Article pubs.acs.org/Langmuir
Fabrication of Mesoporous Polystyrene Films with Controlled Porosity and Pore Size by Solvent Annealing for Templated Syntheses Mohan Raj Krishnan, Yu-Cheng Chien, Chung-Fu Cheng, and Rong-Ming Ho* Department of Chemical Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan, ROC S Supporting Information *
ABSTRACT: Herein, we aim to develop a facile method for the fabrication of mesoporous polystyrene (PS) films with controlled porosity and pore size by solvent annealing. A PS polymer film is solvent-annealed using N,N-dimethyl formamide (DMF) vapor for the development of phase separation, followed by rapidly cooling to the preset cryogenic temperature. Subsequently, a nonsolvent (methanol) is introduced to extract the crystalline DMF from the DMF-swollen PS, giving mesoporous PS with a network structure after the removal of DMF. The porosity of the mesoporous PS films can be controlled by the degree of swelling. Most interestingly, the phase separation between PS and DMF at the thin-film state under solvent annealing can be regulated by the annealing time through the spinodal decomposition, giving the development of nanonetwork structure with controlled structural features (i.e., framework size and interframework spacing) at invariant porosity. Consequently, after the removal of DMF, mesoporous PS films with controlled porosity and pore size can be obtained and then used as a template for the fabrication of a variety of nanoporous inorganics by templated syntheses, such as nanoporous SiO2, TiO2, and Ni, providing a cost-effective way to fabricate a range of nanoporous materials with controlled porosity and pore size as well as large specific surface area for aimed applications.
1. INTRODUCTION Thin films containing nanometer-sized pores find a wide range of potential applications as optical elements,1−4 low-dielectric interconnects in semiconductors,5,6 and mesoscopic chemical reactors.7,8 The pore morphology including pore size, shape, size distribution, and interconnectivity has a dramatic effect on the film properties. Therefore, the fabrication of mesoporous films with precise structural control is indispensable for applications. Moreover, mesoporous polymer films can be used as templates for the synthesis of various nanohybrids and nanoporous materials with high industrial impact.9−15 Materials with meso- and macropores have been produced by the microphase separation of block copolymers (BCPs),15−20 supercritical carbon dioxide foaming,21−23 and freezing techniques24,25 as well as conventional phase separation methods.26,27 Thermally induced and solvent-induced phase separation methods have long been used in industries as they are considered as the simplest and most well-established methods to produce porous polymers.28−31 The key factors affecting the mechanism of pore formation include the phase separation of the solvent and gelation of the polymer solution in classical phase separation methods.32,33 Freeze drying of polymer solutions has also been examined as an effective method in controlling the porous structure,33 but it remains very challenging to prepare mesoporous materials through the phase separation of common polymers.34 Although there are many advanced methods for the fabrication of mesoporous polymers, an effective method to fabricate mesoporous © XXXX American Chemical Society
materials with controlled pore size and precise shape as well as narrow pore size distribution in a facile way is strongly needed; especially, it is highly demanded to acquire network texture from the microphase separation of BCPs or the phase separation of polymer blends and solutions, in particular in the thin-film state, because of its characteristic three-dimensional (3D) co-continuous structure.15,35−37 With the removal of one continuous network, mesoporous polymers can be obtained and used as templates to create a range of nanoporous materials with high porosity and large specific surface area through templated syntheses, followed by the removal of the polymer template.15,38 Accordingly, there is a strong motivation to develop new or improved strategies for the fabrication of mesoporous polymer thin films as templates, in particular with precisely controlled porosity and pore size. Moreover, in templated material research, addressing the cost efficiency and sustainability issues will be central in the near and long-term future. As a result, it is indispensable to focus on developing lower cost and environmentally friendly techniques and materials which can be employed in templating approaches. Herein, we aim to develop a facile and cost-effective method for the fabrication of a mesoporous polymer film with welldefined structural characteristics by a solvent annealing technique and then use it for templated syntheses. In an Received: June 26, 2017 Revised: August 7, 2017
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DOI: 10.1021/acs.langmuir.7b02195 Langmuir XXXX, XXX, XXX−XXX
Article
Langmuir
Figure 1. Schematic illustration of the fabrication of mesoporous PS film by the spinodal decomposition of DMF-swollen PS for phase separation: (a) spin-coated PS film onto a wafer; (b) PS film swollen by DMF under saturation vapor pressure to reach the constant thickness; (c) solventswollen PS film is subjected to annealing at the same chamber, giving co-continuous DMF and PS through the spinodal decomposition; (d) cryogenic condition for the crystallization of DMF; (e) extraction of crystalline DMF network using methanol to give a mesoporous PS film with controlled porosity and pore size; and (f) templated syntheses for the preparation of various nanoporous ceramic or metallic films using the fabricated mesoporous PS film as a template. annealing. To anneal the PS film under DMF saturation vapor, about 20 mL of DMF was taken in the solvent chamber and then the preformed PS film was kept roughly about 2 cm above the solvent level in the chamber. The PS film initially underwent isotropic swelling with DMF vapor and later reached the constant thickness. After annealing the PS film in DMF vapor in the solvent chamber for the required time, the chamber as a whole was transferred to liquid N2 to freeze the PS/DMF mixture. The frozen PS/DMF film was later transferred to a refrigerator which was preset at −80 °C after the addition of methanol (Macron Fine Chemicals, 99.8% GC). The added methanol was exchanged with the phase-separated DMF, yielded a mesoporous texture of PS film. The prepared mesoporous PS film was later dried under vacuum at room temperature. The thickness profile of the PS film during solvent swelling and in the subsequent annealing process was obtained by scanning electron microscopy (SEM) observations for the quenched sample after solvent swelling and annealing at different stages in the homemade solvent chamber. After quenching, the PS film was fractured at cryogenic conditions to acquire the cross section of the PS film for SEM imaging to measure the corresponding thickness. 2.2. Field-Emission Scanning Electron Microscopy. Fieldemission scanning electron microscopy observation was performed on a JEOL JSM-7401F using accelerating voltages of 5 keV. The samples were mounted on brass shims using a carbon adhesive and then sputter-coated with 2−3 nm of platinum. The platinum coating thickness was estimated from a calculated deposition rate and experimental deposition time. 2.3. Small-Angle X-ray Scattering. Small-angle X-ray scattering (SAXS) measurements were conducted at the synchrotron X-ray beamline X27C at the National Synchrotron Radiation Research Center in Hsinchu, Taiwan. The wavelength of the X-ray was 0.155 nm. A MarCCD X-ray detector (Rayonix LLC, Evanston, IL, USA) was used to collect the two-dimensional (2D) SAXS patterns. A onedimensional linear profile can be obtained by the integration of the 2D pattern. The scattering angle was calibrated using silver behenate, with the first-order scattering vector q of 1.076 nm−1. 2.4. Pore Size and Porosity Measurements. The pore size and porosity of the fabricated mesoporous polymer film were analyzed using a volumetric gas adsorption apparatus (BET Sorptometer, CBET-201A, PMI, USA). A glass sample tube was filled with small pieces of dried sample and thoroughly dried overnight under vacuum at 30 °C. The specific surface area of the sample was calculated from the Brunauer−Emmett−Teller (BET) method, and the pore size distribution was derived from Barrett−Joyner−Halenda (BJH) analysis. Some hysteresis was observed between adsorption and
exemplary system, a well-interconnected mesoporous polystyrene (PS) film has been fabricated by initial solvent swelling of the preformed PS film, followed by controlled solvent annealing to develop phase separation on mesoscale through the spinodal decomposition mechanism and subsequent solvent removal from the phase-separated film. Figure 1 illustrates the fabrication process of the mesoporous PS film. A PS film is prepared by spin-coating the PS chlorobenzene solution onto a wafer (Figure 1a). Subsequently, the PS film is subjected to solvent uptake under N,N-dimethyl formamide (DMF) saturation vapor condition to acquire a DMF-swollen PS film with the constant thickness (Figure 1b). The resultant thickness of the DMF-swollen PS film is well-controlled by regulating the vapor pressure of the solvent chamber. After reaching the constant thickness, the DMF-swollen PS film is subjected to solvent annealing for phase separation through the spinodal decomposition to acquire a co-continuous structure of PS and DMF (Figure 1c). By controlling the annealing kinetics, the formation of co-continuous DMF and PS with well-defined structural features can be achieved. Subsequently, the phaseseparated PS film with a precisely controlled DMF network structure is quenched in liquid nitrogen to give crystalline DMF (Figure 1d). The crystalline DMF network is then extracted with methanol for the fabrication of mesoporous PS film (Figure 1e).
2. EXPERIMENTAL SECTION 2.1. Mesoporous PS Film Preparation. A typical preparation process of mesoporous PS films consists of three steps including the preparation of preformed PS films and crystalline solvent uptake, followed by annealing for subsequent phase separation to form a cocontinuous PS and crystalline solvent network. Subsequently, the phase-separated film was frozen under cryogenic conditions to crystallize the solvent network, followed by the extraction of solvent network with methanol. To prepare a PS (Scientific Polymer, Mn = 280 000 gmol−1) film with the thickness of 850 nm, 7 wt % of PS solution in chlorobenzene (Alfa Aesar, 99% GC) was prepared and then spin-coated onto a Si wafer (with native oxide layer) at 2000 rpm for 1 min. The prepared polymer film will be kept under a vacuum oven to remove any residual solvent in the film. The preformed polymer film was kept under DMF saturation vapor (JT Baker, 99.8% GC) in a homemade solvent chamber for different time intervals from 5 to 240 min for solvent uptake and subsequent B
DOI: 10.1021/acs.langmuir.7b02195 Langmuir XXXX, XXX, XXX−XXX
Article
Langmuir desorption processes, which is often the case with capillary condensation in mesoporous materials.
nitrogen. Under cryogenic conditions, the DMF molecules in the PS film will be crystallized and subsequently extracted with methanol, giving a mesoporous PS film for SEM observations. As shown in Figure 3a (top view) and Figure 3d (crosssectional view), a well-interconnected network skeleton with mesoporous voids can be found for the PS film prepared at the condition of 5 min annealing, reflecting the formation of a 3D co-continuous structure after the swelling stage. Most interestingly, with the increase of solvent annealing, the top-view and cross-sectional view images clearly show the increase of structural feature size of the mesoporous PS film as represented for the samples after annealing of 30 min (Figure 3b,e) and 60 min (Figure 3e,f); the observed morphological development is a typical behavior of the spinodal decomposition kinetics. Spinodal decomposition in polymer blends or solutions is a spontaneous phase separation process that occurs when an infinitesimally small fluctuation in the system from homogeneity provokes an exponential growth of the starting fluctuations because of a lowering in the free energy of the system resulting from the phase separation process.32,39−45 The polymer film is metastable under saturated swelling condition and hence undergoes a rapid phase separation when there is an infinitesimal compositional fluctuation. The solvent annealing of the DMF-swollen PS film results in a rapid evolution of features on a certain length scale, followed by a long-time evolution of larger feature size, a characteristic co-continuous network formation of PS and DMF through the spinodal decomposition. We speculate that the spinodal decomposition behaviors during solvent annealing at which the DMF-swollen PS film confined by the air with DMF saturation vapor and the SiO2 substrate can be attributed to the confinement effect of thickness on the solvent-swollen polymer film. Also, note that there is a thin top layer of PS and it can be easily removed by UV exposure; the formed featureless top layer of PS (
