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Interface Components: Nanoparticles, Colloids, Emulsions, Surfactants, Proteins, Polymers

Expanded Kinetic Control for Persistent Micelle Templates with Solvent Selection. Amrita Sarkar, Laurel Evans, and Morgan Stefik Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b00417 • Publication Date (Web): 27 Apr 2018 Downloaded from http://pubs.acs.org on April 27, 2018

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Expanded Kinetic Control for Persistent Micelle Templates with Solvent Selection Amrita Sarkar†, Laurel Evans†,‡ and Morgan Stefik†* † Department of Chemistry and Biochemistry, University of South Carolina, Columbia, South Carolina 29208, United States ‡Present Address: Spring Valley High School, 120 Sparkleberry Ln, Columbia, South Carolina 29229, United States

ABSTRACT

The precision control of nanoscale materials remains a challenge for the study of nanostructureperformance relationships. Persistent micelle templates (PMT) are a kinetic-controlled self-assembly approach that decouples pore and wall control. Here, block copolymer surfactants form persistent micelles that maintain constant template size as material precursors are added, despite the shifting equilibrium dimensions. Prior PMT demonstrations were based upon solvent mixtures where kinetic rates were adjusted with the amount of water cosolvent. This approach is however limited since everhigher water contents can lead to secondary porosity within the material walls. Herein, we report an improved method to regulate PMT kinetics via the majority solvent. This enables a new avenue for expansion of the PMT window to realize templated materials with a greater extent of tunability. In addition, we report a new SAXS-based log-log analysis method to independently test micelle templated series for consistency with the expected lattice expansion with increasing material:template ratio. The PMT window identified by log-log analysis of SAXS data agreed well with independent SEM measurements. The combination of improved micelle control with solvent selection along with SAXS validation will accelerate the development of a myriad of nanomaterial applications.

INTRODUCTION

The controlled self-assembly of surfactants1 and block copolymers (BCPs) has led to a wide range of demonstrated feature sizes in porous materials2-22 that are applicable to numerous electrochemical devices.13,16,19-20 Generally, amphiphilic BCPs are combined with material precursors e.g. metal salts and

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the combination is organized via evaporation-induced self-assembly where the material selectively associates with one polymer block, often poly(ethylene oxide).23 Despite great developments with the number of accessible compositions and morphologies, the simple independent adjustment of pore or wall dimensions while holding the other constant has remained elusive. This challenge has persisted due to the widespread use of equilibrium-based approaches where each feature dimension is subject to the “tyranny of the equilibrium”.24 Kinetic-based approaches such as Persistent Micelle Templates (PMT)25 overcome this limitation by using kinetically-trapped (i.e. persistent) micelles that do not change their size during changes to the solution conditions, e.g. the addition of material precursors. The PMT concept thus separates the formation of a kinetically trapped micelle dimension from the templating of material precursors. We note that tunable wall-thickness has been observed without consideration of kinetic control.26,27 PMT was recently combined with a one-pot titration of material precursors to enable continuously adjustable wall dimensions.28 Since all measurements of electrochemical performance convolve multiple transport processes, it is crucial to broadly realize independent control of each feature dimension in order to deconvolve concomitant processes. Furthermore, the realization of a clearlydefined and predictive synthesis approach opens new opportunities to realize nano-optimized devices where each transport pathway is fully optimized for performance.

The precision control of template materials relies upon precision control of the micelle template. Micelle formation is driven by solvent selectivity where the solvophobic blocks aggregate to form micelle cores, each surrounded by the corresponding solvophilic corona blocks. The equilibrium diameter of a micelle results from the balance of interfacial enthalpy with the entropy associated with chain stretching, as well as other terms. In contrast, the actual diameter of a micelle is a combination of the processing history and the kinetics of chain exchange, in addition to the above thermodynamic considerations. For example, the rate of single chain exchange between micelles is well studied29-39 where the rate scales with a doubleexponential function of the energetic barrier to chain exchange, χN or f(χ)N.40 Here, N scales with the


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molar mass of the solvophobic-core block, f is a monotonic function, and χ is the effective interaction parameter that embodies the enthalpy associated with interface formation but also includes some noncombinatorial entropy.41 Please note that in this context, the relevant χ term is for the interaction of the core block with the solvent. Thus, high-χN conditions can lead to considerably slower exchange rates where micelles become kinetically trapped.42 Such kinetically trapped micelles are the basis of PMT where the high-χN barrier to chain exchange maintains a constant micelle diameter during the addition of material precursors.25,28,43 To date, all PMT demonstrations have relied upon water content alone to regulate χ within THF-rich solutions. With that limited approach the achievement of persistent micelle conditions with low molar mass polymers is particularly challenging where a large volume fraction of water would be needed for sufficiently high-χN conditions. This approach would however cause other deleterious effects during film processing such as secondary porosity within the material walls, vide infra. We present here a method to significantly increase χ during PMT processing via rational solvent selection. A small-angle X-ray scattering (SAXS) based geometric model was previously shown to well-fit persistent micelles during a titration of material precursors.28 There, a natural outcome of constant template size and increasing wall material was a quasi-cube root dependence of d-spacing on the material:template ratio. However, that approach required the input of real-space electron microscopy measurements to enable fitting. Here we present a new approach based upon a log-log coordinate space that is independent of other measurements and enables the direct testing of SAXS data for consistency with PMT lattice expansion.

EXPERIEMNTAL SECTION

Materials. Anhydrous, inhibitor free tetrahydrofuran (THF, 99%, Aldrich) and niobium(V) ethoxide (99.9%, Fisher) were stored inside a glove box and used as received. Hydrogen chloride, 0.5M solution in methanol (Acros Organics) was stored at 2-8 ºC and used without further purification. Ethanol (EtOH,


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200 proof, 100%, Fisher) and methanol (MeOH, 99.8%, Fisher) were dried at room temperature with storage over 50% w/v of molecular sieves (3Å, 8-12 mesh, Acros Organics) for a week.44 37% w/w conc. HCl (ACS grade, VWR), poly(ethylene glycol)methyl ether (PEO-OH, Mn = 5000 gmol-1, Aldrich), 2bromopropionic acid (>99%, Aldrich), and 4-(dimethylamino) pyridine (99%, Aldrich) were used as received. The ligand, tris-(2-dimethylaminoethyl) amine (97%, Aldrich) and catalyst, copper(I) bromide (99.99%, Aldrich) were stored inside a glove box and used as received. Hexyl acrylate (96%, VWR) monomer was passed through basic alumina column just before use. Chloroform (>99%, Aldrich), hexane (>98.5%, Fisher) and dimethylformamide (97%, Aldrich) were used as received.

Polymer Synthesis and Characterization. A poly(ethylene oxide-b-hexyl acrylate) diblock copolymer was used in this study and termed OH. The OH polymer was synthesized by a two-step procedure using a Steglich esterification followed by atom transfer radical polymerization (ATRP). The polymerization procedure was described elsewhere in detail.28 The molar mass of the PHA was determined by comparison to the PEO using a Bruker Avance III HD 300 1H NMR. The molar mass dispersity (Ð) was characterized by a Waters gel permeation chromatography (GPC) instrument equipped with a 515 HPLC pump, a 2410 refractive index detector, and three styragel columns (HR1, HR3 and HR4 in the effective molecular weight range of 0.1-5, 0.5-30, and 5-600 kgmol-1, respectively). THF was used as eluent at 30 ºC temperature and with a flow rate of 1 mLmin-1. The GPC was calibrated with polystyrene standards (2570, 1090, 579, 246, 130, 67.5, 34.8, 18.1, 10.4, 3.4, 1.6 kgmol-1) obtained from Polymer Laboratories. GPC samples were prepared in THF with a concentration of 2-5 mgml-1 and were filtered through 0.2 μm filter media just prior to injection.

Micelle Preparation and Measurements. Solutions were prepared using 100 mg of dried OH polymer in 10 mL of dry solvent, either THF, EtOH, or MeOH at room temperature. The polymer readily dissolved in THF, however, more time and mild shaking were needed for polymer dispersion in alcohols. Next,


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aqueous HCl was added dropwise to a total of 200 μL, i.e. 1.96 vol% for all solutions. The resulting micelle solution was sonicated for 5 min at room temperature to enable chain exchange under kinetically limited conditions.43 Separately, an OH micelle solution was prepared in MeOH-HCl(aq) and was monitored by DLS for 8 hrs to check for micelle rearrangement while quiescent. Dynamic light scattering (DLS) measurements of micelle hydrodynamic diameter were obtained using a Zetasizer Nanoseries ZEN3690 instrument. Solutions for DLS measurement were prepared with OH at a concentration of 10 mgml-1 and were filtered through 0.2 μm filter media just prior to measurement. All measurements were run at least 3 times to confirm reproducibility. All DLS measurements were performed at 25 ºC. For DLS data analysis the viscosities of 0.455 cP, 1.082 cP, 0.547 cP and refractive indices of 1.405, 1,359, and 1.326 were used for the pure solvents, THF, EtOH and MeOH, respectively. For the solvent-water mixtures, viscosities of 0.515 cP, 1.153 cP and 0.607 cP and refractive indices of 1.405, 1.359 and 1.326 were used for 1.96vol% water solutions with THF, EtOH or MeOH, repectively.45-49 We note that a systematic error of ~10% would result by approximating these solutions as the pure solvents. Lognormal size distributions were well fit for each DLS measurement. The corresponding means and standard deviations were reported in Table 2, S1, and S2.

Micelle Templating. The formed micelles were then used to template materials using a titration approach. A predetermined amount of niobium ethoxide was added under near air-free conditions, followed by minor agitation and spin coating. This procedure was repeated to produce samples across a range of material:template (M:T) ratios. Here the M:T mass ratio compares the anticipated final oxide mass (Nb2O5) to the polymer mass. Each aliquot was spin coated for 20s at 1000 RPM under 15%RH as described in detail elsewhere.28,50 Both glass coverslips and silicon wafers were used as substrates. Immediately after spin coating, each sample was removed from the humidity-controlled chamber and placed on a hot plate for 30 minutes at 200 ᵒC for coverslip glass and 8-12 hours at 100 ᵒC for silicon substrates, respectively, to crosslink the material, termed as “aging”. The longer aging period for silicon


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substrates was used since those samples were next calcined to 500 ᵒC to remove the polymer for SEM imaging. The spin coater chamber (Tupperware) was replaced after each coating to avoid solvent residues and improve reproducibility.

X-ray Measurements. X-ray experiments were conducted using a SAXSLab Ganesha at the South Carolina SAXS collaborative (SCSC). A Xenocs GeniX 3D microfocus source was used with a copper target to produce monochromatic beam with a 0.154 nm wavelength. The instrument was calibrated just before measurement, using the National Institute of Standards and Technology (NIST) reference material, 640c silicon powder with the peak position at 2θ = 28.44 ᵒ, where 2θ is the total scattering angle. A Pilatus 300k detector (Dectris) was used to collect the two-dimensional (2D) scattering pattern with nominal pixel dimensions of 172x172 μm. The SAXS data were acquired with an X-ray flux of ~4.1 M photon per second incident upon the sample and a detector-to-sample distance of 1040 mm. Transmission SAXS data were measured to observe the purely in-plane morphology. The 2D images were azimuthally integrated to yield the scattering vector and intensity. Peak positions were fitted using custom MATLAB software. SAXS measurements were reported as the average ± the standard deviation. The error bars for log scale were approximated as 0.434 times the relative error.

Scanning Electron Microscopy (SEM). Top-view images of calcined films were acquired with a Zeiss Ultraplus thermal field emission SEM using an acceleration voltage of 5 keV and an in-lens secondary electron detector. The working distance was maintained at ~3 mm as well as a constant magnification of 400k. Hundreds of SEM measurements were made on each sample to yield statistically significant metrics of pore diameter, and wall-thickness. Data are presented as average values with the error-of-the-mean. Samples with particularly thick walls visually occlude the view of the interior pore diameters and yield pore measurements that are smaller than the actual pore dimensions. With this challenge in mind, a wall:


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pore ratio was defined as a metric to exclude underestimated pore measurements. Here pore size measurements by SEM were considered reliable only if the wall:pore ratio was 0.75 or less.

RESULTS AND DISCUSSION

Thermodynamic Considerations and Solvent Selection. Persistence, that is the lack of chain exchange between micelles, is a quintessential aspect of PMT. Persistence is maintained by using high-χN solution conditions that suppress chain exchange. Hildebrand solubility parameters (δ) provide a semiquantitative method to interpret changes to χ as being proportional to the square of the separation of two δ values. Here, the relevant χ parameter for single chain exchange is between the core block and the solvent mixture. The solubility parameter of solvent mixtures is simply the sum of each solubility parameter weighted by the respective volume fraction. Prior demonstrations showed predictive power with THF-water solutions and various molar mass PEO-b-PHAs.25,28 However, the solubility parameter of THF (18.5-19.53 MPa0.5) is quite similar to that of PHA (16.64 MPa0.5).51 This small separation of solubility parameters implies that the χ between PHA and THF is expected to be quite small and prior studies detected only unimers for PEO-b-PHA in THF without observable micelles (unpublished). Thus, both the micellization and the maintenance of persistent micelles in THF rely upon sufficient water content to raise χ. PMT with low molar mass polymers is particularly challenging where a large volume fraction of 8.09% water was needed for PMT processing.28 This approach however has limited extensibility since further increases to χ with water addition lead to the deleterious formation of secondary porosity within the material walls (Figure S1, SI). Here we instead rationally select alternative solvents based on solubility parameters to enhance χ and slow chain exchange kinetics (Scheme 1). The design of an ideal solvent for PMT is influenced by numerous considerations. 1) The above thermodynamic considerations suggest that χ and thus micelle persistence may be enhanced by selecting solvents with higher Hildebrand solubility parameters that increase the solubility parameter separation between the core block and the solvent.


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Scheme 1: Solubility parameters guide the identification of high-χN conditions to form Persistent Micelle Templates. For example, MeOH (δ~29.7 MPa0.5) and EtOH (δ~26.2 MPa0.5) are considerably more separated from PHA than THF.51 2) The material precursors, often a metal oxide sol, must be soluble in the processing solvent. Hydrogen bonding is one of the most prolific mechanisms for the selective association of sols with the corona block. It follows that solubility of the sol nanoparticles within the solvent often relies upon the possibility to also hydrogen bond with the processing solvent. Thus, ethers and alcohols have been extensively employed elsewhere for the solution processing of sols with block copolymers.52,53 3) The polymer itself must also be dispersible in the processing solvent, an aspect that is subtlety distinct from simply being soluble. Solubility is predictable by selecting solvents with similar solubility parameters to a particular polymer block. The hazard of selecting a solvent that is good for both blocks is that the block copolymer may be dispersed as unimers without aggregating to form micelles. To target micelle formation, the processing solvent should be good for the corona block and poor for the core block. Solvation of the corona is critical for micelle dispersion. 4) For evaporation induced self-assembly, such as PMT, the solvent boiling point and processing conditions of the films must also be considered. Excessively high boiling points >140 ᵒC take considerable time to dry after spin/dip coating. This extended time period makes the maintenance of kinetic control more difficult where regulation of humidity and temperature must be further optimized. In contrast, solvents with too low of boiling points tend to yield less homogeneous films. Finally, within the context of maintaining high-χ conditions one


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must consider the trajectory of mixed-solvent compositions during evaporative processing. Many ethers and alcohols form an azeotrope with water and can lead to concentration/removal of water depending on the details. 5) Finally, the substrate wetting by the processing solution is significantly tuned by the solvent composition. Often substrate surface energies are not ideally matched to the used solvents where slow evaporation can lead to dewetting and the formation of nanostructured islands rather than a continuous film. The issue of wetting is also often addressed by modification of the substrate surface e.g. with plasma cleaning or functionalized silane coatings, depending on the nature of the substrate-solvent pair. When ideal solvents satisfying all these parameters are not feasible then further care is needed to maintain kinetic control throughout processing.

Micellization of OH in Different Solvents. A hallmark sign of dynamic (non-persistent) micelles is the presence of free unimers in solution. This stable unimer population enables continuous exchange of chains between micelles, supporting equilibration. Simple laboratory DLS measurements provide a direct method54,55 to probe for the presence of unimers, with typically a hydrodynamic diameter 1-pentanol (χ=0.76) > THF (χ=0.34). These


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values correspond to a ~3 times higher magnitude of χ for PMA with EtOH than PMA with THF. Similarly, ~5-7 times higher χ values were reported for poly(vinylethylene) (PVE), poly(ethylethylene) (PEE), poly(ethylacrylate) (PEA) and poly(n-butylmethacrylate) (PBMA) in EtOH compared to THF.64,65 One significant difference in terms of intermolecular interactions is that alcohols are able to hydrogen bond with themselves whereas THF is only a hydrogen bond acceptor. Thus THF is unable to hydrogen bond with itself. This significantly reduces the enthalpic cost for chain exchange where PHA-THF interactions are temporarily made by breaking THF-THF interactions. By comparison, the increased strength of EtOH-EtOH interactions via hydrogen bonding thus poses a much larger enthalpic barrier to chain exchange. This difference is qualitatively shown by the different values of solubility parameters in Scheme 1. Thus, for a given water content during micelle templating, it is reasonable to expect EtOH solutions much better inhibit chain exchange than THF solutions. This advantage allows EtOH solutions to maintain micelle persistence over wider titration ranges.

Micelle Templating in MeOH (highest-χ). Lastly, the effect of solvent selection on PMT was examined for the solvent having the highest anticipated χ, MeOH. We note that reported χ values for PMA, PVE PEE, PEA and PBMA increased with lowered carbon content,63-65 highlighting the expectation for MeOH to have a higher χ value with PHA than EtOH. Notably, MeOH was the only solvent in this study where DLS measurements indicated both the presence of micelles and an undetectable presence of free-unimers in the pure, anhydrous solvent. The same 1.96 vol% water was used to maintain similar sol-gel chemistry as the other solvent mixtures examined here. Multiple titration series were carried out and the SAXS profiles for OH-MeOH-Series1 are shown in Figure S8. Like the other solvents, the MeOH series all increased in d-spacing with material additions and these data were plotted in a log-log coordinate space where the lattice expansion with titration followed the expected slope of 1/3 through M:T~2.15-2.40 (Figure 6a). Again, the variability in PMT exit point was attributed to a continuous transition from persistent to dynamic micelles that is sensitive to time and temperature. The SAXS profiles again


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consisted of 2 peaks with an approximate 1:2 q-ratio and the SEM images contained short and medium range ordering. Notably the samples processed from MeOH exhibited improved ordering and contained limited regions with 2-fold and 4-fold symmetry (Figure 7). The combined 3 MeOH series were fitted within the apparent PMT window using the same paracrystalline PMT model and yielded a goodness of fit R2=0.932 (Figure 6b). Separate consideration of the SEM data alone indicated a similar PMT window with a relatively constant average pore size of 12.94 nm until a decrease to 12.75 nm at M:T=2.07 for OHMeOH-Series1 and a continued decrease to 12.44 at M:T=2.16 (~4% decrease). Again, the PMT region identified by SEM and log-log analysis were in close agreement (4% difference). Curiously, the average pore size initially decreased in the dynamic region and then later increased, similar to the EtOH series (Figure 6c, Table S5, SI). Here both the SAXS and SEM data indicate that OH does not form persistent micelles in pure-MeOH, but rather requires a small portion of water to be present. Also, the Nb2O5 wallthickness increased monotonically from 6.24 to 9.33 nm during material titration and followed the PMT model with R2=0.974 (Figure 6d). Compared to THF, the increased χ from using MeOH significantly expanded the PMT window and shifted the exit point by 0.64 ∆M:T, as compared to THF. However, in comparison to EtOH, the switch to MeOH resulted in a similar PMT exit point within the uncertainty of the methods. Earlier discussed χ values for hydrophobic polymers with lower alcohols decreased with carbon content and were significantly reduced for THF.63-65 Considering the reported hypersensitivity of single change exchange to minor changes in χN,33 it is surprising that MeOH and EtOH solutions yielded similar PMT windows. Estimated χ values from solubility parameters (Scheme 1) are at best semiquantitative, however, even experimentally measured χ values for analogous systems suggest larger difference are expected for EtOH and MeOH. The observed similar PMT character may be due to other contributing factors, including free-water content and the corona configuration. Although all of the PMT experiments here used the same 1.96 vol% water, the quantity of free-water in solution is different owing to different rates of niobium ethoxide reactions in these solvents: hydrolysis depletes water and


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subsequent condensation releases water. An earlier study66 of trialkoxysilane reported a slower hydrolysis rate in EtOH than MeOH and was attributed partially to solvent viscosity.67

Figure 6. Analysis of micelle templates processed from a MeOH solution. The d-spacings obtained from the principal scattering peaks were plotted in a log-log coordinate space to identify consistency with PMT lattice expansion when the slope is 1/3 (a). The identified region was fitted with a PMT SAXS model (b). The average pore size was calculated from independent SEM measurements to confirm the PMT region by a second method (c). The average wall thickness was also measured by SEM and compared to model predictions (d). A similar slower hydrolysis would be expected for niobium ethoxide in EtOH due to the doubled viscosity in comparison with MeOH. Furthermore, in MeOH the expected ligand exchange to form niobium methoxide will also increase the rate of hydrolysis.66,68 Furthermore, EtOH has a lower relative


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permittivity than MeOH and is expected to yield stronger charge stabilization that more effectively slows the condensation rate. Each of the above described effects of relative hydrolysis and condensation rates would yield additional free-water in solution for PMT in EtOH compared with MeOH. This extra freewater would increase the χ value for EtOH-PMT solutions and could thus compensate for the lower χ value for PHA with pure-EtOH. Lastly, we note that the PEO corona is more elongated in EtOH than MeOH, as noted by DLS and earlier discussion (Figure 1). This could have several effects for hindered chain exchange in EtOH solutions, 1) it increases the volume fraction of solvent in the corona region which would increase the local χ value for extraction of a core block into the corona region and 2) it increases the diffusion time required for chain extraction into the solvent. The latter criteria is consistent with the classical theory of Halperin and Alexander29 as well as recent observation of chain exchange with variable corona environments.69 Regardless, these overall trends highlight a significant role of solvent selection upon micelle behavior and the enabled PMT processing window.

Figure 7. SEM images of OH-MeOH-Series1 in order of increasing Material:Template ratio, 1.23 (a), 1.50 (b), 1.60 (c), 1.68 (d), 1.73 (e), 1.83 (f) 2.07 (g), and 2.39 (h). CONCLUSIONS

In this work the effect of solvent selection on PMT kinetic-control was guided by solubility parameter considerations. Simple DLS measurements confirmed that OH in pure-THF and pure-EtOH yielded dynamic micelles with detectable unimers. In contrast, DLS of OH in pure-MeOH was consistent with


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micelles without detectable unimers, suggesting it as a promising candidate for improved kinetic control. Series of PMT materials were prepared to compare the micelle persistence using detailed SAXS models and independent SEM observations. Here, a new log-log analysis technique was developed using SAXS data alone to test for sample consistency with the PMT model of lattice expansion. The combination of this analysis with a one-pot titration-approach enabled efficient confirmation of PMT conditions before further measurements and model refinement. The PMT exit point was expanded by selecting solvents with higher solubility parameters. Using a constant 1.96 vol% of water, PMT control with THF spanned up to M:T~1.6, with EtOH up to M:T~2.3, and with MeOH up to M:T~2.3. The findings highlight a new avenue to tune the processing window of persistent micelle templates. Continued development in this direction may enable future PMT processes from simple single-solvent systems. These developments support the predictable synthesis of highly tunable nanomaterials that are important for a wide range of applications.

ASSOCIATED CONTENT

Supporting Information

This Supporting Information is available free of charge on the ACS Publication website. SEM of waterrich film, 1HNMR and GPC of PEO-b-PHA, derivation of log-log coordinate space, SAXS, SEM and measurement summaries for samples series from THF, EtOH, and MeOH.

AUTHOR INFORMATION

Corresponding Author

*Email: [email protected]

Notes


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The authors declare no competing financial interest.

ACKNOWLEDGEMENTS

MS and AS acknowledge support by the National Science Foundation (DMR-1752615). This work made use of the South Carolina SAXS Collaborative, supported by the NSF Major Research Instrumentation Program (DMR-1428620). A.S. acknowledges partial support from the Leon Schechter Endowed Fellowship for Polymer Nanocomposites. L. E. thanks ACS Project SEED for financial support.

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