Enlarged Pore Size in Mesoporous Silica Films Templated by Pluronic

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Enlarged Pore Size in Mesoporous Silica Films Templated by Pluronic F127: Use of Poloxamer Mixtures and Increased Template/SiO2 Ratios in Materials Synthesized by Evaporation-Induced Self-Assembly Darren R. Dunphy, Pratik H. Sheth, Fred L Garcia, and C. Jeffrey Brinker Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/cm5031624 • Publication Date (Web): 10 Dec 2014 Downloaded from http://pubs.acs.org on December 14, 2014

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Chemistry of Materials

Enlarged Pore Size in Mesoporous Silica Films Templated by Pluronic F127: Use of Poloxamer Mixtures and Increased Template/SiO2 Ratios in Materials Synthesized by EvaporationInduced Self-Assembly

Darren R. Dunphya,*, Pratik H. Shetha, Fred L. Garciaa, C. Jeffrey Brinkera,b,* a

University of New Mexico/NSF Center for Micro-Engineered Materials, Department of Chemical and Biological Engineering, Albuquerque, NM 87131, United States b

Sandia National Laboratories, Advanced Materials Laboratory, Albuquerque, NM 87106, United States

*To whom correspondence should be addressed: [email protected], [email protected]

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Although evaporation-induced self-assembly (EISA) has proven to be a convenient method for synthesizing nanoporous silica films (and particles), accessing material structures with pore sizes larger than ca. 10 nm remains experimentally inconvenient. The use of pore swelling agents (SAs), commonly used during the hydrothermal synthesis of mesoporous silicas, results in little or no pore size expansion due to evaporation or phase separation. Moreover, diblock copolymer templates can yield large pores, but are quite expensive and generally require the addition of strong organic cosolvents. Here, we hypothesized that pores templated by the Pluronic triblock polymer F127 could be successfully enlarged, without phase separation, by using a chemically similar, non-volatile, secondary Pluronic (P103) as the SA. We find pore size increased up to 15 nm for a spherical pore morphology, with a phase transition to a multilamellar vesicle (MLV) based nanostructure occurring as the P103/F127 ratio is further increased. This MLV phase produces even larger pore sizes due to collapse of concentric silica shells upon template removal. Remarkably, F127 alone exhibits expansion of pore size (up to ca. 16 nm) as the template/silica ratio is increased. We find appearance of the MLV phase is due to geometric packing considerations, with expansion of F127 micelle size a result of favorable intermolecular interactions driven by the large polyethylene oxide content of F127. Other Pluronic polymers with this feature also exhibit variable pore size based on the template/silica ratio, enabling the synthesis of mesoporous films with 3D pore connectivity and truly variable pore size of ca. 4.5 to almost 20 nm.

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Particles and films of mesoporous silica (MPS) templated by condensation of solutionphase sol-gel precursors around self-assembled surfactant mesophases, either through solution processing1,2 or evaporation-induced self-assembly (EISA),3,4 have become an important class of nanostructured materials, with potential applications ranging from catalytic supports5 and adsorbents for environmental remediation6 to drug delivery7,8 and nanoporous molecular separation membranes.9,10 A prominent feature of these materials is the ease in which pore size can be engineered through the identity of commonly available off-the-shelf surfactant templates, starting at ca. 2 nm,11,12 though the use of alkylammonium surfactants such as cetyltrimetylammonium bromide (CTAB),1 up to approximately 10 nm with amphiphilic block co-polymer templates such as the poloxamer series of poly(ethylene oxide)- poly(propylene oxide)- poly(ethylene oxide) triblocks (PEO-PPO-PEO, exemplified by the SBA series of mesoporous silicas).13-15 However, a need for greater pore size (pushed in part by the use of MPS to deliver large biomolecular cargos such as DNA or functional proteins)16-19 has driven research into methods for expanding the upper limit of this range. One strategy that has been successful in synthesizing porous materials with pore sizes of even greater than 40 nm20 has been the use of specialized block copolymers as templates.19-24 We note that these templates are often not commercially available,19,22 or are rather expensive specialty materials.20,21,23,24 Furthermore, limited solubility in alcohols necessitates the addition of strong cosolvents such as tetrahydrofuran21,24 or even benzene.20 For the synthesis of porous thin films, these issues negate an important feature of the EISA process (the use of low cost and environmentally/ toxicologically benign materials and solvents), spurring the exploration of other methods to access large pore size.. For example, hydrothermal restructuring has been used to transform the pore size distribution of preformed MPS with added cosolvents (forming 20 nm pores)25 or salts



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(yielding pores of ca. 50 nm in size).26 Overall, however, the most common strategy for expanding pore size in MPS, has been the use of swelling agents (SAs), non-polar molecules12,13,19,27-29 or polymers30-34 that partition into the hydrophobic core of surfactant aggregates or mesophases, increasing the thickness or diameter of micelle or mesophase templates and consequently yielding larger pore dimensions after removal of the organic phase. For example, the SA trimethylbenzene (TMB) has been used in conjunction with Pluronic P123 (Pluronic being a trade name for a series of poloxamer polymers) to yield mesoporous foam-like particles with pore size distributions extending beyond 30 nm,29 an increase of over. 3.5x over pores templated with P123 alone (9 nm).15 For polymeric SAs, poly(propylene glycol) (PPG) has been shown to double pore size in particles of organically modified silica (also templated with P123).35 Extension of the SA strategy to mesoporous films (or particles) produced by EISA has had limited success, however. In the EISA process, self-assembly of the surfactant mesophase is driven by evaporation of solvent; increasing the concentration of the template past the critical micelle concentration (CMC) induces aggregation, with subsequent condensation of the molecular precursor around the mesophase stabilizing the material toward removal of this organic phase to form an open porous network. In parallel with solvent evaporation, however, low molecular weight SAs such as TMB are also volatilized, inhibiting the expansion of surfactant aggregates. We note that it has been reported that addition of the SA n-butanol, which has a low vapor pressure, can increase pore size during the EISA synthesis of porous silica films templated by Pluronic F127.28 We have qualitatively replicated this result, increasing pore size by up to ca. 15% upon the addition of n-butanol (Figure 1A), but find that this additive does not act as a SA in that it (or any other alcohol) is not detectable by FTIR (with an estimated volume 4 ACS Paragon Plus Environment

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Figure 1. Use of standard pore SAs in EISA processing. A) Pore spacing (obtained from x-ray scattering data, see discussion for Figure 5) for F127-templated films synthesized with ethanol partly or completely replaced by alcohols of higher molecular weight and lower volatility, consistent with an increase in pore size like that reported in the literature.24 Data point for 100% n-butanol is not present due to poor film quality, B) Transmission FTIR data of assynthesized films (before calcination) synthesized with ethanol replaced with other alcohols. Data, normalized to the ca. 1100 cm-1 Si-O stretch band, shows no significant differences in the C-H region between films made with different alcohols replacing ethanol, indicating that nbutanol and n-propanol are not retained in the film during the EISA process and thus cannot be expanding pore size based on a simple SA mechanism. C) TEM data (from a scraped sample) for F127-templated SiO2 films with added PPG, showing phase separation of PPG (large ca. 200 nm voids) from F127 (smaller pores) during EISA. detection limit >0.5%) in films after coating (Figure 1B). Rather, as has been measured for Pluronic F87,36 addition of cosolvents, even at a few volume percent, can drastically alter poloxamer aggregation behavior; given that the presence of n-butanol favors aggregate (i.e., 5 ACS Paragon Plus Environment


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micelle) formation relative to ethanol or water (as manifested by a lowered CMC,)36 it is likely that the larger pores result from an increase in polymer micelle aggregation number and not from partitioning of this solvent into the hydrophobic core of the F127 phase. We do note that the relatively non-volatile small molecule SA 1,3,5 triisopropylbenzene has been incorporated into CTAB templated films synthesized via EISA,37 but rather than pore size expansion, a distortion of pore morphology was observed. For non-volatile SAs (polymers), rapid film formation during EISA generally results in partial or full phase separation between the SA and template mesophase,3,30,31,33,34 a phenomenon confirmed by us in spin-coated films templated by mixtures of PPG and F127 (Figure 1C). Phase separation was avoided in one report where PPG was added to F127-templated sols,38 but instead of dip- or spin-coating, a porous material was produced by slow evaporation of the sol, a procedure that does not yield oriented thin films. Moreover, addition of PPG in this procedure was primarily used to control the mesophase morphology, with the average pore size of the calcined material actually decreasing relative to templating by F127 itself. We thus began a search for a new class of SAs for expanding the pore size of F127/silica films (with emphasis on F127 as the primary template due to the already large baseline of 8 nm pore size obtained by using this polymer alone, without other additives), seeking a non-volatile polymeric material with more favorable molecular interactions with the primary PEO-PPO-PEO template (thus reducing or eliminating phase separation). Seeing that Pluronic surfactants can form mixed micelles,39-41 binary blends of Pluronic surfactants have been successfully used to vary pore size in a range between that obtained with pure mesophases of either surfactant (keeping the total mass of template constant),42-44 and that the pore size of mesoporous carbon templated by F127 can be increased by the addition of Brij non-ionic surfactants,45 we 6 ACS Paragon Plus Environment

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hypothesized that large-pore mesoporous silica films could be synthesized using EISA with the template F127 and the addition of a chemically similar secondary Pluronic polymer that functions as a SA, without inducing phase separation. Our results confirm this hypothesis, with a ca. 2x increase in pore diameter from 8 to 15-16 nm possible with the addition of the Pluronic P103. Interestingly, above a P103:F127 ratio of ca. 0.8 (keeping the F127/SiO2 mass/mole ratio constant), we observe a phase transition to a mesostructure templated by multilamellar vesicles (MLVs); normally, lamellar structures formed in EISA are oriented parallel to the substrate and collapse upon calcination, but because MLVs contain isotropic lamellae, film structure is isotropic and is stable to template removal by pyrolysis. The pore size distribution (PSD) in MLV templated films is significantly broadened and shifted to larger pore sizes (with a maximum at 30 nm), a result of shifting of concentric silica shells during template removal. Comparison of F127 or P103 templating behavior by themselves versus templating behavior by mixtures of the two Pluronics yields another surprising finding; variation of F127/silica ratio alone can result in increased pore size, again up to ca. 15 nm; P103 does not exhibit this same property. Systematic studies show that the formation of the MLV phase in F127/P103 templated silica films is guided by geometric polymer packing considerations (with a minimum PEO content needed to avoid phase separation), and the expandability of F127 aggregates during EISA appears to be a result of favorable intermolecular interactions largely driven by the large hydrophilic PEO block. Similar to F127, other poloxomers with large PEO content also exhibit pore size expansion as the mass template/moles SiO2 ratio is increased, with Pluronic F108 yielding films with pore sizes of almost 20 nm as determined by N2 adsorption. The remarkable results of these studies show that it is possible, using low cost materials and simple procedures, to synthesize porous silica films using EISA with 3D pore networks with pore sizes from 4.5 to



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almost 20 nm (spherical pores) or 30 nm (irregular pores formed by MLV-templated films), with expected impact in areas such as supports for nanostructured catalysts or (upon extension to aerosol-generated particles) drug delivery vehicles.7,16

Methods To synthesize Pluronic-templated silica films via EISA, the polymer was first weighed out into a glass vial and dissolved by the addition of 0.6 ml of absolute ethanol and 0.16 ml of 0.05 M HCl. Typical synthesis conditions used 25 mg of F127 or other Pluronic, plus 0 to 65 mg of the second polymer. The formulas and molecular weights of the Pluronic templates used in this work (obtained as gifts from BASF Corp., or purchased from Sigma-Aldrich) are given in Table S1. After polymer dissolution, 0.122 ml of tetraethylorthosilicate (TEOS, obtained from Sigma-Aldich at 98% purity) was added, followed by brief shaking and aging for 10 minutes at room temperature. Films were spin-coated at 2000 rpm at 15-20% relative humidity onto polished Si substrates. Template removal was achieved by heating at 350° C for 30 minutes (with complete pyrolysis of template under these conditions confirmed by FTIR measurements). For F127-templated films with a secondary Pluronic added, the F127/SiO2 ratio was kept constant at 45.4 mg/mmol, with the mass ratio of P103/F127 specified in the text or figures. All other Pluronic formulations are listed in units of mg polymer/mmol SiO2 to facilitate comparison of films templates with polymers of disparate molecular weights. Grazing-incidence small-angle x-ray (GISAXS) measurements were performed either on beam line 8-ID at the Advanced Photon Source at Argonne National Labs46 using a wavelength of 1.6868 Å, a sample-to-detector distance of 1200 mm, and a 2048 x 2048 Marr CCD detector, or in-house using a Bruker NANOSTAR SAXS instrument using a Cu Kα (1.54 Å) microfocus


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source, high flux pinhole set (400 µm aperture), a sample-to-detector distance of 100 cm, and a 2048 x 2048 Vantec area detector. The analysis angle (0.18-0.20°) was selected following a reflectivity measurement to determine a value greater than the critical angle of the film but less than that of the substrate (i.e. the grazing incidence condition). All GISAXS data is plotted using a logarithmic scale to accentuate weak scattering features, with qz zero set on the transmitted beam. N2 adsorption isotherms were collected by deposition and processing of films as above on 8 x 11 mm 96 MHz surface acoustic wave (SAW) devices,47 with isotherms collected by a combination of a Micromeritics ASAP 2020 surface area and porosimetry analyzer to control pressure and a homebuilt oscillator circuit to monitor mass adsorption via measurement of resonant frequency.47 TEM imaging, performed in under-focus conditions, was with a JEOL 2010 operating at an accelerating voltage of 200kV. Samples for TEM were prepared by scraping the film of interest with the corner of a razor blade and transferring the fragments to a carbon-coated copper grid (Pacific Grid Tech Cu-200HN). SEM imaging was at 2 kV using a Hitachi S-5200 field emission SEM. Raman spectra were obtained on scraped films using a Thermo DXR Raman spectrometer operating at 780 nm, 24 mW laser power, and 50 µm circular spot size, with signal averaged acquisition times typically 20-30 minutes.

Results and Discussion Figure 2 shows the initial results of our studies into using a secondary Pluronic surfactant as a swelling agent in F127-templated films. In Figure 2A, TEM indicates a nanostructure formed by packing of spherical micelles for films templated by F127 alone, consistent with previous literature,48 with pore size of ca. 8 nm.. As P103 is added to the precursor sol, the pore size increases to ~15 nm while maintaining a spherical morphology up to a P103/F127 mass ratio 9 ACS Paragon Plus Environment


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Figure 2. TEM images for F127 templated (A, pore size = ca. 8 nm) and mixed F127/P103 templated films after calcination at B) a mass ratio of P103:F127 = 0.78 (with spherical pores after template removal by pyrolysis, average pore size = ca. 15 nm), and C) a P103:F127 mass ratio of 1.0, showing an unoriented lamellar (3D MLV) phase. Inset (at 2x scale of the main panel) highlights the mechanism of larger pore formation by shift of concentric spherical silica layers after polymer removal. F127/SiO2 = 45.4 mg/mmol for all films. of 0.78 (Figure 2B) without any observed phase separation, affirming our hypothesis regarding the use of a secondary Pluronic as a SA (compare to Figure 1C). However, further increases in the P103/F127 ratio above this results in the appearance of a new phase morphology (Figure 2C), consistent with a nanostructure templated by onion-like lamellar MLVs of F127 and P103 (still without the presence of any macroscopic phase separation). Typically, in films formed by an EISA process, lamellar phases are oriented parallel to the substrate, resulting in structural collapse after removal of the surfactant template.4 Here, however, curvature of the MLV preserves 3D integrity of the film, yielding nested silica shells that produce stable porosity. Furthermore, these shells, because they are not mechanically connected before the removal of template, shift with respect to one another upon film pyrolysis, forming a polydisperse population of pore galleries where the largest pore size exceeds that of the thickness of the original Pluronic layers (Figure 2C, inset).


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Figure 3. Structural characterization data for calcined silica films templated with a mixture of F127 and P103. A) N2 adsorption/desorption isotherms obtained using a SAW technique as described in the text. Magenta numbers are the F127:P103 mass ratios for each film (with the F127:SiO2 ratio kept constant at 45.4 mg/mmol). B) PSDs calculated from experimental isotherms of Panel A, showing increasing pore size and PSD width as the amount of added P103 is increased, C) GISAXS data illustrating the presence of bcc packing at high F127/P103 ratios, transforming to a lamellar structure with increasing P103. In C, the ring at q = ca. 0.02 Å-1 is due to parasitic background scattering and not related to the sample nanostructure. GISAXS images are plotted as the log of signal counts.

Figure 3 contains data further characterizing the pore structure of silica films templated by mixtures of F127 and P103. Panel A shows N2 adsorption isotherms obtained using films deposited onto SAW substrates47 but otherwise processed identically to films deposited onto silicon. Without P103, the first isotherm is typical of data for other types of porous materials produced through templating by F127, including the presence of a significant desorption



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hysteresis that is generally interpreted as arising from narrow constrictions of openings between adjacent larger pores.49 As P103 is introduced, and the P103/F127 ratio increased, the N2 adsorption isotherm is altered in two ways, with both the adsorption step produced by capillary condensation of liquid N2 within the pore network of the film and the desorption hysteresis shifting to higher relative pressure, with the former indicating increasing pore size and the latter reduced constrictions between pores. From the adsorption branch of this data, pore size distributions (PSDs) were calculated using a model for oxide materials with cylindrical pores constructed from a combination of empirical data and density functional theory calculations;50 although the films in this study have different pore morphologies from this model (oblate spheroids for spherical pores compressed during uniaxial shrinkage during film processing, irregular shapes for MLV-based pores), TEM, when used to calculate PSDs, has even more severe limitation, including sample preparation distorting both pore shape and size (see below), and measured pore dimensions sensitive to focus conditions and sharpness of the pore/silica interface. Most importantly, however, TEM is a local analysis technique, with measured values not necessarily reflective of the overall macroscopic sample, a drawback not shared by N2 adsortion. Nonetheless, we do note that our calculations using the DFT method produce similar values for pore size as those estimated from TEM, and that we are focused on examination of trends in relative pore size rather than absolute quantification of this dimension, with any relative errors in the PSD not altering the overall conclusions of our studies. Finally, the PSDs calculated with this method can exhibit fine structure that is an artifact of the fitting process; because these features are not significant, we report an average pore size obtained by fitting of the PSD with a Gaussian distribution. From F127:P103 ratios of 0 to 0.79, within the range where we see spherical pores, the pore size increases along with the width of the PSD distribution. P103:F127


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= 0.79 appears to be the transition point at which the structure transforms to the MLV-based phase; the width and most likely pore size of the PSD at this point suggests that the film analyzed here may have both types of nanostructure morphologies present. Above this ratio, the PSD widens dramatically, and the modal (most likely) pore size increases to ca. 30 nm. This behavior is consistent with pores formed between shifting of nested silica shells with respect to each other; the maximum pore size will be approximately twice the thickness of the polymer aggregate (estimated to be 15 nm here, based on the maximum spherical pore diameter observed), and the PSD will widen due to the tapering width of the galleries formed in this manner (see Figure 1C, inset) along with the irregular shape of the pore volume. The reduced desorption hysteresis observed in the isotherms of Figure 3A indicates that accessibility of N2 at -196° K (the temperature of the adsorption experiment) to the pore volume increases with addition of the P103, consistent with reduced constriction between pores (with the relationship between the hysteresis measured under these conditions and the overall accessibility of the pore volume to larger molecules at higher temperatures in porous materials formed by EISA remaining a general question.) In our TEM analysis of these MLV-templated films, we do not see any evidence of ‘windows’ appearing between adjacent pore galleys where the silica thickness is zero. Rather, we attribute the accessibility of N2 to the internal pore volume of F127/P103 films to defects in the silica shell nanostructure, but note that further study is required to positively identify the source of the reduced desorption hysteresis. GISAXS was also used to characterize film nanostructure;51,52 in this technique, an x-ray beam is incident upon a sample at an angle (ca. 0.2° for silica on Si) between the critical angles of the film/air and film/substrate interfaces, resulting in penetration of the beam into the film with a reflection at the surface of the substrate, maximizing the scattering volume of the 13 ACS Paragon Plus Environment


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Figure 4. Structural characterization data for calcined silica films templated with either F127 or P103. A) GISAXS data for F127 and B) P103 templated films, showing the presence of bcc packing of pores with decreased long range order as the F127:SiO2 ratio is increased in A, and bcc packing of pores transforming to a lamellar structure as the P103:Si is increased in B. C) and D) SAW-based N2 adsorption/desorption data and calculated PSDs for F127 (C) and P103 (D) templated films. While adding more F127 can significantly increase pore size (up to a factor of ca. x2), films templated with P103 do not exhibit any significant expansion of most likely pore size. interrogating beam and allowing scattering analysis of films as thin as a few nm. GISAXS data for F127/P103 templated films is shown if Figure 3C. Without P103, F127 forms an ordered nanostructure based upon body centered cubic (bcc) packing of spherical micelles (see data for 45.4 mg F127/mmol SiO2 in Figure 4a: although the structure of this film from TEM does not 14 ACS Paragon Plus Environment

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appear to have long-range order, this GISAXS data indicates that the appearance of disorder is a result of distortion during TEM sample preparation, suggesting that caution must be made when TEM is used to examine long-range order in this type of nanomaterial). Upon addition of P103, a bcc phase is maintained at low P103/F127 ratios, but with lowered long range order as evidenced by the disappearance of higher order diffraction spots and the appearance of a {110} ring as orientational order of the nanostructure with respect to the substrate is reduced as more P103 is added (first image, Figure 3C). Formation of the MLV phase shows one distinct spot corresponding to correlation of silica layers in the direction perpendicular to the substrate (last panel), with reduced correlation in the plane of the substrate. Importantly, we note that the correlations seen in this x-ray data are not necessarily the source of the porosity measured with N2 adsorption, a result of silica shells moving relative to one another in the horizontal plane during template pyrolysis to form galleries of irregular shape and size; we attribute the observed scattering in Figure 3C to regions of minimal structure shift, preserving the periodicity found in the original lamellar MLV morphology before template removal.. From the GISAXS data in Figure 3C, using qy= 0.04 for the -110 reflection and the relationship that d110 = 1/ √2 x d100, a unit cell parameter of ca. 22 nm can be calculated for the (undistorted by uniaxial shrinkage) bcc lattice formed by the mixture of F127 and P103 at a mass ratio of 1.00:0.60; given that one unit cell is equal to one pore diameter plus the width of the silica wall for this nanostructure, we find the x-ray scattering data to be consistent with the large pore size (ca. 15 nm) measured by N2 adsorption. Appearance of the MLV phase is unique to films templated by a mixture of F127 and P103, as determined by a study of the templating behavior of each polymer alone over Pluronic/silica volume ratios identical to that studied for the binary system (Figure 4). At low 15 ACS Paragon Plus Environment


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polymer/SiO2 mass/mole ratios, both F127 and P103 yield films with bcc packing of spherical pores (Figure 4A and B, top panels). As the Pluronic/silica mass/mole ratio is increased, the film nanostructure becomes less ordered, as seen by the transformation of the GISAXS scattering pattern from discrete reflections to a ring indicative of loss of packing order. At the highest template/silica ratios studied, F127 still forms a 3D network of interconnected pores. However, P103 produces a lamellar phase; because TEM clearly shows that this is oriented with respect to the substrate, we attribute the faint ring seen in this GISAXS pattern to vestigial unoriented 3D film structure. Parallel to GISAXS characterization of F127 and P103 templated materials, N2 adsorption isotherms were obtained for identical films deposited onto SAW devices for determination of PSDs (Figure 4C and D). At an F127/SiO2 ratio of 45.4 mg/mmol, the N2 isotherm is identical to data obtained in previous studies,48 yielding an average pore size of ca. 8 nm with significant desorption hysteresis present from constricted necks or channels between pores. Surprisingly, however, as this volume ratio is increased, the PSD is shifted to higher average pore size, up to ca. 16 nm at a ratio of almost 120 mg F127/mmol SiO2(Figure 4C). (Above this value, the average pore size decreases, an effect we attribute to pore collapse above a critical size during template removal by pyrolysis, as evidenced by the increase in desorption hysteresis.) This variable pore size, produced by expansion of F127 micelle templates (i.e., an increase in aggregation number), has not been previously reported, and is a significant advance in the EISA synthesis of nanoporous silica films as it enables true tuning of pore size over a significant range with only a simple modification (template/silica precursor ratio) to the film fabrication procedure. In contrast, there is no expansion of pore size in films templated by P103 as the polymer/silica ratio is increased, even before the phase transition to lamellar structures. 16 ACS Paragon Plus Environment

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Similar to the discussion of GISAXS data for bcc packing of pores in Figure 3, unit cell parameters calculated using the -110 spacing obtained from the scattering data in Figure 4A (and B, for 45.4 mg/mmol P103/SiO2) are consistent with the large pore sizes measured using N2 adsorption data. For F127, as the polymer/silica ratio is increased from 45.4 to 118.4 mg/mmol, the overall unit cell does not increase significantly, indicating that expansion of the pore size is accompanied by a decrease in pore wall thickness (from ca. 9 to only 4 nm, based on a combination of GISAXS and N2 adsorption data). To explore the underlying mechanisms for the phase behavior of F127 described here, both alone and mixed with P103, a series of studies were conducted to examine three critical aspects of self-assembly: geometric considerations of template packing, intermolecular interactions, and dynamics of self-assembly. First, for packing geometry, a generalized phase diagram for silica films synthesized using EISA with binary mixtures of F127 (maintaining the F127/SiO2 ratio at 45.4 mg/mmol) and a secondary Pluronic template was measured to investigate the origin of the MLV phase when using a mixture of F127 and P103 to template porosity in silica films. This data, plotted in Figure 5A (with TEM data used to construct this diagram given in Figure S1) as a function of secondary Pluronic PEO and PPO block molecular weight, shows two important trends. At sufficiently low secondary Pluronic/F127 ratios, after template pyrolysis, all films exhibit a nanostructure comprised of open pores formed by spherical aggregates of polymer (e.g. Figure 2B). However, as this ratio is increased, a diagonal line separates secondary template behavior into a region where only spherical pores are formed, and a region where a phase transformation to a lamellar-type nanostructure occurs. This phenomenon can be explained by simple geometric considerations as outlined in the concept of the critical packing parameter (cpp);53 incorporation of secondary Pluronics with large hydrophilic PEO 17 ACS Paragon Plus Environment


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Figure 5. Data collected to help understand the templating behavior of F127, P103, and mixed F127/P103 in mesoporous films synthesized via EISA. A) Phase behavior of F127 mixed with a secondary Pluronic template as a function of secondary polymer composition, keeping F127/SiO2 constant, as determined by TEM of calcined films (with TEM data contained in Figure S2), NOL = non-oriented lamellar, ps = phase separated, OL= oriented lamellar. F127 is included as a point to show that increased F127/SiO2 ratios also yield phase data consistent with the addition of a secondary polymer with high PEO content. B) Raman scattering data for the C-H stretch region obtained on as-deposited films, scraped from Si substrates, with polymer/SiO2 ratios from 45.4 to 74.0 mg/mmol (key given in middle panel). C) Spacing of pores in the plane of the substrate for different Pluronic templates measured using GISAXS of calcined films, with slopes of linear fits given for each template. blocks into F127 micelles maintains the high positive curvature, and thus the spherical shape, of the template aggregate, while secondary Pluronics with low PEO content enrich the hydrophobic aggregate core with more PPO, reducing overall aggregate curvature and driving a phase transformation to a mesophase with lamellar geometry (we note that under the conditions studied, we do not observe the appearance of pores templated by cylindrical aggregates of intermediate cpp.) The second trend observed in this phase diagram, also related to the PEO content of the secondary Pluronic, is the presence of phase separation at very low PEO/PPO 18 ACS Paragon Plus Environment

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ratios (the green region of Figure 5A). This indicates that miscibility between mixed Pluronics during EISA is not related to the size of the hydrophobic domain, but rather to the molecular weight of the PEO block, an unexpected result based on the overall energetics of poloxomer selfassembly being dominated by the PPO component.54 The lamellar (non-phase separating) area of the phase diagram in Figure 5A can be further subdivided into two zones based on the orientation of the nanostructure with respect to the substrate, with L35 and P65 yielding lamellae oriented parallel to this plane (the orange area), and larger secondary Pluronics (F84, P103, P123) producing nonoriented phases (the red region). Here, we postulate that phase orientation is related to the timing of final mesophase assembly during the EISA process. For template systems that undergo a progression of mesophase morphologies as the volume percentage of polymer increases during solvent evaporation, lamellar phases should appear near the end of the film formation process where the influence of the substrate on overall mesophase orientation should be the greatest. In contrast, for templates or template mixtures that aggregate directly into lamellar mesophases early in the assembly pathway (at lower volume percentage), the role of substrate interactions is minimized, allowing the formation of unoriented lamellar phases. Assessment of this hypothesized mechanism awaits appropriate in situ GISAXS studies; although GISAXS has been used to follow the pathway of mesophase formation during EISA, these studies have been focused on the formation of 3D (micellular and minimal surface based cubic) and 2D (hexagonal)55,56 phases as lamellar structures have, until this report, been of little interest in relation to the synthesis of porous films due to collapse of oriented lamellar structures upon template pyrolysis. We do note, however, that consistent with this mechanism, we predict that films synthesized using a single Pluronic (L121) with a very low PEO/PPO ratio (and thus an increased propensity to directly 19 ACS Paragon Plus Environment


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assemble into low curvature mesophases, skipping the formation of round aggregates) would also exhibit unoriented lamellar morphology; this was confirmed by TEM analysis of calcined films (Figure 6), with SEM analysis of this film (inset) showing surface topography consistent with a mechanism of film assembly from preformed MLVs. Although the MLV structure is not as apparent in this TEM as that seen in Figure 2, examination of the edges of the film fragment (as highlighted by the yellow arrow in Figure 6) show that the silica walls terminate in a ‘fanlike’ structure consistent with an unoriented lamellar morphology (with similar features observed for the films analyzed in Figure S1). We also note that an alternate interpretation of the features in Figure 6, that of the presence of local 2D hexagonal packing of pores, is ruled out by the lack of scattering features attributable to such a nanostructure within GISAXS data of this film (not shown). To examine the effect of intermolecular interactions on aggregate formation, Raman spectroscopy of Pluronic/silica films (before template pyrolysis) was employed as a probe of polymer ‘ordering’, a qualitative measure we relate to the overall favorability of poloxomer intermolecular interactions. Specifically, the C-H stretch region at ca. 3700-3000 cm-1 has been correlated to the degree of interchain in ordering poloxamer micelles, with the intensity ratio of the PPO and PEO asymmetric methylene stretches at 2875 and 2885 cm-1, respectively, to the asymmetric PPO methyl stretch at 2930 cm-1 increasing with degree of polymer ordering within micelle aggregates.57 Although nanoscale ordering can also be related to assembly dynamics,58 we note that, even as the polymer/silica ratio is increased, all films investigated here follow an identical self-assembly pathway, minimizing the effects of assembly rate and mechanism in proportion to the thermodynamics of intermolecular interactions.


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Figure 6. TEM image of a calcined (porous) silica film templated by L121 (45.4 mg/mmol SiO2) showing the presence of a non-oriented lamellar phase stable towards calcination. The inset is a plan view (contrast enhanced) SEM image of the surface of the same film showing evidence of templating by unoriented MLVs. In the TEM image, an arrow points to the edge of the film fragment, showing the termination of the dark silica layers with a ‘fan’ structure consistent with a MLV-based nanostructure.

If changes in molecular interaction strength have a role in driving the observed phase transition from spherical to MLV aggregate morphologies in F127/P103 templated films, or in the increase in F127 aggregate size as the template/silica ratio is increased, then we expect this ratio to shift (with increasing values indicating more favorable interactions). Data from this study is presented in Figure 5B. In the top panel, as the F127/silica ratio is increased from 45.4 to 73.0 mg/mmol, there is a substantial increase in both the 2875/2930 cm-1 and 2885/2930 cm-1 ratios (20.6% and 19.0%, respectively), indicating a substantial increase in order for both the PEO and PPO blocks of the polymer. For P103/SiO2 films (middle panel), there is also an increase in these ratios over this template concentration range, but it is significantly less (+9.2% for 2875/2930 cm-1, and +3.3% for 2885/2930 cm-1), suggesting less thermodynamic driving force for increasing aggregate size. A similar trend is observed for F127/P103 templated silica films (last 21 ACS Paragon Plus Environment


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panel), with 2875/2930 cm-1 increasing by 4.9% and 2885/2930 cm-1 increasing by 2.9% as the mass ratio of F127/P103 (keeping F127/SiO2 constant at 45.4 mg/mmol) is raised from 0.5 to 1.0; these numbers are consistent with formation of the MLV phase being driven by packing considerations as discussed above. Comparing the results for F127 and P103, we hypothesized that the larger increase in order seen for the former as the template/silica ratio was increased is due to a greater PEO content, and that other Pluronic templates with similar PEO/PPG ratios will also exhibit an increase in pore size as template/silica is raised, conclusively linking polymer composition with expansion of micelle size. Raman spectra were collected for F84, P123, F68, and F87/silica films at template/SiO2 ratios between 45.4 and 73.0 mg/mmol, as in Figure 5B; this data, plotted in Figure S2, confirms that poloxmers with PEO content > 50% do indeed exhibit significantly increased ordering. Pore size vs. template/silica ratios for silica films templated by all of these polymers was then measured, with pore center-to-center spacing indirectly assessed with GISAXS using the (-110) in-plane reflection (Figure 5C). For ordered films with bcc packing, this planar spacing represents the closest contact between pores; as the nanostructure becomes disordered, scattering at this position becomes indicative of average closest pore-to-pore spacing. In-plane scattering is used to measure pore size as it is not affected by shrinkage perpendicular to the film/substrate interface driven by silica condensation during film drying or heating during template removal. We note that the values obtained from GISAXS (which include wall thickness) are less than pore size measurements obtained from N2 adsorption. However, as noted previously, the PSD model used in this study is approximate, with emphasis in data analysis given to trends in pore size as the template/SiO2 ratio is increased, rather than absolute values.


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Figure 7. Characterization data for Pluronic-templated films synthesized via EISA. A) GISAXS data for porous silica films templated by various Pluronics at a ratio of 45.4 mg/mmol, after calcination, showing the presence of a well-ordered pore nanostructure consistent with bcc packing of spherical pores (first image indexed to Im3m with (110) oriented parallel to the film substrate) except for P108, which exhibits reduced long range order as evidenced by fewer diffraction spots. B) N2 adsorption isotherms for films deposited onto SAW devices under identical conditions as used for the films of panel A, C) PSDs calculated from the adsorption data of panel B using a model as described for Figure 3 Importantly, we observe that observed pore size trends obtained from GISAXS, N2 adsorption, and TEM are all consistent with one another. Slopes of the data in Figure 5C do show that pore size increase in Pluronic-templated films can be qualitatively correlated with increase in polymer ordering (as determined by the Raman data in Figure S2) as the template/SiO2 ratio is increased. Linear fits yield slopes of ca. 0.03 or greater for all high-PEO polymers, and < 0.02 (or even negative) for low-PEO polymers, 23 ACS Paragon Plus Environment


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consistent with the above hypothesis regarding the relationship between intermolecular interactions and ability of a Pluronic template to produce variable sized pores. We note that these high-PEO Pluronics all yield similar phase behavior for silica films synthesized via EISA, with well-ordered bcc phases present at lower template/silica ratios (Figure 7A) losing long range ordering as this ratio is increased (with F108 generally producing films of lower order); N2 adsorption isotherms are also similar for all of these materials, with PSDs showing a continuous range of average pore size, providing a system of porous silica films with true tunability of pore size between 4.5 and 15 nm, keeping the nanostructure morphology constant (albeit with reduced long range ordering as the template/silica ratio is increased). Moreover, the slopes of the data in Figure 5C suggests that F108 may be more efficient at expanding pore size than F127 (perhaps a result of greater PEO content); as the data in Figure 8 demonstrates, this does appear to be the case, with a ca. 20% increase in maximum pore size for F108 relative to that obtained for F127 (as previously discussed, although the model used to calculate the PSD is not strictly applicable to materials with spherical pores; however, the relative error in average pore size should be similar for F127 and F108 templates given identical pore morphologies). GISAXS data for this film, shown as an inset in Figure 8, is consistent with the presence of a highly correlated unoriented packing of spheres of ca. 17-18 nm, showing a similar trend in pore size as that seen in the N2 adsorption data. The observed increase in pore size for Pluronic templates with high PEO content suggests that other types of PEO-containing surfactant templates used in EISA, such as the non-ionic surfactants Brij C10 and C20 (C16H33(OCH2CH2)~10OH and C16H33(OCH2CH2)~20OH, respectively) should also exhibit enlarged pore size with increasing template/SiO2. However, as data in Figure 5C shows, pore size in silica films templates with either of these surfactants do not 24 ACS Paragon Plus Environment

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Figure 8. N2 SAW adsorption, PSD, and TEM data for a F108 templated SiO2 film (118.4 mg F108/mmol SiO2) showing more than a 2x increase in pore size compared to films made at a ratio of 45.4 mg F108/mmol SiO2. In the inset of GISAXS data, scattering from the direct and reflected beams are highlighted with black and yellow lines, respectively. exhibit any significant variation over the template/SiO2 ratios studied here, indicating that swelling of micelle size during EISA is not due to the PEO hydrophile alone, but appears to be a synergistic property of both PEO and PPO blocks, with high PEO content being a necessary but insufficient condition for increased pore size. Finally, to examine possible effects of assembly dynamics on the formation of MLV phases, or expansion of pore size, we coated F127 and F127/P103 films with spin speeds of < 500 to 5000 rpm to vary the rate of film formation. We observed no significant differences in film morphology after calcination in these films by TEM or GISAXS, consistent with thermodynamic control of assembly. Conclusions We have demonstrated two routes to increasing the pore size of silica films synthesized via EISA using the polyoxomer Pluronic F127 as the primary template. In the first method, a second Pluronic polymer is used as a ‘swelling agent’ for F127 micelles, producing larger pores



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without phase separation or evaporation of the additive, both issues seen in EISA-derived films produced with standard hydrophobic polymer or small molecule swelling agents, respectively. Addition of the secondary Pluronic P103 enables the synthesis of films with spherical pores of up to ca. 15 nm in diameter with a composition ratio of 45.4 mg F127/35.4 mg P103/mmol SiO2. Increasing the amount of P103 above this induces a phase transition to a non-oriented multilamellar vesicle structure, where pores of up to ca. 30 nm are produced by off-center shifting of intact concentric MLV layers after template removal. This phase transition is induced by geometrical considerations of micelle shape. Secondary Pluronics with larger PEO mass percentages only produce spherical micelles when mixed with F127, while Pluronics with less PEO are not miscible with F127, producing significant phase separation, even with the presence of the MLV phase. Significantly, we find that pores of about 15 nm in diameter can be produced with F127 alone by increasing the F127/SiO2 ratio; for P103 alone, increasing P103/SiO2 does not yield greater pore size. We attribute this behavior to more favorable molecular interactions in poloxomers with > 50% by mass of PEO relative to poloxomers with < 50%, as evidenced by increased ‘order’ in F127 aggregates as the F127/SiO2 ratio is increased, as determined by Raman spectroscopy. Overall, this work is significant in that it is the first report of true tunablity of pore size in mesoporous films with 3D pore networks synthesized by EISA, enabling the selection of pore size between 4.5 and almost 20 nm (for films with spherical pores) or 30 nm (for irregular pore networks formed by the collapse of MLVs), using only simple modifications to the EISA process (addition of a secondary Pluronic or increase of the Pluronic/SiO2 ratio). Future work will address the applicability of these approaches to production of particles using EISA;3 although the mechanism of assembly is similar for films and particles, the use of higher


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temperatures in particle generation may modify the results as poloxamer self-assembly is highly temperature-sensitive.54



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Supporting Information Available Composition of Pluronic polymers used in these studies, TEM data for Figure 5A, Raman data for Pluronic/SiO2 films. This information is available free of charge via the Internet at http://pubs.acs.org/.

Acknowledgments This research was supported by Chemical Sciences, Geosciences and Biosciences Division, Office of Basic Energy Sciences, Office of Science, U.S. Department of Energy under Grant DEFG02-02-ER15368 as well as U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering Grant DE-SC22.2, and the Laboratory Directed Research and Development program at Sandia National Laboratories, a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy, under contract DE-AC04-94AL85000. Use of the Advanced Photon Source 8-ID-E beamline facility was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract DE-AC0206CH11357.


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Table of Contents Figure


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Enlarged Pore Size in Mesoporous Silica Films Templated by Pluronic

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