Beads-on-String-Shaped Poly(azomethine) Applicable for Solution

Letter Cite This: ACS Macro Lett. 2018, 7, 641−645

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Beads-on-String-Shaped Poly(azomethine) Applicable for Solution Processing of Bilayer Devices Using a Same Solvent Shunichi Fujii,† Saori Minami,‡ Kenji Urayama,‡ Yu Suenaga,§ Hiroyoshi Naito,§ Orito Miyashita,† Hiroaki Imoto,† and Kensuke Naka*,† †

Faculty of Molecular Chemistry and Engineering and ‡Department of Macromolecular Science and Engineering, Kyoto Institute of Technology, Goshokaido-cho, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan § Department of Physics and Electronics, Graduate School of Engineering, Osaka Prefecture University, Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531, Japan S Supporting Information *

ABSTRACT: Solvent-based deposition techniques for fabrication of organic field-effect transistors (OFETs) generally require orthogonal solvents for deposition of a conjugated polymer layer on a polymer gate insulator layer. Here, we found significantly reduced dissolution rate of the polymeric film in the same solvent after casting a homegeneous polymerization solution of para-bis(3-aminopropyl)hexaisobutyl-substituted T8 cage (1) with terephthalaldehyde. The limited dissolution rate in the solvent provided enough chance for fabrication of a regioregular poly(3-hexylthiophene-2,5-diyl) (P3HT) layer on the present polymer films without using an orthogonal solvent. The rheological properties indicate that physical interaction between the polymer chains provides the significantly reduced dissolution rate after the deposition onto a substrate without any cross-linking treatments. olymeric thin films have been widely applied to energy and electronic devices.1 Among various ways for fabrication of the polymeric thin films, solution processes, such as spray, dip, or spin-coatings, have the advantage for rapid and low-cost processing, fabrication on a large size scale and can be used for all soluble materials. This advantage has a limitation for fabrication of bilayer devices such as organic field-effect transistors (OFETs) due to difficulties in the deposition of two organic layers serially through a solution process without dissolution or swelling of the underlayer.2 In particular, conjugated polymers or solution-processed small molecules are dissolved in similar solvents for typical polymer insulators. Solvent-based deposition techniques have been exploited so far only for the fabrication of bilayer devices consisting of two polymeric layers soluble in orthogonal solvents. Cross-linkable films are usual approach to provide solvent resistant polymeric thin films after thermo- or photo-cross-linking or chemical treatments.3 Catalyst-induced cross-linking approach also gives the insolubility of polymer films in a solvent.4 These methods also requires external chemical reagents and additional process after the deposition. Polymers incorporating cage silsesquioxanes, denoted as (RSiO1.5)n or labeled Tn cages, in the main chain provide beadson-string-shaped architectures, which are expected to possess significantly improved hydrophobicity as well as significantly reduced dynamics of the polymer segments.5 The cage silsesquioxane units hinder the interfacial dynamics of the

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polymer segments.6 We recently reported that para-bis(3aminopropyl)hexaisobutyl-substituted T8 cage (1) was successfully synthesized.7 Incorporation of the T8-unit in the main chains significantly improves hydrophobicity as well as increase their glass transition temperatures.8,9 Poly(azomethine)s require mild polymerization condition and require no additional reagents or heating.10 Their synthetic methodology consists in condensation polymerization of diamines with dialdehydes or diketones, without external catalysts, leading to high yields and purity products. Here, we show that the incorporation of the T8-unit in poly(azomethine) main chains provided soluble polymers just after the solution polymerization. On the other hand, significantly reduced dissolution rate of the polymeric films in the same polymerization solvent were observed after casting the polymer solution. The limited dissolution rate in the solvent would provide enough chance for fabrication of a conjugated polymer layer on the present poly(azomethine) films without using an orthogonal solvent. para-Bis(3-aminopropyl)hexaisobutyl-substituted T8 cage (1) was polymerized with terephthalaldehyde (2) at the feed ratio of 1.0 in toluene for 2 h at room temperature to produce poly(azomethine) (3; Scheme 1). The polymerization Received: April 16, 2018 Accepted: May 15, 2018

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DOI: 10.1021/acsmacrolett.8b00271 ACS Macro Lett. 2018, 7, 641−645

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ACS Macro Letters

1090 cm−1, suggesting that no decomposition of the T8 cage structure occurred. A free-standing film was obtained by casting the polymer solution on a Teflon sheet by pealing off. The film showed excellent transparency. The optical transmittance of the colorless film of the polymer was over 90% in the visible region between 780 and 400 nm with a film thickness of 0.1 mm (Figure S3). The XRD pattern of the polymer showed broad peaks centered at near a 2θ value of 20°, indicating an amorphous polymer (Figure S4). The poly(azomethine) film under N2 and air showed 5% weight loss at 311 and 290 °C, respectively (Figure S5). DSC analysis of the polymers showed no glass transition and melt behavior between room temperature and 200 °C (Figure S6). The film was statically immersed in toluene for 3 days at room temperature, wiped and dried. A first visual observation allows evaluating the effect of the solvents on the materials and found that no visual and weight changes were observed (Figure S7). Similarly, no visual and weight changes were observed when the films were statically immersed in chloroform and THF (Figure S8), although the polymerization of 1 and 2 in these solvents also proceeded in homogeneous solutions and gave polymeric products with similar molecular weights of the case in toluene. These observations suggest that the present poly(azomethine) acquired solvent resistant property just casting the polymerization solution on a substrate. The film (17.1 mg) was gradually dissolved in toluene (3 mL) by stirring at room temperature (Figure S9). After 13 days stirring, 99 wt % of the film was dissolved. GPC analysis of the dissolved solution showed similar Mn and Mw/Mn values of the initial polymerization solution. This observation suggests that physi-

Scheme 1. Polymerization of para-Bis(3aminopropyl)hexaisobutyl-Substituted T8 Cage (1) with Terephthalaldehyde (2)

proceeded in a homogeneous solution. GPC analysis (THF, PSt standards) of the polymerization solution showed that number-average molecular weight (Mn) and molecular weight distribution (Mw/Mn) are 3.2 × 103 and 4.45, respectively. According to the 1H NMR analysis, 1 contains 13 mol % of the monofunctionalized compound, 3-aminopropyl-heptaisobutylsubstituted T8 cage octasilsesquioxane, resulting in the relatively low molecular weight poly(azomethine) (see Supporting Information and Figure S1). FT-IR analysis of the polymer after the solvent was removed under the reduced pressure showed peaks characteristic of the imine structure at 1644 cm−1 in addition of 1705 cm−1 (CO stretching) corresponding to the aldehyde group (Figure S2). The CO stretching band of 2 is observed at 1685 cm−1, indicating that no 2 remained. The asymmetric stretching of the Si−O−Si of the cage framework at

Figure 1. (a) Concentration (c) dependence of zero shear relative viscosity (η0,r) and plateau shear modulus (G0). Critical gelation concentration (cg) is estimated as the lowest c where a nonzero value of G0 is observed. (b) The storage modulus (G′) and loss modulus (G″) as a function of angular frequency (ω) for the concentrated solutions. The imposed strain-amplitude is sufficiently small within the linear response regime. (c) G′ and G″ as a function of strain amplitude (γa) for the concentrated solution of c = 0.98 g mL−1. The second run was conducted immediately after the first run was finished. Inset image in (a): Inset images are appearances of the polymerization solution (c = 0.080 g mL−1) and the gel after concentration to one-tenth (c = 0.98 g mL−1) under reduced pressure. 642

DOI: 10.1021/acsmacrolett.8b00271 ACS Macro Lett. 2018, 7, 641−645

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ACS Macro Letters

Figure 2. Fabrication of bottom-gate top-contact organic field-effect transistors (OFETs) with the present poly(azomethine) as gate insulator on glass substrates and transfer characteristics of bottom-gate top-contact OFET using the poly(azomethine).

strain and stress (γc and σc, respectively) for the onset of flowing (yielding) were evaluated to be γc = 0.080 and σc = 21 Pa at c = 0.98 g mL−1, and γc = 0.028 and σc = 0.29 Pa at c = 0.66 g mL−1. The stress σc increases significantly with an increase in c. The reversible yielding feature supports that the network structure is formed by physical interaction between the polymer chains. This feature is also advantageous for the coating purposes: The materials can be coated as a fluid under finite stresses beyond σc, but they do not flow in the quiescent state. The beads-on-string shaped polymer composed of the bulky hydrophobic frameworks in the main chain may promote entanglement of the polymer chains, which provides significantly reduced dissolution rate after the deposition onto the substrate without any cross-linking treatments. The bottom-gate top-contact organic field-effect transistors (OFETs) with the present poly(azomethine) as a gate insulator were fabricated according the process showing in Figure 2. A toluene solution of the poly(azomethine) was spin-coated on a substrate and dried at 100 °C for 30 min in ambient air. A toluene solution of regioregular poly(3-hexylthiophene-2,5diyl) (P3HT) was spin-coated onto the polymer gate insulator. Finally, Au source-drain electrodes were deposited on the P3HT semiconductor layer. Figure 2 shows the typical p-type drain current (ID)−gate voltage (VG) (transfer) characteristics of the fabricated P3HT OFETs in the saturation regime (see Figure S10). No hysteresis is observed in the transfer characteristics (ID does not depend on the sweep direction of VG). Hysteresis is an unwanted feature and must be avoided in standard integrated circuit. No hysteresis observed in Figure 2 shows that the interface traps between P3HT and the polymer gate insulator are very low.11 The field-effect mobility (μ) was calculated from a transfer characteristic in the saturation regime using

cally cross-linking provides the reduced dissolution rate of the film. To elucidate the reduced dissolution rate of the polymer after the casting, rheological measurements were performed for concentrated polymer solutions. The polymerization was conducted in xylene instead of toluene, due to avoid evaporation of the solvent during the measurements. When the polymerization solution was concentrated sufficiently under reduced pressure, gel formation was observed at the high concentrations (c) beyond cg ≈ 0.66 g mL−1 (Figure 1a). In the range of c < cg, zero shear relative viscosity (η0,r) of the polymer solutions increases with an increase in c, and tends to diverge at cg, which is a typical sol−gel transition behavior (Figure 1a). When c exceeds cg, the systems evidently exhibit solid-like behavior: Over the entire range of angular frequency (ω), the storage modulus (G′) is independent of ω, and G′ is more than 1 order of magnitude higher than loss modulus (G″; Figure 1b). The plateau value of G′ (G0) steeply increases with an increase in c: G0 = 10 and 2300 Pa for c = 0.66 and 0.98 g mL−1, respectively. The effects of the strain amplitude (γa) on the dynamic viscoelasticity for the gel with c = 0.98 g mL−1 are investigated, in order to characterize the mechanical strength (Figure 1c). In this experiment, the γa scan started from the lowest value (0.001). In the sufficiently small γa regime, the linear elastic response, that is, a relation of G′ ≫ G″ and the independency of G′ and G″ on γa, is observed. When γa exceeds a critical value (γc) of 0.080, a steep drop of G′ occurs. In the high γa regime of γa > 0.3, G′ becomes less than G″. This result indicates that the gel flows at sufficiently high strain. The recovery of the network structure after the destruction was examined by the second run of the γa scan, which was performed immediately after the measurement at highest γa. The agreement of the data in the first and second runs confirms the reformation of the original network structure after the destruction. The critical values of 643

DOI: 10.1021/acsmacrolett.8b00271 ACS Macro Lett. 2018, 7, 641−645

ACS Macro Letters ⎛ d ID μ = ⎜⎜ ⎝ dVG

⎞2 2L ⎟⎟ · ⎠ WC i

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*E-mail: [email protected]. ORCID

Kenji Urayama: 0000-0002-2823-6344 Kensuke Naka: 0000-0002-4516-6296 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study is a part of a Grant-in-Aid for Scientific Research on Innovative Areas “New Polymeric Materials Based on ElementBlocks (No. 2401)” (24102003) of The Ministry of Education, Culture, Sports, Science, and Technology, Japan.

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REFERENCES

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where q is the elementary charge, S is the subthreshold swing (=dVG/d log(ID)), k is the Boltzmann constant, and T is temperature.13 The interface trap density determined from the transfer characteristic in Figure 2 was 9.3 × 1010 cm−2. The value of the interface trap density is much lower than that of OFETs with conventional gate insulators in bottom-gate configuration (2.0 × 1011 ∼ 1.5 × 1012 cm−2),16 and is consistent with no hysteresis in the transfer characteristics (Figure 2). Exhibiting such high OFET performance clearly indicates successful fabrication of the bilayer polymer films without using an orthogonal solvent. In summary, we prepared the poly(azomethine) containing the isobutyl-substituted T8 cages in the main chain. Although the present poly(azomethine) provided soluble polymer just after the solution polymerization, significantly reduced dissolution rate of the polymeric thin film was exhibited after the deposition on a substrate. The present new concept for fabricating solvent resistant polymeric films by solution processes requires no selection of orthogonal solvents or additional process after the deposition. P3HT OFETs with the poly(azomethine) gate insulator exhibit high OFET performance and thus the present polymer is a promising material for organic printed electronics. Detail mechanism of the gel formation, and optimization of OFET performance are now underway.

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AUTHOR INFORMATION

Corresponding Author

where L is the channel length, W is the channel width, Ci is the capacitance of the gate insulator per unit area.12 The hole fieldeffect mobility of the fabricated OFET calculated from the plot of the square root of ID in the saturation regime versus VG in Figure 2 using eq 1 was 4.3 × 10−3 cm2 V−1 s−1 (the linear relation between the square root of ID and VG in Figure 2 gives VG, independent mobility). It was reported that the hole fieldeffect mobilities of P3HT fabricated on gate insulators increase with increasing water contact angles, due to the increase in the crystallinity of P3HT.13,14 The water contact angle of the present polymer gate insulator surface was 97°. Hydrophobic character of the present poly(azomethine) also provides advantage for increasing the field-effect mobility. The fieldeffect mobility of P3HT obtained from Figure 2 is relatively lower than that in a literature.15 Optimization of the coating process of P3HT is now underway. The surface trap density (Ntrap) was determined by ⎤C ⎡ qS log(e) Ntrap = ⎢ − 1⎥ i ⎦q ⎣ kT

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.8b00271. Figures showing experimental details, characterization data for the model reaction, FT-IR, UV−vis spectrum, XRD data, TGA and DSC traces, solvent-resistant tests for the poly(azomethine), and output characteristics of the OFETs (PDF). 644

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