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Langmuir 1990,6, 263-268
Surface Monolayers of Well-Defined Amphiphilic Block Copolymer Composed of Poly(acry1ic acid) or Poly(oxyethy1ene) and Poly (styrene).' Interpolymer Complexation at the Air-Water Interface Masazo Niwa,' Takehiro Hayashi, and Nobuyuki Higashi Department of Applied Chemistry, Faculty of Engineering, Doshisha University, Kamikyo-ku, Kyoto 602, Japan Received March 17, 1989. In Final Form: August 2, 1989 Amphiphilic block polymers (2, 3) composed of poly(acry1ic acid) (PAA) or poly(oxyethy1ene) (POE) and chain length controlled poly(styrene) (PSt) have been prepared by using a catalytic system of tribromomethyl-terminated oligomer and manganese carbonyl. All the amphiphilic materials formed wellbehaved surface monolayers, and the E A curves for them expanded systematically with an increase of the PSt chain length. The monolayers for 2 were considerably affected by varying the pH in the subphase, due to a conformational change of the PAA segment. Interpolymer complexes between 2 or 3 and poly(vinylpyrro1idone) or PAA in the subphase, respectively, were formed at the air-water interface at an appropriate pH value. In the case of 3-PAA complexed monolayers, the limiting area was markedly expanded below pH 5, which corresponds to the apparent dissociation constant (pK,) of PAA, suggesting a formation of interpolymer complex. Above pH 5, the limiting area was close to that on water (in the absence of PAA) due to the suppression of such complexation by the ionization of the carboxylic acid groups. Such interpolymer complexation was also reversibly controllable at the air-water interface by varying the pH in the subphase.
Introduction Langmuir-Blodgett (LB) films have received much attention in recent years due to the fact that the control of the molecular aggregation state is readily achieved by the preparation technique of the films as developed by Langmuir' and B l ~ d g e t t . ~ For most practical applications, LB films will need to be mechanically and thermally stable. From this point, research efforts on LB films consisting of monolayers of synthetic polymers are rapidly expanding. Most of those polymeric materials are, however, homopolymers4 or copolymers5-' with hydrophilic comonomers prepared from long alkyl chain containing vinyl monomers attached along the resulting polymer backbone, and they are mainly discussed in terms of lateral fixation. On the other hand, there has been little study of surface monolayers of synthetic amphiphilic block polymers composed of a hydrophilic and a hydrophobic segment. Roberts et al.8reported the preparation and dielectric properties of a quaternary ammonium salt terminated polybutadiene. Ikada et al.9 reported on surface monolayers of graft and block copolymers consisting of poly(styrene) and poly(viny1acetate) or poly(viny1 alcohol), which were prepared by a radiation-induced copolymerization. Unfortunately, the (1) Preliminary reports: (a) Niwa, M.; Katsurada, N.; Higashi, N. Macromolecules 1988,21, 1878. (b) Niwa, M.; Higashi, N. Macromolecules 1989,22, 1OOO. (2) Langmuir, I. J. Am. Chem. SOC.1917, 39, 1848. (3) Blodgett, K. J. Am. Chem. SOC.1935,57, 1007. (4) Mumby, S. J.; Swalen, J. D.; Rabolt, J. F. Macromolecules 1986, 19. 1054. (5) Winter, C. S.; Tredgold, R. H.; Vickers, A. J. Thin Solid Films 1985. 134. ~ .,. .-,49. ~(6) Laschewsky, A.; Ringsdorf, H.; Schmit, G.; Schneider, J. J. Am. Chem. SOC.1987,109, 788. (7) Watanabe, M.; Kosaka, Y.; Sanui, K.; Ogata, N. Macromolecules 1987,20, 452. (8) Christie, P.; Petty, M. C.; Roberts, G. G.; Richards, D. H.; Service, D.; Stewed, M. J. Thin Solid Films 1985, 134, 75. (9) Ikada, Y.; Iwata, H.; Nagaoka, S.; Horii, F.; Hatada, M. J. Macromol. Sci.-Phys. 1980, B17(2), 191. ~
~
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Scheme I
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3 Inz9.7)
TBE
chain length of each segment in those copolymers seemed to be uncontrollable and too long to allow precise discussion of the monolayer behavior. We have reported that a well-defined block copolymer can be readily prepared by making use of the catalytic system'09'' of halocontaining macroinitiator and manganese carbonyl (Mn,(C0)l,)'2~'3 as shown in Scheme I. We describe here the preparation of well-defined amphiphilic block copolymers (2 and 3) with systematically different chain length and their peculiar monolayer properties. Also, the effects of varying pH and of addition of poly(vinylpyrro1idone) (PVP) or poly(acry1ic acid)(PAA) in the subphase on the monolayer behavior were investigated in order to obtain information regarding the conformational change of amphiphilic polymer and the polymer-polymer interaction at the air-water interface. (10) Bamford, C.; Finch, C. Trans. Faraday SOC.1963,59,540. (11) Bamford, C.; Xiao-zu, H. Polymer 1981,22, 1299. (12) Niwa, M.; Katsurada, N.; Matsumoto, T.; Okamoto, M. J.Macromol. %.-Chem. 1988, A25, 445. (13) Niwa, M.; Higashi, N.; Okamoto, M. J. Macromol. Sci.-Chem. 1988, A25, 1515.
0 1990 American Chemical Society
Niwa et al.
264 Langmuir, Vol. 6, No. 1, 1990
Experimental Section Preparation of a-Tribromoacetyl-o-methoxypoly(oxyethylene) (TBOE). Tribromoacetyl Chloride. A mixture of 25 g (84 mmol) of tribromoacetic acid, 11 g (92 mmol) of SOCl,, and 0.6 g (8.2 mmol) of DMF as a catalyst was refluxed for 4 h. The mixture was distilled twice to give a colored liquid: yield 18 g (69%);bp 80 "C at 25 mmHg [lit.14bp 70-74 "C at 16 mmHg]; IR (film) 1772 cm-' (C=O). TBOE. Sodium w-methoxypoly(oxyethy1ene) (numberaverage molecular weight, 480; Nippon Syokubai Co. Ltd.; 26 g, 54 mmol) was dissolved in dry THF, and 18 g (57 mmol) of tribromoacetyl chloride in dry THF was added dropwise in 60 min with ice cooling. The mixture was additionally stirred for 15 h at room temperature,and the NaCl precipitate was removed. After solvent removal, the oily residue was washed with petroleum ether several times to give a viscous oil: yield 36 g (81% ); IR (film) 1750 cm-' (C=O); NBr(number of terminal bromine atoms per polymer molecule) 2.8, estimated by bromine analysis. Materials. Monomers (ethyl acrylate (EA) and styrene (St)), solvents, and other reagents were used after conventional purification. PAA (molecular weight 168 000) and PVP (molecular weight 360 OOO) were purchased from Wako Pure Chemical Industries, Ltd., and Nacalai Tesque, Inc., respectively. Polymerization and Hydrolysis. Tribromo-terminated poly(ethyl acrylate) (TBE) was prepared by radical polymerization of EA in the presence of carbon tetrabromide initiated with cup'-azobis(isobutyronitrile)(AIBN)as described before.15 Block polymerizations of St with tribromo-containing macroinitiators (TBE, TBOE) in the presence of Mn,(CO),, were carried out under dry N, atmosphere. The detailed procedure is given elsewhere.' After polymerization,the crude products were purified by a reprecipitation method from a benzene-methanol (or petroleum ether) system. Blocking efficiencies for the copolymerizations were estimated by selective extraction with suitable solvents. PAA-PSt block copolymers (2) were obtained by complete hydrolysis of the corresponding PEA-PSt block copolymers (1) in dioxane for 12 h at 100 "C in the presence of an excess ethanolic KOH and neutralization with aqueous HC1. Measurement. Characterizationof the Polymers. Number-average molecular weights for the macroinitiators and the block copolymers were measured by means of a Knauer vapor pressure osmometer with acetone solutions at 37 "C and a Shimadzu high-performance liquid chromatograph, LC-3A (GPC), with THF solutions a t 40 "C. The chemical compositions of the copolymers were estimated by 'H NMR (400 MHz) and I3C NMR (100 MHz) spectroscopy (JOEL JNM-GX400 FTNMR spectrometer) with TMS as internal standard, using CDCl, solutions. Spreading Experiments. The monolayers were obtained by spreading benzene-chloroform (8/2 in volume) and/or benzene solutions of 2 and 3. The concentrations of the spreading solutions were about 1.5 mg/mL. The temperature of the subphase (Milli-&water) was kept constant at 20 f 0.5 "C. Ten minutes after spreading, the gaseous monolayer was compressed continuously. The compressional velocity was 1.20 cm2 s-'. Below this value, the effect of compression rates on the molecular area was within experimental error. Wilhelmy's plate (filter paper plate) method and a Teflon-coated trough with a microprocessor-controlled film balance (San-Esu Keisoku Ltd., Fukuoka, Japan) with a precision of 0.01 mN/m were used for surface pressure measurements. The measurements of IIA curves for all samples were repeated several times to check their reproducibility. The pH was adjusted with either NaOH or HCl as required. For some experiments, PAA or PVP was unit M). added to the subphase (2 X Langmuir-Blodgett multilayers were prepared by means of a microprocessor-controlled film balance (FSD-21, San-Esu Keisoku Ltd., Fukuoka, Japan). The dipping speed of a substrate was kept constant at 15 mm/min for both downward and upward strokes. Multilayers could be deposited on various mate(14) Randhawa, H. s.;Walter, W. J . Mol. Struct. 1976, 35,303. (15) Niwa, M.; Hayashi, T.; Matsumoto, T. J . Macromol. Sci.Chem. 1986, A23, 433.
rials, such as quartz and CaF, plates. Characterizations of multilayers were performed by UV and FTIR spectroscopy (Shimadzu UV-2100 and Shimadzu FTIR-4200, respectively).
Results and Discussion Preparation of Amphiphilic Block Copolymers (2, 3). Macroinitiator (TBE) was prepared by polymerization of EA (0.5 M) initiated with AIBN (0.5 wt % of monomer) in the presence of carbon tetrabromide (chaintransfer agent, 1.67 M) as described before.15 The number-average degree of polymerization (n)of the polymer thus obtained was estimated to be 7.2 on the basis of vapor pressure osmometry. The mean number of bromine atoms per polymer molecule was determined to be 4.0, based on the result of elemental analysis and the molecular weight. This implies that a macroinitiator having a monobromo group and tribromomethyl group at each end of the polymer molecule (TBE) is obtained. Block polymerizations of St with macroinitiators (TBE or TBOE) were carried out in bulk in the presence of Mn,(CO),, a t 80 "C (Table I). Blocking efficiencies for both cases were almost 100% independent of the monomer conversion, resulting from selective extraction with appropriate solvents. The M , (GPC) values, denoting the molecular weight a t peak top of GPC curves calibrated with a polystyrene standard, were consistent with the number-average molecular weight (M,) calculated from the peak intensity assigned t o each blocking unit in 'H NMR spectra. The M , (calculated) values for TBE-St block copolymers were estimated by using the feed composition of St relative to TBE and the monomer conversion (X,) and were also very close to the values of M g (GPC), independent of chain length (m). A similar situation was observed for TBOE-St block copolymers, as shown in Table I. These indicate that the well-defined block copolymers with controlled chain length, which are readily changeable, were prepared by varying the conversion of St and the feed composition of macroinitiator and St. After hydrolysis of TBE-St block copolymers with ethanolic KOH in dioxane and neutralization with aqueous HC1, they were transformed completely to the amphiphilic form containing the poly(acry1ic acid) (PAA) chain instead of the PEA chain. Such complete transformation was confirmed on the basis of 13C NMR spectroscopy. Thus, our two types of amphiphilic block copolymers (2,3) consisted of a PAA chain ( n = 7.2) or a poly(oxyethy1ene) (POE) chain ( n = 9.7) as the hydrophilic moiety and a PSt chain ( m = 6.7-112.7) as the hydrophobic moiety. Monolayer Behavior of Amphiphilic Block Copolymers at the Air-Water Interface. Figure 1 shows surface pressure (II)-area ( A ) isotherms for a series of 2 and 3 on pure water (pH 6.5) a t 20 "C. The mean area was calculated by using the number-average molecular weight of the block copolymers. The limiting area (A,) and collapse pressure based on II-A isotherms (Figure 1) are listed in Table 11. The II-A curves for both 2 and 3 strongly depend on the hydrophobic chain length ( m ) , and the limiting area (A,) estimated by extrapolating the steepest region to zero pressure tends clearly toward expanding with increasing m. When m becomes larger, the hydrophobic polymer chain may be unable to orient perpendicularly to the interface, due to the formation of a random coil structure on the water surface as shown schematically in Figure 2. Kumaki" proposed an empirical equation for the relationship between the limiting area and molecular weight of polymers of polystyrene ~~
~~
(16) Kumaki, J. Macromolecules 1988, 21, 749.
Langmuir, Vol. 6, No. 1, 1990 265
Surface Monolayer of Amphiphilic Block Copolymer
Deposition of the surface monolayer of 2 ( m = 6.7) was examined by using a quartz plate a t 20 "C under a surface pressure of 20 mN/m. In the first downward stroke, the monolayer could not be deposited, but in the further strokes the monolayer was readily transferred in both the upward and downward strokes. The formation of a Y -type multilayer was assured, because decreases in the surface area were the same in the downward and upward strokes. UV absorption spectra also supported the formation of multilayer. Figure 4 shows the absorbance a t 262 nm, which corresponds to PSt, plotted against the number of layers (N) deposited. The absorbance was proportional to N a t least up to 20 depositions. A similar trend was observed for the other monolayers (2 and 3).
Polymer-Polymer Interactions at the Air-Water Interface. Polyion complexations from bilayer-
Mean Area (nm2/Molecule)
Figure 1. II-A isotherms for a series of 2 (a) and 3 (b) on pure water at 20 "C.
monomolecular particles a t an air-water interface, on the basis of 11-A isotherms and transmission electron microscopy. For a molecualrweight range above 4000, the limiting areas were proportional to the molecular weight, and the relation was represented by
A , = 4 X 10"'M (1) where A,, is the limiting area per molecule (nm2/ molecule) and M is the molecular weight of the polymer. The calculated A, values for 2 ( m = 110) and 3 ( m = 113) with the longest PSt chain used in this study are given by eq 1 as 4.4 and 4.7, respectively, which are relatively close to the observed A, values (5.2 for 2 and 4.9 for 3 ) in Figure 1. This suggests that 2(m = 110) and 3 ( m = 113) form monolayers on water even though they have a long PSt chain and a random coil structure. In the shorter chain length case, polymer chains seem to be oriented perpendicularly and packed more tightly. Since 2 has a PAA segment, its monolayer forming properties are likely to depend on the pH of the subphase. Figure 3 shows the II-A isotherms of 2(m = 6.7), a typical polymer, a t different pH values. The A , values and collapse pressures obtained from the isotherms are summaried in Table 111. The II-A curves have a tendency to expand with pH, and the collapse pressures decrease slightly. The Figure 3 inset shows the pH dependence of the area a t a constant surface pressure of 20 mN/m. The pK, of PAA is 5.6," so the fraction of ionized carboxyl groups will vary considerably in the range of pH investigated. It is expected that ionization of the carboxyl groups would introduce ionic repulsion, which causes a conformational change in the PAA chain from a globular coil in acidic solution to an expanded conformation a t high subphase pH. This probably explains the pH sensitivity of 11-A curves. The lI-A curves for a series of 3 did not show a significant pH dependence, as described later. (17) Ikawa, T.; Abe, K.; Honda, K.; Tsuchida, E. J. Polym. Sci., Polym. Ed. 1975, 13, 1505.
forming amphiphiles and oppositely charged polyions a t the air-water interface have been reported to be useful for the stabilization and facilitated deposition of surface monolayer~.'~J~ It is well-known that an interpolymer complex is formed from PAA and POE or PVP in an aqueous medium, through hydrogen bonding.'*'' However, polymer-polymer interaction a t the air-water interface has not yet been established. The amphiphilic block copolymers ( 2 , 3 )prepared in this study contain PAA or POE segments, respectively, as a hydrophilic part. Therefore, it is expected that our amphiphiles would be useful for clarifying a polymer-polymer interaction a t the airwater interface. Figure 5 displays typical II-A curves for 2(m = 6.7) on pure water and on aqueous PVP. The II-A curve was remarkably affected by the presence of PVP in the subphase, and the limiting area increased dramatically, compared with that on pure water (see Table IV). This suggests that there is a interaction between the PAA segment of 2 and PVP a t the air-water interface. Figure 6 shows the pH dependence of the limiting area, together with the data obtained on pure water (in the absence of PVP), for comparison. The mean area a t 10 mN/m on aqueous PVP decreases wllth pH a t the lower pH region (pH
