Article pubs.acs.org/Langmuir
Morphology Control in Mesoporous Carbon Films Using Solvent Vapor Annealing Zhe Qiang, Jiachen Xue, Kevin A. Cavicchi, and Bryan D. Vogt* Department of Polymer Engineering, University of Akron, Akron, Ohio 44325, United States S Supporting Information *
ABSTRACT: Ordered mesoporous (2−50 nm) carbon films were fabricated using cooperative self-assembly of a phenolic resin oligomer with a novel block copolymer template (poly(styrene-block-N,N-dimethyl-n-octadecylamine p-styrenesulfonate), (PS-b-PSS-DMODA)) synthesized by reversible addition−fragmentation chain transfer (RAFT) polymerization. Due to the high Tg of the PS segment and the strong interactions between the phenolic resin and the PSS-DMODA, the segmental rearrangement is kinetically hindered relative to the cross-linking rate of the phenolic resin, which inhibits longrange ordering and yields a poorly ordered mesoporous carbon with a broad pore size distribution. However, relatively short exposure (2 h) to controlled vapor pressures of methyl ethyl ketone (MEK) yields significant improvements in the long-range ordering and narrows the pore size distribution. The average pore size increases as the solvent vapor pressure during annealing increases, but an upper limit of p/p0 = 0.85 exists above which the films dewet rapidly during solvent vapor annealing. This approach can be extended using mesityl oxide, which has similar solvent qualities to MEK, but is not easily removed by ambient air drying after solvent annealing. This residual solvent can impact the morphology that develops during cross-linking of the films. These results illustrate the ability to fine-tune the mesostructure of ordered mesoporous carbon films through simple changes in the processing without any compositional changes in the initial cast film.
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well established for many sol gel materials.14 In these cases, the substrate tends to align the mesostructure into parallel layers19 unless appropriate modifications are performed.20 Additionally, the mesostructure is generally fixed after film drying due to the condensation of the precursors into cross-linked networks21 except for certain precursors22 and using swelling agents such as CO2 to expand the hydrophobic domains.23 This constraint is even more restrictive when the mesostructure is strongly dependent upon the processing conditions24 and solution aging.25 The concepts associated with the fabrication of mesoporous metal oxide films have been extended to mesoporous carbons through the condensation of carbonizable precursors such as resorcinol-formaldehyde (RF),26−29 and interest in these porous carbons has increased as the synthetic methodologies become more established and their utility in applications such as sensors,30 supercapacitors,31,32 batteries,33 electrocatalysis,34 and membranes35 has been demonstrated. However, the relatively rapid condensation of RF sols leads to limited processing windows for fabrication of mesoporous carbons; alternatively, Zhao and co-workers have demonstrated that the self-assembly of phenolic resin oligomers (resol) with nonionic surfactants can yield analogous ordered mesoporous
INTRODUCTION The synthetic flexibility1−3 in tuning the pore size and matrix chemistry of templated ordered porous materials4,5 has ignited significant efforts in the development of these materials for a host of widely varying applications including catalysis,6 adsorbants,7 controlled drug release,8 water purification,9 energy storage,10,11 and solar cells.12,13 Two distinct families of ordered mesoporous materials exist; the first is soft templated materials that involve the cooperative assembly of the desired framework precursors with an amphiphilic surfactant or block copolymer and subsequent removal of the surfactant template to yield the desired ordered mesoporous material.14,15 Yuan and co-workers have recently reviewed the soft-templating process for the synthesis of ordered mesoporous carbons.16 In some cases, direct synthesis by soft templating is challenging and a hard template (typically silica or carbon) is infiltrated with the precursor to produce a replica with the template subsequently removed to yield the ordered mesoporous material.17,18 These two routes provide for facile synthesis of a wide range of mesoporous materials, but there are a number of applications where thin film coatings or freestanding membranes are preferred.1,19 In thin films, these templating approaches can also be applied, but there are additional constraints that arise due to the simultaneous mesostructure formation and film drying through the evaporation induced self-assembly (EISA) route, which is © 2013 American Chemical Society
Received: December 12, 2012 Revised: February 5, 2013 Published: February 8, 2013 3428
dx.doi.org/10.1021/la304915j | Langmuir 2013, 29, 3428−3438
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Article
linking of the carbon precursor resol. Solvent vapor annealing is used to improve the long-range order and control the spacing of the mesostructure, which can be kinetically trapped by complete removal of the solvent and thermal cross-linking. The flexibility of this approach is demonstrated using a higher boiling point solvent, which is not completely removed after annealing and plasticizes the films, allowing for reordering of the template during thermal cross-linking. These results demonstrate that the judicious choice of template and annealing solvent are useful to generate and replicate nonequilibrium morphologies in mesoporous carbon. This approach has potential for controlled fabrication of other nonequilibrium morphologies, such as perpendicular (throughfilm) cylindrical mesopores or other morphologies (such as hexagonally perforated lamellae) that are difficult to obtain through standard thermal processing, which simply involves thermally annealing the films to order the template and crosslink the carbon.
carbons after thermally cross-linking the resol and subsequent pyrolysis,36−38 including large pore sizes through the use of block copolymer templates.39−41 This methodology is readily extendable to thin films,42,43 but the ordered mesostructure only develops during the thermal cross-linking and not by an EISA process.44 This thermally driven self-assembly is reminiscent of assembly of neat block copolymers in thin films,45 but for these mesoporous carbons, the interplay between cross-linking and ordering kinetics can lead to a poorly ordered film in some cases.46 These prior results suggest that the morphology of the mesoporous carbon films can be modulated by postprocessing and then kinetically arrested if the rate of cross-linking of the resol significantly exceeds the rate of chain rearrangement associated with reordering of the template. This approach could prove extremely useful in producing mesoporous films with difficult to obtain morphologies such as perpendicular cylinders or bicontinuous gyroid by enabling processing steps generally associated with neat block copolymer (BCP) systems to be extended to the fabrication of mesoporous carbon films. For neat, nonreactive BCP films, there are cases where the ordering kinetics are extremely sluggish due to slow chain dynamics, such as systems with at high molecular weights, large segmental interaction parameters, and high glass transition temperatures. In these cases, the ordering can be improved by swelling the BCP by a solvent vapor.47 This solvent vapor annealing (SVA) approach has also been utilized to fabricate a mesoporous carbon film by initially ordering a poly(styreneblock-4-vinylpyridine) (PS-b-P4VP)−resorcinol mixture by exposure to solvent vapor and subsequent exposure to formaldehyde vapor to cross-link the system,26 but this approach required two long solvent vapor exposures. However, in this case, both BCP segments exhibit relatively high glass transition temperatures (Tg) in comparison to the typical Pluronic surfactants. We have previously shown that kinetically arrested mesostructures are obtained in thin films using poly(styrene-block-ethylene oxide) (PS-b-PEO) as the template,46 but some rearrangement still appears to occur during the thermally induced cross-linking as the ethylene oxide segments exhibit a Tg below ambient temperature. A recent review by Zhao and co-workers more extensively describes the use of amphiphilic block copolymers for templating of mesoporous materials. 48 Moreover, reports of ordered mesoporous carbons via soft templating routes have almost exclusively utilized nonionic surfactants36 or BCP amphiphiles.41 Ionic surfactants lead to porous carbon without longrange order and poorly defined porosity.49 Electrostatic interactions between the resol and an ionic template may act to limit the mobility required for ordering. Therefore, there is potential to combine the molecular design of the block copolymer template to tune its chain dynamics and relative incompatibility (solubility) with processing techniques developed for BCPs, such as solvent vapor annealing, to exert greater control over the soft templating of mesoporous carbons. Here, we demonstrate that a new BCP template of poly(styrene-block-N,N-dimethyl-n-octadecylamine p-styrenesulfonate) (PS-b-PSS-DMODA) synthesized by reversible addition−fragmentation chain transfer (RAFT) polymerization provides a template that can be selectively swollen with resol to obtain mesoporous carbons. The PS-b-PSS-DMODA exhibits a combination of chain dynamics and solubility that is conducive to fabrication of highly ordered mesoporous carbon films through solvent vapor annealing and subsequent thermal cross-
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EXPERIMENTAL SECTION
Materials. Sodium p-styrenesulfonate (NaSS), chlorobenzene (>99.8%), and styrene (>99%, stabilized with 10−15 ppm 4-tertbutylcatechol) were purchased from Alfa Aesar. N,N-Dimethyl-noctadecylamine (DMODA) was purchased from Tokyo Chemical Industry Co. Ltd. (>85%). Mesityl oxide (>90%), tetrahydrofuran (THF) (>99%), phenol (>99%), and formaldehyde (ACS reagent, 37 wt % in H2O, contains 10−15% methanol as stabilizer) were obtained from Sigma-Aldrich. Methyl ethyl ketone (MEK) (>99%) and hexane (>99%) were purchased from Fisher Scientific. Sodium hydroxide (>99%) was purchased from EMD Chemicals Inc. Nitrogen gas (>99%) was purchased from Praxair. Styrene was purified by filtering through a basic alumina column to remove the polymerization inhibitor. All other reagents were used as received. A low-molecular weight soluble phenolic resin (resol) was synthesized as described previously by Zhao and co-workers50 and utilized as the carbonizable precursor. The chemical structure of the resol is illustrated in Scheme 1A. Briefly, the resol was synthesized by NaOH-catalyzed condensation of phenol and formaldehyde (37 wt % in H2O) that was neutralized by dilute HCl following reaction. The water was removed by rotary evaporation; low temperatures (
