Self-Assembly of a Midblock-Sulfonated Pentablock Copolymer in

Jan 4, 2019 - Ionic, and specifically sulfonated, block copolymers are continually gaining interest in the soft materials community due to their uniqu...
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Self-Assembly of a Midblock-Sulfonated Pentablock Copolymer in Mixed Organic Solvents: A Combined SAXS and SANS Analysis Kenneth P. Mineart,*,† Justin J. Ryan,‡ Marie-Sousai Appavou,§ Byeongdu Lee,∥ Michael Gradzielski,*,⊥ and Richard J. Spontak‡,# †

Department of Chemical Engineering, Bucknell University, Lewisburg, Pennsylvania 17837, United States Department of Materials Science & Engineering and #Department of Chemical & Biomolecular Engineering, North Carolina State University, Raleigh, North Carolina 27695, United States § Forschungszentrum Jülich, Outstation at MLZ, Jülich Centre for Neutron Science, Garching D-85747, Germany ∥ Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, United States ⊥ Stranski Laboratorium für Physikalische und Theoretische Chemie, Institut für Chemie, Technische Universität Berlin, Berlin D-10623, Germany

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‡

S Supporting Information *

ABSTRACT: Ionic, and specifically sulfonated, block copolymers are continually gaining interest in the soft materials community due to their unique suitability in various ionexchange applications such as fuel cells, organic photovoltaics, and desalination membranes. One unresolved challenge inherent to these materials is solvent templating, that is, the translation of self-assembled solution structures into nonequilibrium solid film morphologies. Recently, the use of mixed polar/nonpolar organic solvents has been examined in an effort to elucidate and control the solution self-assembly of sulfonated block copolymers. The current study sheds new light on micellar assemblies (i.e., those with the sulfonated blocks comprising the micellar core) of a midblock-sulfonated pentablock copolymer in polar/nonpolar solvent mixtures by combining small-angle X-ray and small-angle neutron scattering. Our scattering data reveal that micelle size depends strongly on overall solvent composition: micelle cores and coronae grow as the fraction of nonpolar solvent is increased. Universal model fits further indicate that an unexpectedly high fraction of the micelle cores is occupied by polar solvent (60−80 vol %) and that partitioning of the polar solvent into micelle cores becomes more pronounced as its overall quantity decreases. This solvent presence in the micelle cores explains the simultaneous core/ corona growth, which is otherwise counterintuitive. Our findings provide a potential pathway for the formation of solventtemplated films with more interconnected morphologies due to the greatly solvated micellar cores in solution, thereby enhancing the molecular, ion, and electron-transport properties of the resultant films.

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backbone. In melts possessing sufficient incompatibility,14 the copolymer composition (f i , the number fraction of i units in the copolymer) primarily dictates interfacial packing and, hence, morphological characteristics. For the simplest bicomponent molecular architectures (AB diblock and ABA triblock copolymers composed of A and B repeat units), increasing fA from near zero in the intermediate to strong segregation regime (12 ≲ χA‑BN ≲ 60) results in a wellestablished morphological progression: A-rich spheres on a body- or face-centered cubic lattice (SPH) to A-rich cylinders on a hexagonal lattice (CYL) to the bicontinuous gyroid (GYR) to alternating lamellae (LAM). The LAM morphology

INTRODUCTION Block copolymers are of considerable scientific and technological interest due primarily to their inherent ability to form well-defined nanoscale structures in both melts1−4 and solutions.5−7 Additionally, their chemically dissimilar blocks, and consequently microdomains, provide beneficial property combinations such as mechanical rigidity and ion transport in, for example, poly[(methyl methacrylate)-b-(1-[(2methacryloyloxy)ethyl]-3-butylimidazolium bis(trifluoromethanesulfonyl)imide)] diblock copolymers8−10 or amphiphilicity in poly[butadiene-b-(ethylene oxide)] diblock copolymers.11−13 The formation of nanoscale structures is driven largely by interblock thermodynamic incompatibility, expressed as χi‑jN,1 where χi‑j is the Flory−Huggins interaction parameter between the i and j species, and N denotes the number of statistical repeat units along the copolymer © XXXX American Chemical Society

Received: November 13, 2018 Revised: January 3, 2019 Published: January 4, 2019 A

DOI: 10.1021/acs.langmuir.8b03825 Langmuir XXXX, XXX, XXX−XXX

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Langmuir exists near compositional symmetry (fA ≈ 0.5). Further increases in fA lead to the reverse progression with discrete B-rich microdomains in the CYL and SPH morphologies.15,16 Additional nanostructures more recently observed in block copolymer melts include the Fddd,17,18 double-diamond,19 and Frank-Kasper σ20,21 morphologies. Similar behavior applies to block copolymer solutions. At copolymer concentrations below the critical gel concentration, the solution exhibits purely liquid behavior.22,23 The presence of solvent molecules in such solutions contributes additional thermodynamic interactions that must be considered specifically in terms of χA−S and χB−S, where S represents the added solvent. Individually, these values dictate block conformation.24 Collectively, they shed light on whether copolymer molecules disperse as unimers (χA−S ≈ χB−S) or form copolymer micelles (χA−S ≠ χB−S) and, if the latter, which blocks comprise the interior (solvophobic) core and exterior (solvophilic) corona. Much like their analogous melts, the equilibrium copolymer nanostructure depends on fA, spanning from unimers to spherical micelles to worm- and rod-like micelles to lamellar morphologies including vesicles (with fA decreasing from unity, where A is the solvophilic block).11,25−27 The degree of solvent selectivity (i.e., the magnitude of χB−S/χA−S) can also be used to controllably alter copolymer morphology due to micellar core/corona + solvent interfacial tension and has most commonly been accessed through miscible mixtures of selective and nonselective solvents.28,29 An elegant example of this concept focuses on amphiphilic poly[styrene-b-(acrylic acid)] (PS-b-PAA) assemblies in water/dioxane solutions (water is selective for PAA, whereas dioxane is nonselective).30−32 The results of this study demonstrate that increasing water content in PS310-b-PAA52 solutions (in which case χPAA‑s/χPS‑s → 0) yields a morphological progression qualitatively identical to that described above for decreasing fA in a single, selective solvent. Nanoscale morphologies in solution and in solid state can be intimately related in practice. Processes that employ solution casting in the absence of thermal annealing, for example, rely to varying extents on copolymer solution assembly for the formation of the final solid morphology. This solution → solid nanostructural dependence, generally referred to as solvent templating, is especially prevalent in materials that cannot be thermally annealed due to experimentally inaccessible glass or order−disorder transition temperatures (Tg and TODT, respectively). Block copolymers containing strongly acidic and nonpolar blocks, which are currently of considerable interest due to their applicability as ion- and electron-exchange membranes,33−37 are typically unaffected by thermal treatment due to high Tg and interblock incompatibility (χA−B directly relates to TODT). Recent studies38−40 of midblock-sulfonated pentablock copolymers have established an intimate connection between solution and bulk film morphologies. An understanding of ionic block copolymer assembly in solution is therefore warranted to control solid-state morphological development, which governs macroscopic properties such as ion or electron transport. Winey and co-workers41 have initiated efforts in this direction through use of a miscible polar/nonpolar (n-propanol/cyclohexane or n-propanol/toluene) solvent mixture to alter solvent selectivity for the nonpolar or ionic blocks on the basis of composition. This constitutes a unique system due to the miscible, counterselective solvent mixture (i.e., χA−S1 < χB−S1 and χA−S2 > χB−S2) made possible by the high interblock incompatibility.

Increasing the polarity of the solvent in this system induces a progression from micelles with sulfonated cores to unimers to reverse micelles with sulfonated coronas, as well as corresponding changes in micelle size.41 Inspired by these results, we examine similar sulfonated copolymer solutions with a focus on the role of the individual solvents. Our aim is to obtain detailed insight into the structure of copolymer micelles as a function of solvent quality.

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EXPERIMENTAL SECTION

Materials. Schematically depicted in Figure 1, the poly[tertbutylstyrene-b-(ethylene-alt-propylene)-b-(styrene-co-styrenesulfo-

Figure 1. (top) The chemical structure of SBI-52 with (red) nonpolar (TS and EP) blocks and a (blue) sulfonated block (S/sS). (bottom) Photographs indicating the color and transparency of a ternary SBI52/T/IPA solution with wp = 4% and ΦT = 90%. nate)-b-(ethylene-alt-propylene)-b-tert-butylstyrene] (TS-EP-S/sSEP-TS) sulfonated ABCBA pentablock copolymer (SBI) employed in this study was kindly provided by Kraton Polymers and consisted of a styrenic midblock with 52 mol % sulfonation (and is hereafter designated as SBI-52 for this reason). The block weights of the parent, unsulfonated copolymer were 15 (TS), 10 (EP), and 28 (S) kDa for a total molecular weight of 78 kDa (as provided by the manufacturer). Upon sulfonation, this molecular weight theoretically increased to ∼89 kDa due to replacement of −H with −SO3H in 52% of the S repeat units. Certified ACS-grade toluene (T) and isopropanol (IPA) were purchased from Fisher-Scientific, and deuterated toluene (dT, 99+ atom % D) and IPA (dIPA, 99+ atom % D) were obtained from Acros Organics. All chemicals were used as-received. Methods. Solvent mixtures were formulated on the basis of volume and were thoroughly mixed before copolymer addition. Solutions are specified by two composition metrics: the vol % toluene in the solvent mixture (ΦT) and the wt % SBI-52 in the solution (wp). Solutions were prepared by first mixing the desired quantities of copolymer and mixed solvent on a magnetic stir plate for 30 min. Resulting solutions were subsequently sonicated in an isothermal bath maintained at ambient temperature for 10 min, after which time all solutions appeared yellowish-orange (cf. Figure 1) and homogeneous to the unaided eye. These solutions were allowed to equilibrate on a benchtop for at least 10 min (for X-ray and light scattering) or 24 h (for neutron scattering). Small-angle X-ray scattering (SAXS) was performed on beamline 12-ID-B at the Advanced Photon Source (APS) at Argonne National Laboratory. Specimens were analyzed at ambient conditions in 2.0 mm quartz capillary tubes. The characteristic wavelength (λ) and flux of the incident X-ray beam were 0.087 nm and 1 × 1012 photons/s, respectively, and samples were exposed for 1 s to a beam with a spot size of 0.5 × 0.5 mm2 at a sample-to-detector distance of 2 m. A B

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Langmuir Pilatus 2 M detector collected two-dimensional (2-D) scattered X-ray information, which was azimuthally integrated to yield one-dimensional (1-D) scattering profiles. Measured data were subtractioncorrected with respect to an empty capillary tube where necessary. Small-angle neutron scattering (SANS) was conducted on beamline KWS-2 operated by the Jülich Centre for Neutron Science (JCNS) at the Heinz Maier-Leibnitz (MLZ) reactor in Garching, Germany.42 Samples were examined at ambient conditions in Hellma quartz cuvettes with a 2 mm path length. Scattering data were collected using three neutron wavelength (nm)/sample-to-detector distance (m) combinations (1.0/20, 0.45/8, and 0.45/1.2) to access a wide q-range from ∼0.02 to 4.5 nm−1 (q is the magnitude of the scattering vector defined as (4π sin θ)/λ, where θ is the scattering half angle). Resulting 2-D scattering patterns were subtraction-corrected for empty-cell scattering and background noise and then azimuthally integrated to yield 1-D scattering profiles. In the case of SANS, transmitted beam intensity was collected for each sample, an empty background, and plexiglass to obtain absolute intensity. All solutions for SAXS and SANS were prepared with wp = 4%. Dynamic light scattering (DLS) experiments were performed on an ALV/CGS-3 precision compact goniometer system utilizing a He−Ne laser with a wavelength of 632.8 nm. Pseudocross-correlation functions were recorded with an ALV/LSE-5004 multiple-τ digital correlator, where τ denotes delay time. Temperature within the sample cell was maintained at 25.0 ± 0.1 °C in a thermally controlled toluene bath. The DLS tests were conducted on solutions with wp = 0.25% and collected over a series of angles ranging from 30° to 110°. The viscosity (η) of each solvent mixture was estimated from ln(η) = ∑i xi ln(ηi), where xi and ηi represent the mole fraction and viscosity, respectively, of the ith constituent species.43

understanding SBI-52 micelles, we henceforth focus the present work on systems with ΦT ≥ 65%. Comparative Micelle Analysis. We begin the current analysis by using SAXS to compare SBI-52/T/IPA solutions varying in ΦT to those previously examined by Winey and coworkers.41 The SAXS profiles displayed in Figure 3 clearly

Figure 3. SAXS profiles collected from SBI-52/T/IPA solutions (wp = 4%) at various ΦT (labeled and color-coded). The solid lines represent fits of the MHS model (with a contribution to account for polydispersity in micelle size) to the data. Data are shifted vertically by a factor of 8n for clarity.

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RESULTS The present study focuses on SBI-52/T/IPA ternary solutions that possess micelles with sulfonated cores. The solvent mixture allows control over self-assembled structures, since increasing toluene (T) fraction decreases the solubility of styrene-co-styrenesulfonate (S/sS), thereby enhancing the tendency for micellization. Initial interpretation of SAXS (Figure 2a) and SANS (Figure 2b) data collected herein

confirm that multiple features change upon increasing ΦT. First, the peak at small q, which is attributed to the micelle− micelle structure factor, shifts to lower q, reflecting an increase in spacing between micelle cores. Second, the features located at intermediate q (∼0.1−1.0 nm−1), which are primarily related to the spherical micelle form factor, also shift to lower q. Taken together, both shifts indicate an increase in individual micelle size. Geometric parameters corresponding to each of these observations can be extracted from the profiles via fitting with an appropriate model. To ensure comparable evaluation with regard to previous findings,41 we use the modified hard sphere (MHS) model with a slight alteration to account for the micelle core polydispersity index (PDI) introduced by a Schulz distribution (PDI was fixed at 0.15). The most physically relevant parameters from the MHS model are the micelle core radius (r) and the hard sphere radius (rHS),44 which are depicted in Figure 4a. The micelle corona (also depicted in Figure 4a), which is not directly represented in the MHS model, imparts little contribution to SAXS profiles due to significantly lower contrast between the TS+EP copolymer blocks and solvent (compared to S/sS and solvent). Fitting the experimental data in Figure 3 with the MHS model captures the signature scattering features, and the resultant fitting parameters follow similar trends to those previously reported,41 namely, r and rHS increase with increasing ΦT, as shown in Figure 4b. These toluene-induced size increases have been attributed to higher incompatibility between the ionic micelle cores and the bulk solvent mixture (T + IPA in the present case), and the interested reader is directed to this prior study for a more detailed discussion. We note, however, a substantial increase in the magnitude of the radii in the current work (cf. Figure 4b). This difference could stem from a number of dissimilarities between the formulations of the two studies, including propyl alcohol isomer (iso- vs n-), copolymer concentration (wp = 4% vs 5−

Figure 2. SAXS (a) and SANS (b) profiles collected from a series of SBI-52/T/IPA solutions with various ΦT (labeled and color-coded). SAXS data are shifted vertically by a factor of 8n (where n is an increasing integer value) for clarity, and SANS samples were prepared with dT/IPA.

suggests that the transition from unimers to micelles occurs at ΦT ≈ 55−60% when wp = 4%. This conclusion, which agrees favorably with previously reported41 results, is based on the decay of scattering features characteristic of spherical micelles, as evidenced by, for example, the broad peaks in the mid-q region (∼0.1−1.0 nm−1). Since this study is dedicated to C

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Table 1. Neutron Scattering Length Density (ρN) and Electron Density (ρX) Values for Each Component in SBI52 Micelle Solutionsa component

ρN (cm−2)

ρX (cm−2)

T dT IPA dIPA S/sS cor (TS+EP)

1.17 × 10 5.39 × 1010 −0.33 × 1010 6.23 × 1010 1.49 × 1010 0.28 × 1010

7.99 × 1010

10

7.55 × 1010 9.58 × 1010 8.66 × 1010

a

Values for S/sS and TS+EP blocks are provided to reflect the contribution of copolymer within micelle cores and coronae (cor). Mass densities (in g/cm3) of 0.87 (T), 0.94 (dT), 0.79 (IPA), 0.89 (dIPA), 1.10 (S/sS), and 0.91 (TS+EP) were used in the calculation of ρn and ρx values.

Figure 4. (a) Schematic illustration depicting a single SBI-52 micelle with geometric parameters relevant to the current work: core radius (r), hard sphere radius (rHS), coronal chain radius of gyration (Rg,cor), and coronal thickness (t). (b) Solvent-composition dependence of r and rHS from SAXS. Each data set is color-coded and labeled, with those from the current study (●) and from Griffin et al.41 (○) displayed side-by-side for direct comparison.

10%), and solution preparation (stirring/sonication vs stirring only). Furthermore, inclusion of polydispersity in the present model enables observation that the model deviates from experimental data at high q (≈0.5−1.0 nm−1), and the deviation esclates with decreasing ΦT. We believe this is a result of unimer presence (based upon qualitative fitting with a combined MHS+unimer model) in SBI-52/T/IPA solutions. Mixed Solvent Distribution. While SAXS possesses a powerful ability to quantitate nanostructure geometry, it is limited in its capacity to measure the composition of targeted spatial regions, although recent efforts45 have attempted such. In the present study, SANS constitutes a complementary scattering technique that can detect components present in each region through controllable contrast variation. Phase scattering in SANS is related to the atomic nuclei present, and therefore isotopic substitution enables simple variation without substantial changes in solution self-assembly. The current SBI52/T/IPA solutions possess four principal contrast variation schemes (at fixed ΦT): SBI-52/T/IPA, SBI-52/dT/dIPA, SBI52/dT/IPA, and SBI-52/T/dIPA. (Note that SBI-52 is not commercially available in partially or fully deuterated form.) On the basis of the neutron scattering length density (ρN) of each constituent species (listed in Table 1), the first case is likely to be ineffective, since it lacks sufficient contrast between the micelle regions illustrated in Figure 4a (and is similar to SAXS with respect to the contrast conditions). Moreover, fully protonated systems frequently generate considerable incoherent background scattering. The latter three cases, however, possess adequate inter-region contrast that varies significantly from case to case and therefore complement each other. This expectation is verified by the SANS data provided in Figure 5, wherein the three contrast schemes provide an assorted set of scattering curves with two (dT/IPA and T/dIPA) appearing similar to the SAXS profiles discussed above and the other (dT/dIPA) displaying dissimilar features. The observed

Figure 5. Schematic depiction of SANS contrast conditions for SBI52 micelles swollen with IPA, or dIPA, (top) and corresponding SANS data (ΦT = 75%, wp = 4%; labeled and color-coded) (bottom). Increasing darkness in the schematic corresponds to decreasing deuteration (increasing protonation). Solid black lines in SANS data represent modified SGP universal fitting.

difference suggests that the micelles are not homogeneously structured (i.e., they are not exclusively composed of SBI-52 molecules). If no solvent penetrates the micelles, then it would be expected that scattering curves vary solely in absolute intensity based upon the level of solvent deuteration. This is clearly not the case due to distinction of the SBI-52/dT/dIPA profile and the fact that SBI-52/dT/IPA solutions exhibit the highest intensity. In contrast to the SAXS data and modeling discussed above, the corona chains of SBI-52 micelles have a non-negligible contribution to SANS scattering profiles as evident from comparing contrast in each experiment type and formulation scheme (Table 1)the relation between contrast of each phase region and its scattering contribution is given by I(q) ∝ (Δρ)2. We therefore selected the spherical GerstenbergPedersen (SGP) model,46 which is described as a spherical microdomain surface-decorated with Gaussian polymer coils as the form factor, but maintain use of a hard-sphere structure factor to capture intermicellar spacing (cf. Figure 4a). In D

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Langmuir addition to r and rHS, this modified SGP model includes the coronal chain radius of gyration (Rg,cor) and SBI-52 aggregation number (Nagg). Here, we must also account for q-dependent scattering from solvent in the dT/IPA and T/dIPA schemes as indicated by Figure S1. The solvent contribution can be effectively described by that of a Gaussian coil (eq S7) and is captured in model fits as such.46 Note that the solvent contribution has minimal impact on micelle modeling due to the much smaller size of solvent molecules (Rg,solv ≈ 0.5 nm) relative to SBI-52 micelles (∼10 nm). The likelihood of solvent-swollen micelle cores, as hinted by the heterogeneous micelle structure mentioned above, should be reflected in a number of places within the modified SGP model. We first must make one assumption to facilitate fitting: only IPA can penetrate the ionic cores. This assumption is supported by the compatibility of polar IPA, as well as the incompatibility of nonpolar T, with the ionic S/sS blocks, in addition to the chemical dissimilarity of the cosolvents. The Hansen solubility parameter distance (Ra), which contributes to χi‑j ,47 provides a quantitative measure of relevant incompatibility levels: 10.2 MPa 1/2 (S/sS-IPA), 13.5 MPa1/2 (S/sS-T), and 19.7 MPa1/2 (IPA-T).48 Incorporation of solvent into the micelle cores is taken into account by a composition-dependent core scattering length density given by ρc = φS/sS,c ρS/sS + φIPA,c ρIPA, where φi,c is the volume fraction of component i in the micelle core. The micelle aggregation number is also related to the quantity of IPA in the micelle core by Nagg = φS/sS,c4πr3/3vc, where vc is the volume of the copolymer block in the core (vS/sS = 59.2 nm3), and φS/sS,c = 1 − φIPA,c. The amount of IPA in the system must be conserved to establish physically meaningful results, in which case the scattering length density of the solvent environment is also subject to correction (since ρs = φT,s ρT + φIPA,s ρIPA, where φi,s is the volume fraction of component i in the solvent). Finally, it is worth mentioning that the SANS data for a given ΦT are fitted globally so that the 2−3 contrast schemes are constrained by the same geometric inputs (fitting parameters: r, rHS, Rg,cor, and φIPA,c) and varied only in terms of their contrast and background. Modeling of the SANS data displayed in Figures 5 and S2 affords results that quantitatively agree with those from SAXS. As is evident from Figure 6a, fitted r and rHS values extracted from both scattering methods not only yield the same trends, but values that lie within 5% (r) and 10% (rHS) of each other (some systematic deviation is to be expected, since different models are employed). The SANS analysis further indicates a strong dependence of φIPA,c on ΦT, as shown in Figure 6b. According to these results, the SBI-52 micelles contain an unexpectedly high level (60−80 vol %) of IPA in their cores and become less IPA-swollen as the overall quantity of IPA in the solvent (i.e., 1 − ΦT) is decreased. Since IPA swelling of SBI-52 micelles is coupled with the composition of the surrounding solvent (through conservation of volume; see Supporting Information for mathematical treatment), it is more informative to examine the IPA partition coefficient between SBI-52 micelle cores and the solvent: KD (=φIPA,c/ φIPA,s). Values of KD shown in Figure 6b exhibit qualitatively opposite behavior to φIPA,c (i.e., they increase monotonically with ΦT), thereby conveying that the preferential segregation of IPA molecules into micelle cores is more pronounced as IPA becomes less abundant. More specifically, the values follow an inverse proportion with the amount of IPA present (i.e., KD ∝ (1 − ΦT)−1) leading to nearly exclusive segregation at small

Figure 6. Parameters extracted from SANS fits using the modified SGP model: r and rHS (a) and φIPA,c and KD (b) (●, labeled and color-coded). Data from SAXS fits with the MHS model (○) are included in (a) for comparison. Solid lines are guides for the eye except for KD, which displays an inverse proportional relationship to (1 − ΦT).

quantities of IPA. This finding supports other studies wherein a small addition of ethanol to SBI-52/cyclohexane solutions translated to a notable increase in viscosity.38 Refined Structural Description. As alluded to above, modified-SGP modeling of SANS data yields other informative structural detail: Rg,cor and Nagg. The corona chain radii of gyration, and similarly the corona thickness (t), are presented in Figure 7 and increase with ΦT as expected from solubility

Figure 7. Corona gyration radii (Rg,cor) and thicknesses (t) from SANS (●) and SAXS (○) (labeled and color-coded). Solid lines are linear regressions to the data and serve as guides for the eye.

considerations of TS+EP in T and IPA. The expansion of corona chains into the solvent, taken alone, should translate into smaller micelles based on chain packing arguments, since the molecular shape parameterυ/(a0 lc), where υ is the volume of an S/sS block, a0 is the solvent/corona interfacial area of a TS+EP corona chain, and lc is the critical length of an S/sS blockdecreases.49 However, this expectation contradicts the observed increases in r and rHS. The divergence of these findings further supports the swollen micelle structure explored above. For swollen micelles, the amount of solvent present in the cores also affects their size. As the quantity of IPA in the system increases, the interfacial energy between a E

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former resulting from increased partitioning of IPA into the cores and growing core/solvent interfacial energy, whereas the latter is simply a consequence of improved solvent quality for the corona blocks. Ultimately, the highly plasticized micelle cores discovered might enable direct casting of films that contain more interconnected morphologies than those prepared by a single selective solvent, therefore making them more desirable for diffusion-dependent applications.54

majority IPA phase and the bulk T/IPA solvent decreases, so that higher surface area-to-volume aggregates form and fewer SBI-52 chains are required for stabilization. The core/corona areal density of SBI-52 (σ), which can be computed directly from model parameters [σ = 2Nagg/4πr2 (note that the prefactor of two reflects the fact that two corona chains are present for each polymer chain)], provides a direct reflection of the interfacial energy argument as shown in Table 2. The

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Table 2. Influence of ΦT on Corona Chain Areal Density (σ) and Radii Ratio (r/RH)

ASSOCIATED CONTENT

S Supporting Information *

ΦT

σ (chains/nm2)

r/RH

65% 70% 75% 80% 85% 90%

0.034 0.042 0.056 0.068 0.076 0.088

0.83 0.77 0.76 0.78 0.73 0.66

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.8b03825. Discussion of small-angle scattering of SBI-52 micelles, SANS data, discussion of component volume conservation, DLS autocorrelation functions, SBI-52 micelle hydrodynamic and core radii (PDF)

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trend in areal density can be independently confirmed through determination of Rg,mic/RH, where Rg,mic and RH correspond to the radius of gyration and hydrodynamic radius of an entire micelle, respectively. The Rg,mic/RH ratio is expected to increase as the areal density of corona chains on a given aggregate decreases.50 The complexity of the current systems, however, makes quantitative extraction of Rg,mic and RH (from lightscattering experiments) challenging. While Rg,mic is unfortunately completely lost to this fact, values of RH can be ascertained from suitable fitting of autocorrelation data (see Supporting Information and Figure S4). The micelle radius of gyration can be approximated by r from SAXS, so the ratio can still be examined, and full independence from SANS can be maintained. The resulting ratios (Table 2) agree favorably with those reported previously for star polymers51 and polymeric micelles52 by decreasing with increasing ΦT, suggesting again that corona chain density increases with ΦT. The overall refined structural picture of SBI-52 micelles enables us to conclude that the contribution of micelle core/bulk solvent interfacial energy far outweighs that of corona chain packing when it comes to final micelle size.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. (K.P.M.) *E-mail: [email protected]. (M.G.) ORCID

Kenneth P. Mineart: 0000-0003-2374-4670 Byeongdu Lee: 0000-0003-2514-8805 Michael Gradzielski: 0000-0002-7262-7115 Richard J. Spontak: 0000-0001-8458-0038 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We graciously thank the Nonwovens Institute at North Carolina State Univ., MANN+HUMMEL GmbH, and the National Science Foundation IRES program (Grant No. 1065466) for financial support. This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. The SANS part of this work is based upon experiments performed at the KWS-2 instrument operated by the Jülich Centre for Neutron Science at the Heinz Maier-Leibnitz Zentrum, Garching, Germany. Allocation of SANS beamtime is gratefully acknowledged. In addition, we want to recognize B. von Lospichl and V. Spiering for valuable discussions and SANS technical support.

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CONCLUSIONS Ion-containing block copolymers remain at the forefront of soft materials research due to their potential in a variety of applications that rely on regulated transport in bulk material or films. One of the most beneficial features of ion-containing block copolymers is their inherent ability to form ionic and nonionic domains that provide solid materials with high ionexchange capacity, molecular/electron permeability, and mechanical resilience. Fabrication of films with controlled morphological characteristics remains challenging due to the physicochemical phenomenon known as solvent templating. While our prior findings53 have demonstrated the possibility of solvent-vapor annealing to achieve near equilibrium morphologies, it would be beneficial to fabricate similar nanoscale structures through a single solvent-casting step. Here, we have established that the self-assembly of an ionic block copolymer in mixed miscible solvents that are counter-selective for the copolymer blocks yields micelles with highly swollen (60−80 vol % polar solvent) cores. A detailed structural analysis indicates that increasing the fraction of nonpolar solvent in the system causes micelle core and corona expansion with the

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