Direct Synthesis of Porous Polyurea Films by Vapor Deposition

Aug 11, 2016 - Most network-structured, porous polymer materials have so far been synthesized in bulk solution processes, which can sometimes be a ...
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Direct Synthesis of Porous Polyurea Films by Vapor Deposition Polymerization in Ionic Liquid Yuya Ohsawa, Rikuto Takahashi, Shingo Maruyama, and Yuji Matsumoto* Department of Applied Chemistry, School of Engineering, Tohoku University, 6-6-07, Aramaki Aza Aoba Aobaku, Sendai, Miyagi 980-8579, Japan S Supporting Information *

ABSTRACT: Most network-structured, porous polymer materials have so far been synthesized in bulk solution processes, which can sometimes be a drawback in their thin-film applications. In this communication, we propose a new route to the direct synthesis of porous polyurea (PU) films by vapor deposition polymerization in ionic liquid (IL), which is based on our original process “IL-assisted vacuum deposition”. When 4,4′-methylenebis (2-chlorophenyl isocyanate) (MBCI) and 2,7diaminofluorene (DAF) monomer molecules were codeposited onto an IL-coated substrate in vacuum, the PU films were found to have a percolated pore and network structure with a high degree of polymerization even without the postannealing treatment, which is required to proceed with the polymerization of PU films in the conventional vapor deposition polymerization. The porous PU film after annealing was still as rigid as the plain PU films and possessed a stronger water repellency, with contact angles exceeding 100°, than the plain PU films. substrate to fabricate micron-sized coral-like polymer films.8 In addition, the codeposition of monomers, cross-linkers, and small molecular porogens was also attempted by Anthamatten et al. for porous polymers in the vapor deposition process.9,10 With this background, we propose a new direct synthesis route to porous polymer films based on our originally developed ionic liquid (IL)-assisted vacuum deposition method. In the IL-assisted vacuum deposition process, IL (stable in vacuum) has been so far used as a solvent in order to assist vapor crystal growth in a vacuum; the examples include not only the growth of single-crystal-phase pentacene11 but also the fabrications of atomically flat epitaxial KBr(111) films12,13 and high-quality C60 epitaxial films.14 Since some porous polymer materials, as already pointed out above, have been synthesized in bulk solution processes, our IL-assisted process would also have much better usability in the vapor deposition polymerization, even for porous polymer films In this communication, we report the synthesis of porous polyurea (PU) films by using this IL-assisted vacuum deposition method. PU is synthesized by copolymerization of diamine and diisocyanate, and its films prepared by vapor deposition polymerization are potentially applicable as a piezoelectric material.15 It is known that a PU film just after deposition at room temperature (RT) exists dominantly in an oligomer state, containing around 4 and 6 monomers,16 and

etwork-structured, porous polymer films, taking the merits of their micropores and the resultant large surface area, are often used as a filter in separation processes,1 as a support material in heterogeneous catalysis,2 and in sensors, etc.3 Most polymer materials have been synthesized in bulk solution processes and can have various porous structures depending on the kinds of solvents used in the process. For example, poly(L-lactic acid) (PLLA) porous membranes are prepared by a solvent-casting particulate-leaching technique with sieved salt particles used as a porogen material,4 and macroporous poly(N-isopropylacrylamide) (pNIPA) gels are prepared by a cryopolymerization method.5 Meanwhile the thermal-induced phase separation phenomenon in the morphological development is theoretically investigated during the fabrication of anisotropic polymer materials.6 However, these porous polymers are usually obtained in bulk, and thus they are not suitable for particular thin-film applications. It is because as long as any solution methods such as spin-coating, dip-coating, and polymer casting are used in making thin films of such a bulky polymer its porous structure will be once broken up. To solve this problem, quite recently some direct syntheses of porous polymer films based on the vapor deposition polymerization have been demonstrated, which is a method noticeable in that there is no need to use a solvent for polymerization. Seidel et al., for one example, fabricated a polymer membrane with a dual-scale porosity by introducing a saturated monomer vapor in the initiated chemical vapor deposition (iCVD) process.7 Another example is that Bradley et al. employed the same iCVD process and used a liquid

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© XXXX American Chemical Society

Received: August 2, 2016 Accepted: August 10, 2016

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DOI: 10.1021/acsmacrolett.6b00594 ACS Macro Lett. 2016, 5, 1009−1013

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ACS Macro Letters thus a postannealing treatment is required in order to complete the polymerization. On the other hand, Zhu et al. reported that a porous bulk PU could be synthesized with an IL via interfacial full polymerization even at RT.17 Therefore, one can expect a direct synthesis of porous PU films with higher degree of polymerization even at RT by the IL-assisted vacuum deposition method, different from those by the conventional vapor polymerization process. In fact, network-structured, porous PU films have been successfully fabricated for the first time by codepositing the monomers of 2,7-diaminofluorene (DAF) and 4,4′-methylenebis(2-chlorophenyl isocyanate) (MBCI), according to the reaction scheme in Scheme S1, into an IL thin layer precoated on a substrate at RT; the unique surface morphology, high degree of polymerization, and strong water repellency of the obtained porous PU films have been found. The ILs used in this study were all from Kanto Chemical: 1methyl-3-octylimidazolium bis(trifluoromethylsulfonyl) amide ([Omim][TFSA], 99%), 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl) amide ([Bmim][TFSA], 99%), and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl) amide ([Emim][TFSA], 99%). The following were also used: the monomers 4,4′-methylenebis(2-chlorophenyl isocyanate) (MBCI, from Sigma Aldrich) and 2,7-diaminofluorene (DAF, 98%, from Tokyo Chemical Industry), the hydrophilic coating agent poly(vinyl alcohol) (PVA, n ≈ 2000, from Tokyo Chemical Industry), and the washing solvent 2-propanol (99.7%, from Wako Pure Chemical Industry). All these chemicals were used as received. Si(100) of 15 mm × 5 mm in size was used as a substrate, cut from a Si single-crystal wafer with SiOx on the surface (≥1000 Ω cm, ⟨100⟩, p-type, Mitsubishi Materials Trading). In order to prepare an IL layer on a Si substrate, a 5% aqueous solution of PVA was first spin-coated (MS-A100, MIKASA) on the Si substrate (3000 rpm, 30 s) and dried at 150 °C for 30 min (about 1 μm in thickness); subsequently IL was spin-coated on the PVA-coated substrate (7000 rpm, 180 s, about 1 μm in thickness) (Figure 1(a)). For electrochemical measurements of PU films, Au was first deposited on a Si substrate instead of the PVA being coated. All these treatments were conducted in air. For fabrication of polyurea (PU) films by vapor deposition polymerization in ionic liquid, in this study, a continuous infrared (CW-IR) laser deposition method was employed for thermal evaporation of each monomer.18 Figure 1(d) shows a schematic drawing of our custom-designed codeposition chamber equipped with two CW-IR laser sources. The pulsed CW-IR laser beams (wavelength is 808 nm) were simultaneously introduced into the vacuum chamber (base pressure is less than 5 × 10−7 Torr) through different quartz-glass windows and irradiated to MBCI and DAF monomer targets, respectively. In order to efficiently absorb the IR laser light, Si powder was added to each monomer container. The powers of the two CW-IR lasers were independently controlled so as for the deposition rates of the monomers to be the same, 20 nm/min, monitored by the quartz crystal microbalance (QCM) sensors. The depositions were done at RT until the nominal total thickness reached 360 nm (Figure 1(b)), followed by the postannealing treatment at 200 °C for 10 min16,19 in a vacuum in order to complete the polymerization as well as to evaporate the remaining IL as shown in Figure 1(c). In the case without the postannealing treatment (PU partially remained in an

Figure 1. (a)−(c) Sample preparation process. (d) Schematic drawing of the CW-IR laser codeposition chamber.

oligomer state, as will be later discussed), the remaining IL on the sample was washed out by 2-propanol. The morphology of the obtained PU films was observed by atomic force microscopy in a tapping-mode (AFM, SPM400, SII Nano Technology) as well as field emission-scanning electron microscopy (FE-SEM, S-4800, Hitachi High-Technologies). In nanoindentation experiments the stiffness (ΔF/ Δx) for each PU film was estimated from AFM force curve measurements. The composition of the films and their degree of polymerization were examined by a Fourier transform infrared spectrophotometer (FTIR, FTIR-8400, Shimadzu) after removing the IL. For cyclic voltammetry measurements, a 0.01 M ferrocene-contained [Emim][TFSA] IL was used as electrolyte; Pt and Ag wires were used as counter and reference electrodes, respectively. The PU films were prepared exactly at the same process conditions except that a Au substrate was used as a working electrode, and its exposed area to IL electrolyte was about 7 mm2. Water contact angles (WCAs) on these PU films were measured by a pendant drop method with a 1 μL volume of water. Figures 2(a) and (b) are AFM images to compare the surface morphologies of as-deposited PU films on [Omim][TFSA] IL layer/PVA-coated and PVA-coated Si substrates (if needed, the remaining IL was removed before the AFM measurement). The PU film obtained in the IL layer shows a network structure surface (Figure 2(a)), and it was still visible even after the postannealing treatment (Figure 2(c)). Such a network structure was never found in the PU films fabricated without IL, regardless of being postannealed (Figure 2(b) and (d)). The insets of Figure 2(c) and (d) are the corresponding SEM 1010

DOI: 10.1021/acsmacrolett.6b00594 ACS Macro Lett. 2016, 5, 1009−1013

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PVA/Si before and after the postannealing treatment, along with a neat PVA/Si substrate for a reference. The appearance of an absorption peak at around 2270 cm−1 in both as-deposited PU films, which is from the NCO group of MBCI, indicates that the polymerization reaction had not been completed yet. After the postannealing treatment, these N CO peaks completely disappeared, accompanied by an increase in the intensity of the adsorption peak at 1650 cm−1, which is from the urea CO group, suggestive of the further polymerization during the annealing treatment.13 Moreover, there was no additional peak found in the spectra of PU films on IL/PVA/Si before and after the postannealing treatment, indicative of their high purity without any byproducts with, for example, IL molecules. It should be pointed out that the peak of the urea CO group is ready to appear in the PU films on IL/PV/Si even before the postannealing treatment, consistent with their relatively smaller peak intensity of the remaining N CO group than that of the as-deposited PU films on PV/Si. This result indicates that IL has a substantial effect to accelerate the polymerization reaction at RT. In fact, as reported by Zhu et al.,17 the full polymerization in the synthesis of a bulk PU with IL could proceed even at RT, and thus, as was expected, the polymerization in the vapor deposition of PU films could proceed in IL at RT as well, though it was not completed. The porous network structures were also obtained with other ILs of [Bmim][TFSA] and [Emim][TFSA]. There is found a tendency that the pore size increases in the order of [Omim][TFSA] < [Bmim][TFSA] < [Emim][TFSA]. The viscosity of these ILs also decreases in this order,20−22 suggestive of the diffusion process being one of the dominant factors in the formation of the network structure (see Supporting Information Figure S3). A similar viscosity dependence of the pore size in the network structure is found for bulk polyurea synthesized with IL.17 Figure 4 is a set of cyclic voltammograms (CVs) for postannealed PU films obtained with and without [Omim]-

Figure 2. AFM images of the surfaces of PU deposited into [Omim][TFSA] IL/PVA/Si (a and c) and PU deposited onto PVA/Si (b and d). (a) and (b) are as-deposited samples, while (c) and (d) are annealed samples.

images, by comparison of which a similar structural difference was also found even on a nanometer scale. The present network structure and porous morphology may be brought about by a similar effect of IL: Zhu et al. found that a PU bulk synthesized via interfacial polymerization with IL also shows a macroporous morphology of aggregated rod-like particles, and thus the present porous polyurea films are also of physical gel.17 From a cross-sectional SEM image (see Supporting Information Figure S1), the porous film thickness was determined to be about 530nm, thicker than the nominal thickness of 360 nm due to the porosification, giving a volume fraction of voids to be 0.32. The porous PU film obtained after annealing is still as rigid as the plain PU films obtained without IL, as indicated by their comparable stiffness values on average independent of whether films have a network structure or not (see Supporting Information Figure S2). Next, we examined the degree of polymerization reaction by FTIR. Figure 3 shows a comparison of FTIR spectra between PU films on PVA/Si and PU films on [Omim][TFSA] IL/

Figure 4. Set of cyclic voltammograms for postannealed PU films obtained (b) without and (c) with [Omim][TFSA] IL on a Au electrode substrate with different thicknesses of 20, 100, 200, and 360 nm and for a bare Au electrode as a reference (a).

[TFSA] IL on a Au electrode substrate with different thicknesses of 20, 100, 200, and 360 nm, along with that for a bare Au electrode as a reference. As shown in Figure 4(b), almost no redox currents are observed in the ferrocenecontained IL for the plain PU films obtained without IL irrespective of their film thickness, as compared with that for the Au electrode (Figure 4(a)). It suggests that the plain PU films are homogeneous enough to completely cover the Au electrode surface, resulting in such a high insulating behavior in

Figure 3. Comparison of FTIR spectra between PU films on PVA/Si and PU films on [Omim][TFSA] IL/PVA/Si before and after the postannealing treatment after removing the IL, along with a neat PVA/ Si substrate for a reference. 1011

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strong water repellency when compared to those for the plain PU films (smaller than 100°). The WCA behavior on the porous polyurea surfaces could be basically understood by the Cassie−Baxter model.23 Since the polyurea surface is essentially hydrophobic and if the pore size will not change so largely for the same IL used, the area ratio of polyurea solid surface becomes smaller when the RMS value increases, giving a positive correlation between the contact angle and RMS roughness, as shown in Figure 5(b). On the other hand, as discussed in Figure S3, the ILs with different viscosity will have different pore sizes, where the average WCA increases in the order of ILs used, [Emim][TFSA] < [Bmim][TFSA] < [Omim][TFSA], irrespective of the postannealing treatment (Table S1). In addition, the WCAs of porous polyurea films for the [Emim][TFSA] are likely to become smaller even when their RMS roughness values are relatively large (Figure S4). This result cannot be understood simply by the present model and is probably because the polyurea surface is weakly hydrophobic, and some pores might be partially filled with water at the water−porous polyurea interface when the pore size becomes larger. In summary, a novel vapor deposition polymerization in IL was demonstrated in the fabrication of PU films as a new route to direct synthesis of polymer films. IL was found to work as a solvent and play similar roles in the vapor deposition process as in the bulk solution processes, yielding network-structured, porous PU films with a high degree of polymerization. The percolated pore, rigid network, and strong water-repellency nature of the PU films may open new applications as a filter in separation processes, as well as a water-repellent coating material.

CV. In contrast, some redox currents are found for the thinner porous PU films obtained with IL, though no redox current for the 360 nm thick porous PU film (Figure 4(c)). This result indicates that the PU films have a percolated pore network so that the redox current can pass through the PU film. However, for porous PU films, at least, thicker than 360 nm the percolation probability seems to become too small to give the redox current path. Finally, we measured the water contact angles (WCAs) and root-mean square (RMS) surface roughness values for the PU films and examined the effect the pore network structure has on the water repellency of the porous PU films. In order to rule out any other possible effects of the postannealing and washing by 2-propanol, including their process sequence, the PU film samples are classified into 4 types, as labeled with “Process 1”, “Process 2”, “Process 3”, and “Process 4”, depending on the process treatment (Figure 5(a)). Figure 5(b) shows the plots of

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.6b00594. Scheme of reaction of DAF and MBCI into PU, Figures S1−S4, and Table S1 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +81-22-985-7266. E-mail: [email protected]. tohoku.ac.jp. Figure 5. (a) Flowchart showing the different sequence of the process treatment. (b) The plots of the WCAs against the RMS values for various PU films prepared with and without [Omim][TFSA] IL.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by the Incorporated Administrative Agency New Energy and Industrial Technology Development Organization (NEDO) under Ministry of Economy, Trade, and Industry (METI).

the WCAs against the RMS values for various PU films prepared with and without [Omim][TFSA] IL. As expected, the PU films obtained without IL are atomically smooth with RMS values less than 3 nm, but the WCA values are scattered even between the PU films classified in the same type, resulting in little correlation with the RMS values. On the other hand, the PU films obtained with IL are relatively rough with RMS values widely ranging from 10 to 100 nm because of their porous network structure. The WCA seems to have a positive correlation with the RMS roughness irrespective of the process treatment, though some of the data points are still scattered. As a result, a significant difference in the WCAs was found between the plain and porous PU films: the values of the WCA for most of the porous PU films exceed 100°, exhibiting a

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