Article pubs.acs.org/Langmuir
Cite This: Langmuir 2018, 34, 8007−8014
Synthesis and Porous SiO2 Nanofilm Formation of the Silsesquioxane-Containing Amphiphilic Block Copolymer Yuya Ishizaki, Shunsuke Yamamoto,* Tokuji Miyashita, and Masaya Mitsuishi* Institute of Multidisciplinary Research for Advanced Materials (IMRAM), Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan
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ABSTRACT: We describe the synthesis, Langmuir−Blodgett (LB) film formation, and photo-oxidation of an organic− inorganic hybrid block copolymer consisting of N-dodecyl acrylamide (DDA) and silsesquioxane (SQ) comonomers [p(DDA/ SQ26)-b-pDDA]. The copolymer was synthesized by reversible addition fragmentation chain transfer polymerization of DDA and SQ. Higher monolayer stability at the air−water interface was confirmed for p(DDA/SQ26)-b-pDDA. The p(DDA/SQ26)b-pDDA monolayer was deposited onto solid substrates with a monolayer thickness of 2.3 nm. The photo-oxidized SiO2 nanofilm revealed its porous structure, which reflects phase-separated structures of p(DDA/SQ26)-b-pDDA, as confirmed using atomic force microscopy, quartz crystal microbalance, and cyclic voltammetry measurements. These results demonstrate that this preparation method using photo-oxidation of the organic−inorganic hybrid block copolymer LB film is promising for manipulating pore formations of inorganic oxide nanofilms.
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INTRODUCTION Bottom-up approaches are necessary from the viewpoint of constructing ordered three-dimensional (3D) nanostructures at a molecular level. The Langmuir−Blodgett (LB) technique is an effective method for creating 3D nanostructures by stacking a two-dimensional (2D) molecular layer at the air− water interface.1,2 Especially, the 2D molecular architecture takes a crucially important role in controlling molecular orientation and packing as well as its 3D nanostructure. For the LB technique, the molecular design of amphiphilic molecules including amphiphilic polymers is indispensable for forming a monolayer that is transferrable from the water surface. In earlier studies, we investigated the polymer Langmuir monolayer formation and its LB films using poly(N-dodecyl acrylamide) (pDDA).3−5 pDDA exhibits excellent monolayer formation capability at the air−water interface because of 2D hydrogen bond formation of acrylamide groups. Dodecyl side chains provide a good balance of hydrophilicity and hydrophobicity. As many as 700 multilayer depositions onto the solid substrates are possible as a result of the 2D nanosheet arrangement.6 Various functional groups were incorporated as a comonomer of amphiphilic DDA-based random copolymers7−10 with functional groups dispersed uniformly in the © 2018 American Chemical Society
monolayer, resulting in high-density functional polymer nanosheets with well-defined layer structures. However, block copolymers are expected to provide unique nanostructures that are not achieved with random copolymers. Particularly, amphiphilic block copolymers exhibit microphase separation at the air−water interface because of the self-assembly combination of different polymer segments.11,12 Nevertheless, few reports have described the studies of amphiphilic block copolymers with good film formation ability and multilayer stacking properties.13 Further challenges are available for functional amphiphilic block copolymers including organic− inorganic hybrid diblock copolymers. Therefore, synthesis of pDDA-based amphiphilic block copolymers with hybrid moieties and their precise integration are important issues to be resolved to enable the molecular-level control of the nanostructures. Polyhedral oligomeric silsesquioxane (SQ) is widely recognized as a promising organic−inorganic hybrid building block because it has high potential for possessing various Received: April 5, 2018 Revised: June 3, 2018 Published: June 25, 2018 8007
DOI: 10.1021/acs.langmuir.8b01114 Langmuir 2018, 34, 8007−8014
Article
Langmuir Scheme 1. Synthesis Route of p(DDA/SQ) and p(DDA/SQ)-b-pDDA by RAFT Polymerization
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properties such as thermal and mechanical stabilities and optical transparency in the visible light wavelength region.8,14−17 Actually, SQs (RSiO1.5)n have a rigid inorganic core of SiO1.5 and organic substituent groups (R) such as hydrogen, alkyl, phenyl, or other functional groups, which can be modified easily. Therefore, SQs are used as building blocks for diverse organic−inorganic hybrid materials such as linear and star-shaped polymers18−22 and network polymers.23−25 Moreover, SQ resin can be oxidized into silicon oxides along with the decomposition of organic moieties by UV light irradiation.26,27 An earlier report described that SQ-containing random copolymer p(DDA/SQ) LB films8 were converted easily from the initial cage structure to ultrathin SiO2 nanofilms by UV irradiation under an ambient atmosphere28,29 with no thermal annealing process. The SiO2 nanofilm obtained from the SQ-containing LB film by photo-oxidation has uniform smoothness and high hardness, shear modulus, and electrical resistance. Furthermore, SQ-containing polymer nanosheets are good precursors for ultrathin SiO2 nanofilms. As explained herein, we specifically examined an SQcontaining amphiphilic block copolymer. Polymer nanosheets prepared from SQ-containing amphiphilic block copolymer monolayers are expected to control the 3D nanostructures of photo-oxidized SiO2 nanofilms because of block copolymer self-assembled nanostructure formation. We synthesized a novel amphiphilic block copolymer, p(DDA/SQ)-b-pDDA, consisting of amphiphilic random copolymer p(DDA/SQ26) (26 mol % SQ content) and pDDA via reversible addition fragmentation chain transfer (RAFT) polymerization. The monolayer behavior of p(DDA/SQ)-b-pDDA was examined at the air−water interface. p(DDA/SQ)-b-pDDA nanosheets were also prepared using the LB technique. The highly ordered layer structure of the polymer nanosheets was characterized using atomic force microscopy (AFM) and UV−vis spectroscopy. Finally, unique porous SiO2 nanofilms were prepared from p(DDA/SQ)-b-pDDA nanosheets by photo-oxidation using a UV−ozone cleaner in an ambient atmosphere.
EXPERIMENTAL SECTION
Materials. All chemicals were used without further purification unless otherwise noted. 3-Methacryloxypropyl-T8-heptaphenyl polyhedral oligomeric SQ was provided by JNC Corp. It was recrystallized from chloroform/n-hexane (Kanto Chemical Co. Inc.) before use. DDA was prepared as reported.3 2,2′-Azobis(isobutyronitrile) (AIBN; Wako Pure Chemical Inds. Ltd.) was recrystallized from methanol (Kanto Chemical Co. Inc.). 2-Cyano-2-propyl dodecyl trithiocarbonate (CTA-Pr) and tris(trimethylsilyl)silane (TTMSS) were purchased from Sigma-Aldrich Co. LCC. Toluene, acetonitrile, and acetone were purchased from Nacalai Tesque Inc. Chloroform-d (CDCl3) and tetrahydrofuran (THF) were purchased from Kanto Chemical Co. Inc. Ultrapure water with resistivity higher than 17.5 MΩ cm was used as a subphase (purified using a RFD240RA and CPW-101 system; Advantec Toyo Kaisha Ltd.). Synthesis. Amphiphilic copolymers p(DDA/SQ)s and block copolymer p(DDA/SQ)-b-pDDA were synthesized by RAFT polymerization (Scheme 1). An example of the RAFT polymerization process is the following: at the first step, DDA (336.16 mg), SQ (648.90 mg), AIBN (3.82 mg), and CTA-Pr (6.47 mg) were dissolved in toluene (10 mL). The solution was subjected to freeze−pump− thaw cycles to remove oxygen before polymerization. Polymerization was conducted at 60 °C for 24 h. After the reaction, toluene was evaporated from the polymer solution. After the crude was dissolved in a small amount of chloroform, it was reprecipitated by pouring in 300 mL of acetonitrile. Subsequently, after the obtained polymer was reprecipitated two more times, it was dried in vacuum overnight. p(DDA/SQ)-macroCTA (472.94 mg, yield = 48%) was obtained as pale yellow powder. Next, DDA (122.63 mg), p(DDA/SQ)macroCTA (139.60 mg), and AIBN (1.27 mg) were dissolved in 5 mL of toluene. After the same polymerization and purification processes of the first step, block copolymer capped by trithiocarbonate groups at the chain end, p(DDA/SQ)-b-pDDA-S, was obtained as pale yellow powder (157.66 mg, yield = 60%). Finally, the trithiocarbonate groups of p(DDA/SQ)-b-pDDA-S were reduced using the radical-induced reduction method.30 Then, p(DDA/SQ)-bpDDA-S (150.30 mg), TTMSS (5.0 μL), and AIBN (1.50 mg) were dissolved in 2 mL of toluene. The reaction was conducted at 60 °C for 8 h after freeze−pump−thaw cycle treatment. After the polymer solution was evaporated and dissolved in a small amount of chloroform, it was reprecipitated in 100 mL of acetonitrile. After 8008
DOI: 10.1021/acs.langmuir.8b01114 Langmuir 2018, 34, 8007−8014
Article
Langmuir Table 1. Polymerization Results of p(DDA/SQ)a run
[monomer]0/M
[DDA]0/[SQ]0
[AIBN]0/mM
F1 F2 F3 R1 R2 R3
0.200 0.199 0.200 0.200 0.201 0.200
90:10 80:20 70:30 90:10 80: 20 70:30
2.12 2.12 2.24 1.99 1.99 2.33
[CTA-Pr]0/mM
time/h
compositionb (DDA/SQ)
2.03 2.03 1.95
24 24 15 24 24 24
92:8 86:14 81:19 91:6 86:14 74:26
Mn/104 (Mw/Mn) 3.58 4.13 3.61 1.47 1.51 1.41
(3.7) (4.1) (3.9) (1.6) (1.7) (1.7)
a F stands for free-radical copolymerization. R corresponds to RAFT copolymerization. bComposition of DDA/SQ was ascertained from 1H NMR spectra by comparing the integral ratio of the relevant peaks of DDA and SQ (Figure S1).
the obtained polymer was reprecipitated two more times, it was dried in vacuum overnight. Finally, p(DDA/SQ)-b-pDDA was obtained as white powder (104.83 mg, yield = 70%). Substrate Preparation. p-Si wafers (≤0.02 Ω cm; Mitsubishi Materials Trading Co.) and quartz substrates were washed by ultrasonication in chloroform, acetone, and 2-propanol for 15 min each and were dried with N2 gas. Then, they were cleaned with a UV−ozone cleaner (NL-UV253S; Nippon Laser and Electronics Lab) for 30 min. The substrates were immersed in a chloroform solution of n-octyltrichlorosilane (Tokyo Chemical Industry Co. Ltd.) for 6 h. Finally, the substrates were dried with N2 gas. CaF2 plates (Pier Optics Co. Ltd.) were washed in acetone and 2-propanol and were dried with N2 gas before use. LB Film Deposition. Surface pressure (π)−area (A) isotherm measurements and LB film deposition were conducted using automatically controlled Langmuir troughs ((π−A) HBM-AP; Kyowa Interface Science Co. Ltd., and (LB) FSD-21; USI Systems Inc.). A dilute chloroform solution of p(DDA/SQ) or p(DDA/SQ)-bpDDA (ca. 1.0 mM) was spread onto the ultrapure water surface at 20 °C. After solvent evaporation, the monolayer was compressed at a rate of 15 cm2 min−1. The surface pressure was monitored using a Wilhelmy plate. The surface pressure was maintained at 25 mN m−1 during deposition. Then, the monolayer was transferred onto hydrophobic substrates using the vertical dipping method with a dipping speed of 10 mm min−1. Measurements. 1H NMR spectra were measured using a 400 MHz NMR spectrometer (ADVANCE III; Bruker Analytik). Size exclusion chromatography (SEC) measurements were taken using a GPC system (GPC-8020; Tosoh Corp.) equipped with a gel column (TSKgel SuperHZM-M; Tosoh Corp.) and a refractive index (RI) and UV detector (RI-8020 and UV-8020) using polystyrene standards. Also, Fourier transform infrared (FT-IR) and UV−vis spectra were measured, respectively, using an FT-IR spectrometer (FT-IR 4200; Jasco Corp.) and a UV−vis spectrometer (U-3000; Shimadzu Corp.). Surface morphologies were characterized using AFM (SPA400; Seiko Instruments Inc.) with an Al-coated silicon cantilever SI-DF20 (16 N m−1, 136 kHz; Seiko Instruments Inc.). Scanning electron microscopy (SEM) images were obtained using field-emission SEM (3.0 kV, S4800; Hitachi Ltd.) after sputtering of Pt. The film density was determined using a 9 MHz quartz crystal microbalance (QCM; USI Co. Ltd.). Considering the mass obtained and the film thickness, the densities of the nanosheet before and after photoirradiation were determined. The ion permeability of the photooxidized SiO2 nanofilms was measured using cyclic voltammetry (CV) using a potentiostat (model 611B; BAS Inc.) equipped with three electrodes. The SiO2 nanofilm prepared onto the Si substrate/Cr (3 nm)/Au (50 nm) was used as a working electrode. Cr and Au layers were deposited onto a Si substrate by thermal vapor deposition. A glassy carbon was used as a counter electrode and a Ag/AgCl electrode as a reference electrode.
synthesized by free-radical and RAFT copolymerization with different monomer feed ratios under a certain monomer concentration. The composition of DDA and SQ in p(DDA/ SQ) was determined using 1H NMR (Figure S1a). In the case of free-radical copolymerization, p(DDA/SQ)s were obtained with large polydispersity index (Mw/Mn = 3.7−4.1) for all monomer feed ratios of DDA and SQ. Actually, RAFT polymerization afforded p(DDA/SQ) with lower Mn and Mw/ Mn (
