Block polymer structure-directing agents (SDA) enable the production of porous nanoscale materials. Most strategies rely upon polymer equilibration where diverse morphologies are realized in porous functional materials. This review details how solvent selectivity determines the polymer SDA behaviors, spanning from bulk-type to solution-type. Equilibrating behavior of either type, however, obscures nanostructure cause-and-effect since the resulting sample series convolve multiple spatial variations. Solution-type SDA behaviors include both dynamic and persistent micelles. Persistent micelle templates (PMT) use high solvent selectivity for kinetic entrapment. PMTs enable independent wall thickness control with demonstrated 2 Å precision alterations. Unimodal PMT pore size distributions have spanned from 11.8 to 109 nm and multimodal pore sizes up to 290 nm. The PMT method is simple to validate with diffraction models and is feasible in any laboratory. Finally, recent energy device publications enabled by PMT are reviewed where tailored nanomaterials provide a unique perspective to unambiguously identify nanostructure–property–performance relationships.
Widely tunable persistent micelle templates via homopolymer swelling
The combination of precision control with wide tunability remains a challenge for the fabrication of porous nanomaterials suitable for studies of nanostructure–behavior relationships. Polymer micelle templates broadly enable porous materials, however micelle equilibration hampers independent pore and wall size control. Persistent micelle templates (PMT) have emerged as a kinetic controlled platform that uniquely decouples the control of pore and wall dimensions. Here, chain exchange is inhibited to preserve a constant template dimension (pore size) despite the shifting equilibrium while materials are added between micelles. Early PMT demonstrations were synthesis intensive with limited 1–1.3× pore size tuning for a given polymer. Here we demonstrate PMT swelling with homopolymer enables 1–3.2× (13.3–41.9 nm) pore size variation while maintaining a monomodal distribution with up to 250 wt% homopolymer, considerably higher than the ∼90 wt% limit found for equilibrating micelles. These swollen PMTs enabled nanomaterial series with constant pore size and precision varied wall-thickness. Kinetic size control here is unexpected since the homopolymer undergoes dynamic exchange between micelles. The solvent selection influenced the time window before homopolymer phase separation, highlighting the importance of considering homopolymer–solvent interactions. This is the first PMT demonstration with wide variation of both the pore and wall dimensions using a single block polymer. Lastly this approach was extended to a 72 kg mol −1 block polymer to enable a wide 50–290 nm range of tunable macropores. Here the use of just two different block polymers and one homopolymer enabled wide ranging pore sizes spanning from 13.3–290 nm (1–22×).
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- Award ID(s):
- 1752615
- NSF-PAR ID:
- 10130905
- Date Published:
- Journal Name:
- Soft Matter
- Volume:
- 15
- Issue:
- 26
- ISSN:
- 1744-683X
- Page Range / eLocation ID:
- 5193 to 5203
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Graphical abstract -
Porous nanoscale carbonaceous materials are widely employed for catalysis, separations, and electrochemical devices where device performance often relies upon specific and well-defined regular feature sizes. The use of block polymers as templates has enabled affordable and scalable production of diverse porous carbons. However, popular carbon preparations use equilibrating micelles which can change dimensions in response to the processing environment. Thus, polymer methods have not yet demonstrated carbon nanomaterials with constant average template diameter and tailored wall thickness. In contrast, persistent micelle templates (PMTs) use kinetic control to preserve constant micelle template diameters, and thus PMT has enabled constant pore diameter metrics. With PMT, the wall thickness is independently adjustable via the amount of material precursor added to the micelle templates. Previous PMT demonstrations relied upon thermodynamic barriers to inhibit chain exchange while in solution, followed by rapid evaporation and cross-linking of material precursors to mitigate micelle reorganization once the solvent evaporated. It is shown here that this approach, however, fails to deliver kinetic micelle control when used with slowly cross-linking material precursors such as those for porous carbons. A new modality for kinetic control over micelle templates, glassy-PMTs, is shown using an immobilized glassy micelle core composed of polystyrene (PS). Although PS based polymers have been used to template carbon materials before, all prior reports included plasticizers that prevented kinetic micelle control. Here the key synthetic conditions for carbon materials with glassy-PMT control are enumerated, including dependencies upon polymer block selection, block molecular mass, solvent selection, and micelle processing timeline. The use of glassy-PMTs also enables the direct observation of micelle cores by TEM which are shown to be commensurate with template dimensions. Glassy-PMTs are thus robust and insensitive to material processing kinetics, broadly enabling tailored nanomaterials with diverse chemistries.more » « less
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Abstract The predictive self‐assembly of tunable nanostructures is of great utility for broad nanomaterial investigations and applications. The use of equilibrium‐based approaches however prevents independent feature size control. Kinetic‐controlled methods such as persistent micelle templates (PMTs) overcome this limitation and maintain constant pore size by imposing a large thermodynamic barrier to chain exchange. Thus, the wall thickness is independently adjusted via addition of material precursors to PMTs. Prior PMT demonstrations added water‐reactive material precursors directly to aqueous micelle solutions. That approach depletes the thermodynamic barrier to chain exchange and thus limits the amount of material added under PMT‐control. Here, an ex situ hydrolysis method is developed for TiO2that mitigates this depletion of water and nearly decouples materials chemistry from micelle control. This enables the widest reported PMT range (M:T = 1.6–4.0), spanning the gamut from sparse walls to nearly isolated pores with ≈2 Å precision adjustment. This high‐resolution nanomaterial series exhibits monotonic trends where PMT confinement within increasing wall‐thickness leads to larger crystallites and an increasing extent of lithiation, reaching Li0.66TiO2. The increasing extent of lithiation with increasing anatase crystallite dimensions is attributed to the size‐dependent strain mismatch of anatase and bronze polymorph mixtures.
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Kinetically trapped (“persistent”) micelles enable emerging applications requiring a constant core diameter. Preserving a χN barrier to chain exchange with low- N requires a commensurately higher χ core–solvent for micelle persistence. Low- N , high- χ micelles containing fluorophobic interactions were studied using poly(ethylene oxide- b -perfluorooctyl acrylate)s (O 45 F X , x = 8, 11) in methanolic solutions. DLS analysis of micelles revealed chain exchange only for O 45 F 8 while SAXS analysis suggested elongated core block conformations commensurate with the contour lengths. Micelle chain exchange from solution perturbations were examined by characterizing their behavior as templates for inorganic materials via SAXS and SEM. In contrast to the F 8 analog, the larger χN barrier for the O 45 F 11 enabled persistent micelle behavior in both thin films and bulk samples despite the low T g micelle core. Careful measures of micelle core diameters and pore sizes revealed that the nanoparticle distribution extended through the corona and 0.52 ± 0.15 nm into the core–corona interface, highlighting thermodynamics favoring both locations simultaneously.more » « less
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/c9sm00484j
