Hybrid Threadlike Micelle Formation between a Surfactant and

The construction of a new type of threadlike micelle called a “hybrid threadlike micelle”, formed by surfactant and polymer molecules, was investi...
1 downloads 0 Views 151KB Size
Download PDF
8654

Langmuir 2003, 19, 8654-8660

Hybrid Threadlike Micelle Formation between a Surfactant and Polymer in Aqueous Solution Kenji Nakamura, Kanae Yamanaka,† and Toshiyuki Shikata* Department of Macromolecular Science, Osaka University, Toyonaka, Osaka 560-0043, Japan Received March 12, 2003. In Final Form: June 10, 2003 The construction of a new type of threadlike micelle called a “hybrid threadlike micelle”, formed by surfactant and polymer molecules, was investigated using NMR and viscoelastic measurements in an aqueous system of cetyltrimethylammonium bromide (CTAB), sodium p-toluensulfonate (NapTS), and sodium poly(p-vinylbezoate) (NaPVB) (CTAB:NapTS:NaPVB/W), varying the pH and composition of components. When the molar concentration of NapTS (CS) is higher than 60% of the concentration of CTAB (CD), the system constructs hybrid threadlike micelles that are long enough to become entangled with each other and that exhibit pronounced viscoelasticity well described by the behavior of a simple Maxwell model, like an ordinary aqueous threadlike micellar system without polymers (CTAB:NapTS/W). At a pH lower than the pKa of the polymer, the viscoelastic relaxation time and strength of the CTAB:NapTS: NaPVB/W system become greater than those of the ordinary threadlike micellar system. These findings suggest that incorporation of long polymers into the hybrid threadlike micelle effectively hinders the crossing-through reaction between two micelles at an entanglement point (which releases the entanglement) and that it causes the rigidity of the hybrid threadlike micelle to be greater than that of the ordinary threadlike micelle.

Introduction Surfactant molecules form many kinds of molecular assemblies in aqueous solution, depending on conditions such as concentration, temperature, pH, and the presence of additives. One such molecular assembly formed by surfactants is a threadlike micelle with an extraordinarily long structure.1-5 Some surfactants, such as cetyltrimethylammonium bromide (CTAB), form long threadlike micelles in aqueous solution with certain additives, including sodium salicylate (NaSal) and sodium p-toluensulfonate (NapTS). Threadlike micelles in aqueous solution form concentrated entanglement networks, similar to those formed by polymer molecules in semidilute to concentrated solutions. Also, threadlike micelles exhibit pronounced viscoelasticity because of entanglement effects between the molecules.6 The viscoelastic behavior of the aqueous threadlike micellar solution is similar to that of the concentrated polymer system in some aspects such as the concentration (CP) dependence of plateau modulus: GN ∝ CP2. However, the relaxation mechanism in the threadlike micellar system is completely different from that of the concentrated polymer system.7 In threadlike micelles, surfactant molecules are not connected by chemical bonding as in polymer molecules; rather, they are gathered up into micelles via intermolecular forces such as hydrophobic and cation-π8,9 interaction. Thus, surfactant molecules are easily rearranged in threadlike micelles, suggesting * To whom correspondence should be addressed. E-mail: shikata@ chem.sci.osaka-u.ac.jp. † Present address: Hitachi Displays, Ltd., 3300 Hayano, Mobara, Chiba 297-8622, Japan. (1) Debye, P.; Anacker, E. J. Phys. Colloid Chem. 1951, 55, 644. (2) Mandel, B. A.; Ray, S.; Biswas, M. A.; Moulik, P. S. J. Phys. Chem. 1980, 84, 856. (3) Leibner, E. J.; Jacobus, J. J. J. Phys. Chem. 1977, 81, 130. (4) Tanford, C.; Nozaki, M.; Rhode, F. M. J. Phys. Chem. 1977, 81, 1555. (5) Gravasholt, S. J. Colloid Interface Sci. 1967, 57, 575. (6) Rehage, H.; Hoffmann, H. Mol. Phys. 1991, 74, 933 (7) Shikata, T.; Hirata, H.; Kotaka, T. Langmuir 1988, 4, 354. (8) Gao, J.; Chou, L. W.; Auerbach, A. Biophys. J. 1993, 65, 43.

that the relaxation mechanism of entanglement release in the threadlike micellar system is governed by a phantom-like crossing-through reaction7 between threadlike micelles at entanglement points. Other models, which take into account the structural features of threadlike micelles,10,11 have been proposed to explain the unique viscoelastic behavior of the aqueous threadlike micellar solution. In “hybrid threadlike micelles”, in which polymer molecules are combined with ordinary surfactant threadlike micelles, some physicochemical features of ordinary threadlike micelles are altered. For example, the passingthrough reaction between threadlike micelles at entanglement points is unlikely to occur when polymer molecules incorporated into the hybrid threadlike micelle are present at the entanglement points. The flexibility (or rigidity) of the hybrid threadlike micelle also differs from that of the ordinary threadlike micelle. Many groups have attempted to construct hybrid threadlike micelles, using various combinations of polymers and ordinary threadlike micellar systems.12,13 Of the surfactants that form threadlike micelles with additives, CTAB is the best known. Accordingly, studies of interactions between CTAB and several polymers have been conducted, to collect basic information about hybrid threadlike micelle formation. Sodium poly(styrenesulfonate) (NaPSS) has been studied as a candidate polymer for hybrid threadlike micelle formation with CTAB, but electrostatic interaction between CTA+ and PSS- is too strong for formation of stable hybrid threadlike micelles. Consequently, an aqueous solution of CTAB and NaPSS becomes highly turbid to precipitate segregation.14-16 Poly(acrylamide) (PAm), another candidate (9) Hedin, N.; Sitnikov, R.; Furo´, I.; Henriksson, U.; Regev, O. J. Phys. Chem. B 1999, 103, 9631. (10) Cates, M. E. Macromolecules 1987, 20, 2289. (11) Turner, M.; Marques, M. E.; Cates, M. E. Langmuir 1993, 9, 695. (12) Brackman, J. C.; Engberts, J. B. F. N. J. Am. Chem. Soc. 1990, 112, 872. (13) Lin, Z.; Eads, C. D. Langmuir 1997, 13, 2647.

10.1021/la030101l CCC: $25.00 © 2003 American Chemical Society Published on Web 09/18/2003

Hybrid Threadlike Micelle Formation Scheme 1

polymer, also does not readily form hybrid threadlike micelles with CTAB. It is likely that PAm and CTA+ dissolve in water independently. The reason for poor hybrid threadlike micelle formation between CTAB and PAm is apparently weak interaction between PAm and CTA+. Although many other polymers12,13 have been tested as candidates for hybrid threadlike micelle formation with CTAB, no successful combinations between CTAB and polymers have previously been found. Recently, Kline reported the polymerization of threadlike micelles.17,18 He synthesized a polymerizable surfactant, cetyltrimethylammonium p-vinylbenzoate (CTAVB), by replacing the Br- of CTAB with p-vinylbenzoate (VB-). After polymerization, an aqueous solution of CTAVB becomes a low viscous liquid. However, in small-angle neutron scattering experiments, he found that a solution of polymerized CTAVB still contains short (hybrid) threadlike micelles. This strongly suggests that poly(pvinylbenzoate) (PVB-) can form hybrid threadlike micelles with CTA+ in aqueous solution but does not form long hybrid threadlike micelles as readily as VB-. Klein’s impressive studies17,18 led us to speculate that hybrid threadlike micelles can be formed by a combination of sodium poly(p-vinylbenzoate) (NaPVB), CTAB, and NapTS in aqueous solution (see Scheme 1). pTS- forms threadlike micelles with CTAB much more readily than PVB-, and the purpose of pTS- in this system is to act as an assistant agent. In the present study, we investigated hybrid threadlike micelle formation in an aqueous solution of NaPVB, CTAB, and NapTS (CTAB:NapTS:NaPVB/W). Because NaPVB bears carboxylate groups, physicochemical features of the hybrid threadlike micelles that are formed are pH-dependent. Structure and some characteristics of the hybrid threadlike micelles formed using this system are discussed in relation to the viscolastic behavior of the system at various pH values and concentrations of NaPVB, CTAB, and NapTS. Experimental Section Materials. CTAB and NapTS were purchased from Wako Pure Chemicals Ltd. (Osaka) and were purified by re-crystallization using a methanol/acetone mixture and a water/methanol mixture, respectively. Highly deionized water with a specific resistance greater than 16 MΩcm, obtained using a MilliQ system, was used as the solvent. Deuterium oxide (D2O) (99.9%, ISOTEC INC, Ohio) was used as the solvent for 1H NMR measurements. Hydrochloric acid (37%, Sigma-Aldrich Japan, Tokyo) and sodium hydroxide were used to control pH. Deuteriumchloric acid (37%, ISOTEC INC, Ohio) was used to control the pH of sample solutions used for 1H NMR measurements. A monomer, p-vinylbenzoic acid (VBA), was synthesized from 4-bromomethylbenzoic acid using the Wittig method.19 Poly(vinylbenzoic acid) (PVBA) was obtained by free radical polym(14) Abuin, E. B.; Scaiano, J. C. J. Am. Chem. Soc. 1984, 106, 6274. (15) Almgren, M.; Hansson, P.; Mukhtar, E.; van Stam, J. Langmuir 1992, 8, 2405. (16) Fundin, J.; Brown, W. Macromolecules 1994, 27, 5024. (17) Kline, S. R. Langmuir 1999, 15, 2726. (18) Kline, S. R. J. Appl. Crystallogr. 2000, 33, 618. (19) Harwood, L. M.; Moody, C. J. Experimenal organic chemistry, principle and practice; Blackwall: Oxford, 1989; p 588.

Langmuir, Vol. 19, No. 21, 2003 8655 Table 1. Number Average Molecular Weight (Mn) and Distribution of Molecular weight (Mw/Mn) for Three Types of Used Poly(p-vinylbezoic acid) (PVBA) in Esterified Form Mn/104

Mw/Mn

1.5 10 36

2.0 1.9 2.8

erization of VBA initiated by AIBN in aqueous solution at 60 °C. Atom transfer radical polymerization (ATRP)20 was also used to obtain PVBA from VBA. To determine the average molecular weight of the synthesized PVBA, we carried out gel permeation chromatography (GPC) measurements using tetrahydrofuran (THF) as eluent. Because PVBA is insoluble in THF, we performed methylesterification of PVB as follows. PVBA was dissolved in a mixture of THF and methanol and was esterified by the addition of sulfuric acid. Degree of esterification of PVBA was determined using data from 1H NMR measurements. All esterified PVBA samples showed a degree of esterification higher than 95%. The GPC apparatus (SL-800, Toso, Tokyo) was equipped with columns connected doubly (Shodex Asahipack KF806M, Showa Denko, Tokyo). It was calibrated using standard polystyrene samples with average molecular weight (Mw) ranging from 104 to 106 and was used to determine the average molecular weight of the esterified PVBA. The average molecular weight (Mn) and molecular weight distribution (Mw/Mn) of the esterified PVBA are shown in Table 1. The molecular weight distribution of the obtained polymers was not as narrow as was expected from ATRP. The obtained PVBA was converted to NaPVB by neutralizing an aqueous PVBA solution by adding sodium hydroxide. The obtained NaPVB was dialyzed against water for more than 1 week. The aqueous micellar system, CTAB:NapTS:NaPVB/W, was prepared by adding preweighed NaPVB and NapTS to aqueous CTAB solutions. The pH of the CTAB:NapTS:NaPVB/W system was controlled by adding hydrochloric acid. Before measurements, the solution was kept at room temperature for more than 10 h to equilibrate. Measurements. 1H NMR spectra were recorded with JEOL GSX-400 spectrometers using deuterium locked mode at 30 °C. Dynamic viscoelastic measurements were performed using a stress-controlled rheometer (Dynalyser 100, Reologica, Lund) equipped with cone-plate at 25 °C. The cone had an angle of 4 degrees and a diameter of 40 mm. Frequency (ω) was measured in the range from 6.28 × 10-3 to 6.28 × 102 rad s-1. Storage and loss moduli (G′ and G′′) that obeyed the linear viscoelastic response were used for analysis.

Results Formation of Hybrid Threadlike Micelles. We determined how much NapTS is necessary for hybrid micelle formation in the CTAB:NapTS:NaPVB/W system. The concentration of CTAB (CD) was fixed at 25 mM, and the concentrations of NapTS (CS) and NaPVB (CP in monomer content) were varied, with the total concentration of NapTS plus NaPVB (CP + CS) remaining fixed at 25 mM ()CD). Table 2 summarizes solute content and also shows apparent turbidity and viscoelasticity of the CTAB:NapTS:NaPVB/W system at 25 °C, on the basis of visual inspection. When CS is greater than 17.5 mM, the system becomes turbid at pH values less than the pKa of PVBA ()4.4).21 In contrast, when CP is greater than 10 mM, the system retains its transparency at pH values less than 7. Moreover, when CP is greater than 20 mM, the system is transparent at all pH values and loses its viscoelasticity. Because an aqueous NaPVB solution with a CP of 25 mM (Mn ) 3.6 × 105) is not viscoelastic, the zero (20) Wang, X. S.; Jackson, R. A.; Armes, S. P. Macromolecules 2000, 33, 255. (21) Gabaston, L. I.; Furlong, S. A.; Jackson, R. A.; Armes, S. P. Polymer 1999, 40, 4505.

8656

Langmuir, Vol. 19, No. 21, 2003

Figure 1. 1H NMR spectra for the CTAB:NapTS:NaPVB/D2O system with CD ) 25, CS ) 17.5, and CP ) 7.5 mM at pH ) 5.8, the CTAB:NaPVB/D2O system with a CP/CD ratio from 0.3 to 4.0 at CD ) 25 mM and a NaPVB/D2O system with CP ) 25 mM (in molar concentration of the monomer unit). Table 2. Turbidity and Viscoelasticity of the CTAB:NapTS:NaPVB/W System at Various Compositions

a

Gray: turbid, hatching: opaque, white: clear.

shear viscosity of the solution is only 0.03 Pas, the strong viscoelasticity observed in a CTAB:NapTS:NaPVB/W system with a CS of 17.5 mM and CP of 7.5 mM results from formation of hybrid threadlike micelles. An aqueous solution of CTAB and NapTS with a CD of 25 mM and CS of 17.5 mM is less viscoelatic because of the low CS. The above findings indicate that the amount of NapTS necessary for long hybrid threadlike micelle formation in clear solution is more than 60% of CTAB content in the CTAB:NapTS:NaPVB/W system. This implies that the ability of NaPVB to participate in threadlike micelle formation is not sufficient to form long hybrid threadlike micelles with CTAB. The greater ability of NapTS to form threadlike micelles with CTAB is clearly necessary to achieve hybrid threadlike micelle formation in the present system. 1 H NMR Spectra. The values of chemical shift for molecules evaluated by NMR are very sensitive to the chemical conditions surrounding the molecules. Proton NMR (1H NMR) spectra are shown in Figure 1 for the following systems: NaPVB/D2O with a CP of 25 mM; CTAB:NaPVB/D2O with a CP/CD ratio ranging from 0 to 4.0 and a CD of 25 mM; and CTAB:NapTS:NaPVB/D2O with a CD of 25 mM, CS of 17.5 mM, CP of 7.5 mM, and pH of 5.8. Comparing the spectrum of the NaPVB/D2O system with that of the CTAB:NaPVB/D2O system reveals

Nakamura et al.

Figure 2. Dependence of G′ and G′′ on ω for the CTAB:NapTS: NaPVB/W system with various polymer molecular weight, Mn, at concentrations of CD ) 25, CS ) 17.5, and CP ) 7.5 mM and at pH ) 3.1.

that 1H NMR signals attributed to aromatic ring protons, especially a proton (a), of PVB- dissolved in bulk D2O phase shift to a higher magnetic field in the CTAB:NaPVB/ D2O system. Additive organic ions capable of threadlike micelle formation in aqueous CTAB solution, such as a salicylate ion (Sal-), are present in the micellar interior, and the NMR signals of ring protons belonging to Salappear at a higher magnetic field than those detected in bulk D2O phase. This suggests that the hydrophobic environment in the threadlike micellar interior provides lower values of chemical shift (higher magnetic field) to ring protons of additives.22,23 The 1H NMR spectra for the CTAB:NaPVB/D2O system in Figure 1 show chemical shift changes of ring protons belonging to PVB- as functions of the CP/CD ratio. When the ratio is less than 1, the signal of proton (a) is found at the chemical shift of a higher magnetic field than that of free PVB- in bulk aqueous phase in the NaPVB/D2O system. However, the signal of proton (a) appears doublepeaked when the ratio is greater than unity. One peak is at a chemical shift identical to that found at a ratio less than unity, and the other peak is found at the chemical shift for free PVB- in bulk aqueous phase. This implies that PVB- in the system does not alternate between bulk aqueous phase and incorporation in the threadlike micellar interior formed by CTA+ and PVB-, on the time scale of NMR measurements. This suggests that interaction between CTA+ and PVB- in short hybrid threadlike micelles is remarkably strong. The fact that similar chemical shift changes of ring protons of PVB- to higher magnetic fields are also observed in the CTAB:NapTS:NaPVB/D2O system (Figure 1) indicates that PVB- in the CTAB:NapTS:NaPVB/D2O system is located in the hydrophobic micellar interior, not in bulk D2O phase, as seen in the CTAB:NaPVB/D2O system. Rheological Behavior. Molecular Weight Dependence. Figure 2 shows the frequency, ω, and dependence of storage and loss moduli, G′ and G′′, for a CTAB:NapTS:NaPVB/W solution with a CD of 25 mM, CS of 17.5 mM, CP of 7.5 mM, and pH of 3.1. The molecular weight, Mn, of the polymer is shown in Table 1. It appears that the hybrid threadlike (22) Nakagawa, T.; Tori, K. Kolloid Z. Z. Polym. 1964, 164, 143. (23) Muller, N.; Birkhahn, R. H. J. Phys. Chem. 1967, 71, 957.

Hybrid Threadlike Micelle Formation

micelles formed in the CTAB:NapTS:NaPVB/W system are fully entangled with each other at the composition and pH shown in Figure 2, because the viscoelasticity was as strong as that observed in ordinary threadlike micellar systems.24-26 The viscoelastic spectra for an ordinary CTAB:NapTS/W threadlike micellar system can be well described by a Maxwell model with a single set of relaxation time and strength.11 If the hybrid threadlike micelles formed in the CTAB:NapTS:NaPVB/W system are long enough to become entangled, and the mechanism for entanglement release is similar to that of an ordinary CTAB:NapTS/W system, viscoelastic spectra for the system can be expected to be well described by the behavior of a single Maxwell element. The ω dependence of G′′ curves around the maxima for the system is slightly broader than that of a single Maxwell element (Figure 2). The relaxation time (τ) estimated from the reciprocal of the maximum frequency of G′′ curves increases and the plateau modulus (GN) strengthens with increasing Mn of PVB- incorporated in the hybrid threadlike micellar system (Figure 2). Because micelles (even threadlike micelles) are constructed by intermolecular interactions among constituent molecules, hybrid threadlike micelles may be more rigid than ordinary threadlike micelles because of incorporation of PVB- in the micelle. In polymer rheology, the magnitude of GN is proportional to the number density (Fe) of entanglement points between polymer chains,27 and the value of Fe is influenced by the flexibility of the polymer chain. The more rigid the polymer chain becomes, the greater Fe becomes in general. It is likely that the presence of covalently bonded PVB- makes the hybrid threadlike micelle more rigid. There is a clear change in the rigidity of the hybrid threadlike micelle as the Mn value increases. pH Dependence. The viscoelasticity of the CTAB:NapTS: NaPVB/W system is highly dependent on pH. Figure 3A shows the dependence of G′ and G′′ on ω for a system with a CD of 25 mM, a CS of 17.5 mM, and a CP of 7.5 mM. The Mn value for PVB- incorporated into the hybrid threadlike micelle was kept at 1.0 × 105, and the pH value was varied widely. Solid and broken lines in the figure indicate the viscoelastic spectrum of a CTAB:NapTS/W system with a CD and CS of 25 mM. The viscoelastic spectra for the CTAB:NapTS/W system were independent of pH in the region examined. At pH values lower than the pKa of PVBA, the hybrid threadlike micelles in the system grow long enough to become fully entangled, and the viscoelasticity of the system becomes stronger than that of the CTAB:NapTS/W system at the same CD (dCS) value. On the other hand, the viscoelasticity of the system at a pH of 6.0 is much weaker than that of the CTAB:NapTS/W system. Figure 3B shows the dependence of G′ and G′′ on ω for the CTAB:NapTS:NaPVB/W system with a CD of 25 mM, CS of 10 mM, and CP of 15 mM. The polymer included in the system is the same as that in Figure 3A, and the pH value was varied widely. Solid and broken lines in the figure have the same meaning as in Figure 3A. The viscoelasticity of the system is apparently weak at a pH of 2.6, whereas the solution shown in Figure 3A shows high viscoelasticity at the same pH value. At pH values (24) Shikata, T.; Hirata, H.; Kotaka, T. Langmuir 1987, 3, 1081. (25) Imai, S.; Shikata, T. Langmuir 1999, 15, 7993. (26) Imai, S.; Kunimoto, E.; Shikata, T. J. Soc. Rheol. Jpn. (Nihon Reoroji Gakkaishi) 2000, 28, 63. (27) Ferry, J. D. Viscoelastic Properities of Polymers, 3rd ed.; John Wiley: New York, 1980.

Langmuir, Vol. 19, No. 21, 2003 8657

Figure 3. (A) Dependence of G′ and G′′ on ω for the CTAB: NapTS:NaPVB/W system with Mn ) 1.0 × 105, CD ) 25, CS ) 17.5, and CP ) 7.5 mM at various pH values. (B) Dependence of G′ and G′′ on ω for the same system as A with CD ) 25, CS ) 10, and CP ) 15 mM.

close to the pKa of PVBA, the system shows relatively strong viscoelastic behavior like that of the ordinary CTAB: NapTS/W threadlike micellar system. On the other hand, the viscoelasticity of the system is very weak at a pH of 6.0, as observed for the solution shown in Figure 3A at the same pH value. NaPVB Concentration Dependence. The change in viscoelasticity of the CTAB:NapTS:NaPVB/W system at an Mn of 3.6 × 105, CD of 25 mM, and CS of 17.5 mM was investigated as a function of CP at 3 pH values: 2.6, 3.6, and 5.2. Because the solution shows weak viscoelasticity at a CP of 0 mM, without a clear plateau in the G′ curve, we could only determine a set of the longest relaxation time (τl) and steady state compliance (Je). The reason for the weak viscoelasticity of this solution is that the threadlike micelles were not long enough to form a fully entangled network.24-26 Some viscoelastic spectra obtained for systems with relatively low CP values also do not show plateaus in G′ curves. In these cases, the values of τl and Je-1 have been accepted as substitutes for τ and GN. The relationships between CP and both τ and GN for the system are shown in Figure 4A and 4B, respectively. At pH lower than the pKa of the polymer, the τ of the CTAB:NapTS:NaPVB/W system increases with increasing CP in the range of CP < 5 mM, reaches its maximum value (dependent on pH) at a CP of ∼5 mM, and slightly decreases as CP increases beyond 5 mM (Figure 4A). The thick solid

8658

Langmuir, Vol. 19, No. 21, 2003

Nakamura et al.

Figure 5. Schematic representation of a phantom crossing model24,26 for an entanglement release mechanism in threadlike micellar systems.

Figure 4. (A) Relationship between relaxation time, τ, and CP for a CTAB:NapTS:NaPVB/W system with Mn ) 3.6 × 105, CD ) 25, CS ) 17.5, and CP ) 7.5 mM at various pH values. Data for the CTAB:NapTS/W system with CD ) 25, CS ) 17.5 mM, and various CS′ are also plotted in the same figure. (B) Dependence of plateaus modulus, GN, on CP for the same systems shown in A.

line in Figure 4A implies dependence of τ on CS′ ()CS -17.5 mM) for the ordinary CTAB:NapTS/W threadlike micellar system. The CTAB:NapTS:NaPVB/W and CTAB: NapTS/W systems exhibit qualitatively similar dependence of τ on CP and CS, but the maximum value of τ in the CTAB:NapTS/W system does not correspond to that of the CTAB:NapTS:NaPVB/W system. The value of GN increases with increasing CP; at low pH, the rate of increase (slopes in Figure 4B) changes at a CP of around 5 mM. The maximum values of τ are as shown in Figure 4A. However, the slope of the relationship between GN and CP is constant at a pH of 5.2. Moreover, solutions at pH values of 2.6 and 3.6 have identical slopes, even at CP values greater than 5 mM; the slope for the solution at a pH of 5.2 provides the relationship GN ∝ CP. Discussion Relaxation Mechanism for Hybrid Threadlike Micelles. The viscoelastic behavior of the system is most striking at pH < pKa (Figure 4A and B). Increases in GN

with increasing CP for the CTAB:NapTS:NaPVB/W system up to a CP of 5 mM implies an increase in the density of entanglements, Fe, between hybrid threadlike micelles caused by the growth of micellar length. Because the viscoelastic spectra for the system are relatively sharp and not very different from that of the Maxwell model at low frequency with a CP greater than 2.5 mM, the relaxation mechanism for entanglement release in the system is likely controlled by a phantom crossing mechanism24-26 between the hybrid threadlike micelles, as is the case with ordinary threadlike micelles. The phantom crossing model schematically depicted in Figure 5 accurately describes the unique Maxwell-modeltype viscoelastic behavior observed in the ordinary CTAB: NapTS/W threadlike micellar system at CD e CS.25,26 In the model, it is assumed that two entangled threadlike micelles form a temporary branch, followed by a crossingthrough reaction after the lifetime of the entanglement point, which is identical to relaxation time, τ.24 According to the model,25,26 free pTS- in bulk aqueous phase acts as a catalyst for the crossing-through reaction of threadlike micelles; thus, the lifetime of the entanglement point is not a function of CD but of the concentration of free pTS-, CS*. In the CTAB:NapTS/W system, τ decreases with increasing CS* (Figure 4A) in the range of CS′ > 5 mM, where CD ) 25, CS + CS′ > 22.5 mM, and free pTS- appears in bulk aqueous phase.25,26 As discussed above, the interaction between CTA+ and PVB- is much stronger than that between CTA+ and pTS-. However, most PVB- is protonated to form PVBA when the pH value of the system is lower than the pKa of the polymer. In preliminary 1H NMR experiments, the chemical shift of ring protons belonging to pTS- moved toward that of free pTS- in bulk aqueous phase with increasing CP in the CTAB:NapTS:NaPVB/D2O system. This suggests that the interaction between CTA+ and PVBA is also stronger than that between CTA+ and pTS-; consequently, the pTS- ions in threadlike micelles are replaced by the added PVBA. Thus, the increase in CP of the CTAB:NapTS:NaPVB/W system at pH lower than the pKa of the polymer induces an increase in the concentration of free pTS- in bulk aqueous phase, CS*, at CP greater than 5 mM and a CD of 25 mM. The decrease in τ of the system at a pH of 2.6 or 3.6 (Figure 4A) is likely controlled by the same mechanism as in the CTAB:NapTS/W system. The maximum value of τ is highly dependent on Mn of PVBA in the CTAB: NapTS:NaPVB/W system, even at the CD, CS, CP (equal to CS*), and pH shown in Figure 2. This suggests that the presence of longer PVBA in the hybrid threadlike micelle increases the value of τ necessary for the micelle to undergo the crossing-through relaxation.

Hybrid Threadlike Micelle Formation

Because the probability of the presence of PVBA is equal for all hybrid threadlike micelles, every entanglement point has the same probability of the presence of PVBA. Therefore, the fact that the incorporation of longer PVBA increases the value of τ strongly suggests that the presence of polymers at entanglements effectively depresses the rate of the crossing-through reaction of the micelles, as predicted above in the Introduction. It is to be expected that the mobility of PVBA along hybrid threadlike micelles is highly dependent on Mn. The longer that PVB- remains at the entanglement point, the greater the value of τ. The strength of interaction between CTA+ and PVBA is another factor controlling the residence time of the polymer at an entanglement point. When the pH is similar to the pKa, the strength of interaction between CTA+ and PVBA (and PVB-) is apparently optimum, and the value of τ at an Mn of 3.6 × 105 is 4 times the value for the ordinary CTAB:NapTS/W threadlike micellar system (Figure 4B). These findings suggest that the presence of longer polymer molecules in hybrid threadlike micelles effectively hinders the crossing-through reaction between hybrid threadlike micelles at entanglement points. The magnitude of GN for the CTAB:NapTS:NaPVB/W system at a CP greater than 5 mM is greater than that of the ordinary CTAB:NapTS/W threadlike micellar system and appears to increase with increasing CP, when the pH is lower than the pKa (Figure 4B). Because all added PVBA is incorporated into the micellar interior in the CP range examined, the increase in the magnitude of GN indicates that the rigidity of hybrid threadlike micelles increases as the amount of PVBA incorporated into micelles increases. Interaction between CTA+ and PVBA at a pH of 3.6 should be stronger than observed at a pH of 2.6, because the latter pH value is closer to the pKa, where the systems segregate precipitation. Consequently, as the interaction between CTA+ and PVBA becomes stronger, the hybrid threadlike micelle becomes more rigid. Moreover, the rigidity of hybrid threadlike micelles increases with increasing Mn of incorporated PVBA and saturates at an Mn of around 1.0 × 105 at a pH of 3.1 (Figure 2). On the other hand, τ does not increase with increasing CP at a pH of 5.2, which is higher than the pKa of the CTAB:NapTS:NaPVB/W system (Figure 4A). The magnitude of GN is apparently proportional to CP at high values of CP (Figure 4B). In a study by Kline,17 polymerized CTAVB formed short hybrid threadlike micelles in aqueous solution. In the CTAB:NapTS:NaPVB/W system at a pH of 5.2 (higher than the pKa), the state of dissociation of PVB- is not very different from that observed in the study by Kline. Consequently, highly dissociated PVBand CTA+ form short hybrid threadlike micelles irrespective of the presence of NapTS at a pH greater than the pKa, because the interaction between CTA+ and dissociated PVB- is very much stronger than that between CTA+ and pTS-. At a pH of 5.2, the CTAB:NapTS:NaPVB/W system likely contains a mixture of two kinds of short threadlike micelles: ordinary threadlike micelles formed by CTA+ and pTS- and hybrid threadlike micelles formed by CTA+ and PVB-. The short hybrid threadlike micelles relax the anisotropy of their orientation via the rotational mode in a viscoelastic matrix formed with ordinary threadlike micelles. The increase in the magnitude of GN, proportional to CP, shown in Figure 4B corresponds well to the increase in the number of short hybrid threadlike micelles. The slight decrease in τ with increasing CP shown in Figure 4A suggests that the decrease in the viscosity of the matrix formed by ordinary threadlike micelles is due to the

Langmuir, Vol. 19, No. 21, 2003 8659

decrease in the number of threadlike micelles caused by plundering of CTA+ by added PVB-. Optimum Interaction for Hybrid Threadlike Micelle Formation. CTAB and NaPSS do not form hybrid threadlike micelles in aqueous solution but segregate precipitation.14-16 The reason NaPSS does not form threadlike micelles with CTAB is apparently the remarkable reduction of molecular motion of monomer units because of polymerization, because the monomer, styrenesulfonate, forms ordinary threadlike micelles with CTA+. The rotational motion of additives and surfactants in threadlike micelles is about 10-9 s.25 Therefore, we speculate that it is necessary to provide sufficient freedom to CTA+ and polymeric additives for quick molecular motion in micelles when constructing stable hybrid threadlike micelles between CTAB and polymeric additives. Because polymerization of monomers reduces the mobility of monomers, the only thing that can be done to provide sufficient mobility to the monomers of polymeric additives and surfactants in micelles is reduction of the strength of interaction between the monomer and surfactant. In the CTAB:NapTS:NaPVB/W system, there is welldefined electrostatic, hydrophobic, and cation-π8,9 interaction between CTA+ and PVBA or PVB-. The cation-π interaction, which is not very strong but is essential in the aqueous threadlike micellar system formed between CTAB and aromatic additives such as NapTS26 and NaSal,24 exists irrespective of the presence of electric charges on the polymeric additives. The electrostatic interaction can easily be governed by controlling pH, because NaPVB bears carboxylate groups. The hydrophobic interaction exists under all conditions and is not controllable. When the pH is much higher than the pKa in the CTAB: NapTS:NaPVB/W system with CS g 7.5 mM, NaPVB completely dissociates into PVB-, so that the system segregates precipitation (see Table 2) because of the strong electrostatic interaction at a pH of 7, as in an aqueous solution of CTAB and NaPSS. On the other hand, when the pH is lower than the pKa (e.g., pH ∼ 3), PVB- is completely protonated to PVBA in the micelle formed; thus, the essential interaction in the micelle other than the hydrophobic interaction is the cation-π interaction between CTA+ and a phenyl ring of PVBA. The fact that the system segregates precipitation at a pH around 3.5 (Table 2) suggests that the magnitude of cation-π interaction is pH-dependent and that it reaches its maximum at a pH value near the pKa. The importance of the cation-π interaction for hybrid threadlike micelle formation is easily demonstrated by the following simple experiment. Sodium poly(acrylate) (NaPA) is a polyelectrolyte bearing carboxylate groups (like PVB-), but it has no phenyl rings. Thus, PA- does not have cation-π interaction with CTA+. Hydrophobic interaction in an aqueous CTAB and NaPA system (CTAB: NaPA/W) is relatively weak because NaPA is soluble in water irrespective of pH. When the pH value of the CTAB: NaPA/W system is greater than 4, the system segregates precipitation; the pKa value of poly(acrylic acid) (PAA) is 5.8.28 On the other hand, at a pH less than 3, the CTAB: NaPA/W system is a perfectly transparent liquid with very low viscosity. Under these conditions, PA- is protonated to PAA. 1H NMR spectra for the CTAB:NaPA/ D2O system at a pH of 3 reveal that PAA dissolves independently without interaction with micelles formed (28) Philippova, O. E.; Hourdet, D.; Audebert, R.; Khokhlov, A. R. Macromolecules 1997, 30, 8278.

8660

Langmuir, Vol. 19, No. 21, 2003

Nakamura et al.

possible to construct “pure” hybrid threadlike micelles consisting only of surfactant and polymer molecules. Formation of such “pure” micelles requires polymeric additives that interact appropriately with surfactant molecules, providing relatively high mobility to monomer units and surfactant molecules in the hybrid threadlike micelles. Concluding Remarks

Figure 6. 1H NMR spectra for the CTAB:NaPA/D2O system at CD ) CP ) 25 mM and pH ) 3, the PAA/D2O system at CP ) 25 mM, the NaPA/D2O system at CP ) 25 mM, and the CTAB/D2O system at CD ) 25 mM. The PAA used here has the number average molecular weight, Mn, higher than 4.5 × 104 and the Mw/Mn ratio higher than 3.0.

by CTAB, because the values of chemical shift for CTA+ and PAA show little change when CTAB and NaPA are mixed in aqueous solution (Figure 6). These findings suggest that the absence of cation-π interaction between CTA+ and PAA is the main factor responsible for the poor hybrid threadlike micelle formation in the CTAB:PAA/W system. Finally, in the CTAB:NapTS:NaPVB/W system, addition of NapTS as an assistant agent and control of pH are necessary to construct hybrid threadlike micelles long enough to form entanglement networks. However, it is

Novel hybrid threadlike micelles were constructed from surfactant and polymer molecules in an aqueous system of cetyltrimethylammonium bromide (CTAB), sodium p-toluensulfonate (NapTS), and sodium poly(p-vinylbenzoate) (NaPVB) (CTAB:NapTS:NaPVB/W). When the NapTS concentration (CS) is greater than 60% of the CTAB concentration (CD), the system constructs hybrid threadlike micelles long enough to become entangled. When the pH is lower than the pKa of the polymer, the viscoelastic relaxation time and strength of the system become greater than those of the ordinary threadlike micellar system without polymers (CTAB:NapTS/W). Incorporation of protonated PVBA into the hybrid threadlike micelle effectively hinders the crossing-through reaction between micelles at entanglement points. The presence of PVBA in the hybrid micelle makes the rigidity of the hybrid threadlike micelle greater than that of the ordinary threadlike micelle. Hybrid threadlike micelles are constructed in the CTAB: NapTS:NaPVB/W system by reduction of electrostatic interaction between CTA+ and incorporated polymer molecules, at pH values lower than the pKa. LA030101L