The dielectric properties of low temperature thermally cross-linked polystyrene and poly(methyl methacrylate) thin films

Abstract

Two azide functionalized polymers were synthesized by free radical copolymerization. Each new polymer was effectively cross-linked with a small molecule cross-linker by a thermally activated reaction at 100 °C. This cross-linking method was compatible with plastic substrates for flexible electronic applications. The new method did not use any catalyst and initiator, and did not produce any by-product which could stay in the dielectric layer. The cross-linked thin-films showed good solvent resistance and smooth surfaces. The dielectric characteristics of the cross-linked polymer films were evaluated via quantitative capacitance and leakage current measurements in a metal–insulator–metal (MIM) test structure. The cross-linked thin-films showed significantly reduced leakage current density compared with uncross-linked thin-films in MIM testing devices.

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Introduction

Organic field-effect transistors (OFETs) based on organic semiconductors and plastic substrates have attracted considerable attention.^1–7 OFETs have many potential applications such as flexible electronics and wearable sensors.^8–10 Both the intrinsic properties of the organic semiconductors and the dielectric properties of gate materials are considered important for the performances of OFET devices.^11–15 Polymers are one type of promising material for gate dielectrics because of their good film-forming property, interfacial compatibility with organic semiconductors, and solution processability.^16–20 The stabilities of polymer dielectric layers are important in the multi-layer solution fabrication of OFET devices because the sequential deposition of two organic layers through solution processes could cause dissolution or swelling of the under-layer. Photo and thermal cross-linking methods have been studied for improving the stabilities of the dielectric layers.^21–29 However, common thermal cross-linking reactions required high curing temperature (above 150 °C).³⁰ Such conditions are incompatible with plastic substrates for flexible electronic applications. Many common polymers such as polystyrene (PS) and poly(methyl methacrylate) (PMMA) thin-films usually show low breakdown voltages due to large free volume. Cross-link not only can enhance solvent resistance, but also can improve the dielectric performances of polymers.^31,32 The azide and alkyne functional groups are kinetically stable at room temperature. However, they can react effectively in solid-state upon heating. Effective cross-linking in the thin-films has been achieved at relatively low temperature (100 °C) by thermal azide–alkyne cycloaddition (TAAC) reaction.^33–36

In this work, we introduce new cross-linkable PS, PMMA, and a small molecule cross-linker, 1,3,5-tris(2-propynyloxy)benzene (TYB). The new systems can be cross-linked at 100 °C and give smooth thin-films (Fig. 1). The new method provided easy control on the degree of cross-linking in thin-films. The cross-linked polystyrene (C-PS) and poly(methyl methacrylate) (C-PMMA) thin-films showed good solvent resistant and displayed significantly improved dielectric properties compared with uncross-linked polymers.

Fig. 1 The cross-linking polymer films fabrication process and the schematic of the metal–insulator–metal (MIM) test structure.

Results and discussion

The azide functionalized polymers synthesis and characterization

The bromide functionalized acrylic ester monomer MMA-Br was synthesized with high yield via the condensation of hydroxyethyl methacrylate and 3-bromopropionyl chloride in pyridine. The structure of MMA-Br was confirmed by the ¹H NMR and ¹³C NMR (Fig. S1†). The polymer PMMA-Br was prepared by free radical copolymerization of methyl methacrylate and MMA-Br at 7 : 3 molar ratios. AIBN was used as initiator for the polymerizations. The azide functionalized PMMA-N₃ was obtained by the reaction of PMMA-Br and sodium azide in dimethylformamide (Scheme 1). The random copolymer displayed good solubility in common organic solvents such as tetrahydrofuran, chloroform and chlorobenzene. The structures of polymers were confirmed by the ¹H NMR (Fig. S2†) and FT-IR spectra (Fig. S3†). About 35 percent of repeating units in the PMMA-Br were functionalized with bromide moieties and about 23 percent of repeating units in PMMA-N₃ were functionalized with azide moieties based on the analysis of ¹H NMR spectra. The molecular weights of polymers were determined by GPC. The number-average molecular weight (M_n) was 23.3 kDa and the polydispersity index (PDI) was 2.6 for PMMA-Br. The M_n was 24.1 kDa and the PDI was 2.7 for PMMA-N₃.

Scheme 1 Syntheses of the azide functionalized polymer PS-N₃ and PMMA-N₃.

The thermal properties of polymer blends and cross-linking reactions in thin-films

Differential scanning calorimeter (DSC) was used to study the reaction kinetic of the PS-N₃ and the blend of PS-N₃/TYB (PS-N₃ and TYB at 7 : 3 weight ratios) in solid states (Fig. 2a). PS-N₃ alone did not show any change until temperature reach 200 °C. The large exothermic process between 200 to 280 °C was ascribed to the thermal decomposition of the azide groups in the PS-N₃.³⁷ The blend of PS-N₃/TYB showed a distinct exothermic process from ∼80 to 180 °C with a peak temperature (T_max) at 140 °C. This transition corresponded to the thermal azide–alkyne cycloaddition (TAAC) reaction between the azides and the alkynes in the blend. There was also an exothermic process between 190 to 280 °C for the blend of PS-N₃/TYB. It corresponded to thermal decomposition of the residual azide moiety on the PS-N₃ in the blend. The reactive species in a solid state cross-linking reaction usually are not completely consumed due to the self-limiting mechanism resulted from restriction on chain movement in solid state.³³ Similarly, the PMMA-N₃ and the blend of PMMA-N₃/TYB (PMMA-N₃ and TYB at 7 : 3 weight ratios) were also studied by DSC (Fig. 3b). There was an exothermic process between 150 to 280 °C for the PMMA-N₃ alone. It corresponded to thermal decomposition of the azide moieties in the PMMA-N₃. The PMMA-N₃/TYB showed a distinct exothermic process from ∼80 to 200 °C with a peak temperature (T_max) at 150 °C. This transition corresponded to the thermal azide–alkyne cycloaddition (TAAC) reaction between the azides and the alkynes in the blend. There was also an exothermic process between 200 to 260 °C for the blend of PMMA-N₃/TYB. It corresponded to thermal decomposition of the residual azide moieties on the PMMA-N₃ in the blend. It is worthwhile to note that cross-linking is started at temperature blow 100 °C in these solid state reactions.

Fig. 2 The DSC curves of the (a) PS-N₃/TYB and (b) PMMA-N₃/TYB.

Fig. 3 The TGA curves of the (a) PS-N₃/TYB and (b) PMMA-N₃/TYB.

Thermo-gravimetric analysis (TGA) was used to study the thermal stability of the polymers and polymer blends (Fig. 3). The PS-N₃ and PS-N₃/TYB showed 5% mass loss at 250 °C and 310 °C, and 10% mass loss at 350 °C and 370 °C, respectively (Fig. 3a); The PMMA-N₃ and PMMA-N₃/TYB showed 5% mass loss at 215 °C and 255 °C, and 10% mass loss at 240 °C and 300 °C, respectively (Fig. 3b). The cross-linked polymer blends showed improved thermal stabilities compared with uncross-linked counterparts.

The thermal activated cross-linking reactions in the blend thin-film were characterized by FTIR spectrometer. The thin-films of the PS-N₃/TYB and PMMA-N₃/TYB (polymer and TYB at 7 : 3 weight ratios) were spin-coated on bare silicon wafers from dichloromethane. The thin-films were heated at 100 °C (Fig. 4). The attenuations of azide (2100 cm⁻¹) and the alkyne (3290 cm⁻¹) peaks were observed with increasing heating time due to the consumption of the reactive species by TAAC reaction. Based on the results of FTIR spectroscopy study, we chose heating at 100 °C for 0.5 h as the typical condition for cross-linking the polymer dielectric layers in device study. This temperature was much lower than the glass transition temperatures of the most plastic substrates for flexible electronic applications.

Fig. 4 FT-IR spectra of (a) the PS-N₃/TYB and (b) the PMMA-N₃/TYB thin films spin-coated on a silicon wafer before and after heating at 100 °C for 0.5 h.

Surface morphology and solvent resistance

The morphologies of cross-linked polymer films were evaluated by atomic force microscopy (AFM) (Fig. 5a and b). The images of the C-PS film and C-PMMA film both exhibited smooth surface without major defects and pinholes. The root mean square (RMS) roughness of the C-PS film and C-PMMA film were 0.505 nm and 0.661 nm, respectively. A smooth surface with few pinholes is essential for uniform dielectric thickness and high insulating performances. The thicknesses of C-PMMA and C-PS films were characterized by profiler before and after dipping in chlorobenzene, a common solvent used for fabrication of organic semiconducting materials. The thicknesses of films showed no change even for the solvent soaked samples (2 min in chlorobenzene) (Fig. 5c and d). The cross-linked films strongly adhered to substrates and did not undergo de-lamination/cracking upon solvent treatment owing to their cross-linked microstructure. The thermal cross-linked thin-films showed good solvent resistant and smooth surfaces. These are desirable material properties of polymer dielectric for flexible organic electronic applications.

Fig. 5 The AFM images (5 μm × 5 μm scan area) of the (a) C-PS and (b) C-PMMA; the films thicknesses of (c) C-PS and (d) C-PMMA were characterized by a profiler before and after dipped in chlorobenzene for 2 min.

Dielectric characteristics

The dielectric characteristics of the polymer thin-films were evaluated with metal–insulator–metal (MIM) devices. The dielectric constant and phase angle as a function of frequency are showed in Fig. 6a and b. The results are summarized in Table 1. There are only small changes on both dielectric constant and phase angle with the increasing of the frequency up to 10⁶ Hz. Both C-PS and C-PMMA based devices showed standard capacitance behaviors which were the prerequisite of gate dielectric materials for OFET applications. The plots of leakage current density as a function of bias voltage are shown in Fig. 6c and d. The uncross-linked PS thin-film (300 nm) showed poor insulating property with high leakage current density of 10⁻⁴ A cm⁻² at a bias voltage of 20 V. The C-PS thin-film (260 nm) showed low leakage current density with a value of 10⁻⁶ A cm⁻² at the same voltage, (Fig. 6c). The uncross-linked PMMA thin-film (200 nm) showed high leakage current density with a value of 10⁻⁴ A cm⁻² at a bias voltage of 20 V. The C-PMMA (180 nm) showed low leakage current density with a value of 10⁻⁶ A cm⁻² at the same voltage (Fig. 6d). The plots of leakage current density vs. electric field for the C-PS and C-PMMA thin-film are showed in Fig. S4 and S5.† These results demonstrate that cross-linking by TAAC reaction can significantly improve dielectric performances of polymers.

Fig. 6 The dielectric constant and phase angle as a function of the frequency, (a) for C-PS and (b) for C-PMMA thin-films. The leakage current density as a function of the bias voltage, (c) for C-PS and (d) for C-PMMA thin-films.

Table 1 Summary of thin-film and dielectric properties of cross-liked polymers

Dielectrics	Film thickness (nm)	Capacitance (nF cm^−2)	Dielectric constant (at 10^3 Hz)
C-PS	260	10	2.9
C-PMMA	180	20	4.0

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Experimental section

Chemicals materials

Polystyrene (PS, M_w = 280 000 g mol⁻¹), polymethylmethacrylate (PMMA, M_w = 120 000 g mol⁻¹) and other chemicals were purchased from Aldrich. Azobisisobutyronitrile (AIBN) was recrystallized in ethanol before use. Toluene was freshly distilled over sodium wire under a nitrogen atmosphere prior to use. Flash chromatography was carried out on silica gel (200–300 mesh). Azide-containing styrenic polymer (PS-N₃) was synthesized according to our previous work.³⁸ 1,3,5-Tris(2-propynyloxy)benzene (TYB) was synthesized according to literature procedures.³⁹

Characterization

Nuclear magnetic resonance (NMR) spectra were recorded on a Mercury plus 400 MHz instrument. The FT-IR spectra of the spin-coated polymer films on single-side polished silicon wafer were recorded on a PerkinElmer Spectrum 100 Fourier transform infrared spectrometer in a transmission mode. Thermogravimetric analyses (TGA) were carried out on a TA instrument Q5000TGA at a heating rate of 20 °C min⁻¹ under nitrogen gas flow. Differential scanning calorimeter (DSC) studies were carried out on a TA instrument Q2000DSC under nitrogen flow. Sample (about 5.0 mg in weight) was heated up to 300 °C at rate of 10 °C min⁻¹. Molecular weights and molecular weight distributions (M_w/M_n) of polymer were determined on a gel permeation chromatograph (GPC, Tosoh Corporation) with DMF as an eluent and polystyrenes as standards. APCI-FTMS spectra were recorded on a SolariX-70FT-MS.

Device fabrication and characterization

The devices with metal–insulator–metal (MIM) structure (Ag/polymer film/Al) were fabricated for the characterization of the dielectric properties of the thin-films. The thermally evaporated Al electrodes were patterned as the bottom metal on the glass substrate through a shadow mask. Then the solutions of polymers (40 mg) and cross-linker TYB (12 mg) in 1 mL chlorobenzene was spin-coated on polished silicon wafer. The thin-films were annealed at 100 °C for 0.5 h in a glove box. Finally, Ag electrodes were also evaporated as the top metal. The area of the capacitor was 1.2 mm × 1.2 mm. The specific dielectric constant and phase angle of the MIM devices were measured using a WK6515B precision impedance analyzer.

Synthetic procedures

MMA-Br. 3-Bromopropionic acid (0.99 g, 6.5 mmol) was dissolved in dichloromethane (10 mL) under nitrogen atmosphere. The solution was then cooled in an ice bath and thionyl chloride (15 mL, 200 mmol) was added dropwise. The solution was stirred under nitrogen overnight at room temperature. The solvent was evaporated to provide 3-bromopropionyl chloride.

Hydroxyethyl methacrylate (1.30 g, 10 mmol) and pyridine (0.8 mL, 10 mmol) were dissolved in dichloromethane (15 mL) under nitrogen atmosphere. The solution was then cooled in an ice bath and 3-bromopropionyl chloride (0.86 g, 5 mmol) obtained in the previous step was added dropwise. A white solid precipitated from the solution during the addition. After the addition of 3-bromopropionyl chloride, the mixture was slowly warmed to room temperature and stirred under nitrogen for 8 h. The suspension was vacuum filtered. The filtrate was washed with deionized water for three times and was dried with anhydrous sodium sulfate. The solvent was evaporated and the crude product was purified by flash chromatography using hexane and ethyl acetate (6 : 1, v/v) as eluent to give MMA-Br 1.21 g (70%) as a light yellow oil liquid. ¹H NMR (δ, CDCl₃): 6.10 (m, 1H), 5.56 (m, 1H), 4.34 (m, 4H), 3.54 (t, 2H), 2.92 (t, 2H), 1.91 (q, 3H); ¹³C NMR (δ, CDCl₃): 170.41, 167.21, 135.97, 126.27, 62.69, 63.32, 37.72, 25.86, 18.41; APCI-FTMS calcd for C₉H₁₃O₄Br, (M + H)⁺ m/z 265.00755, found 265.00728.

PMMA-Br. Methyl methacrylate (0.70 g, 7.0 mmol), MMA-Br (0.80 g, 3.0 mmol), AIBN (0.035 g, 0.20 mmol), and toluene (25 mL) were combined in a Schlenk flask (100 mL) equipped with a stir bar and argon was bubbled through the solution for 30 min. The solution was subsequently heated to 70 °C for 14 h. The reaction was cooled to room temperature and was precipitated into anhydrous methanol (250 mL). The mixture was filtered and the product was dried in vacuo to afford the PMMA-Br as a white powder (1.2 g, 80% conversion, M_n = 23 309 g mol⁻¹, PDI = 2.6). ¹H NMR (δ, CDCl₃): 4.4–4.3 (m, 2H), 4.3–4.1 (m, 2H), 3.8–3.5 (m, 7.5H), 3.1–2.9 (m, 2H), 2.1–0.7 ppm (m, 19H); FTIR 2998, 2944, 1727, 1588, 1440, 1270, 1154, 960, 604 cm⁻¹.

PMMA-N₃. PMMA-Br (0.60 g), sodium azide (1.0 g, 14.0 mmol), and dimethylformamide (10 mL) were combined in a round bottom flask (50 mL) equipped with a stir bar under nitrogen atmosphere. The solution was heated at 60 °C for 48 h. The reaction was cooled to room temperature, and deionized water (10 mL) and dichloromethane (20 mL) were added. The organic layer was separated and was washed with deionized water for three times, was dried with anhydrous sodium sulfate and was precipitated into anhydrous methanol (250 mL). The precipitate was collected by filtration and was re-dissolved in acetone (10 mL). The solution was transferred into a dialysis tube (M_w cutoff 3500), which had been washed thoroughly with deionized water prior to use. The tubing was then closed and was placed in a beaker containing acetone (500 mL). After 48 h, the dialyzed solution was concentrated and was dried in vacuo to afford the product the PMMA-N₃ as a white powder (0.76 g, 48% conversion, M_n = 24 095 g mol⁻¹, PDI = 2.7). ¹H NMR (δ, CDCl₃): 4.5–4.3 (m, 2H), 4.3–4.0 (m, 2H), 3.8–3.5 (m, 8H), 2.8–2.5 (m, 1.2H), 2.1–0.7 (m, 22H); FTIR 2998, 2944, 2091, 1735, 1456, 1247, 1139, 999, 642 cm⁻¹.

Conclusions

In summary, we have demonstrated that polystyrene and poly(methyl methacrylate) can be effectively thermally cross-linked at relatively low temperature. This new cross-linking method is compatible with plastic substrates for flexible electronic applications. The cross-linked thin-films show good solvent resistant and smooth surfaces. The cross-linked thin-films showed significantly reduced leakage current density compared with uncross-linked thin-films in MIM testing devices. The current cross-linking method will have wide applications in fabrication of OFET devices on flexible substrates.

Acknowledgements

This work was supported by National Natural Science Foundation of China (NSFC Grant no. 21174084 and 21274087), and was supported by the Doctoral Fund of Ministry of Education of China (Grant no. 20120073110032).

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Footnote

† Electronic supplementary information (ESI) available: NMR data the monomer and polymers; FT-IR data of the polymers and cross-linker. See DOI: 10.1039/c5ra00848d

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