#2009 The Society of Polymer Science, Japan

Micellization Behavior of an Amphiphilic Statistical Copolymer in -Methanol

By Takuya MORI,y Akihito HASHIDZUME, and Takahiro SATOÃ

Sedimentation equilibrium measurements were made on an amphiphilic statistical copolymer p(AMPS/C6) made of sodium 2-(acrylamido)-2-methylpropane-sulfonate (AMPS) and n-hexyl methacrylate (C6), dissolved in water-methanol mixtures with 0.2 M LiClO4 and different methanol contents over a wide polymer range. The experimental results were analyzed in terms of a micellar theory for amphiphilic polyelectrolytes to estimate the aggregation number m of the micelle and the internal free energy change Ám=m (per polymer chain) due to micellization. Both m and Ám=m decreased with increasing the methanol content in the . In addition, dynamic light scattering for the same copolymer system indicated that p(AMPS/C6) formed a uni-core star-like micelle in water-methanol with the methanol fractions of 0 and 0.06. KEY WORDS: Micelle / Amphiphilic Polymer / Statistical Copolymer / Sedimentation Equilibrium / Dynamic Light Scattering /

Amphiphilic copolymers form micelles in aqueous media solution theory, we have estimated m and also the internal free due to the strong hydrophobic interaction among the polymer energy change Ám=m due to micellization as functions of the chains.1 The micellization is important for their applications to methanol content, from the sedimentation equilibrium data. On the rheology modifier, emulsifier, drag delivery system, and so the other hand, we have analyzed the dynamic light scattering on,2–4 because the micellization changes remarkably their results to elucidate the micellar structure in aqueous and properties. In the practical applications for those methanol . It was found that the micellization behavior purposes, their aqueous solutions are often mixed with various of p(AMPS/C6) was very sensitive to the addition of small additives, e.g., low molar organic compounds, salts, amount of methanol, which weakened the hydrophobic surfactants, and other polymers. Those additives can modify interaction among hexyl groups on copolymer chains. the micellization behavior of the polymers in aqueous media by interacting with the hydrophobic or hydrophilic moieties of the EXPERIMENTAL copolymer chain, so that it is essentially important to under- stand the effect of additives on the micellization behavior in p(AMPS/C6) Sample and Test Solutions aqueous solutions of amphiphilic copolymers. A statistical copolymer sample of p(AMPS/C6) was Recently, Hashidzume et al.5 investigated the micellar obtained by the reversible addition-fragmentation chain trans- structure of an amphiphilic statistical copolymer p(AMPS/C6), fer (RAFT) polymerization,7–9 as reported previously.5 The made of sodium 2-(acrylamido)-2-methylpropane-sulfonate sample was purified by reprecipitation and dialysis, and then (AMPS) and n-hexyl methacrylate (C6), in aqueous salt freeze-dried from aqueous solution. solution. The copolymer aggregation number m of the micelle The weight-average molecular weight Mw, the ratio Mz=Mw and the number of hydrophobic cores per micelle depended on of the z-average molecular weight to Mw, and the second the copolymer molecular weight and hydrophobic monomer virial coefficient A2 of the copolymer sample obtained were  content. Furthermore, their hydrodynamic radius data indicated determined in methanol with 0.2 M LiClO4 at 25 Cby that the copolymer formed a uni-core star-like micelle at low sedimentation equilibrium (cf. below). The x of copolymer molecular weights. the hydrophobic monomer units in the copolymer sample was 6 1  5 Afterward, Nojima et al. refined the data analysis for the estimated by H NMR using D2O as the solvent at 30 C. All p(AMPS/C6) micelle in aqueous salt solution by taking into the results are listed in Table I. The Mz=Mw value indicates a account the association-dissociation equilibrium in the micellar narrow molecular weight distribution of the sample prepared solution. Using a micellar solution theory for amphiphilic by the RAFT polymerization. polyelectrolytes, they estimated more precise m for the Water, methanol, and their mixtures with different compo- p(AMPS/C6) micelle in aqueous salt solution. sitions all of which contained 0.2 M LiCliO4 were used as In the present study, we have made sedimentation equilibrium . The freeze-dry sample of p(AMPS/C6) was directly and dynamic light scattering measurements on p(AMPS/C6) dissolved in those solvents. In what follows, the solvent dissolved in water-methanol mixtures with 0.2 M LiClO4 and composition is expressed in terms of the mole fraction xmethanol different methanol mole fraction xmethanol. Applying the micellar of methanol in the solvent.

Department of Macromolecular Science, Osaka University, 1-1 Machikaneyama-cho, Toyonaka 560-0043, Japan yPresent Address: Sumitomo Chemical Co. Ltd., 1-98, Kasugadenaka 3-chome, Konohana-ku, Osaka 554-8558, Japan ÃTo whom correspondence should be addressed (Tel: +81-6-6850-5461, Fax: +81-6-6850-5461, E-mail: [email protected]).

Polymer Journal, Vol. 41, No. 3, pp. 189–194, 2009 doi:10.1295/polymj.PJ2008223 189 T. MORI,A.HASHIDZUME, and T. SATO

Table I. Molecular characteristics of copolymers in methanol with 0.2 M LiClO4

4 a) a) b) À3 3 À2 a) sample Mw=10 Mz=Mw x A2=10 mol cm g p(AMPS/C6) 1.69 1.17 0.15 1.00

The above data were determined by a) sedimentation equilibrium or b) 1H NMR.

Measurements Sedimentation equilibrium measurements on p(AMPS/C6)  solutions with different xmethanol were performed at 25.0 C using an Optima XL-I type ultracentrifuge (Beckman-Coulter) equipped with a Rayleigh interference optical system with a 675 nm light emitting from a diode laser. The concentration distribution in the solution under the centrifugal field was determined by the interferometry using the specific refractive index increment ð@n=@cÞ at constant solvent chemical poten- tial. The apparent molecular weight Mapp was calculated from10,11

2 2 2 À1 ! ðrb À ra Þc0ð@=@cÞ Mapp ¼ 2RTðcb À caÞ

where rb and ra are the distance from the center of revolution to the cell bottom and meniscus, respectively, cb and ca are polymer mass at rb and ra respectively, ! is the angular velocity, c0 is concentration of solution before centrifugation, and ð@=@cÞ is the specific increment at constant solvent ; RT is the Figure 1. Concentration dependence of the apparent molecular weight Mapp obtained by sedimentation equilibrium for the amphiphilic statis- multiplied by the absolute . In dilute methanol tical copolymer p(AMPS/C6) dissolved in water-methanol mix- solutions where p(AMPS/C6) is molecularly dispersed,5 the tures with 0.2 M LiClO4 and different methanol mole fraction x . (a) Data in a dilute region; (b) data in a wide concen- À1 methanol equation Mapp ¼ 1=Mw þ 2A2c holds where c ¼ðca þ cbÞ=2, tration range; Curves: the micellar solution theory for amphiphilic 12 polyelectrolytes (see the text) to determine the aggregation and we can estimate Mw and A2. Using the established number m and the internal free energy change Ám =m due to the procedure, we can also estimate Mz=Mw from sedimentation micellization. equilibrium data for dilute solutions.13 Values of ð@n=@cÞ and ð@=@cÞ of p(AMPS/C6) at different xmethanol were measured using solutions dialyzed a Nd:YAG laser was used as an incident light, and the scattered against their solvents at 25 C for a week. A modified Schulz- light was measured with no analyzer. Test solutions for light Cantow type differential refractometer (Shimadzu) and a scattering measurements were optically purified by filtration DMA5000 densitometer (Anton Paar) were used for refrac- through a 0.8 mm membrane filter, followed by a 3 h centrifu- tometry and densitometry. The results summarize in Table II. gation at 9000 rpm prior to measurements. The intensity The partial specific  of p(AMPS/C6) at each xmethanol, autocorrelation function obtained by dynamic light scattering calculated from  ¼½1 Àð@=@cÞŠ=0 (0: the solvent den- was analyzed by a CONTIN program to estimate the spectrum sity) is also listed in Table II. Að; kÞ of the relaxation time  in the logarithmic scale at each Dynamic light scattering measurements for copolymer scattering angle or the magnitude of the scattering vector k. solutions at xmethanol ¼ 0 and 0.06 were performed using an ALV/DLS/SLS-5000 light scattering system equipped by an RESULTS ALV-5000 multiple  digital correlator at 25 C. Vertically À1=2 polarized light with the wavelength 0 of 532 nm emitted from Figure 1 shows plots of Mapp vs. c obtained by sedimentation equilibrium for p(AMPS/C6) in water-methanol mixtures with 0.2 M LiClO4. In Panel a showing a dilute Table II. Values of ð@n=@cÞ and ð@=@cÞ for p(AMPS/C6)   region, data points at xmethanol ¼ 1 (filled circles) almost follow in methanol-water mixtures at 25 C the straight line, from which we have determined Mw and A2, x 0 0.06 0.13 0.31 1 methanol already shown in Table I. With decreasing xmethanol, data points À1 3 @n=@c (675 nm)/g cm 0.128 0.142 0.147 0.146 0.146 shift downward at xmethanol < 0:31, indicating the micellization @=@c 0.211 0.320 0.334 0.403 0.599 of p(AMPS/C6) chains. Although data points at xmethanol ¼ 0 3 À1 /cm g 0.782 0.684 0.682 0.637 0.499 (filled squares) slightly scatter, they seem to obey a curve

190 #2009 The Society of Polymer Science, Japan Polymer Journal, Vol. 41, No. 3, pp. 189–194, 2009 Micellization Behavior of Amphiphilic Statistical Copolymers

"#"# X  X wslowMslow  Mapp lim Að; kÞ Að; kÞ k!0 2slow 2all

Here Mslow is the of the slow relaxation component, the lower bound of which may be estimated from the hydrodynamic radius of the slow relaxation component (obtained from Að; kÞ of the component) in the procedure 5 applied in the previous study. The upper bound of wslow obtained by the above method was less than 0.5% both at xmethanol ¼ 0 and 0.06. Therefore, although the slow relaxation component was detectable by light scattering due to its very high scattering power, it may not affect sedimentation equilib- rium results. At present, we have no information about the origin of the coexistence of a tiny amount of large aggregtaes.

DISCUSSION

Theoretical Basis In previous publications,5,6 we have already demonstrated that p(AMPS/C6) forms a uni-core micelle in aqueous media if the molecular weight is low enough. We here adopt the same uni-core micelle model for our p(AMPS/C6) sample; the suitability of the model will be checked in the following. For star-like micelles, the internal free energy change Ám=m (per polymer chain) due to micellization is written as15 2 2 Ám=m ¼ðÀm þ m þ 4Rc Þ=m ð1Þ Figure 2. (a) Relaxation time spectra Að; kÞ obtained by dynamic light scattering for a p(AMPS/C6) solution with xmethanol ¼ 0:06 and m  c ¼ 0:0018 g/cm3 at three different scattering angles; (b) Con- where is the aggregation number, is the free energy gain centration dependence of the diffusion coefficient (of the fast when a hydrophobic moiety of a polymer chain enters in the relaxation component) for p(AMPS/C6) dissolved in water-meth- micelle core from the solvent,  is the free energy due to anol mixtures with 0.2 M LiClO4 at xmethanol ¼ 0 and 0.06. interaction between two coronal chains of the micelle, and the third term is the interfacial free energy of the micelle core (Rc: convex downward. In Panel b of a wider concentration range, the radius of the micellar core; : the interfacial tension). The À1=2 2 1=2 Mapp monotonically increases with c. Even at c > 0:1 g/ optimum aggregation number is equal to ð4Rc =Þ , and in cm3, solution viscosities were still low and gelation was not what follows, we consider the monodisperse micellar solution observed for all solutions examined. with this optimum aggregation number. Figure 2a illustrates examples of the relaxation time spectra Under the micellization equilibrium, the association con- Að; kÞ obtained by dynamic light scattering (xmethanol ¼ 0:06 stant Km is determined by the equilibrium condition m ¼ 3 and c ¼ 0:018 g/cm ). The spectra are bimodal, being similar m1, where m and 1 are the chemical potentials of m-mer to those reported previously for other p(AMPS/C6) samples in and unimer, respectively. Thus, we need chemical potential 0.1 M aqueous ,5 and indicating the existence expressions applicable over a wide polymer concentration of two scattering components with fast and slow relaxation range. Here, we use the single-contact approximation to the 11 times. Most of solutions with xmethanol ¼ 0 and 0.06 exhibited micellar system, and apply Sato et al.’s expression of m similar bimodal Að;kÞ. given by The mutual diffusion coefficient Dfast of the fast   m ¼ m þ ln C þ F0 ðÞþmM F~ðÞð2Þ relaxationP component was estimated by Dfast ¼ m 5 1 À2 À1 14 kBT kBT limk!0 k 2fast  Að;kÞ. Figure 2b shows the results of  Dfast for solutions with xmethanol ¼ 0 and 0.06, where kB is the where, m and Cm are the standard chemical potential (or the , 0 is the solvent viscosity coefficient, and internal free energy) and the molar concentration of m-mer, 0 kBT=60Dfast corresponds to the apparent hydrodynamic respectively, M1 is the molecular weight of unimer, and F 5ðÞ radius. It can be seen that kBT=60Dfast is a slowly decreasing and F~ðÞ are known functions of the total volume frcation  0 function of c at xmethanol ¼ 0, but almost independent of c at of the polymer; F 5ðÞ necessary in what follows is defined xmethanol ¼ 0:06. by On the other hand, we may estimate (the upper bound of) the 5 À 32 weight fraction w of the slow relaxation component in F0  À lnð1 À Þð3Þ slow 5 3ð1 À Þ2 dilute solution using the approximated equation14

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(We have neglected an attractive interaction term of the chain where M 0 is the average monomer molar mass, x is the content 11 end. ) The chemical potential of unimer 1 can be calculated of hydrophobic monomer unit, and el and v are the binary by eq 2 with m ¼ 1. The above expression of m was cluster integrals with respect to the electrostatic and hydro- originally derived on the basis of the scaled particle theory phobic interactions, respectively. The theory of polyelectro- 16,17 for polydisperse wormlike spherocylinders, and may be a good lytes provides us the following equation of el approximation to polymer systems with lower molar mass and 2h2 lower degree of branching. ¼ RðyÞð9Þ el  Applying this m to the equilibrium condition, we have the following equation Here, h is the contour length per monomer unit,  is the reciprocal of the Debye screening length, and y  Cm ð1 À w1Þ=m K  ¼ 2 Àd m m m mÀ1 2effQe = with the effective charge density eff and the C1 w1 ð1000c=M1Þ ð4Þ Bjerrum length Q. In this study, eff was calculated based on ¼ exp½ÀÁ Àðm À 1ÞF0 ðފ m 5 the Philip-Wooding theory18 for charged cylinder model. The Here, w1 is the weight fraction in the total polymer, M1 ¼ function RðyÞ is given in ref 16. Taking into account the 4   1:69  10 in our system, and Ám ðm À m1 Þ=kBT is the contribution of counter ions of the copolymer to the ionic internal free energy change due to micellization being identical strength, we calculate  by with that given in eq 1 except for a constant term. For given m,  2 Cs 1 À x Ám, c, and , we can calculate w from the above equation.  ¼ 8QNA þ c ð10Þ 1 1000 2M Sato et al.11 also formulated the apparent molecular weight 0 Mapp obtained by sedimentation equilibrium, which is written as The mutual diffusion coefficient Dfast in micellar solutions at 15,19 À1 À1 finite concentrations may be written as Mapp ¼ Mw þ 2ðÀ2;0 þ A2,wÞc ð5Þ k T M ð1 þ k0 c=cÃÞ where M is the weight-average molar mass, À is the B ¼ app H R ð11Þ w 2;0 6 D M ð1 À cÞ H apparent second virial coefficient for the hard-core potential, 0 fast w 0 and A2,w is the second virial coefficient with respect to the where  is the polymer partial specific volume, k H is the electrostatic and hydrophobic interactions. (The original paper strength of the intermolecular hydrodynamic interaction (cor- dealt with the quantity ZðrÞ, which can be identified with responding to the Huggins coefficient for the polymer solution À1 à Mapp in a good approximation.) The coefficient À2;0 is viscosity), c is the overlap concentration, and RH is the calculated by average hydrodynamic radius given by   2 À1 d NA F6 F2 F7 RH w1M1 ð1 À w1ÞmM1 À2;0 ¼ þ þ ¼ þ ð12Þ 4M dM M M M R g R ðlinearÞ L L w n  ð6Þ w H;1 H;m H;m 1 1 À À F ð1 þ F Þ Here, RH;1 and RH;m (linear) are the hydrodynamic radii of the M M 2 2 n w linear chains with the molar of M1 and mM1, where d are ML are the hard-core diameter and the molar mass respectively, and gH;m is the g-factor with respect to the per unit contour length of the polymer chain, respectively, Mn hydrodynamic radius for the m-mer, depending on the branch- à is the number average molar mass, and NA is the Avogadro ing architecture. The overlap concentration c may be 15 constant. The functions F2, F6, and F7 are defined by calculated by À  þ 2 à 3 8 7 3 c ¼ 3Mw=4NAR ð13Þ F2  ; H 3ð1 À Þ3 1 þ  F6  2 ; ð7Þ Analysis of Experimental Results ð1 À Þ4  Using the above theoretical expressions, we analyze our 2 4 þ  þ 2 experimental Mapp and Dfast results for p(AMPS/C6) solutions. F7  4 3 ð1 À Þ To calculate Mapp as a function of c, we need the following The of the total polymer can be calculated parameters: m, Ám, d, ML, h, eff, and v. Among them, we 2 20 by  ¼ d NAc=4ML, while the average molar masses by have h ¼ 0:25 nm for vinyl polymers, and ML ¼ M 0=h ¼ À1 À1 Mw=M1 ¼ w1 þð1 À w1Þm and M1=Mn ¼ w1 þð1 À w1Þm . 835 nm . The effective charge density eff can be calculated À1 À1 It is noted that the equality between Mapp and the reciprocal from the linear charge density [¼ð1 À xÞ=h ¼ 3:4 nm ] and À1 18 of the osmotic compressibility ðRTÞ @Å=@c does not strictly d. Thus the remaining unknown parameters are m, Ám, d, hold for multicomponent polymer solutions. and v. On the other hand, A2,w for amphiphilic copolymers may be At xmethanol ¼ 1 and 0.31, where the micelle is not formed, calculated by Mapp is determined only by d and v.Ifd is calculated from the experimental partial specific volume  (cf. Table II) NA A ¼ ½ð1 À xÞ2 þ x2 Šð8Þ 1=2 2,w  2 el v by d ¼ð4ML=NAÞ , we can fit eq 5 to the Mapp data 2M0 3 at xmethanol ¼ 1 and 0.31 with v ¼À0:3 nm . Even at

192 #2009 The Society of Polymer Science, Japan Polymer Journal, Vol. 41, No. 3, pp. 189–194, 2009 Micellization Behavior of Amphiphilic Statistical Copolymers

Table III. Values of parameters determined from Mapp data

3 xmethanol d/nm v /nm m Ám =m 0 1.17 À0:84À6:87 0.06 1.5 À0:32À4:35 0.13 1.5 À0:32À3:80 0.31 & 1 0.96 À0:3— —

xmethanol ¼ 0 where micelle is formed, Mapp in a concentrated region is little dependent on m and Ám, so that we can similarly determine values of d and v by fitting high concentration Mapp data. Values of m and Ám were determined afterward by fitting lower concentration Mapp data. At xmethanol ¼ 0:06 and 0.13, we had to choose d values Figure 3. Comparison of experimental data of the diffusion coefficient for larger than those estimated from  to fit eq 5 to high p(AMPS/C6) micelles at xmethanol ¼ 0 and 0.06 with the theory for the f -arm star-like micelle. concentration data of Mapp. This may have something to do with the of methanol to the p(AMPS/C6) chain. 1=4 À À ð À Þ f ffiffiffi 1 0:068 0:0075 f 1 Here, we assumed that v at xmethanol ¼ 0:06 and 0.13 is equal gH;m ¼ p ½2 À f þ 2ð f À 1ފ1=2 1 À 0:068 to that at xmethanol ¼ 1 and 0.31, and determined d to fit eq 5 to ð14Þ Mapp data at high c. Values of m and Ám were determined from Mapp data at low c. Finally, linear flexible homopolymers in good solvents take 0 As shown by solid curves in Figure 1, we can satisfactorily values of 4–7 for the strength k H of the intermolecular 15 0 fit the theoretical curve to experimental data at every xmethanol. hydrodynamic interaction. Choosing k H ¼ 8 which is slight- Values of d, v, m, and Ám=m used in the fittings are listed in ly larger than those for linear polymers, we obtain solid and Table III. The aggregation number m is 4 at xmethanol ¼ 0, but dashed curves in Figure 3. Here, the solid and dashed curves reduces to 2 at xmethanol ¼ 0:06 and 0.13, and micellization does correspond to theoretical values for f ¼ m and 2m, respec- not take place at xmethanol > 0:3. Furthermore, Ám=m (the free tively. Experimental data points at both xmethanol ¼ 0 and 0.06 energy change per polymer chain due to micellization) are between the solid and dashed curves. This demonstrates increases with increasing xmethanol. Destabilization of the that the micelle of p(AMPS/C6) is star-like and its hydro- micelle by addition of methanol arises from diminishing the phobic core consists of hexyl groups attaching to random hydrophobic interaction. From the same reason, we can expect positions of the copolymer chain. an increase of the repulsion among coronal chains and then of  (the interaction free energy between two coronal chains) in Comparison with the Corresponding Low-Molar Mass eq 1 as xmethanol increases, which may lead the reduction of the Amphilile 2 1=2 23 optimum aggregation number [¼ð4Rc =Þ ]. Recently, Ruso et al. investigated the micellization The diffusion coefficient Dfast for the micellar solution is behavior of n-hexyltrimethylammonium bromide (C6TAB) in calculated by eqs 11–13, which include RH;1, RH;m (linear), aqueous 0.2–0.7 M NaBr, of which hydrophobic moiety is 0 gH;m, and k H. In general, the hydrodynamic radius RH of linear the same as p(AMPS/C6). The aggregation number reported polymers has a power-law dependence on the degree of for C6TAB (3–4) is close to our aggregation number of 0 aH 0 polymerization N0: RH ¼ K HN0 with two parameters K H p(AMPS/C6) chains in aqueous LiClO4, but C6TAB forms the and aH characteristic to the polymer-solvent system. The N0 micelle only at high concentrations (0.4–0.6 M of C6TAB). For dependence of RH of p(AMPS/C6) with x ¼ 0:15 at xmethanol ¼ p(AMPS/C6), the micelle is formed at concentration as low as 0 and 0.06 should be similar to that for the AMPS 0.002 g/cm3, which corresponds to 10À3 M of the hexyl group. homopolymer in aqueous solution,5 but due to the hydrophobic The remarkable stabilization of the p(AMPS/C6) micelle may interaction among hexyl groups within the chain the solvent be owing to (1) hexyl groups are concentrated within the 0 quality may be more or less reduced. We have used K H copolymer chain domain and (2) the charged sulphate group is (¼ 0:22 nm) and aH (¼ 0:58) values of the AMPS homopol- apart from the hexyl group. Although the aggregation number ymers in 0.5 M aqueous NaCl which may be a slightly poorer of p(AMPS/C6) chains is four, the core of the copolymer 21 solvent than 0.2 M aqueous LiClO4. micelle may consist of hexyl groups more than four, which can If near one of p(AMPS/C6) chain ends are stabilize the copolymer micelle more. incorporated into the core, our uni-core micelle of the m-mer Since the p(AMPS/C6) micelle is star-like, not all hexyl can be regarded as an m-arm star polymer. On the other hand, groups attaching to the copolymer chain are incorporated into when hydrophobes in the middle portion of the chain are the hydrophobic core. The high critical aggregation concen- included into the core, the uni-core micelle should be a 2m-arm tration for C6TAB indicates that hydrophobicity of the hexyl star polymer. It is known that the g-factor of the f -arm star group is not so strong that the hexyl group is completely helped polymer with respect to the hydrodynamic radius is given by22 exposing to aqueous medium.

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5. A. Hashidzume, A. Kawaguchi, A. Tagawa, K. Hyoda, and T. Sato, CONCLUSION Macromolecules, 39, 1135 (2006). 6. R. Nojima, A. Hashidzume, and T. Sato, Macromol. Symp., 249–250, 502 (2007). We have investigated the micellization equilibrium of the 7. E. Sprong, D. De Wet-Roos, M. P. Tonge, and R. D. Sanderson, J. statistical copolymer p(AMPS/C6) in water, methanol, and Polym. Sci., Part A: Polym. Chem., 41, 223 (2003). their mixtures. In aqueous 0.2 M LiClO4, p(AMPS/C6) forms a 8. J. Chiefari, Y. K. Chong, F. Ercole, J. Krstina, J. Jeffery, T. P. T. Le, uni-core star-like tetramer, and both stability and aggregation R. T. A. Mayadunne, G. F. Meijs, C. L. Moad, G. Moad, E. Rizzardo, and S. H. Thang, Macromolecules, 31, 5559 (1998). number of the micelle sharply decrease by addition of methanol 9. Y. Mitsukami, M. S. Donovan, A. B. Lowe, and C. L. McCormick, which reduces the hydrophobic interaction among hexyl groups Macromolecules, 34, 2248 (2001). attaching to copolymer chains. The micellization equilibrium 10. H. Fujita, ‘‘Foundation of Ultracentrifugal Analysis,’’ Wiley-Inter- and also micellar conformation were successfully described by science, New York, 1975. the proposed theory. 11. T. Sato, Y. Jinbo, and A. Teramoto, Macromolecules, 30, 590 (1997). 12. T. Kawata, A. Hashidzume, and T. Sato, Macromolecules, 40, 1174 (2007). Received: September 20, 2008 13. L. Wu, T. Sato, H.-Z. Tang, and M. Fujiki, Macromolecules, 37, 6183 Accepted: November 20, 2008 (2004). Published: January 15, 2009 14. M. Kanao, Y. Matsuda, and T. Sato, Macromolecules, 36, 2093 (2003). 15. Y. Matsuda, R. Nojima, T. Sato, and H. Watanabe, Macromolecules, REFERENCES 40, 1631 (2007). 16. M. Fixman and J. Skolnick, Macromolecules, 11, 863 (1978). 1. A. Hashidzume, Y. Morishima, and K. Szczubialka, in ‘‘Handbook of 17. K. Kawakami and T. Norisuye, Macromolecules, 24, 4898 (1991). Polyelectrolytes and Their Applications,’’ S. K. Tipathy, J. Kumar, 18. J. R. Philip and R. A. Wooding, J. Chem. Phys., 52, 953 (1970). and H. Nalwa, Ed., American Scientific Publishers, Stevenson Ranch, 19. T. Kanematsu, T. Sato, Y. Imai, K. Ute, and T. Kitayama, Polym. J., CA, 2002, vol. 2, pp 1–63. 37, 65 (2005). 2. ‘‘Hydrophilic Polymers. Performance with Environmental Accept- 20. Y. Matsuda, Y. Miyazaki, S. Sugihara, S. Aoshima, K. Saito, and ability,’’ J. E. Glass, Ed., American Chemical Society, Washington, T. Sato, J. Polym. Sci., Part B: Polym. Phys., 43, 2937 (2005). DC, 1996. 21. L. W. Fisher, A. R. Sochor, and J. S. Tan, Macromolecules, 10, 949 3. ‘‘Associative Polymers in Aqueous Solutions,’’ J. E. Glass, Ed., (1977). American Chemical Society, Washington, DC, 2000. 22. J. F. Douglas, J. Roovers, and K. Freed, Macromolecules, 23, 4168 4. ‘‘Stimuli-Responsive Water Soluble and Amphiphilic Polymers,’’ (1990). C. L. McCormick, Ed., American Chemical Society, Washington, 23. J. M. Ruso, D. Attwood, P. Taboada, and V. Mosquera, DC, 2001. Polym. Sci., 280, 336 (2002).

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