Molecular Assemblies of Fluorinated Silicon Oligomers with

Engineering, Nara Technical College, 22 Yada-cho, Yamatokoriyama, Nara 639-11, Japan, ... Institute of Colloid and Interface Science, Science Universi...
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Langmuir 1998, 14, 2061-2067

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Molecular Assemblies of Fluorinated Silicon Oligomers with Carboxylic Acid Groups: Effects of Chemical Oligomer Structure on Assembly Shape Junpei Nakagawa,† Keiji Kamogawa,‡ Nobuyuki Momozawa,†,‡ Hideki Sakai,†,‡ Tokuzo Kawase,§ Hideo Sawada,| Yoh Sano,⊥ and Masahiko Abe*,†,‡ Faculty of Science and Technology, Science University of Tokyo, Yamazaki 2641, Noda-shi, Chiba 278, Japan, Institute of Colloid and Interface Science, Science University of Tokyo, 1-3 Kagurazaka, Shinjuku, Tokyo 162, Japan, Faculty of Household Sciences, Osaka City University, 3-3-138 Sugimoto, Sumiyoshi, Osaka 558, Japan, Department of Chemical Engineering, Nara Technical College, 22 Yada-cho, Yamatokoriyama, Nara 639-11, Japan, and National Food Research Institute, 2-1-2 Kannondai, Tsukuba, Ibaraki 305, Japan Received November 3, 1997. In Final Form: February 2, 1998 Examinations were conducted by the static light scattering method on the aggregation number, shape, and size of molecular assemblies formed in aqueous solution of copolymers of acrylic acid and trimethylvinylsilane with a fluoroalkyl group (RF) on both terminals (fluorinated silicon oligomers having carboxylic acid groups; RF-(CH2CH(COOH)x-(CH2CHSiMe3)y-RF; RF ) C3F7, C2F5(CF2OCFCF3), C2F5(CF2OCFCF3)2; abbreviated hereafter to fluorinated silicon oligomers). Molecular assemblies of the fluorinated silicon oligomers with an RF group on both terminals were found to have an ellipsoidal shape while those of the silicon oligomers with no RF were spherical or ellipsoidal in shape. Moreover, molecular assemblies of the fluorinated silicon oligomers were found to change their shape from an ellipsoidal to a spherical one with decreases in the length of the RF chain and the proportion of CH2CH(SiMe3) segments to CH2CH(COOH) segments or an increase in the number of CH2CH(COOH) segments.

1. Introduction In general, polymer surfactants are better as dispersing agents and flocculants but worse in other surface active properties than low-molecular-weight surfactants.1 Introduction of fluoroalkyl groups into polymer surfactants, however, is expected to improve their surface activity, such as the ability to reduce the surface tension of water, even though it may cause a decrease in their solubility in water and hydrocarbon oils.2 We have synthesized acrylic acid oligomers with a fluoroalkyl group (RF) introduced onto both terminals and examined the surface chemical properties of the fluorinated oligomers.3 The oligomers have been found to be highly soluble in polar solvents such as water and methanol and lower the surface tension of water to 20 mN/m. We have further synthesized copolymers of acrylic acid and trimethylvinylsilane (fluorinated silicon oligomers with carboxylic acid groups (RF-(CH2CHCOOH)x(CH2CHSiMe3)-RF): RF ) C3F7, C2F5(CF2OCFCF3), C2F5(CF2OCFCF3)2) to give oil solubility to the fluorinated oligomers.4 The fluorinated silicon oligomers obtained have been found to be easily soluble in not only polar * To whom all correspondence should be addressed at Faculty of Science and Technology, Science University of Tokyo, E-mail: [email protected]. † Faculty of Science and Technology, Science University of Tokyo. ‡ Institute of Colloid and Interface Science, Science University of Tokyo. § Osaka City University. | Nara Technical College. ⊥ National Food Research Institute. (1) Tanizaki, Y. J. Jpn. Oil Chem. Soc. 1985, 34(11), 973. (2) Yoshida, T., et al., Ed. In Handbook of Surfactant; Kougaku, Tosho: Tokyo, 1991. (3) Sawada, H.; Minoshima, Y.; Gong, Y.-F.; Matsumoto, T.; Kosugi, M.; Migita, T. J. Jpn. Oil Chem. Soc. 1992, 41 (8), 649. (4) Sawada, H.; Ohashi, A.; Oue, M.; Abe, M.; Mitani, M.; Nakajima, H.; Nishida, M.; Moriya, Y. J. Jpn. Oil Chem. Soc. 1994, 43, 1097.

solvents such as water and methanol but also aromatic solvents such as benzene, toluene, and xylene5 and possess an ability to strongly hinder HIV virus from proliferating, that is, a high anti-AIDS activity.6,7 In fact, the antiAIDS activity of the fluorinated silicon oligomers which are chemically stable is equivalent to or even higher than that of dextran sulfate, which has attracted attention as a polymeric anti-AIDS drug, and hence, the oligomers are expected to be promising as an anti-AIDS drug with low side effects.7 It has also been revealed that these fluorinated oligomers form molecular assemblies larger in size than ordinary surfactant micelles above a certain concentration that depends on the chemical oligomer structure, namely, the kind of RF group, the number of CH2CH(COOH) segments, and the average monomer ratio, x:y, in copolymers.8 In this study, the effects are examined by the static light scattering method of the chemical oligomer structure (RF, CH2CH(COOH) segment number, average monomer ratio) on the aggregation number, shape, and size of molecular assemblies (micelles) formed in aqueous solution of the fluorinated silicon oligomers. 2. Experimental Section 2.1. Materials. The fluorinated silicon oligomers used in this work are copolymers of acrylic acid and trimethylvinylsilane with a fluoroalkyl group (RF) on both terminals. Their syntheses are not given here because the details of the syntheses are described in a previous paper.7 (5) Sawada, H.; Gong, Y.-F.; Minoshima, Y.; Matsumoto, T.; Nakayama, M.; Kosugi, M.; Migita, T. J. Chem. Soc., Chem. Commun. 1992, 537. (6) Baba, M.; Kira, T.; Shigeta, S.; Matsumoto, T.; Sawada, H. J. Acquired Immune Defic. Syndr. 1994, 7 (1), 24. (7) Sawada, H.; Ohashi, A.; Oue, M.; Baba, M.; Abe, M.; Mitani, M.; Nakajima, H. J. Fluorine Chem. 1995, 75, 12. (8) Nakagawa, J.; Kamogawa, K.; Sakai, H.; Kawase, T.; Sawada, H.; Manosroi, J.; Manosroi, A.; Abe, M. Langmuir 1998, 8, 2055.

S0743-7463(97)01201-8 CCC: $15.00 © 1998 American Chemical Society Published on Web 03/24/1998

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Table 1. Fluoroalkyl Groups (RF), Number Average Molecular Weight (Mn), Polydispersity (Mw/Mn), and Copolymerization Ratio (x:y) of Fluorinated Silicon Oligomers with Carboxyl Groups sample

RF

x

y

x:y

M h n (M h w/M h n)

PAA R(16-1) RF(13-2) RF(85-1) RFO(19-6) RFO(34-7) RFO(95-6) RFO(25-9) RFO(42-3) RFO(156-0) RFO2(167-0)

none none C3F7 C3F7 C2F5(CF2OCFCF3) C2F5(CF2OCFCF3) C2F5(CF2OCFCF3) C2F5(CF2OCFCF3) C2F5(CF2OCFCF3) C2F5(CF2OCFCF3) C2F5(CF2OCFCF3)2

28 16 13 85 19 34 95 25 42 156 167

0 1 2 1 6 7 6 9 3 0 0

100:0 92.2:7.8 87.5:12.5 98.6:1.4 76.0:24.0 82.3:17.7 94.0:6.0 74.0:26.0 74.0:26.0 100:0 100:0

2000 (-) 1250 (2.76) 1480 (1.45) 6600 (1.94) 2510 (1.25) 3770 (1.55) 8000 (1.76) 3300 (1.41) 3300 (1.41) 12000 (1.54) 12970 (1.90)

In Table 1 are shown the fluoroalkyl group (RF), the copolymerization ratio, the number average molecular weight, the polydispersity (Mn/Mw), and the mean segment number of each of the oligomers. Here, the following abbreviations are used to designate each of the oligomers. When RF is C3F7 and the numbers of CH2CH(COOH) and CH2CH(SiMe3) segments are 13 and 2, and 85 and 1, respectively, these oligomers are called RF(13-2) and RF(85-1), respectively. When RF is C2F5(CF2OCFCF3) with an ether bond and the numbers of the two segments are 19 and 6, 34 and 7, 95 and 6, 25 and 9, 42 and 3, and 156 and 0, respectively, these oligomers are designated as RFO(19-6), RFO(34-7), RFO(95-6), RFO(25-9), RFO(42-3), and RFO(156-0), respectively. If RF is C2F5(CF2OCFCF3)2 having two ether bonds and the numbers of the two segments are 167 and 0, this oligomer is denoted as RFO2(167-0). R(16-1) is the abbreviation for the silicon oligomer that has 16 CH2CH(COOH) segments and one CH2CH(SiMe3) segment but no RF on either of the terminals. For comparison, poly(acrylic acid) (PAA; (CH2CHCOOH)x; Mn ) 2000 (Aldrich)), a polyelectrolyte with a chemical structure similar to that of the fluorinated silicon oligomers with no RF and no CH2CH(SiMe3) segment, was also used. Water for injection (Ohtsuka Pharmaceutical Co., Ltd.) was used as the solvent. 2.2. Methods. 2.2.1. Static Light Scattering. The intensities of scattered light between the scattering angles 45° and 135° were measured with a light scattering measuring apparatus (Malvern, 4700-Submicron Particle Analyzer) equipped with an argon laser as the light source (Coherent, Innova 90, maximum output power 5000 mW) using a cylindrical glass cell at the wavelength 488 nm and at 30 °C. Aqueous oligomer solutions at different concentrations were prepared, and dusts were removed from the solutions by passing them through a membrane filter (Nomura Microscience, Tokyo, Japan) before the measurement. The angular dependence was examined by calculating the ratio of the intensities of scattered light at 45° and 135°, i.e., the asymmetry coefficient (Z45) for the molecular assembly. Benzene of UV absorption spectrum grade (Dojin Chemical Laboratory, Kumamoto, Japan) was used as the standard substance. To examine the aggregation number, shape, and size of molecular assemblies from the experimental data, the following equation was employed9

K(c - c0)/(Rθ - R0) ) 1/MP(θ) + 2A2(c - c0)

(1)

where K is the optical constant and is given by

K ) 4π2n02(dn/dc)2/λ04NA

(2)

weight of the oligomer assembly, A2 is the second virial coefficient, and P(θ) is the intramolecular interference factor. When θ is small enough, eq 1 can be written as

K(c - c0)/(Rθ - R0) ) 1/M(1 + 16π2/3λ2RG2 sin2(θ/2)) + 2A2(c - c0) (3) where RG is the radius of gyration and λ is the wavelength of laser light. Experimentally, the scattered intensity was measured by changing the scattering angle from 30° to 135° through 15° intervals on aqueous oligomer solutions of various concentrations. First, values of K(c - c0)/(Rθ - R0) at each scattered angle were plotted as a function of c - c0. These plots give a straight line; thus, the least-squares method is employed for the data to fit the straight line, and the extrapolation c f c0 ([K(c - c0)/(Rθ - R0)]CfC0) is obtained. [K(c - c0)/(Rθ - R0)]CfC0] can be explained from eq 3 as

[K(c - c0)/(Rθ - R0)]CfC0] ) 1/M + 16π2/3Mλ2RG2 sin2(θ/2) (4) Next, [K(c - c0)/(Rθ - R0)]CfC0 is plotted against sin2(θ/2). When θ is small enough, this plot can be fit to a straight line and the least-squares method is employed to determine the slope and the intercept on the ordinate. The weight of the oligomer assembly (M) and the radius of gyration (RG) are determined from the intercept on the ordinate and the slope of the line. Finally, the weight of the oligomer assembly determined above was divided by the molecular weight of the oligomer, giving the aggregation number. A differential refractometer (Ohtsuka Electronics, RM-102) was used to measure the differential refractive index, which is essential in analyzing the data. 2.2.2. Determination of Shape and Size of Molecular Assemblies. The intramolecular interference factor (P(θ)) obtained from the Zimm plots shows the angular dependence of the intensity of light scattered by particles, which depends on the size and shape of the particles.9 In this work, the shape and size of oligomer assemblies were determined by comparing the angular dependence of P(θ) obtained experimentally with the calculated values of P(θ) for various models. The theoretical equations of P(θ) are given as follow as a function of scattering angle for various models such as sphere, rod, monosized random coil, disk, and ellipsoid. The shape and size of oligomer assemblies were determined by comparing the most suitable theoretical values of P(θ) with the values of P(θ) experimentally obtained. For a sphere10

where λ0 is the wavelength of laser light in a vacuum (nm), NA is Avogadro’s number, n0 is the refractive index of the solvent, dn/dc is the refractive index increment, c is the oligomer concentration (g/mL), c0 is the critical concentration at which oligomer assemblies are formed (g/mL), Rθ is the reduced scattered intensity, R0 is the reduced scattered intensity at the critical concentration of oligomer assembly formation, M is the

where r is the radius of the sphere (nm) and λ is the wavelength of laser light in solution (nm).

(9) Nakagaki, M., Inagaki, H., Eds. Experimental Method of Light Scattering; Nankoudo: Tokyo, 1965; pp 38-42.

(10) Burchard, W.; Patterson, G. D. Light Scattering from Polymers. In Advances in Polymer Science; Springer-Verlag: New York, 1983.

P(θ) ) [3(sin x - x cos x)/x3]2

(5)

x ) 4πr(sin θ/2)/λ

(6)

Silicon Oligomers with Carboxylic Acid Groups

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For a thin rod10

error ) [

P(θ) ) 1/y

∫

2y

0

2

2

sin t/t dt - (sin y/y)2

y ) 2 πl(sin θ/2)/λ

(7) (8)

where l is the length of the rod. For a monosized random coil10

(9)

z ) 16π2/λ2(R2/6) sin2 θ/2

(10)

where R is the mean distance between the ends (nm). For a disk10

P(θ) ) 2{1 - J1(2X)/X}/X2

(11)

X ) 4πr(sin θ/2)/λ

(12)

where J1(t) is the Bessel cylindrical function of the first kind and r is the radius of the disk. The equation for the ellipsoid is10,11

∫

π/2

0

J3/22(V)/V3 cos dβ

(13)

J3/2(V) ) x2V/π(sin V - V cos V)/V2

(14)

V2 ) (4π/λ sin θ/2)2(a2cos2 β + b2 sin2 β)

(15)

where 2a and 2b are the lengths of the minor and major axes of the ellipsoid (nm), respectively. The reciprocal of P(θ), P-1(θ), is obtained by c f c0 extrapolation of eq 1 as

P-1(θ) ) [K(c - c0)/(Rθ - R0)]cfc0M

(22)

If the error defined by eq 22 for each model gives minimum values, then the model is recognized as the desired geometry for the molecular assembly.

3. Results and Discussion

P(θ) ) 2(z - 1 + e-z)/z2

P(θ) ) 9π/2

∑(theoretical values - observed values) / ∑(observed values) ] × 100

(16)

Namely, the values of P-1(θ) were calculated at each scattering angle using eq 16, and the values thus obtained were compared with the theoretical curves for each of the geometries obtained from the corresponding theoretical equations (eqs 5-15). Since the radius of gyration (RG) is given for each geometry (eqs 1721) as shown below,10 the size of each geometry was determined by substituting the experimentally obtained value of RG into the corresponding equation. The size determined for each geometry was then substituted into the corresponding theoretical equation for P(θ).

RG2 ) 3r2/5 (sphere)

(17)

RG2 ) l2/12 (thin rod)

(18)

RG2 ) R2/6 (random coil)

(19)

RG2 ) r2/2 (disk)

(20)

RG2 ) (2a2 + b2)/5 (ellipsoid)

(21)

For an ellipsoid, there are two variables (a,b), and P(θ) was calculated by changing the two variables while RG was kept constant. Computer simulation was then performed to fit the theoretical P(θ) curve with the experimental P(θ) values until the error defined by the following equation gave minimum values. (11) Saito, N.; Ikeda, Y. J. Phys. Soc. Jpn. 1951, 6, 305. (12) Tani, H.; Sekiguchi, H. Monnerie’s Introduction of Polymer Science; Kyouritsu-Shuppan: Tokyo, 1973 in Japanese.

3.1. Aggregation Number and Radius of Gyration (RG) for Molecular Assemblies. The ratio of the scattered light intensities at 45° and 135°, the dissymmetry factor Z45 (I45/I135), is generally known to become larger than 1 when the size of the particle is larger than about 1/20 of the wavelength of irradiated laser light, since the intensity of scattered light decreases due to an interference effect.13 The relationship between Z45 and concentration was first examined for RF(13-2), a fluorinated silicon oligomer, and PAA, a polyelectrolyte (Figure 1). The abscissa indicates the ratio of the polymer concentration to the critical concentration for molecular assembly formation (critical assembly concentration). The value of Z45 for PAA, the reference sample, remained almost constant (about 1) independent of concentration, and the intensity of scattered light showed no angular dependence. On the contrary, the value of Z45 for RF(13-2) was found to be considerably larger than 1, whereas it remained almost unchanged under all concentrations. This suggests that the size of oligomer assemblies is so large that it causes the reduction of the intensity of scattered light at a higher scattering angle. A similar tendency was also observed for the other oligomers (data are not shown). Hence, measurements of the detailed aggregation number of oligomer assemblies were conducted using the Zimm plot method, taking into consideration the angular dependence, instead of the Debye plot method in which the measuring angle is fixed at 90°. In Table 2 are shown the aggregation numbers and RG values of molecular assemblies of the fluorinated silicon oligomers. When an inspection was made of the data for RF(13-2) and RF(85-1) or for RFO(19-6), RFO(34-7), and RFO(95-6), the oligomers with the same RF, nearly the same number of CH2CH(SiMe3) segments (y in Table 1), and a different number of CH2CH(COOH) segments (x in Table 1), a decrease in both the aggregation number and RG value was found as the number of CH2CH(COOH) segments increases. This would be interpreted as showing that an increase in the number of CH2CH(COOH) segments hinders oligomer molecules from aggregating because of the increased hydrophilicity and electrostatic repulsion between the carboxylic acid groups. Comparison of RFO(156-0) with RFO2(167-0), both of which have nearly the same number of CH2CH(COOH) segments, a different length of RF chain, but no CH2CH(SiMe3) segment, revealed that the latter with a longer RF chain is larger in both the aggregation number and the RG value than the former. Also, a comparison was made among the oligomers RFO(25-9), RFO(34-7), and RFO(42-3), which possess the same RF and a different copolymerization ratio, to show that the aggregation number and RG decrease with increasing proportion of CH2CH(COOH) segments (x). This would be brought about by the decrease in the hydrophobicity of the molecule due to the increase in the electrostatic repulsion produced by an increase in the proportion of CH2CH(COOH) segments (x). (13) Oseki, S. Hyomen 1982, 20, 632.

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Figure 1. Relationship between Z45 values and the concentrations of RF(13-2) and PAA at 30 °C: (O) RF(13-2); (0) PAA. Table 2. Aggregation Number (N) and Radius of Gyration (RG) of Molecular Assemblies Formed with Fluorinated Silicon Oligomers with Carboxyl Groups at 30 °C sample

N

RG (nm)

RF(13-2) RF(85-1) RFO(19-6) RFO(34-7) RFO(95-6) RFO(42-3) RFO(25-9) RFO(156-0) RFO2(167-0) R(16-1) PAA

8990 11 2810 177 16 24 4180 8 52 43 12

84.5 64.0 123.8 107.5 67.8 82.5 161.1 95.3 118.9 64.5 23.7

Moreover, the effect of RF was investigated on the aggregation number and RG value of fluorinated silicon oligomers by comparing PAA and R(16-1), both of which have nearly the same molecular weight but no RF on either terminal, with RF(13-2), having an RF on both terminals. The comparison showed that the aggregation number and RG value are appreciably larger for RF(13-2) than for the other two oligomers. This suggests that since the presence of RF increases the hydrophobicity of the oligomer, thereby making it easier to be excluded by water molecules, the aggregation number as well as the RG value becomes larger, accordingly. 3.2. Effects of Chemical Structure on the Shape and Size of Oligomer Assemblies. 3.2.1. Oligomers with Nearly the Same Number of CH2CH(COOH) Segments, a Different RF Chain Length, but no CH2CH(SiMe3) Segment. The shape and size of molecular assemblies were examined with the oligomers RFO(156-0) and RFO2(167-0), both of which have nearly the same number of CH2CH(COOH) segments, a different length of RF chain, but no CH2CH(SiMe3) segment. The theoretical curves of possible geometries for RFO(156-0) are shown in Figure 2A as a typical example, in which the ordinate and abscissa are the reciprocal of P(θ), P-1(θ), and sin2(θ/2), respectively. Table 3 shows the sizes and fitting errors defined by eq 22 between the theoretical and observed values of the possible geometries. The sizes of the geometries in the table are denoted by radius (r), length (l), mean distance between the ends (R), radius (r), and ratio of the length of major axis (2b) to that of minor axis (2a) for sphere, rod, monosized random coil, disk, and ellipsoid, respectively. The errors are calculated using eq 22 as described before. The smallest error was found between the theoretical and observed values for the ellipsoid, as is shown in the

table. This means that the shape of molecular assemblies formed in aqueous solution is most likely ellipsoidal. A similar result was obtained for RFO2(167-0). On the basis of these findings, calculations were made of the ratios of the length of the major axis to that of the minor axis for both oligomers and values of 3.38 and 3.06 were obtained for RFO2(167-0) and RFO(156-0), respectively. This suggests a more slender shape for RFO2(167-0) with a longer RF chain. Next, estimation of the geometry was performed for molecular assemblies of R(16-1) and PAA, the reference samples, both of which have no RF on their terminals. The theoretical curves for possible geometries are shown in parts B and C of Figure 2 for R(16-1) and PAA, respectively. In Table 3 are shown the sizes of the possible geometries and the fitting errors between the theoretical and observed values. The error was found to be smallest for R(16-1) when the equation for a sphere or an ellipsoid was used, while the use of the equation for a sphere gave the least error for PAA with a radius of 30.7 nm. This would indicate that molecular assemblies of oligomers take a spherical shape if they have no RF on both terminals. 3.2.2. Oligomers with the Same RF, Nearly the Same Number of CH2CH(SiMe3) Segments, and a Different Number of CH2CH(COOH) Segments. The effect of the CH2CH(COOH) segment number was examined on the shape and size for the fluorinated silicon oligomers with the same RF, nearly the same number of CH2CH(SiMe3) segments, and a different number of CH2CH(COOH) segments, that is, RF(13-2) and RF(85-1), both of which have C3F7 as RF, and RFO(19-6), RFO(34-7), and RFO(95-6), all of which have C2F5(CF2OCFCF3) as RF. Figure 2D shows the theoretical curves of the possible geometries for RFO(19-6) as an example, indicating that molecular assemblies of the oligomer have an ellipsoidal shape. Similar results were obtained with the other oligomers. This supports the inference described before that the presence of RF makes the shape of oligomer assemblies ellipsoidal. The 2b/2a ratios were then calculated for the five oligomers on the basis of the above findings that molecular assemblies of all the oligomers are ellipsoidal in shape, to examine the effect of the number of CH2CH(COOH) segments on the geometry of the molecular assemblies. In Table 3 are shown the 2b/2a ratios, together with the lengths of the major and minor axes of the ellipsoids of the oligomers. Oligomer assemblies seem to change their shape from a slender to a round one, since the value of the 2b/2a ratio decreased with increasing number of CH2CH(COOH) segments, as seen from the table. The size and RG value for oligomer assemblies decreased as the number of CH2CH(COOH) segments (x) increased (Table 2). Polymeric compounds are reported to take, in general, an expanded structure in polar solvents such as water when their hydrophilicity is high.12 Hence, the results obtained with the oligomers seem to contradict the above report. In view of this, calculations were made about the volume per molecule in a molecular assembly for all oligomers used in this work to check whether the assembly structure becomes expanded when the hydrophilicity of the silicon oligomer molecule is high (Table 4). Here, V/N in the table denotes the volume per silicon oligomer molecule (nm3). The volume per molecule increased with increasing number of CH2CH(COOH) segments. Meanwhile, the aggregation number decreased as the number of CH2CH(COOH) segments increased, as mentioned before. Then, the following reason is possible for the finding that the size of oligomer assemblies decreases with the increase in the number of CH2CH-

Silicon Oligomers with Carboxylic Acid Groups

Langmuir, Vol. 14, No. 8, 1998 2065

Figure 2. Determination of (A) RF(156-0), (B) R(16-1), (C) PAA, and (D) RFO(19-6) micelle geometry by comparing experimental plots with theoretical curves for spherical, rod-like, random-coiled, disklike, and ellipsoidal geometries at 30 °C. Table 3. Size of the Molecular Aggregates Formed with RFO(156-0), R(16-1), and PAA and Fitting Error between Experimental Plots and Theoretical Curves RFO(156-0) R(16-1) PAA

length (nm) fit error (%) length (nm) fit error (%) length (nm) fit error (%)

sphere (r)

rod (l)

random coil (R)

disk (r)

ellipsoid (2b/2a)

123 4780 83.2 0.261 30.7 0.0401

330 21.9 223 14.0 82.0 0.0707

233 1.67 158 11.3 58.0 0.0719

135 8.58 91.2 2.54 33.5 0.0498

387/126 0.285 186/156 0.250 61.2/61.0 0.0434

Table 4. Size, (2b/2a) Ratio, and Volume per Oligomer-Molecule (V/N) in a Molecular Assembly Formed with Fluorinated Silicon Oligomers with Carboxyl Groups at 30 °C sample

ellipsoid (2b/2a)

2b/2a

V/N (nm3)

RF(13-2) RF(85-1) RFO(19-6) RFO(34-7) RFO(95-6) RFO(42-3) RFO(25-9) PAA R(16-1)

349/103 238/112 549/49.6 457/106 250/121 326/122 703/111 186/156

2.84 2.12 11.0 3.49 1.78 2.24 6.33 1.18

216 143000 251 15000 120000 107000 1080 10100 5500

(COOH) segments. First, an increase in the number of CH2CH(COOH) segments causes a silicon oligomer molecule to take an expanded structure while it produces an increase in the electrostatic repulsion which prevents molecular aggregation, thereby making the size of the molecular assemblies small. In the case of low-molecularweight ionic surfactants, it is known that the size of their micelles is kept at a certain level because there is a balance between the hydrophobic interaction between long chain

alkyl groups, the driving force of micelle formation, and the increased electric potential due to a decrease in the distance between ionic head groups, the suppressing force of molecular aggregation. In other words, an increase in the number of ionized groups can be regarded as a prime factor to control the size of molecular assemblies, since the presence of many ionized groups brings about a rise in the electric potential to make the system unstable.14 Similarly, in this work, an increase in the number of hydrophilic CH2CH(COOH) segments arouses the electric potential to considerably rise, thereby making the size of oligomer assemblies smaller. 3.2.3. Oligomers with the Same RF, Nearly the Same Molecular Weight, and a Different Copolymerization Ratio. As is seen from Table 4, since the 2b/2a ratio decreased with increasing proportion of CH2CH(COOH) segments (x) for RFO(25-9), RFO(34-7), and RFO(42-3), the fluorinated silicon oligomers with the same RF, nearly the same molecular weight, and a different copolymerization ratio, the structure of their molecular assemblies had changed from a slender one to a round one. The size of the molecular (14) Tanaka, M. J. Jpn. Oil Chem. Soc. 1985, 34, 206.

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assemblies as well as RG increased as the proportion of CH2CH(COOH) segments (x) decreased (Tables 2 and 4). These findings also seem to contradict the behavior of ordinary polymeric compounds in solution.12 If the effect is taken into consideration about the number of CH2CH(COOH) segments on the volume per molecule in an oligomer assembly, the oligomers are supposed to take a more expanded structure when the proportion of CH2CH(COOH) segments increases. Calculations were then made of the volume per oligomer molecule in a molecular assembly for the oligomers (Table 4). As a result, the volume per molecule was found to become smaller as the proportion of CH2CH(COOH) segments (x) decreased. The finding that the size of oligomer assemblies increases with decreasing proportion of CH2CH(COOH) segments (x) would, therefore, be interpreted as being caused by the increased intermolecular cohesive forces because the electrostatic repulsion decreases and the hydrophobicity of oligomer molecules increases even though the volume per molecule decreases. 3.3. Mechanism of Molecular Assembly Formation. 3.3.1. Penetration of Water into the Interior of Molecular Assemblies. Since the fluorinated silicon oligomers used in the present work are random copolymers, they are likely to form molecular assemblies in which their hydrophilic and hydrophobic parts are not separated from each other but intermingled. If hydrophilic carboxylic acid groups are present in the interior of molecular assemblies, it is possible that water can penetrate into the assembly interior. Hence, the micropolarity of the assembly interior was examined by the pyrene fluorescent probe method. Measurements of the I1/I3 value for pyrene in aqueous solutions of RF(13-2) at different concentrations revealed that it has a nearly constant value of 1.55 below the cac while it decreased to about 1.45 above the cac. A similar tendency was observed with other fluorinated silicon oligomers. The decrease in the I1/I3 value above the cac would be ascribed to the reduced polarity in the surroundings of the pyrene molecules that are solubilized in molecular assemblies. Because the I1/I3 value above the cac is higher than the value of about 1.01 in micelles of sodium dodecyl sulfate, a typical low-molecular-weight surfactant, the interior of molecular assemblies of the fluorinated silicon oligomers used in this work is supposed to be less hydrophobic than that of micelles of lowmolecular-weight surfactants, thus allowing water to penetrate into it. Especially with those silicon oligomers which have many CH2CH(COOH) segments, the interior of their molecular assemblies is presumed to be hydrophilic enough to permit water to easily penetrate into it, thus making the assemblies take a loose structure. 3.3.2. Oligomers without RF on Terminals. The mechanism of molecular assembly formation was examined for the oligomers with no RF on both terminals. PAA and R(16-1), both of which have no RF on their terminals, were found to be spherical and spheroidal in shape, respectively. Since the volume per molecule in a molecular assembly for these oligomers was noticeably larger than that obtained from the molecular model, the structure of molecular assemblies of the oligomers would be rather loose. Hence, the formation of molecular assemblies of the oligomers would be through the van der Waals forces (cohesive forces) between the hydrocarbon chains of oligomer molecules acting at nonspecific sites on their principal chains, thereby making the assembly shape random coil-like. In view of the fact that the shape of molecular assemblies of polymeric compounds in aqueous solution is, in most cases, spherical as reported in many

Nakagawa et al.

papers,15-19 the oligomers without RF, such as PAA and R(16-1), are supposed to be similar to ordinary polymeric compounds in their behavior. 3.3.3. Oligomers with RF on Both Terminals. The mechanism of assembly formation was then examined for the oligomers with RF on both terminals. As mentioned before, molecular assemblies of the oligomers with RF on both terminals are all ellipsoidal in shape and their formation is characterized as follows: (1) Electrostatic repulsion acts even when molecular assemblies are formed, and it decreases with increasing oligomer concentration, since the aggregation number was found to depend on the number of CH2CH(COOH) segments. (2) The interior of molecular assemblies is rather loose in structure. (3) Water penetrates into the interior of molecular assemblies. (4) RF is strongly excluded by water molecules because introduction of RF onto both terminals was found to cause a steep rise in the aggregation number of molecular assemblies. (5) For polymeric compounds with a hydrophobic hydrocarbon group on both terminals, the terminal hydrocarbon groups cohere with each other when the compounds form molecular assemblies.19 (6) Since the oligomers used in this work are random copolymers, they form molecular assemblies in which their hydrophilic and hydrophobic parts are intermingled, differing from those assemblies formed by block copolymers in which the hydrophobic parts make up the core and the hydrophilic parts encircle it as corona.20 From what has been mentioned above, the reason molecular assemblies of the oligomers with RF on both terminals have an ellipsoidal shape would be described as follows. The silicon oligomer molecule is supposed to take a somewhat extended structure in aqueous solution because of the electrostatic repulsion acting between the dissociated carboxylic acid groups of the molecule. Electrostatic repulsion also acts between oligomer chains, and strong cohesive forces act between RF groups. Hence, the oligomers would form molecular assemblies in such a way that their chains are located at a certain distance apart from each other to make the electrostatic repulsion as weak as possible while their RF groups cohere with each other through the van der Waals forces, as shown in Figure 3A. It may also be possible for the oligomers to form molecular assemblies in which their molecules are arrayed nearly perpendicularly to each other. This possibility is ruled out, however, because RF groups, which are perfluoroalkyl groups, are hardly bendable and rigid,21 and a more stable assembly is obtained when their contact area is larger. Molecules of the silicon oligomers are, therefore, supposed to array horizontally when they form molecular assemblies. According to this model, highly hydrophobic RF groups are situated on the periphery of molecular assemblies. Hence, the RF groups on the assembly periphery are presumed to be covered with the neighboring CH2CH(COOH) chains. 3.3.4. Effect of CH2CH(COOH) Segment Number. Next, the effect was examined of the number of CH2CH(COOH) segments on the formation of molecular assemblies. The (15) Proch, K. Macromolecules 1996, 29, 6518. (16) Guenoum, P.; Davis, H. T.; Tirrell, M. J.; Mays, W. Macromolecules 1996, 29, 3965. (17) Hickl, P. M.; Ballauff, M. Macromolecules 1996, 29, 4006. (18) Peiqiang, W.; Siddiq, M.; Huiying, C.; Di, Q.; Wu, C. Macromolecules 1996, 29, 277. (19) Yekta, A.; Xu, B.; Duhamel, J.; Adiwiwidjaja, E.; Winnik, M. A. Macromolecules 1995, 28, 956. (20) Alexandridis, P.; Nivaggioli, T.; Hatton, T. A. Langmuir 1995, 11, 1468. (21) The Society of Polymer Science. Fluorinated Polymers; Kyouritou-Shuppan: Tokyo, 1990; pp 2-4, (in Japanese).

Silicon Oligomers with Carboxylic Acid Groups

Figure 3. (A) Schematic model of micelles formed with oligomers with RF. (B) Effect of (CH2CHCOOH) segment number on the geometry of the molecule and effect of the copolymerization ratio on the geometry of the molecule.

shape of the molecular assemblies formed changed from a slender one to a round one as the number of CH2CH(COOH) segments increased. In searching for the reason the shape of molecular assemblies is dependent on segment number, the following findings were taken into consideration. (1) The volume per molecule (V/N) in an oligomer assembly rose with increasing number of CH2CH(COOH) segments. (2) A linear relation was obtained when V/N was plotted against segment number for RFO(19-6), RFO(34-7), and RFO(95-6), indicating that the increase in V/N with segment number was not brought about by expansion of oligomer molecules but by an increase in their chain length. On the basis of the above considerations, a model for molecular assembly formation was proposed as shown in Figure 3B. Thus, as the segment number increases, the length of the principal chain also increases, making the probability of principal chains (hydrocarbon chains) to aggregate higher while making that of RF groups to cohere lower. As a consequence, molecular assemblies are supposed to change their shape to a round one, since the cohesion of RF groups shown in Figure 3A becomes less likely to occur. 3.3.5. Effect of Chain Length of RF. Next, the effect of the chain length of RF was examined on the formation of molecular assemblies. The shape of molecular assemblies was found to be more slender for the oligomers with longer RF chains. This would be ascribed to a higher probability

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of longer RF chains to cohere with each other to make it easier for the type of assembly formation shown in Figure 3A to take place. 3.3.6. Effect of Copolymerization Ratio. The effect of the copolymerization ratio is interpreted as follows. As mentioned before, the shape of molecular assemblies changed from a slender one to a round one with an increase in the proportion of CH2CH(COOH) segments. A model for this change is shown in Figure 3C. Since V/N increased with an increase in the proportion of CH2CH(COOH) segments, that is, an increase in the hydrophilicity of the molecule, oligomer molecules are presumed to take a more expanded structure. The increased length of principal chains would lead to a higher probability for the hydrocarbon chains to adhere to each other and a lower probability for oligomer molecules to form the type of assemblies described above. Consequently, the shape of molecular assemblies changes from a slender one to a round one. 4. Conclusions The effects were examined of RF, the number of CH2CH(COOH) segments, and the copolymerization ratio on the aggregation number, shape, and size of molecular assemblies of copolymers of acrylic acid and trimethylvinylsilane with an RF group on both terminals (fluorinated silicon oligomers), and the following results were obtained. The aggregation number increases with introduction of RF, an increase in the length of the RF chain, a decrease in the number of CH2CH(COOH) segments, and an increase in the proportion of CH2CH(SiMe3) segments to CH2CH(COOH) segments. The presence of the RF group on both terminals makes the shape of molecular assemblies slender, since those oligomers with no RF on the terminals form nearly spherical molecular assemblies. The ratio of the lengths of the major and minor axes, (2b/2a), of molecular assemblies decreases with increases in both the number of CH2CH(COOH) segments and the proportion of CH2CH(COOH) segments to CH2CH(SiMe3) segments and a decrease in the chain length of the RF group. The size of molecular assemblies increases with an increase in the length of the RF chain, a decrease in the number of CH2CH(COOH) segments, and an increase in the proportion of CH2CH(SiMe3) segments to CH2CH(COOH) segments. LA971201S