaces

Figure 1. DODA monolayer isotherm on a pure water subphase. Compression rate: 4 × 10-2 nm2‚molecule-1‚min-1. T ) 20 °C. BAM images recorded at 5, 12, and 42 mN‚m-1 are inserted. The bar represents 100 µm.

(LC) phase for the smallest areas is then observed on the π-A isotherm.32 This “plateau” characterizing the LE/LC transition is not perfectly flat. This phenomenon is certainly due to a kinetic effect related to the strong repulsion forces between polar heads. Indeed, the “plateau” position depends on the compression speed (results not shown). In addition, from 0.80 nm2‚molecule-1 (12 mN‚m-1), small bright circular domains are observed on BAM images. This observation confirms a first-order LE/LC phase transition, in spite of the nonzero slope of the “plateau”. Upon further compression of the monolayer (until 42 mN‚m-1), the size and the number of domains increase; however, they become star-shaped. Thus, the LE and LC phases coexist during all the monolayer compression. This peculiar phenomenon has been already reported.28,29 The DODA monolayer is then characterized on SSt monomer solutions whose concentrations are varied between 4 × 10-6 and 1.1 × 10-3 mol‚L-1. 3.2. DODA Monolayer on an SSt Monomer Subphase. Two types of experiments were carried out. In the first one, the DODA monolayer is spread onto SSt monomer solutions and characterized by isotherms and BAM. The concentrations are chosen above and below the polymerization threshold concentration (5 × 10-4 mol‚L-1). In the second type of experiment, the DODA monolayer is spread onto a pure water subphase, compressed up to a surface pressure of π ) 20 mN‚m-1; the monomer is subsequently injected under the monolayer, thus leading to final subphase concentrations below and above the threshold concentration. The possible modifications in the DODA monolayer morphology are then followed in real time by means of BAM. Finally, since sodium bicarbonate was shown to be an inhibitor of the polymerization, BAM characterizations of a DODA monolayer spread on monomer solutions (at concentrations above the threshold) containing NaHCO3 (i.e., where the polymerization is known to be inhibited) have been performed. 3.2.1. SSt Monomer Solution. The influence of the monomer concentration in the subphase on the π-A isotherm of a DODA monolayer is shown in Figure 2. These isotherms significantly differ from those recorded on a pure water subphase. Indeed, the pseudo surface pressure plateau evidenced on pure water corresponding to the LE/LC transition disappears. Below π ) 42 mN‚m-1, the isotherm shape is very similar, whatever the monomer concentration in the studied range. The surface pressure increases slowly during compression, so that the monolayer can be considered disordered and described as an LE phase. This assumption is confirmed by the BAM images. The DODA monolayer appears to be homogeneous, and no phase transition (32) Kozarac, Z.; Ahuja, R.; Mobius, D. Langmuir 1995, 11, 568.

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Figure 2. Effect of SSt monomer concentration in the subphase on the DODA π-A isotherm at (a) 4 × 10-6, (b) 1.6 × 10-5, (c) 5 × 10-4, (d) 6 × 10-4, and (e) 1.1 × 10-3 mol‚L-1. Compression rate: 4 × 10-2 nm2‚molecule-1‚min-1. T ) 20 °C. BAM images recorded at 20, 42, and 53 mN‚m-1 are inserted. The bar represents 100 µm.

is detected at surface pressures below 42 mN‚m-1, whatever the monomer concentration. For a 4 × 10-6 mol‚L-1 monomer concentration, a surface pressure plateau is observed at 42 mN‚m-1 for a molecular area below 0.60 nm2‚molecule-1. A two-dimensional (2D) network of small bright aggregates is observed by BAM on this plateau (Figure 2a), indicating the monolayer collapse, at the same surface pressure as that over pure water. When the SSt monomer concentration lies between 1.6 × 10-5 and 6 × 10-4 mol‚L-1, the monolayer shows the same morphology as that at the lowest monomer concentrations, and a plateau at 42 mN‚m-1 is also observed. However, when the molecular area is decreased again, the surface pressure increases further up to 53 mN‚m-1 at a molecular area depending on the monomer concentration in the subphase. For example, the surface pressure increase occurs at a molecular area of 0.22 and 0.36 nm2‚molecule-1 when the monomer concentration in the subphase is equal to 1.6 × 10-5 and 5 × 10-4 mol‚L-1, respectively. Whatever the monomer concentration, a second surface pressure plateau is then detected at 53 mN‚m-1. At this surface pressure, a large part of the monolayer keeps the same morphology as the one described for the lowest monomer concentration: the 2D network of small bright aggregates is always observed, but some large brighter domains appear. The surface covered by these large aggregates depends on the monomer concentration: the higher the monomer concentration, the higher the number of these bright domains, and the higher the molecular area where it occurs. This second structure detected by BAM makes it possible to conclude that the second surface pressure plateau corresponds to a second monolayer collapse. Finally, when the monomer concentration is above 6 × 10-4 mol‚L-1, the surface pressure plateau at 42 mN‚m-1 is replaced by a small inflection on the isotherm. However, the second plateau at 53 mN‚m-1 is always observed. The BAM image recorded at this surface pressure (Figure 2e) no longer shows the 2D network of small bright aggregates, but only many large bright domains. The existence of two collapse pressures (42 and 53 mN‚m-1) whose values are independent of the monomer concentration indicates the presence of two phases in the monolayer, the compositions of which do not depend on the monomer concentration. The lower collapse pressure value (42 mN‚m-1) is close to the one measured for a DODA monolayer spread onto a pure water subphase. A 2D network of small bright aggregates is evidenced by BAM. The proportion of the corresponding phase

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Figure 3. BAM images of a DODA monolayer stabilized at π ) 20 mN‚m-1 (a) 0, (b) 1, (c) 3, and (d) 15 min after the injection of SSt monomer in the subphase (final SSt concentration ) 6 × 10-4 mol/L). The bars represent 100 µm.

in the monolayer decreases with increasing monomer concentration. The second surface pressure collapse appears for the highest monomer concentrations as very large and bright patches. Simultaneously, the 2D network of small bright aggregates disappears when the monomer concentration increases. Consequently, and according to previous results,5 it is likely that the lowest collapse pressure (42 mN‚m-1) might correspond to a DODA phase in which the amphiphilic molecule is associated with the SSt monomer, whereas the second one might be attributed to another DODA phase in which the SSt monomer has polymerized under or inside the monolayer. These results bring new information complementary to our previous studies. Indeed, when the SSt monomer concentration is lower than 5 × 10-4 mol‚L-1, we concluded that spontaneous polymerization did not occur (well-organized LB films with more than one layer cannot be built up, and there was no typical variation in the A-t curves after the monomer injection in the subphase) and that the SSt monomer acted as a salt in the subphase. The present results confirm this conclusion and provide a more precise value of the concentration threshold, close to 10-5 mol‚L-1, corresponding to the appearance of the large bright spots at the 53 mN‚m-1 surface pressure collapse. 3.2.2. SSt Monomer Injection. In this part, the DODA monolayer is spread onto a pure water subphase and compressed up to a surface pressure of 20 mN‚m-1, and the monomer is subsequently injected under the monolayer, leading to final aqueous concentrations below or above the threshold concentration. Before the SSt monomer injection, the BAM images recorded on the pure water subphase show a monolayer with bright circular or star-shaped domains (LC phase dispersed in a continuous LE phase) as previously described (Figure 3a). BAM images recorded 1, 3, and 15 min after SSt monomer injection in the subphase (final SSt concentration in the subphase, 6 × 10-4 mol‚L-1) are shown in Figure 3. Significant changes are observed within a few minutes. Indeed, after 1 min (Figure 3b), in most places, the contrast between the LE and LC phases decreases, and the dense domain distribution becomes less regular. However, in some regions, the contrast remains unchanged, and the morphology is similar to that of a DODA monolayer onto

Cantin et al.

pure water. After 3 min, the LE phase (dark on the image) is no longer visible, and the LC phase (white on the image) is less and less detectable (Figure 3c). Finally, a few minutes later, the whole surface is completely homogeneous (Figure 3d). The described phenomenon does not depend on the final SSt concentration in the studied range (7 × 10-5 to 2 × 10-3 mol‚L-1), that is, below and above the concentration threshold (5 × 10-4 mol‚L-1). However, the time necessary to reach a homogeneous layer after monomer injection varies from a few minutes for the highest concentration up to more than 1 h for the lowest concentration. Whatever the monomer concentration, BAM images show that the layer becomes homogeneous after the monomer injection (Figure 2). This observation is in agreement with the results obtained for monomer solutions used directly as subphase and a monolayer surface pressure of 20 mN‚m-1. Then, once the monolayer homogeneity is obtained, the film is compressed. The π-A isotherm recorded starting from 20 mN‚m-1 again shows two surface plateaus at 42 and 53 mN‚m-1, and the BAM images recorded at these plateaus are very similar to those previously described for different monomer solutions used as the subphase. This study shows that the method of monomer introduction into the subphase does not seem to affect the DODA monolayer morphology on the microscopic scale. In addition, the monomer adsorption under or inside the monolayer is very quick (some minutes). This hypothesis put forward in previous studies is now confirmed. However, it is not possible, starting from these images recorded in real time, to make any unambiguous conclusions about the SSt polymerization. 3.2.3. SSt Monomer Solution with Sodium Bicarbonate (NaHCO3) Addition. Sodium bicarbonate was shown to be an inhibitor of this spontaneous polymerization. Thus BAM characterizations of the DODA monolayer were performed on monomer solutions at concentrations above which the polymerization occurs, but, in this case, sodium bicarbonate (5 × 10-3 mol‚L-1) was inserted in the subphase in order to inhibit the polymerization. The results were the same as those obtained on a 4 × 10-6 mol‚L-1 SSt monomer solution without sodium bicarbonate. Indeed, only one collapse pressure at π ) 42 mN‚m-1 is detected, and BAM shows a 2D network of small bright aggregates as previously described. The large bright domains attributed to the collapse of a DODA phase associated with polymerized SSt are no longer observed at high monomer concentrations. From previous work and the experiments presented above, these results confirm that sodium bicarbonate inhibits the SSt spontaneous polymerization and are in agreement with a DODA monolayer associated with an adsorbed monomer. In order to confirm that the observed phenomenon is due to the SSt polymerization and not to simple electrolyte adsorption, TSt solutions were then used as the subphase. The chemical structure of TSt is very close to that of SSt , but TSt does not bear any polymerizable function. 3.3. DODA Monolayer on a TSt Monomer Subphase. Isotherms of DODA monolayers spread onto TSt solutions at different concentrations are shown in Figure 4. The subphase concentration was varied between 4 × 10-6 mol‚L-1 and 1.2 × 10-3 mol‚L-1. The isotherms appear independent of the TSt concentration. A surface pressure plateau is detected at a surface pressure close to 42 mN‚m-1. This plateau probably corresponds to the monolayer collapse, at the same surface pressure as for a DODA monolayer onto pure water. By means of BAM, the monolayer is observed to be homogeneous until the surface pressure reaches 42 mN‚m-1. Indeed, the LE/LC phase transition of the DODA monolayer over pure water is no longer evidenced.

Spontaneous Polymerization at Air-Water Interfaces

Figure 4. Effect of TSt concentration in the subphase on the DODA π-A isotherm at (a) 4 × 10-6 and (b) 1.1 × 10-3 mol‚L-1. Compression rate: 4 × 10-2 nm2‚molecule-1‚min-1. T ) 20 °C. BAM images recorded for a TSt concentration of 1.1 × 10-3 mol‚L-1 at 0.24 and 0.58 nm2 are inserted. The bar represents 100 µm.

At the beginning of the surface pressure plateau, very bright domains are observed to nucleate, in agreement with the monolayer collapse (Figure 4). Their size grows upon further compression. The surface covered by these domains increases with the monomer concentration. These results indicate that TSt and SSt have significantly different behaviors with respect to the DODA monolayer. Indeed, whatever the TSt concentration, the DODA monolayer displays a behavior similar to that on low-concentration SSt solutions (below the threshold). The main difference is thus the appearance of two collapse pressures in the case of the SSt monomer. The first collapse pressure is evidenced at 42 mN‚m-1, in the same way as in the presence of TSt. This means that, in both cases, the DODA molecule is associated to the salt. The second collapse pressure, only detected in the presence of SSt monomer at concentrations above about 10-5 mol‚L-1, indicates a new phenomenon. In the light of previous studies5,6 and keeping in mind that TSt bears no polymerizable function, the formation of polymerized material is thus evidenced at the air-SSt solution interface covered with a DODA monolayer. Finally, the DODA monolayer is characterized on atactic PSSt polymer solutions. Indeed, when SSt spontaneously polymerizes at the air-solution interface, the monomer is probably preorganized under or inside the DODA monolayer. It is accordingly possible that the resulting polymer synthesized at the interface would show some different microstructure, which could be evidenced by comparison with the DODA monolayer structure on an atactic PSSt polymer solution. 3.4. DODA Monolayer on a PSSt Polymer Solution. The exact concentration of the polymer formed at the air-solution interface is unknown; this is why the DODA monolayer structure has been characterized on PSSt solutions with various concentrations. The DODA isotherms recorded on commercial polystyrene sulfonate (PSSt) solution, which leads to a complex formation (Figure 5), show two different regimes according to PSSt concentration. Therefore, the polymer influences the way in which the amphiphilic molecules are arranged in the monolayer.33 In the first regime, that is, when the polyelectrolyte concentration is equal to 4 × 10-7 mol‚L-1, the DODA monolayer isotherm is very similar to that recorded on pure water. Thus, at this concentration, the surface pressure collapse is not modified by the addition of polyelectrolyte in the subphase. The BAM images (33) Lackmann, H.; Engelking, J.; Menzel, H. Mater. Sci. Eng., C 1999, 8-9, 127.

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Figure 5. PSSt polymer concentration effect on DODA monolayer surface pressure versus area per molecule (π-A isotherm): pure water (a), 4 × 10-7 (b), 10-5 (c), 5 × 10-5 (d), and 10-4 mol‚L-1 (e). Compression speed: 4 × 10-2 nm2‚molecule-1‚min-1. T ) 20 °C. BAM image recorded for a 5 × 10-5 mol‚L-1 PSSt concentration at π ) 53 mN‚m-1 is inserted. The bar represents 100 µm.

highlight a first-order phase transition between the LE and LC phases very similar to the one observed on the pure water subphase. However, the contrast between the LE and LC phases is much lower on the polymer solution than on pure water. When the surface pressure collapse is reached (42 mN‚m-1), the shape of the aggregates looks like the one obtained for DODA spread onto a pure water subphase. Thus the DODA monolayer presents the same morphology on a very low polymer concentration solution as on a pure water subphase. In the second regime, that is, when the PSSt solution concentration is greater than or equal to 10-5 mol‚L-1, a shift in the molecular area toward lower values (compared to DODA on pure water) is observed. The isotherms showing an expanded phase, a plateau region, and a condensed phase are identical for all polymer concentrations. For example, when PSSt concentration is equal to 10-5 mol‚L-1, the isotherm shows a lift off just above 1.00 nm2‚mol-1, corresponding to the LE phase. Compressing the film, the pressure increases continuously and reaches about 8 mN‚m-1 at 0.85 nm2‚mol-1, then a change in the slope indicates a transition plateau whose end is located at about 0.60 nm2‚mol-1 (15 mN‚m-1). The monolayer is then in an LC phase that is composed of stretched alkyl chains tilted from ca. 45° relative to the surface normal,34 leading to a minimum area of 0.55 nm2. The lower molecular area in the complexed monolayer compared to the monolayer on pure water can be ascribed to the charge compensation, which significantly reduces the Coulombic interaction of the head groups. Finally, the monolayer collapse is detected at 60 mN‚m-1. Thus, when the PSSt concentration is higher than 10-5 mol‚L-1, the surface pressure collapse is increased by more than 10 mN‚m-1 compared to that measured on the pure water subphase. For this concentration range, the isotherms are in agreement with those reported in the literature.35 In this concentration range, BAM observations are independent of the PSSt concentration. Whereas a pseudo plateau is observed on the compression isotherm curves, no LE/LC phase transition is visualized by BAM. Thus, the contrast between the LE and LC phases is strongly reduced when polymer is added in the subphase. This phenomenon can be due to the presence of a thick polymer layer adsorbed under the DODA monolayer, which is confirmed by the strong rigidity of the monolayer at surface pressures higher than 15 mN‚m-1; indeed, no layer movement is detected even when blowing over it. The monolayer thus (34) Kunitake, T. Angew. Chem. 1992, 104, 692. (35) Engelking, J.; Ulrich, D.; Meyer, W. H.; Schenk-Meuser, K.; Duschner, H.; Menzel, H. Mater. Sci. Eng., C 1999, 8-9, 29.

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remains homogeneous up to the collapse, which is visualized at very high pressure (>55 mN‚m-1) and appears as very bright stripes parallel to the compression barrier (Figure 5). Consequently, the morphologies of the DODA monolayer at the collapse are different on PSSt solution and on SSt monomer solutions at a concentration higher than 10-5 mol‚L-1. This means that the polymer synthesized at the air-water interface does not have the same interaction with the amphiphilic molecule as the commercial polymer. Both polymers with either different molecular weight or a different tacticity can explain this difference. Indeed, the molecular weight of the polymer synthesized at the air-solution interface cannot be exactly determined due to the small amount of compound. Fan and Miyashita have shown that the π-A isotherms of poly(N-dodecylacrylamide) changed drastically with the molecular weight of the polymer. The monolayer becomes more stable and more condensed with increasing molecular weight.36 Knowing that the molecular weight of the adsorbed polyelectrolyte can affect the monolayer structure, different PSSt polymers have been synthesized under experimental conditions nearly identical to those of the interface. For that, water is used as solvent, and the polymerization is carried out in concentrated medium (0.50 mol‚L-1) and under air atmosphere. PSSt solutions used as the subphase are prepared directly by dilution of these concentrated solutions. Thus, DODA monolayer has been characterized on solutions (4 × 10-7, 10-5, 5 × 10-5, and 10-4 mol‚L-1) of PSSt with different molecular weights. No difference compared with commercial PSSt solutions was identified on the compression isotherms and BAM images, whatever the synthesized PSSt polymer introduced into the subphase. Thus, the molecular weight of the polymer adsorbed under or in the DODA monolayer does not seem to be responsible for the morphology differences evidenced by BAM. With the polymer molecular weight effect being isolated, it is now reasonable to hypothesize that the SSt monomer is probably oriented under or in the DODA monolayer at the interface before its polymerization, leading to the formation of a polymer showing a possible tacticity.

4. Conclusion When a DODA monolayer is spread onto styrene SSt aqueous solution, this monomer undergoes a spontaneous polymerization process.5,6 The polymer synthesized in the DODA monolayer (36) Fan, F.; Miyashita, T. Chem. Lett. 1999, 7, 669.

Cantin et al.

cannot be characterized in situ by classical characterization techniques, thus BAM has been used as a complementary method in order to study this spontaneous polymerization. Previously, the spontaneous polymerization has been highlighted only when the SSt monomer concentration is above 5 × 10-4 mol‚L-1 (concentration threshold). By BAM, two phases whose proportions vary with SSt monomer concentration were clearly evidenced. One probably corresponds to SSt monomer associated to DODA, whereas the other one results from polymerized SSt under the DODA monolayer. From the π-A isotherms and BAM images presented in this paper, it was thus possible to determine a more precise value of the threshold above which polymerized material is formed at the interface (10-5 mol‚L-1). Moreover, the results were compared with those obtained with TSt, differing from SSt by the absence of polymerizable function. The significantly different behavior of the two species at the highest studied concentrations confirms the SSt spontaneous polymerization. The previous studies have shown no difference on the resulting LB film characterizations when the monomer is directly introduced in the subphase as solution and when the monomer is injected (at the same final concentration) under the DODA monolayer stabilized onto a pure water subphase. BAM observations confirm this result. Moreover, this technique evidenced the very fast (a few minutes) monomer adsorption process under the DODA monolayer supposed in our previous paper. In the same way, BAM images have confirmed that sodium bicarbonate acts as an inhibitor of this spontaneous polymerization. Finally, BAM experiments have been performed on different subphases containing PSSt polymers with various molecular weights. The behavior strongly differs from that obtained on monomer solutions above the concentration threshold. In particular, a very high rigidity can be noticed as soon as the surface pressure reaches about 15 mN‚m-1. The morphology of the collapse is also different. After checking that these differences were not due to the molecular weight of the polymer, it is possible to conclude that the polymer synthesized in solution is different from the polymer synthesized at the air-monomer solution interface. Further experiments will be performed to analyze the possible tacticity of the polymer at the air-monomer solution interface. LA7020534