Structural Analysis of Lipophilic Polyelectrolyte Solutions and Gels in

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Structural Analysis of Lipophilic Polyelectrolyte Solutions and Gels in Low-Polar Solvents Kengo Nishi,#,† Saki Tochioka,#,† Takashi Hiroi,‡ Taihei Yamada,§ Kenta Kokado,§ Tae-Hwan Kim,∥ Elliot Paul Gilbert,⊥ Kazuki Sada,§ and Mitsuhiro Shibayama*,† †

Institute for Solid State Physics, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8581, Japan Department of Chemistry, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan § Department of Chemistry, Graduate School of Science, Hokkaido University, Kita 10, Nishi 8, Kita, Sapporo 060-0810, Japan ∥ Korea Atomic Energy Research Institute, Daejeon, 1045 Daedeok-daero, Yuseong-gu, 305-353, Korea ⊥ Bragg Institute, Australian Nuclear Science and Technology Organization, Locked Bag 2001, Kirrawee DC, NSW 2232, Australia ‡

S Supporting Information *

ABSTRACT: Lipophilic polyelectrolyte gels capable of large swelling in low-polar solvents (3 ≤ ε ≤ 10) were developed by Ono et al. (Nature Mater. 2007), where ε is the dielectric constant. These gels were prepared by introducing tetraphenylborate as a lipophilic anion (tetrakis(3,5-bis(trifluoromethyl)phenyl)borate; TFPB−) and tetraalkylammonium with long alkyl chains as a lipophilic cation (tetra(nbutyl)ammonium; TBA+) into a poly(octadecyl acrylate) (pODA) backbone chain. Here, we investigated the structure of the lipophilic polyelectrolyte gels and corresponding polymer solutions in CH2Cl2 with small-angle neutron scattering (SANS) and dynamic light scattering (DLS). From SANS, it was revealed that individual pODA chain is regarded as a rod with the cross-section radius of 15 Å and the length of ca. 160 Å and is little changed by introduction of charges or cross-linking. In addition to this, it was revealed from SANS measurements that the second virial coefficient of pODA in CH2Cl2 was positive. In combination with DLS measurements, we observed several characteristic features similar to polyelectrolyte aqueous systems such as (i) the clear appearance of slow diffusional motion in polymer solutions, (ii) an increase of diffusion coefficient in gels, and (iii) an increase of osmotic modulus in solutions and gels when ionic groups are incorporated in pODA. These experimental findings clearly show that [TBA+][TFPB−] dissociates enough and pODA, accompanying these ionic groups, acts as a polyelectrolyte even in a low-polar solvent such as CH2Cl2 (ε = 8.9). It is concluded that the good compatibility of pODA with CH2Cl2 and the introduction of dissociable ionic groups into pODA result in high-swelling capability of the lipophilic polyelectrolyte gels.

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INTRODUCTION Polyelectrolyte gels absorb a greater amount of solvent than nonionic gels due to their large osmotic pressure generated by the Donnan potential, i.e., the osmotic pressure of freely mobile counterions in the polyelectrolyte hydrogels.1 For example, poly(acrylic acid) hydrogels (PAAc gels) absorb water up to several hundred or thousand times their dry weight. Thus, they are widely used in industrial applications, such as diapers, fillers in cosmetics, separation media, drug delivery systems, biomedical devices, sensors, inks, and display devices.2−4 Polyelectrolyte gels can be fabricated by introducing ionizable groups into polymer networks. However, such absorbency is limited to aqueous systems because the ionizable group can dissociate only in a solvent with large dielectric constant. There have been many attempts to synthesize polymers that exhibit similar swelling capability in organic solvents to polyelectrolyte hydrogels as reviewed by Jin et al.5 Kim et al.6 © 2015 American Chemical Society

synthesized cross-linked poly(cinnamoyloxyethyl methacrylate (CEMA)-stearyl methacrylate(SMA)) by UV light irradiation. However, the absorbency for crude oil was limited to 6.10 g/g. An improvement of oil absorbency was made by Jang et al. by tuning the hydrophobicity of the copolymer and the degree of cross-linking.7,8 They found that the copolymer with longer alkyl acrylate possessed higher oil absorbency, but the oil absorbency was limited to 15.0 g/g. On the other hand, the introduction of ionizable groups to lipophilic polymers have also been explored. It was reported that some ionic polymers composed of hydrophilic anions or cations behave as polyelectrolytes in high-polar organic media, such as THF and DMF.9−13 Jousset et al. prepared ionic polymers by Received: April 11, 2015 Revised: May 14, 2015 Published: May 20, 2015 3613

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Figure 1. Chemical structure of pODA and [TBA+][TFPB−] (left) and its schematic illustration emphasizing the bulkiness of the polymer chains and ions (right). methane and passed through a column filled with silica gel. The solvent was removed under reduced pressure. Copper bromide (CuBr) was purified with glacial acetic acid and washed with ethanol and then dried under reduced pressure. Preparation of Lipophilic Polyelectrolyte. The ion monomer, (N-(3-(acryloyloxy)propyl)-N,N,N-trihexylammonium tetrakis(3,5bis(trifluoromethyl) phenyl)borate were synthesized according to the reported methods.15 In a typical polymerization, 15.9 mg (0.1 mmol) of CuBr and 18.4 mmol of acryloyl monomer (ion monomer/ODA = 0/100 or 3/97 (mol/mol)) were added to a dry two-necked round flask with a stirrer bar. The flask was degassed and backfilled with nitrogen more than three times and then left under nitrogen. Subsequently, 12 mL of dry anisole, 19.2 mg (0.1 mmol) of N,N,N′,N′,N-pentamethyldiethylenetriamine (PMDETA), 450 mg (1.1 mmol) of tin(II) 2-ethylhexanoate (Sn(EH)2), and 18.0 mg (0.09 mmol) of ethyl 2-bromoisobutyrate (EBiB) were added into the flask. The flask was placed in an oil bath at 70 °C for 22 h. The solution was cooled at room temperature and reprecipitated into methanol. The precipitate was collected with filtration and dissolved in dichloromethane. The solution was passed through a column filled with neutral alumina to remove the copper complex, evaporated and the residue was dissolved in dichloromethane. The solution was subsequently reprecipitated into methanol/THF = 9/1 and dried under reduced pressure to obtain a white solid. The number-average molecular weight (Mn) and polydispersity (Mw/Mn) of the prepared samples were determined by size-exclusion chromatography (SEC) (toluene, polystyrene standard). The raw data and the characterization results with a sample ID are shown in Figure S1 and Table 1. Here,

introducing tetramethylammonium 1,1,2,3,3-pentacyanopropenide. It was revealed from viscosity measurements that there are polyelectrolyte effects in organic solvents more polar than acetone (ε ≥ 20.6), but not in lower polar solvents such as cyclopentanone (ε = 13.6) and triethyl phosphate (ε = 10.8),14 where ε is the dielectric constant. Therefore, a large swelling capability was not achieved for these ionic polymers in lowpolar solvents (ε ≤ 10), e.g., CH2Cl2 and CHCl3, because the ionic groups could not dissociate in these solvents. In 2007, Ono et al. developed lipophilic polyelectrolyte gels undergoing large swelling in low-polar solvents (3 < ε < 10).15 The important improvement in their work is the incorporation of tetra(n-butyl)ammonium (TBA + ) tetrakis(3,5-bis(trifluoromethyl)phenyl)borate (TFPB−) ions into long alkyl acrylate polymer (poly(octadecyl acrylate); pODA). Because both TBA+ and TFPB− are surrounded by bulky lipophilic groups as shown in Figure 1, these ions themselves (when neither of them are bound to a polymer chain) dissociate when ε > 3.16 It should be noted here that TFPB− belongs to the class of weakly coordinating anions and dissociates to free ions or loosely bound ion pairs even in low-polar organic solvents. As a result, this lipophilic polyelectrolyte exhibits similar behavior to polyelectrolytes in aqueous system, such as high swelling ability of gels,15,17,18 volume phase transition,19 and large intrinsic viscosity16 in low-polar solvents. In response to this success, several new types of lipophilic polyelectrolyte gels were developed by combining a long alkyl acrylate polymer and ion pairs which dissociate in low-polar solvents.20,21 By taking advantage of these characteristics, we anticipated that these gels would be a key material for protective barriers of volatile organic compounds (VOCs). Although several macroscopic properties of lipophilic polyelectrolytes were examined in previous works,15−19 the microscopic structures were not extensively investigated. In this study, we investigate the structure of this lipophilic polyelectrolyte in detail with small-angle neutron scattering (SANS) and dynamic light scattering (DLS). In addition, we discuss the mechanism of high swelling ability of lipophilic polyelectrolyte gels and present guidelines for the preparation of high-swelling lipophilic polyelectrolyte gels.

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Table 1. Characterization of Polymer Samples sample ID

molar fraction of ions (mol %)

Mn

Mw/Mn

NP EP3

0 3

14100 15300

1.7 1.7

EP3 is the polymer synthesized by the above-mentioned method, where the number 3 represents the mol % of ions in the polymer chain. NP is the polymer prepared without incorporation of ion groups. Preparation of Lipophilic Polyelectrolyte Gels. All gelation conditions are shown in Table 2. The lipophilic ion monomer, octadecyl acrylate (ODA), ethylene glycol dimethacrylate (EGDMA) and azobis(isobutyronitrile) (AIBN) were dissolved in benzene. The solution was degassed and the synthesis allowed to occur at 60 °C for 24 h. The feed ratio was adjusted to lipophilic ion monomer:ODA:EGDMA = x:100 − x:1, and the resultant gels are denoted as EGx. The octadecyl acrylate gel (NG) was prepared under the same copolymerization conditions (ODA: EGDMA = 100:1) without

EXPERIMENTAL SECTION

Materials. The following solvents and chemicals were obtained commercially. Octadecyl acrylate (ODA) was dissolved with dichloro3614

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Macromolecules Table 2. Sample ID and Gelation Compositions sample ID

ion monomer (mg)

ODA (mg)

EGDMA (mg)

AIBN (mg)

benzene (μL)

NG EG1 EG3 EG5

0 25 75 125

649 643 630 617

3.96 3.96 3.96 3.96

3.37 6.75 6.75 6.75

150 150 150 150

lipophilic ion monomer. The resultant gels were washed by soaking in chloroform for 10 h, air-dried at room temperature and subsequently dried in vacuo at 40 °C. Equilibrium Swelling Degree Measurements. Samples were swollen in the following organic solvents at room temperature: toluene (ε = 2.2), carbon tetrachloride (ε = 2.4), THF (ε = 7.6), and dichloromethane (ε = 8.9). After the gels were immersed in these solvents for 2 days, the equilibrium swelling degree Q of the gels was measured; this is defined as Q = (Wwet − Wdry)/Wdry and where Wdry and Wwet are the weights of the dried and wet gels, respectively. Dynamic Light Scattering (DLS). DLS measurements were carried out by using a DLS/SLS-5000 compact goniometer (ALV, Langen), coupled with an ALV photon correlator. A 22 mW He-Ne laser (wavelength, λ = 632.8 nm) was used as the incident beam and the scattering angle θ was 90°. Each measurement time was 30 s. For solution samples, each specimen was poured into a DLS test tube through a filter (pore size: 5.0 μm). For the gel samples, swollen gels were cut into small pieces of about 5 mm in size. After that, small pieces of gels were set into DLS test tubes with an adequate amount of solvent. Small-Angle Neutron Scattering (SANS). Two series of measurements were conducted assessing the influence of (i) concentration on polymer conformation in solution and (ii) molar fraction of [TBA+][TFPB−], f ion, dependence on the network structure for gel samples. In the former, polymer solutions with concentrations from 0.5 to 10 wt % for NP and EP3 in CD2Cl2 were prepared. In the latter, gels in CD2Cl2 were prepared which are denoted by NG (f ion = 0 mol %), EG1 ( f ion = 1 mol %), EG3 ( f ion = 3 mol %) and EG5 (f ion = 5 mol %). Solution samples were poured into Banjo cells of 2 cm in diameter and 1 mm in thickness. As for the gel samples, a swollen gel in CD2Cl2 was cut into small pieces of about 1 mm in size. After that, small pieces of gels were crushed into sample cells with CD2Cl2. Because of the uncertainty in precise sample thickness, this was estimated from the neutron transmission as described in the previous work.22 The SANS measurements were performed on the 40 m SANS instrument at the High-Flux Advanced Neutron Application Reactor (HANARO), Korea Atomic Energy Research Institute (KAERI), and QUOKKA,23 at the OPAL reactor at the Australian Nuclear Science and Technology Organization (ANSTO). On QUOKKA, two instrument configurations were used to yield a q range from 0.3 Å−1 where q is the magnitude of the scattering vector defined as q = (4π/λ) sin θ and 2θ is the scattering angle. These configurations were: (i) source-to-sample distance (SSD) = 16.0 m, sample-todetector distance (SDD) = 16.1 m and (ii) SSD = 5.9 m, SDD = 3.1 m with a 300 mm detector offset. Both configurations used a wavelength of 5.0 Å and 10% wavelength resolution. Source and sample aperture diameters were 50 mm and 10 mm, respectively. On the 40 m HANARO SANS, a wavelength of 6.00 Å with 10% wavelength resolution was used with instrument configurations of SSD = 17.9 m, SDD = 17.5 m and SSD = 3.7 m and SDD = 2.0 m. On both instruments, the scattered neutrons were counted on a 2D detector. Neutron data were corrected for background scattering (empty sample holder), transmission, and detector efficiency and set to absolute units using a calibrated attenuated direct-beam measurement. Each measurement temperature was 25 °C.

Figure 2. Swelling behavior of lipophilic polyelectrolyte gels (EG) and nonionic gels (NG) in various organic solvents.

polyelectrolyte gels in various solvents. The value of Q increases with increasing mole fraction of ion in pODA, f ion. It should be noted that the greater the dielectric constant, ε, the greater the value of Q. However, it should be also noted that Q decreases with any further increases in ε beyond that used in the current study as reported by Ono et al.15 because the solubility of pODA becomes lower in solvents with higher ε. This means that there is an optimum condition with respect to ε for given types of polymers. In the case of pODA, CH2Cl2 (ε = 8.9) is the best solvent for large swelling. To clarify the relationship between the swelling capability (Q) and the polymer structure, we carried out a series of DLS and SANS measurements by using CH2Cl2 and CD2Cl2 as a solvent, respectively. 2. Structural Analysis of Lipophilic Polyelectrolyte Solution. In order to study the effect of ionic groups on polymer dynamics, we carried out DLS measurements on NP and EP3 in CH2Cl2. Parts a and b of Figure 3 show the intensity−intensity time correlation functions, g(2)(τ,q)s, where τ is the decay time and q is the scattering vector, and the corresponding plots of the characteristic relaxation time distribution function analyzed by the CONTIN method, respectively. As shown in Figure 3a, two relaxation modes are observed in NP solution. The fast mode is attributed to the translational diffusion of individual polymer chains, and the slow mode is to cluster mode, which can also be observed in neutral polymer solutions.24,25 As for EP3, the amplitude of the fast mode is very small though two relaxation modes are also observed for EP3. The prominent appearance of the slow mode in polyelectrolyte solutions is commonly observed and has been reported several times for different systems, such as DNA,26−32 NaPSS,33−35 and partially hydrolyzed polyacrylamide.36 Several interpretations of the slow mode have been given; the most common is that it is due to scattering from aggregates and interchain domains,37−40 or a reptation motion of the chain.33,34 The physical meaning of this slow mode is still controversial and it is not discussed further here. However, it is noteworthy that the introduction of the ionic group, [TBA+][TFPB−], strongly affects polymer dynamics in the same way as would be expected for an aqueous polyelectrolyte solution. The concentration dependence of the hydrodynamic radius, Rh, from DLS measurements for the NP solution is plotted in Figure 4; these values have been evaluated from the fast mode as follows. The intensity−intensity time correlation function,

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RESULTS AND DISCUSSION 1. Swelling Behavior of Lipophilic Polyelectrolyte Gel. Figure 2 shows the degree of swelling, Q, for lipophilic 3615

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Figure 3. (a) Normalized intensity-time correlation functions for NP and EP3 in CH2Cl2. (b) Corresponding characteristic decay time distribution functions, G(Γ−1).

Figure 4. Concentration dependence of Rh in NP solution.

g(2)(τ,q) is related to the first-order electric field time correlation function, g(1)(τ,q) via the Siegert relation:41 g(2)(τ , q) = 1 + β |g(1)(τ , q)|2

(1)

where β is the coherence factor of the apparatus (≈ 0.95). For polymer solutions, the g(1)(τ,q) is given by g(1)(τ , q) = exp( −Dq2τ )

(2)

where D is the diffusion coefficient. According to the Stokes− Einstein equation, the hydrodynamic radius, Rh, is related to D by Rh =

kBT 6πηD

(3)

Here, kB is Boltzmann’s constant, T is the absolute temperature, and η is the viscosity of the solvent. By using equations from eq 1 to 3, Rh can be evaluated. As shown in Figure 4, the hydrodynamic radius, Rh increases slightly for C ≤ 4 wt %, followed by a sharp decrease for C ≥ 4 wt % with increasing concentration. This fact implies that the solution in the range from 4 to 8 wt % is in the transition region (≈C*) from dilute to semidilute solutions, where C* is the chain overlap concentration. Figure 5 shows the SANS results for (a) NP and (b) EP3 in CD2Cl2. A marked difference is observed in the low-q region (q < 0.01 Å−1). That is, the scattering curves show an intensity plateau for the NP solution (Figure 5a), while a slight increase in the scattering intensity appears for the EP3 solution (Figure 5b) with increasing polymer concentration. The latter can be

Figure 5. Concentration dependence of SANS profiles for (a) NP and (b) EP3 in the range of 0.5 to 10 wt % in CD2Cl2.

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associated slope of this linear dependence. The values of Rc obtained by linear least-squares fitting of the data are shown in Figure 7.

attributed to cluster formation. The other characteristic feature in the scattering curves is a “tail” observed in q > 0.2 Å−1, which are not observed in flexible polymer solutions. Taking account of the existence of the “tail” in the scattering profiles and the molecular structure of pODA as described in Figure 1, pODA should be considered to be a bottlebrush polymer represented by a “rod”. Thus, in the following section, we carried out Zimmplot analysis in the range of 0.01 Å−1 ≤ q ≤ 0.03 Å−1 and crosssectional Gunier plots42 in the range of 0.05 Å−1 ≤ q ≤ 0.1 Å−1 for dilute solutions.

Figure 7. Cross-sectional radius of gyration Rc as a function of concentration for NP and EP3 in CD2Cl2.

As shown in Figure 7, Rc is independent of the polymer concentration and of the existence/absence of the ionic groups with a value of approximately 14 Å; this result is reasonable as Rc arises mainly from the excluded volume effect of the side chains. It is valuable to consider the conformation of the side chain in the context of this value. The maximum alkyl chain length approximating to an all-trans conformation, lmax, can be estimated from Tanford’s formula, i.e., lmax ≈ 1.54 + 1.265nc (Å), where nc is the number of carbon atoms in the alkyl chain. In this study, the number of carbon atom is 18, thus lmax can be estimated to be 24.3 Å. The experimental value of Rc is evidently smaller than lmax and thus the side chain is not a fully stretched. Next, we carried out Zimm analysis for dilute solutions. At the limit of low concentration, the scattering intensity I(q) can be described in terms of the following Zimm equation ⎛ R g 2 2⎞ Kc −1⎜ q ⎟ + 2A 2 c = M w ⎜1 + I(q) 3 ⎟⎠ ⎝ 2

where K = (Δρ) is the contrast factor (Δρ is the difference in scattering length densities between polymer and solvent, dpoly is the mass density of the polymer, NA is the Avogadro’ s number, Rg is the radius of gyration, and A2 is the second virial coefficient.). Figure 8 shows the Zimm plots for 0.5−2 wt %. Mws are estimated to be 22.0 and 20.6 kg/mol for NP and EP3, respectively. These values are reasonable when we consider the SEC results in Table 1 (Mws are estimated to be 24.0 and 26.0 kg/mol for NP and EP3, respectively, from the corresponding Mn values and Mw/Mn = 1.7, i.e., Mn = 14.1 and 15.3 kg/mol, respectively for NP and EP3). This confirms the validity of the experiment and the analysis. The Rg values evaluated from Zimm plots for NP and EP3 in CD2Cl2 are 48.7 and 48.1 Å, respectively. Assuming that the backbone of pODA is in an all-trans conformation and the weight-average polymerization degrees are 68 and 62 for NP and EP3, respectively, the total lengths of rods, Ls, may be estimated to be 170 and 160 Å, respectively for NP and EP3 from Tanford’s formula. In this case, the radius of gyration Rg is calculated from Rg = (L2/12 + Rc2/2)1/2 to be 50 and 47 Å, respectively for NP and EP3. These Rg values are consistent

Figure 6. Cross-sectional Guinier plots of ln(qI(q)) as a function q2 for (a) NP and (b) EP3 in CD2Cl2. The solid straight lines were obtained by linear least-squares fitting of the data.

Figure 6 shows cross-sectional Guinier plots (i.e., ln(qI(q)) as a function q2) for (a) NP and (b) EP3 in CD2Cl2. The crosssectional radius of gyration Rc is given by ⎛ 1 ⎞ qI(q) ∝ exp⎜ − Rc 2q2⎟ ⎝ 2 ⎠

(5)

/NAdpoly2

(4) 2

According to eq 4, the plot of ln(qI(q)) versus q should be linear in the q-range where this approximation applies, and the cross-sectional radius of gyration Rc can be obtained from the 3617

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Figure 9. Concentration dependence of the osmotic modulus, Kos, for NP and EP3. The lines are guides for the eye.

behaves as a polyelectrolyte. The high swelling ability of the lipophilic polyelectrolyte gel takes its origin from two factors described above, namely the good solubility of pODA in CH2Cl2 and an increase in the osmotic modulus due to the dissociation of [TBA+][TFPB−]. 3. Structural Analysis of Lipophilic Polyelectrolyte Gel. We conducted DLS measurements on equilibrium-swollen gels in toluene (ε = 2.2), CCl4 (ε = 2.4), THF (ε = 7.6), and CH2Cl2 (ε = 8.9) in order to evaluate the effect of ionic group on the dynamics of polymer network. The scattering intensity from a gel comprises two parts; namely (i) the fluctuating component originating from the dynamics of polymer network, IF(τ, q), and (ii) the constant component associated with network inhomogeneity, Ic:

Figure 8. Zimm plots for dilute solutions of (a) NP and (b) EP3 in CD2Cl2.

with the experimental results and suggests that the pODA chain takes a rod conformation. Taking account of Rh values obtained from DLS in Figure 4, the ratio of Rg/Rh ∼ 1.15 indicates that pODA does not take a thin rod conformation (Rg/Rh ≈ 2) but rather a thick rod conformation (e.g., Wu et al.43). The radius of gyration of EP3 is almost the same as that of NP. This suggests that the introduction of [TBA+][TFPB−] does not significantly affect the static conformation of pODA. This is because the pODA chain is not a flexible polymer chain but rather a rigid thick rod (or a bottle brush polymer). To characterize the solvent quality, the values of the second virial coefficient, A2, in CD2Cl2 are determined from the Zimm plots. The obtained A2 value of NP is 5.60 × 10−4 mol·m3·kg−2 and takes a positive value suggesting that CD2Cl2 is a good solvent for NP. As for EP3, A2 is estimated to be 1.09 × 10−3 mol·m3·kg−2 and it becomes higher than that of NP. This indicates that the affinity of EP3 for CD2Cl2 becomes higher because of the introduction of ionic groups. The thermodynamic property, osmotic modulus, Kos, may be evaluated from I(0) using a following equation: (Δρ)2 kBTϕ2 I(0) = KOS

I = IF(τ , q) + Ic

(7)

Note that Ic depends on the sample position at observation. According to Joosten et al.,44 the intensity−intensity time correlation function at an arbitrary sample position, p, gp(2)(τ, q) can be deconvoluted as follows, gp(2)(τ , q) =

⟨I(t )I(t + τ )⟩T , p ⟨I(t )⟩T , p 2

= [X pgF(1)(τ , q)]2 + 2X p(1 − X p)gF(1)(τ , q) + 1 (8)

gF(1)(τ,

Here, we define Xp and q) as the ratio of the intensity of the thermal fluctuations to the total intensity, and the field correlation function of the fully fluctuating component, respectively. Xp can be evaluated from the initial amplitude of gp(2)(τ, q), i.e., gp(2)(0, q), as follows: gp(2)(0, q) = X p(2 − X p) + 1

(9)

Therefore, by using eqs 8 and 9, we can obtain gF(1)(τ, q) from gp(2)(τ, q). The cooperative diffusion coefficient of the gel network, D, can be evaluated by postulating that gF(1)(τ, q) exhibits a single-exponential intensity time correlation function:

(6)

where kB, ϕ, and T are the Boltzmann constant, the polymer volume fraction, and absolute temperature, respectively. The concentration dependence of Kos is shown in Figure 9. As the polymer concentration increases, Kos increases for both systems. Furthermore, as one can see in Figure 9, Kos of EP3 is significantly larger than that of NP. This increase indicates that the ion pair, [TBA+][TFPB−] incorporated in EP3 dissociates in CD2Cl2 large enough that the polymer effectively

gF(1)(τ , q) = exp( −Dq2τ )

(10)

In Figure 10a, the cooperative diffusion coefficients of lipophilic polyelectrolyte gels are plotted against f ion. When ε < 3 (toluene and CCl4), the diffusion coefficients do not change 3618

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so as to keep ϕ = 0.196. An excess scattering due to crosslinking inhomogeneity is observed in the low-q region (q < 0.03 Å−1). On the other hand, in q > 0.1 Å−1, the scattering curves for gels look similar to those of solutions, which indicates that gels consist of a network with bottlebrush polymer chains. To evaluate the network structure quantitatively, the correlation length, ξ, of gels and the longitudinal modulus, Mos = Kos + (4/ 3)μ, were determined from Ornstein−Zernike (OZ) plots and eq 11, where μ is the shear modulus. I(0) =

(Δρ)2 kBTϕ2 MOS

(11)

The OZ plot for gels is shown in Figure 11b. In addition, the f ion dependence of the correlation length ξ and the osmotic modulus Mos obtained from the OZ plots are plotted in Figure 11, parts c and d. Though the polymer concentration is constant for all samples, ξ decreases with increasing f ion, which indicates that the correlation length is neither a mesh size nor a simply geometric length parameter but rather a spatial measure of thermal fluctuations; this tendency can be interpreted as a suppression of a thermal fluctuations due to the electrostatic repulsion between the ionic groups. According to D = kBT/ 6πηξ, the diffusion coefficient D increases with increasing f ion. This tendency is consistent with DLS measurements. As shown in Figure 11d, the longitudinal modulus MOS increases with increasing f ion. These data clearly shows that this lipophilic polyelectrolyte gel gains a high swelling ability because of the introduction of dissociable [TBA+][TFPB−] ions.

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CONCLUSIONS We prepared lipophilic polyelectrolyte solutions and gels, pODA carrying bulky lipophilic cations and anions, i.e., tetra(nbutyl)ammonium (TBA+) and tetrakis(3,5-bis(trifluoromethyl)phenyl)borate (TFPB−), respectively, and carried out structural characterization with SANS and DLS. It was revealed that the polymer chains are represented by a bottlebrush polymer with its cross-section radius of 14 and 160 Å long, and the conformation is rather unchanged against introduction of ionic groups or cross-linking. Furthermore, several characteristic features in the structure as well as in dynamics were observed similar to polyelectrolyte aqueous solutions: (i) From DLS measurements on EP3 solutions of CH2Cl2, a prominent appearance of the slow mode was observed, indicating cluster formation by ionic association. This clustering is supported by the observation of an increase in the SANS intensity in the low-q region of the SANS profiles. (ii) An increase of osmotic modulus by the introduction of [TBA+][TFPB−] was observed from SANS measurement on NP and EP3 solutions. As for the gel samples, it is revealed from DLS and SANS measurements that the diffusion coefficients increase and the correlation length decreases with increasing the amount of ionic groups in pODA, f ion. An increase of the osmotic modulus was also detected from SANS measurements as compared to neutral pODA solutions. These experimental findings clearly indicate that [TBA+][TFPB−] dissociates in CH2Cl2 and the ion-introduced pODAs behave as polyelectrolytes. (iii) SANS analysis demonstrates that the A2 value of pODA in CH2Cl2 is positive. It is concluded that the introduction of dissociable charges and good miscibility between the solute polymer and the solvents are the key factors for the high swelling capability of lipophilic polyelectrolyte gels while ionic association may

Figure 10. (a) f ion dependence of diffusion coefficients, D, evaluated from DLS measurement at equilibrium swollen state. (b) Diffusion coefficients as a function of the swelling ratio (Q). All dotted lines are guides for the eye.

despite the increase of f ion because the ionic groups do not dissociate in these solvents as confirmed from our previous works.16 In contrast, in the case of ε > 3 (THF and CH2Cl2), the diffusion coefficients increase with increasing f ion. In order to clarify the relationship between the cooperative diffusion coefficients and the swelling ratio, the diffusion coefficients are replotted as a function of the swelling ratio (Q) in Figure 10b. With increasing swelling ratio, the diffusion coefficients do not change in ε < 3 (toluene and CCl4) but increase in ε > 3 (THF and CH2Cl2). The increase of diffusion coefficients with Q can be observed in previous polyelectrolyte hydrogel systems, such as pAAc gels,45 pNIPA/AAc gels,46 and poly(acrylamide)/AAc gels.47 It is revealed from our experiments that the increase in D with increasing f ion is not limited to polyelectrolyte hydrogels, but a common feature of polyelectrolyte gels in any solvent with reasonable values of ε. As mentioned in the previous works,38,47 the increase of diffusion coefficients with Q is opposite to the case in nonionic gel systems, i.e., diffusion coefficients decrease as the swelling ratio increases in nonionic gel systems. This tendency cannot be explained by existing theories,48 and there is a need for new theoretical approaches for describing the elastic modes of polyelectrolyte gels. Figure 11a shows f ion dependence of SANS profiles for NG, EG1, EG3, and EG5. Here, all samples are swollen in CD2Cl2 3619

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Figure 11. (a) f ion dependence of SANS profiles for lipophilic gels in CD2Cl2. Polymer volume fractions of all samples are set to be constant at ϕ = 0.196. (b) Ornstein−Zernike plots of I(q) as a function of q2. Solid lines are the fitting results by linear lines. (c) Correlation length, ξ, as a function of f ion evaluated from OZ plots. (d) f ion dependence of the longitudinal modulus, Mos = Kos + (4/3)μ, where μ is the shear modulus.48

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reduce the capacity for large swelling. Further investigations will be conducted in the near future by varying the main chain and/ or chargeable groups.

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ASSOCIATED CONTENT

S Supporting Information *

SEC charts of NP and EP3. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b00753.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*(M.S.) E-mail: [email protected]. Author Contributions #

K.N. and T.S. equally contributed to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been financially supported by Grant-in-Aids for Scientific Research from the Ministry of Education, Culture, Sports, Science, and Technology (No. 25248027). SANS experiments were performed on the 40 m SANS at HANARO, KAERI, Daejeon, South Korea and QUOKKA at the OPAL reactor at ANSTO, Australia. K.N. acknowledges the support from Research Fellowship for Young Scientists of the Japan Society for the Promotion of Science. This work was carried out under the Joint-use Research Program for Neutron Scattering, Institute for Solid State Physics (ISSP), the University of Tokyo, at the Research Reactor JRR-3, JAEA (Proposal No. 14587). 3620

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