Water-Induced Formation of Reverse Micelles from Diblock Copolymer

Jun 9, 2015 - Department of Macromolecular Science, Graduate School of Science, Osaka University, 1-1 Machikaneyama-cho, Toyonaka, Osaka 560-0043, Jap...
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Water-Induced Formation of Reverse Micelles from Diblock Copolymer of Styrene and N‑Isopropylacrylamide in 1,2Dichloroethane Tomoe Arai, Akihito Hashidzume,* and Takahiro Sato Department of Macromolecular Science, Graduate School of Science, Osaka University, 1-1 Machikaneyama-cho, Toyonaka, Osaka 560-0043, Japan S Supporting Information *

ABSTRACT: Water-induced formation of reverse micelles from polystyrene-b-poly(N-isopropylacrylamide) (PS(x)−PN(y), where x and y were the degrees of polymerization of PS and PN blocks, respectively) in 1,2-dichloroethane (DCE) was investigated mainly by light scattering. Four PS(x)−PN(y) samples with different degrees of polymerization x and y were prepared by the reversible addition−fragmentation chain-transfer (RAFT) radical polymerization technique. While PS(x)− PN(y) was molecularly dispersed in DCE, the addition of water remarkably enhanced scattering light intensity from the DCE solutions of all the PS(x)−PN(y) samples, indicative of the formation of the reverse micelle having a water pool as the micellar core. Static light scattering (SLS) data for PS(x)−PN(y)/water/DCE ternary systems were analyzed using a model of spherical reverse micelle to estimate structural parameters, which were dependent on the hydrophilic−hydrophobic balance, i.e., y/x.

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INTRODUCTION Water is the most popular and important liquid because it is utilized as a solvent for chemical reactions in biological systems. Chemical reactions in biological systems proceed in water phases of nanometer size.1−3 It is likely that such water phases of nanometer size act as a special reaction medium different from bulk water phase presumably because of confined water molecules in the interfacial layer.4−6 In artificial systems, such water phases of nanometer size are available as water pools inside reverse micelles formed from low molecular weight surfactants, e.g., aerosol OT.7,8 A number of examples of chemical reactions in water pools of reverse micelles have been reported so far.9−11 These reports are indicative of the importance of interfacial layer.12 However, low molecular weight surfactants often make it difficult to recover the products. To overcome this difficulty, polymeric surfactants, i.e., amphiphilic polymers, may be useful. Amphiphilic polymers possessing solvophilic and solvophobic units (i.e., hydrophilic and hydrophobic units in aqueous media) on the same polymer chain form various types of aggregates in selective solvents depending on their chemical structure.13 Their aggregation behavior is of considerable importance not only because they are used in various fields of applications including cosmetics, drug delivery systems, paints, coatings, and personal care goods,14−16 but also because © XXXX American Chemical Society

they are considered as simple models for the formation of higher order structures of biological macromolecules. Therefore, the aggregation behavior of amphiphilic polymers has been studied by a number of research groups in recent two or more decades. On the basis of these studies, the aggregation behavior of amphiphilic polymers can be controlled by changing the chemical structure. There have been only a few reports on polymeric reverse micelles containing a water pool in their inside, apart from block copolymers of poly(ethylene oxide) (PEO) and polystyrene (PS) and of PEO and poly(propylene oxide) (PPO).17,18 Gast et al.19−21 studied the formation of reverse micelles from PS−PEO block copolymer in cyclopentane or cyclohexane upon addition of a small amount of water by static and dynamic light scattering (SLS and DLS), small-angle X-ray scattering (SAXS), and small angle neutron scattering (SANS). Chu et al.22,23 reported water-induced micelle formation of a PEO−PPO−PEO block copolymer in o-xylene as studied by several techniques, including NMR, viscometry, SLS, and SANS. Alexandridis et al.24−28 have studied in detail the phase behavior of ternary mixtures of PEO−PPO−PEO block Received: March 5, 2015 Revised: May 26, 2015

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DOI: 10.1021/acs.macromol.5b00480 Macromolecules XXXX, XXX, XXX−XXX

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St, CMEDTB, and AIBN were dissolved in toluene using a 100 mL round-bottom flask equipped with a three-way stopcock under an argon atmosphere, and the flask was then warmed with an oil bath thermostated at 80 °C for 24 h. The polymerization was terminated by rapid cooling with an ice water bath. The reaction mixture was slowly poured into a large excess of methanol. The polymer obtained was recovered by filtration and finally dried under reduced pressure at room temperature. The polystyrene (PS) obtained (macro-CTA), NIPAM, and AIBN were dissolved in DMF (for PS(51) and PS(25)) or toluene (for PS(68) and PS(54)) using a test tube equipped with a three-way stopcock under an argon atmosphere, and then the tube was warmed with an oil bath thermostated at 80 °C for 24 h. The polymerization was terminated by rapid cooling with an ice water bath. The reaction mixture was slowly poured to a large excess of methanol (for PS(51)−PN(16)), water (for PS(25)−PS(18)), or hexane (for PS(68)−PN(80) and PS(54)−PS(75)). The polymer obtained was recovered by filtration and dried under reduced pressure at room temperature. Measurements. Size exclusion chromatography (SEC) measurements for PS were carried out at 30 °C on a system consisting of a Tosoh DP-8020 pump, a Viscotek TDA 302 detector, and two Tosoh TSKgel GMHXL columns connected in series, using THF as eluent at a flow rate of 1.0 mL min−1. Molecular weights were calibrated with polystyrene standards (Tosoh TSK standard POLYSTYRENE). For PS and PS(x)−PN(y) samples, SEC measurements were carried out to determine the weight and number-average molecular weights Mw1 and Mn1 at 35 °C on a system consisting of a Shodex GPC-101, a Shodex RI-71 detector, a Wyatt DAWN HELEOS-II detector, and two TOSOH TSKgel GMHXL columns connected in series, using THF as eluent at a flow rate of 1.0 mL min −1. Before the measurements, a solution was filtrated by a 0.20 μm PTFE membrane filter. To determine Mw1 and z-average molecular weight Mz1 of PS(x)− PN(y) samples, sedimentation equilibrium experiments were also performed for the copolymers dissolved in DCE at 25.0 °C. Data of sedimentation equilibrium were obtained using a Beckman-Coulter Optima XL-I type ultracentrifuge equipped with a Rayleigh interference optical system and a diode laser operating at 675 nm, and were analyzed according to the established method.32 1 H NMR spectra were measured on a JEOL JNM ECA500 or ECS400 spectrometer using dimethyl sulfoxide-d6 (DMSO-d6) or CDCl3 as a solvent at 30 or 50 °C. Water-added DCE solutions of the PS(x)−PN(y) samples used for light scattering measurements were prepared as follows. A predetermined amount of PS(x)−PN(y) was dissolved in DCE. The solution of PS(x)−PN(y) in DCE was optically cleaned by filtration with a 0.50 μm PTFE membrane filter and poured into a vial containing a predetermined amount of water. The ternary mixture was sonicated overnight. Static light scattering (SLS) measurements were performed for solutions of copolymers in DCE at 25.0 °C using an ALV/SLS/DLS-5000 light scattering instrument equipped with a Nd:YAG (532 nm) laser to obtain the excess Rayleigh ratio Rθ over that of DCE and the scattering intensity time-correlation function g(2)(t) for each solution. Specific refractive index increments ∂n/∂c of PS(x)−PN(y) samples in DCE at 25 °C were determined with a Shimadzu modified SchulzCantow type differential refractometer. The results were plotted against the weight fraction wPN of the PN block in Figure S1 in Supporting Information. By extrapolating the weight fraction to zero and unity, we determined ∂n/∂c of PS and PN to be 0.165 and 0.038 cm3 g−1, respectively. To analyze the sedimentation equilibrium data, specific density increments ∂ρ/∂c were also determined by an AntonPaar DMS5000 oscillation U-tube densitometer to be −0.176. Composition of the Ternary Solution. The composition of water added to the DCE solution of PS(x)−PN(y) is expressed in terms of the mass concentrations of the copolymer cP and water cW in the ternary solution, in what follows. The mass concentrations cP and cW were calculated from their weight fractions in the ternary solution, multiplied by the density of pure DCE (=1.253 g cm−3). In the light scattering experiment, the copolymer and water in the water pool are

copolymers, p-xylene, and water by several techniques, including SAXS and SANS, and demonstrated that lamellae, cylinder and spherical micelles, and gyroid are formed in the presence of water, depending on the composition of the ternary mixture and the block length. It is important to provide other examples of polymeric reverse micelles containing water pools for application of the water phase in reverse micelles in a wide variety of fields. Recently, we have thus investigated the formation of reverse micelles from amphiphilic diblock copolymers of PS and ionic blocks, i.e., poly{(ar-vinylbenzyl)trimethylammonium chloride} and poly(2-acrylamido-2-methylpropanesulfonic acid), in 1,2dichloroethane (DCE) mainly by light scattering.29 Characterization data have indicated that these amphiphilic diblock copolymers form reverse micelles in DCE depending on the ionic block and the ratio of block lengths. However, it was difficult to incorporate water into the polar core of the reverse micelle by adding water to micellar solutions in DCE presumably because PS coronal chains of the micelle prevented water from approaching the polar core. We have thus switched less amphiphilic block copolymers, which are molecularly dispersed in DCE and form the reverse micelle by addition of water. In the present study, we have chosen N-isopropylacrylamide (NIPAM) as a nonionic hydrophilic monomer. This is because poly(NIPAM) is well soluble not only in less polar organic solvents but also in water, and thus poly(NIPAM) exhibits a characteristic hydrophile−lipophile balance, similar to PEO. We have studied water-induced formation of reverse micelles from polystyrene-b-poly(N-isopropylacrylamide) (PS(x)−PN(y), where x and y are the degrees of polymerization of styrene and N-isopropylacrylamide, Scheme 1) in DCE mainly by light scattering. On the basis of the light scattering data, we will discuss the structure of reverse micelles. Scheme 1. Chemical Structure of PS(x)−PN(y)

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EXPERIMENTAL SECTION

Materials. Styrene (St) was purchased from Wako Pure Chemical Industries and purified by distillation under reduced pressure just prior to use. N-Isopropylacrylamide (NIPAM) was purchased from Tokyo Chemical Industry Co., Ltd. and recrystallized from hexane prior to use. 2,2′-Azobis(isobutyronitrile) (AIBN) was purchased from Kanto Chemical Co. and purified by recrystallization from methanol. 1Cyano-1-methylethyl dithiobenzoate (CMEDTB), used as a chain transfer agent (CTA) in this study, was prepared according to the procedure reported previously.30,31 Toluene and N,N-dimethylformamide (DMF), used for polymerization, were distilled over calcium hydride under reduced pressure just prior to use. 1,2-Dichloroethane (DCE), used for measurements, was purified by distillation over calcium hydride. Water was purified by a Millipore Milli-Q system. Other reagents were used without further purification. Preparation of PS(x)−PN(y) Samples. Four PS(x)−PN(y) samples used in the present study were prepared by the reversible addition−fragmentation chain transfer (RAFT) radical polymerization technique. B

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Macromolecules Table 1. Molecular Characteristics of the PS(x)−PN(y) Samples Used in This Study parameters

PS(51)−PN(16)

PS(25)−PN(18)

PS(68)−PN(80)

PS(54)−PN(75)

xa ya y/xa Mw1/103 b,c Mw1/Mn1b Mz1/Mw1c Mw1,PS/103 e Mw1,PN/103 e ∂n/∂c/cm3 g−1

51 (62d) 16 (20d) 0.31 8.7b 1.2

25 (31d) 18 (23d) 0.72 5.8b (6.3c) 1.3 1.1 3.3 2.6 0.111

68 (210d) 80 (240d) 1.2 49b 3.0

54 (100d) 75 (140d) 1.4 26b (26c) 1.8 1.3 10 16 0.067

6.5 2.2 0.145

22 28 0.084

a Determined by 1H NMR and elemental analysis. bDetermined by SEC-MALS in THF at 35 °C. cDetermined by sedimentation equilibrium in DCE. dWeight-average degree of polymerization, calculated from the results x/y and Mw1 listed in the fourth and fifth lines. eWeight-average molecular weight calculated from the weight-average x or y in parentheses of the second or third lines.

Figure 1. (a) Angular dependencies of Rθ for PS(54)−PN(75) with cW = 0.00252 g cm−3 (circle), PS(25)−PN(18) with cW = 0.00171 g cm−3 (square), and PS(54)−PN(75) with cW = 0 g cm−3 (diamond) at c ∼ 1 × 10−2 g cm−3. (b) Autocorrelation functions (unfilled symbols) and relaxation time distributions (filled symbols) for PS(54)−PN(75) with cW = 0.00252 g cm−3 and PS(25)−PN(18) with cW = 0.00171 g cm−3 measured at θ = 90° and c ∼ 1 × 10−2 g cm−3. regarded as the scattering components. The total concentration of the scattering components is denoted as c (=cP + cW), in what follows.

polymerization x and y range 25−68 and 16−75, respectively, and y/x ranges 0.31−1.4, where PS(x)−PN(y) of a larger y/x is more hydrophilic. Mw1 and Mw1/Mn1 range (5.8−49) × 103 and 1.2−3.0, respectively. Table 1 also contains Mw1 for PS(25)−PN(18) and PS(54)− PN(75) determined by sedimentation equilibrium in DCE at 25 °C. The results almost agree with those obtained in THF by SEC-MALS, indicating that the PS(x)−PN(y) samples are molecularly dispersed in DCE at 25 °C. Light Scattering Results. Figure 1a shows excess Rayleigh ratios Rθ for DCE solutions of PS(54)−PN(75) with cW = 0.00252 and 0 g cm−3 as well as of PS(25)−PN(18) with cW = 0.00171 g cm−3, where the mass concentration c (=cP + cW) of the whole scattering component is fixed at ca. 1.0 × 10−2 g cm−3. The abscissa is the square of the scattering vector k2. The light scattering measurement was not made on the DCE solution of PS(25)−PN(18) with cW = 0 g cm−3, because Rθ was too small to be measured. By addition of water, Rθ enormously increases, indicative of the formation of reverse micelles in the DCE solutions of PS(x)−PN(y). Both wateradded DCE solutions of PS(x)−PN(y) were close to the solubility limit of water, and further addition of water induced macroscopic phase separations. Figure 1b shows scattering intensity time-correlation functions g(2)(t) at the scattering angle θ = 90°, and relaxation time spectra A(τ) obtained from g(2)(t) using the CONTIN

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RESULTS AND DISCUSSION Molecular Characteristics of PS(x)−PN(y) samples. The PS(x)−PN(y) samples used in this study were prepared by RAFT radical polymerization. In the presence of a chain transfer agent, CMEDTB, St was first polymerized. Table S1 in Supporting Information lists the conditions and results of the RAFT radical polymerization of St. As can be seen in Figure S2 in Supporting Information, SEC data indicate unimodal signals with a narrower molecular weight distribution for all the PS samples obtained. Using these PS samples as macro-CTA, NIPAM was then polymerized as the second monomer. The conditions and results of RAFT radical polymerization of NIPAM are listed in Table S2 in Supporting Information. Figure S2 in Supporting Information also demonstrates that the PS(x)−PN(y) samples obtained show unimodal signals with a rather narrow molecular weight distribution. These observations are indicative of successful preparation of the PS(x)− PN(y) block copolymer samples. Table 1 summarizes the basic characteristics of the PS(x)− PN(y) samples prepared, i.e., the degrees of polymerization (x and y) and the ratio of degrees of polymerization (y/x) determined by NMR, as well as Mw1 and Mw1/Mn1 determined by SEC-MALS using THF as eluent. The degrees of C

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are stronger than those for the solutions of PS(68)−PN(80) and PS(54)−PN(75) with longer PN blocks, indicating that the PS(x)−PN(y) samples of smaller y/x form much larger reverse micelles in water-added DCE than do the polymer samples of larger y/x. On the other hand, the scattering intensity itself is stronger for PS(68)−PN(80) and PS(54)−PN(75) than that for PS(51)−PN(16) and PS(25)−PN(18), indicating that the molar concentration of reverse micelles formed from the PS(x)−PN(y) samples of larger y/x is much higher than that of reverse micelles formed from the polymer samples of smaller y/ x. Combining with the dynamic light scattering results, we may say that the reverse micellization behavior of PS(51)−PN(16) and PS(25)−PN(18) with shorter PN blocks is quite different from that of PS(68)−PN(80) and PS(54)−PN(75) with longer PN blocks. A more quantitative argument will be made in the next subsection. The water contents cW of the four DCE solutions of PS(x)− PN(y) shown in Figure 2 were all close to the solubility limit of water to the solutions, and definitely larger than the solubility limit to pure DCE. As mentioned above, with increasing y/x, the size of the reverse micelle decreases, but its molar concentration increases. As the result, cW is not so much dependent on y/x of the copolymer. Water-Uptake Reverse Micelle Model. Let us consider the reverse micelle model depicted in Figure 3. The micelle

analysis for the same water-added DCE solutions of PS(54)− PN(75) and PS(25)−PN(18). In the abscissa, kBT is the Boltzmann constant multiplied by the absolute temperature, ηs is the viscosity coefficient of DCE, and τ is the relaxation time. Thus, (kBT/6πηs)k2τ means the apparent hydrodynamic radius. While A(τ) for the water-added DCE solution of PS(54)− PN(75) is unimodal, that for the water-added DCE solution of PS(25)−PN(18) is bimodal. From the apparent hydrodynamic radius, the fast and slow relaxation components of the latter solution can be ascribed to the molecularly dispersed copolymer chain and reverse micelle, respectively. For the former solution, we cannot observe the scattering component corresponding to the molecularly dispersed copolymer chain, because the scattering from the component is too weak to be observed, in comparison with the scattering from the reverse micelle. As can be seen in Figure S3 in Supporting Information, A(τ) of water-added DCE solutions of PS(51)−PN(16) and PS(68)−PN(80) were bimodal and almost unimodal, respectively. The ternary solutions of PS(51)−PN(16) and PS(25)− PN(18) that show a bimodal A(τ) distribution contain a much larger fraction of molecularly dispersed polymer chains than do the ternary solutions of PS(68)−PN(80) and PS(54)−PN(75) that exhibit an unimodal A(τ) distribution. This is because the latter polymers are more hydrophilic than the former polymers.

Figure 3. Schematic diagram of the water-uptake reverse micelle model. Here the micelle consists of m copolymer chains and water of which molar mass is MH, and a denotes the area of the water pool interface per copolymer chain. Figure 2. Angular dependency of (K′c/Rθ)1/2 for water-added DCE solutions of PS(x)−PN(y) samples: PS(51)−PN(16) at c = 0.769 × 10−2 g cm−3 and cW = 0.00177 g cm−3 (circle), PS(25)−PN(18) at c = 1.02 × 10−2 g cm−3 and cW = 0.00171 g cm−3 (square), PS(68)− PN(80) at c = 1.31 × 10−2 g cm−3 and cW = 0.00373 g cm−3 (triangle), PS(54)−PN(75) at c = 1.01 × 10−2 g cm−3 and cW = 0.00252 g cm−3 (diamond). Solid curves indicate the best fits by the water-uptake reverse micelle model explained in the text.

consists of m copolymer chains and water of which molar mass is MH. The total molar mass M of the micelle is given by M = MH + mMP

The radius of the water pool RW in the micelle is related to the partial specific volumes of water υH̅ and of the PN block chain υ̅PN by 4π NAR W 3 = υH̅ MH + mυPN ̅ MPN (3) 3

Figure 2 shows angular dependencies of (K′c/Rθ)1/2 for water-added DCE solutions of four PS(x)−PN(y) samples. Here, K′ is the optical constant defined by K′ =

where MPN is the molar mass of the PN block chain. The radius RW is also related to the area a of the water pool interface per copolymer chain by 4πRW2 = ma, so that we have the following cubic equation for m:

4π 2n0 2 NAλ 4

(2)

(1)

NA 2a3 3 2 2 m − (υPN ̅ MPN) m − 2υH̅ υPN ̅ MHMPNm 36π

where n0 is the refractive index of DCE, NA is the Avogadro constant, and λ is the wavelength of the incident light in vacuum. The angular dependencies for the solutions of PS(51)−PN(16) and PS(25)−PN(18) with shorter PN blocks

− (υH̅ MH)2 = 0 D

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Macromolecules For given values of MH and a, we can solve this equation to obtain m and then RW, where experimental values are available for υH̅ , υ̅PN, and MPN. As shown in Figure 3, the interface of the water pool is covered with block copolymer chains, and radial concentration distributions of water, PN, and PS are written as

the reverse micelle, and express the weight fraction of the reverse micelle with MH in the form w(MH)dMH =

x≡

⎧ 0 (0 < r < R−) ⎪ c PS(r ) = ⎨ 0 (R − < r < R W ) ⎪c ̃ R < r < R ⎩ PS W +

lim

c→0

(5)

1/2

R+ = R W + ⟨R2⟩PS

(6)

3MH − 4πNAυH̅ −1R −3

c PN ̃

Rθ = K ′c

∫

Fk(R ) ≡ 4π c PS ̃ =

3

4πNA(R+ − R W ) (7)

The reverse micelle in the ternary solution may have a size distribution. We assume the log-normal distribution for MH on

(11)

sin(kR ) − kR cos(kR ) k3

Rθ ν 2w M P (k) + ν2 2w2M 2P2(k) + 2ν1ν2w1w2M1P1(k)M 2P2(k)(A11 + A 22 − 2A12 )c = 1 1 11 K ′c [1 + 2w1M1P1(k)A11c][1 + 2w2M 2P2(k)A 22 c] − 4w1w2M1P1(k)M 2P2(k)(A12 c)2

where νi, ci, Mi, and Pi(k) are the refractive index increment, weight fraction, molar mass, and particle scattering function of the component i (=1, 2), respectively, and Aij is the second virial coefficient between the component i and j (= 1, 2). Experimental values are available for ν1 and M1, and we can assume P1(k) = 1 because sizes of PS(x)−PN(y) chains are much smaller than the wavelength of light. For the polydisperse reverse micelle, ν2 is calculated by ν2 =

∫ w(MH)

(10)

(12)

and νi (i = H, PN, PS) being the refractive index increment ∂n/ ∂c of i. Actually, the ternary solution of PS(x)−PN(y), water, and DCE may contain free PS(x)−PN(y) chains (component 1) as well as polydisperse reverse micelles (component 2). Thus, the excess Rayleigh ratio Rθ may be given by

3mMPS 3

w(MH)(NAFk)2 dMH MH + m(MN + MS)

with Fk(R) defined by

,

4πNA(R W 3 − R −3) 3mMPN , = 4πNA(R W 3 − R −3)

(9)

+ νPSc PS ̃ [Fk(R+) − Fk(R W )]

Furthermore, the local concentrations of water, PN, and PS in the above equations are given by c H̃ =

2ln(MH,w /MH,n)

where Fk is the amplitude of the electric field of scattered light with the magnitude k of the scattering vector, calculated by ν Fk = H Fk(R −) + (νPNc PN ̃ + νHc H̃ )[Fk(R W ) − Fk(R −)] υH̅

Using root-mean-square end-to-end distances of the PN chain ⟨R2⟩PN1/2 and of the PS chain ⟨R2⟩PS1/2, we approximately express ,

ln(MH / MH,w MH,n )

with the weight and number-average molar masses MH,w and MH,n of water, respectively. According to the light scattering theory, Rθ/K′c for an infinitely dilute solution of the polydisperse concentric sphere of which radial concentration distribution is given by eq 5, is represented by

⎧ 0 (0 < r < R −) ⎪ c PN(r ) = ⎨ c PN (R − < r < R W ) ̃ ⎪0 R W < r < R+ ⎩

1/2

(8)

where x is defined by

⎧ υ −1 (0 < r < R ) − ⎪ H̅ c H(r ) = ⎨ c H̃ (R − < r < R W ) ⎪0 R W < r < R+ ⎩

R − = R W − ⟨R2⟩PN

1 exp( −x 2) dx π

vH = υH̅ (nH − nDCE)

(13) (15)

Comparison with Experimental Results. To calculate Rθ/K′c for the PS(x)−PN(y)/water/DCE ternary solutions by the above equations, we have to choose values of many parameters. Some of the parameters may be estimated from the polymer solution theory. The end-to-end distances of the PN and PS block chains, ⟨R2⟩PN1/2 and ⟨R2⟩PS1/2, can be calculated by assuming that the PN and PS chains are the wormlike chain with the persistence length of 1 nm and the contour length per monomer unit of 0.25 nm.34 The interfacial area a of the water pool per copolymer chain may be estimated from the radius of gyration ⟨S2⟩PS1/2 of the PS block chain by a = 4π⟨S2⟩PS, where ⟨S2⟩PS1/2 is calculated from the same wormlike chain model. Moreover, the second virial coefficient A22 of the reverse micelle is estimated by

vHMH + m(νPNMPN + νPSMPS) dMH MH + m(MPN + MPS) (14)

and M2P2(k) is calculated from the right-hand side of eq 10, divided by ν22. The refractive index increment νH of water in DCE is necessary to calculate the scattering function, but water and DCE are immiscible, so that the direct measurement is difficult. Using the equation of Gladstone and Dale for the specific refraction,33 νH may be calculated to be −0.111 cm3 g−1 from the refractive indices of water (nH = 1.334) and of DCE (nDCE = 1.445) and the specific volume of water (υ̅H = 1 cm3 g−1) using the equation

A 22 =

16πNA 3M w

2

1/2 3

(R̅ W + ⟨R2⟩PS

)

(16)

where R̅ W is the average radius of the water pool (cf. Figure 3) calculated by E

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Macromolecules Table 2. Structural Parameters of Spherical Reverse Micelles Formed from PS(x)−PN(y) parameters

PS(51)−PN(16)

PS(25)−PN(18)

PS(68)−PN(80)

PS(54)−PN(75)

c/10−2 g cm−3 cP/10−2 g cm−3 cW/10−2 g cm−3 ⟨R2⟩PS1/2 (⟨S2⟩PS1/2)/nma ⟨R2⟩PN1/2/nma a/nm2 b w2 MH,w/g mol−1 MH,w/MH,n mwc Mw/g mol−1 d R̅ w/nme Z° f A22/cm3 mol g−2g ν2 = (∂n/∂c)2h

0.769 0.592 0.177 5.4 (1.8) 2.8 11 1.9 × 10−4 1.5 × 109 5 7100 1.6 × 109 77 600 2.3 × 10−9 −0.095

1.02 0.849 0.171 3.7 (0.92) 3.1 2.7 8.3 × 10−3 6.0 × 107 30 2950 7.7 × 107 25 50 4.1 × 10−8 −0.011

1.31 0.937 0.373 10 (3.9) 11 48.5 1 1.3 × 107 35 58 1.6 × 107 15 51 6.3 × 10−7 −0.027

1.01 0.758 0.252 7.0 (2.5) 8.2 20 1 2.5 × 106 45 53 3.9 × 106 9.2 19 2.85 × 10−6 0.019

a

Calculated by assuming that the PN and PS chains are the wormlike chain with the persistence length of 1 nm and the contour length per monomer unit of 0.25 nm.34 bCalculated by a = 4π⟨S2⟩PS. cCalculated from eqs 4 and 8 with MH,w and MH,w/MH,n. dCalculated by eq 2 with MH,w and mw. eThe average radius of the water pool (cf. Figure 3) calculated by eq 17. fThe molar ratio of water to the NIPAM unit in the reverse micelle, calculated by (MH,w/18)/(ymw). gSecond virial coefficient of the reverse micelle is estimated by eq 16. hSpecific refractive index increment of the reverse micelle is estimated by eqs 14 and 8.

R̅ W =

⎛ m w a ⎞1/2 ⎜ ⎟ ⎝ 4π ⎠

of water to the NIPAM unit in the reverse micelle, listed in Table 2. Figure 5 indicates the y/x dependencies of the adjustable parameters, i.e., w2, MH,w, and MH,w/MH,n. Figure 5a indicates

(17)

Here, the weight-average molar mass Mw and aggregation number of the copolymer chain mw for the reverse micelle can be calculated by eqs 2, 4, and 8. It turned out that Rθ/K′ was not sensitive to values of A11 and A12 under experimental conditions of our PS(x)−PN(y)/water/DCE ternary solutions examined. Thus, we assumed both A11 and A12 to be zero, in the following fitting. The remaining adjustable parameters are w2, MH,w, and MH,w/ MH,n. The solid curves in Figure 2 indicate the best fits by the water-uptake reverse micelle model explained above. The parameters used in the fitting are listed in Table 2. On the basis of the parameters in this table, it is concluded that PS(x)− PN(y) forms different types of reverse micelles depending on the hydrophilicity, i.e., y/x, as can be seen in Figure 4. PS(x)−

Figure 4. Conceptual illustration of different types of spherical reverse micelles formed from PS(x)−PN(y) samples of y/x ≤ 0.72 (a) and of y/x ≥ 1.2 (b).

Figure 5. Copolymer hydrophilicity dependencies of the adjustable parameters w2 (a), MH,w (b), and MH,w/MH,n (c) chosen for the fitting shown in Figure 2c.

PN(y) of y/x ≤ 0.72 forms reverse micelles containing a hydorophilic core of R̅ W markedly larger than the end-to-end distance ⟨R2⟩PN1/2 of PN block (cf. Table 2). On the other hand, PS(x)−PN(y) of y/x ≥ 1.2 forms star-shape reverse micelles, in which R̅ W is almost the same as ⟨R2⟩PN1/2 (cf. Table 2). This trend is supported also by values of the molar ratio Z°

the fraction w2 of PS(x)−PN(y) chains participating in the reverse micelles as a function of y/x. At y/x ≤ 0.72, w2 is smaller than 1%, indicating that most of PS(x)−PN(y) chains are molecularly dispersed in water-added DCE. At y/x ≥ 1.2, on the other hand, w2 is unity, indicating that all the PS(x)− PN(y) chains take part in the formation of reverse micelles. Since PS(x)−PN(y) chains of smaller y/x possess longer PS and shorter PN blocks and are more hydrophobic, these F

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Macromolecules polymer chains tend to be molecularly dispersed in the wateradded DCE solution. It may be possible that only PS(x)− PN(y) chains of y/x larger than a certain level in the sample may form reverse micelles in the ternary solutions, but because of narrow dispersities of y and x for samples PS(54)−PN(18) and PS(25)−PN(16) (cf. Table 1), this effect may be minor. Since PS(x)−PN(y) chains of larger y/x are hydrophilic, all the polymer chains form the reverse micelles in the water-added DCE solution. On the basis of Figure 5a, it is likely that there is a critical y/x in the y/x region 0.72−1.2 for the behavior of reverse micelle formation. Figure 5b shows the values of molar mass of water pool MH,w plotted against y/x. This semilogarithmic plot indicates that MH,w decreases with increasing y/x. It is well-known that large particles are formed in hydrophobe-uptake micellar solutions of low-molar-mass surfactants near the critical micellar concentration (cmc), although the origin of this phenomenon has not been explained theoretically.35 Because the concentrations cP of samples PS(54)−PN(18) and PS(25)−PN(16) are close to the cmc at cW examined by light scattering, the large MH,w for the micelles of these samples may be related to this phenomenon for low-molar-mass surfactant systems. For the two samples with larger y/x, the cmc must be much lower than cP examined, so that their water-uptake reverse micelles with small MH,w are too stable to capture more water during the solution preparation. Figure 5c depicts the molar mass distribution MH,w/MH,n of water pool as a function of y/x. This figure demonstrates that MH,w/MH,n increases with y/x, indicating that the reverse micelle of a smaller MH,w possesses a larger distribution MH,w/ MH,n. Since there may be no thermodynamically optimum size of the reverse micelle, the size distribution of the reverse micelle may be determined kinetically. Since the ternary solutions were prepared with sonication in the present study, it is likely that the sonication is the major factor determining the size and size distribution of reverse micelle. Sonication may be more effective for larger reverse micelle,36 so that it makes narrower the size distribution of the reverse micelle with larger MH,w.37

added to the solution due to some kinetic reason, so that the micelle cannot grow to a large reverse micelle.

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ASSOCIATED CONTENT

S Supporting Information *

Conditions and results of the RAFT radical polymerization, specific refractive index increments ∂n/∂c as a function of the weight fraction wPN of the PN block for PS(x)−PN(y) samples, SEC data for the samples of PS(x)−PN(y) and macro-CTA, and DLS data for PS(54)−PN(75) with cW = 0, PS(51)− PN(16) with cW = 0.00177 g cm−3, and PS(68)−PN(80) with cW = 0.00373 g cm−3. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b00480.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +81-6-6850-5462. Fax: +81-6-6850-5461. E-mail [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partly supported by Grants-in-Aid for Scientific Research Nos. 23350055, 262880610, and 26620184 from the Japan Society for the Promotion of Science.

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REFERENCES

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CONCLUSION The water-induced formation of reverse micelles from DCE solutions of PS(x)−PN(y) was investigated mainly by light scattering. Four PS(x)−PN(y) samples with different degrees of polymerization x and y were prepared by the RAFT radical polymerization technique. While PS(x)−PN(y) was molecularly dispersed in DCE, the addition of water remarkably enhanced scattering light intensity from the DCE solutions of all the PS(x)−PN(y) samples, indicative of the formation of the reverse micelle having a water pool as the micellar core. The fraction of PS(x)−PN(y) participating in the reverse micelle strongly depended on the copolymer hydrophilicity represented by y/x. When PS(x)−PN(y) was more hydrophobic (i.e., y/x ≤ 0.72), only a small fraction of the polymer chains participated in the reverse micelle, and the major fraction was still in DCE-rich phase even after addition of water. However, when PS(x)−PN(y) was more hydrophilic (i.e., y/x ≥ 1.2), most of the polymer chains took part in the reverse micelle, because the longer PN block chain tended to be inserted into the water pool of the reverse micelle. The reverse micelle formed from more hydrophilic PS(x)−PN(y) is more stable, and then may be more difficult to capture water further G

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DOI: 10.1021/acs.macromol.5b00480 Macromolecules XXXX, XXX, XXX−XXX